Natural Killer Cells || Developmental stages and pathways of NK cell maturation

Bartosz Grzywacz, Jeffery S. Miller, Michael R. Verneris

1Developmental stages and pathways of NK cell maturation

Chapter One

chApter contents

The early events in hematopoiesis . . . . . . . . . . . . . . . 3

NK cells as a distinct cell type . . . . . . . . . . . . . . . . . . 4

Lineage specific growth factors . . . . . . . . . . . . . . . . . 4

Sites of NK development: the importance of the developmental environment . . . . . . . . . . . . . . . . . . . . 5

Fate determining interactions with stroma . . . . . . . . 6

Transcription factors involved in NK cell differentiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Second messenger signalling in NK cell development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

The NK cell ontogeny—lessons from evolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Lessons from embryogenesis . . . . . . . . . . . . . . . . . . 12

Lessons from NK cell immune reconstitution after hematopoietic cell transplantation . . . . . . . . . 12

Stages of NK cell development . . . . . . . . . . . . . . . . 13

Acquisition of inhibitory receptors during NK cell development . . . . . . . . . . . . . . . . . . . . . . . . . 14

Linear and branching models of human NK cell development . . . . . . . . . . . . . . . . . . . . . . . . . 15

Boundaries of NK cell lineage . . . . . . . . . . . . . . . . . . 16

Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Yes, there are two paths you can go by, but in the long run, there is still time to change the road you are on….Robert Plant of Led Zeppelin (1970)

AbstrAct

Hematopoietic stem cells (HSCs) by definition can differentiate into all types of blood cells. There are several factors and events that promote HSC differentiation into the natural killer (NK) cell lineage. These include soluble factors, with a prominent role for interleukin 15, as well as contact- or gradient-dependent signals, such as Gas6/Tyro family of ligands and factors activating Wnt pathway. A complete understanding of the factors that control NK cell differentiation may allow for manipulation of NK cell reconstitution following hematopoietic cell transplantation and efficient ex vivo generation of NK cells for adoptive immune therapy.

The early events in hematopoiesis

Hematopoiesis sustains the production of blood cells throughout life and is the best understood model of stem cell differentiation. By way of asymmetric cell division, hematopoietic stem cells (HSCs) give rise to two daughter cells. One of these daughter cells main-tains HSC characteristics, while the other goes on to generate progeny that ultimately develop into mature blood cells (Orkin and Zon, 2008). During stem cell differentiation two parallel processes are imposed upon the cell: (1) the gradual acquisition of features of a given cell lineage, and (2) the loss of the ability to give rise to

Key words

NK cells, Development, Hematopoiesis, Transcription factors, Differentiation, Lineage CHOICE, Cytokines and receptors, Morphogens, Innate immunity

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other cell lineages. Differentiation entails the selective expression of proteins that determine the identity of a particular lineage. This is accompanied by the repres-sion of genes that direct differentiation towards other lineages.

Hematopoiesis has been schematically depicted as a series of binary choices faced by a multipotent HSC. The most widely accepted model, according to Weissman and colleagues, describes the initial develop-mental choice between the myelo-erythroid vs. lym-phoid fates. Accordingly, common lymphoid precursors (CLPs) have no myeloid potential (Kondo et al., 1997), and common myeloid progenitors (CMPs) lack lym-phoid generating capacity (Akashi et al., 2000). This model is based on the prospective isolation of CLPs that give rise to lymphocytes but not myeloid cells upon transplantation. Similarly, CMPs can replenish the mye-loid and erythrocyte compartments but do not generate lymphocytes after adoptive transfer. The simplicity of this model has been called into question by the dem-onstration of progenitors that lack erythroid potential but retain myeloid and lymphoid capacity (Adolfsson et al., 2005; Katsura, 2002). Thus, there is still some ambiguity regarding the order of choices during early hematopoiesis.

NK cells as a distinct cell type

The observations of natural killing (cytotoxicity with-out prior antigen priming) by a population of non-T lymphocytes and non-B lymphocytes have led to the identification of the natural killer (NK) cell lineage (Herberman et al., 1975; Kiessling et al., 1975). The advent of monoclonal antibodies and flow cytometry allowed for a phenotypic definition of NK cells (i.e. CD56CD3) (Lanier et al., 1986). Although a lym-phoid vs. myeloid origin of NK cell development had been debated early in their discovery based on early application of monoclonal antibody typing (Li et al., 1994; Ortaldo and Herberman, 1984), the demonstra-tion of a common T/NK precursor in human (Sanchez et al., 1994) and murine thymic tissue (Carlyle et al., 1997) placed NK cells developmentally close to T cells. The lymphoid origin of NK cells was more formally demonstrated by Akashi who isolated murine CLPs and demonstrated their ability to give rise to NK cells (in addition to T cells and B cells) upon transplantation into congenic animals (Kondo et al., 1997). Thus, NK cells can be derived from lymphoid progenitors.

Unlike the other progeny of CLPs (i.e. T cells and B cells), NK cells do not mediate conventional adaptive immunity. This is due to the lack of rearranged antigen-specific receptors, such as the T cell and B cell receptors that are generated following somatic recombination.

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Instead, NK cells express diverse sets of germline-encoded receptors responsible for antigen recognition (Lanier, 2005). This strategy (expression of multiple nonrearranged receptors) is commonly employed by the innate immune system. Recently however, NK cells have been shown to be closer to the adaptive immune system than previously appreciated. In this regard, NK cells can mediate recall or secondary immune responses, including contact dependent hypersensitivity to sec-ondary challenge by chemical irritants (O’Leary et al., 2006). Recall responses by murine NK cells express-ing Ly49H have also been observed after infection with mCMV (Sun et al., 2009). In human NK cells studies, expansion of NK cell clones expressing inhibi-tory receptors specific for ligands missing in the host have also been observed following HSC transplantation (Ruggeri et al., 1999, 2002). Despite this evidence for the expansion of reactive NK cell clones mediating sec-ondary immune responses, NK cells lack the fine and exclusive specificity for the challenging antigen that is conferred by the B cell and T cell receptors. Likewise, questions still remain as to how long NK cell ‘memory’ persists.

While NK cells do not fully conform to the defini-tion of adaptive immunity, they also differ from mem-bers of the innate immune system. For instance, NK cells do not mediate phagocytosis and lack bactericidal enzymatic systems. Rather, they express intracellular proteins associated with effector functions also used by cytotoxic T cells, including granzymes and per-forin. As well, NK cells rapidly release a wide array of cytokines upon activation, including IFN- and TNF-, which serve to shape adaptive immune responses. Con-sequently, owing to their CLP derivation, NK cells are developmentally close to the adaptive immune system, while functionally they retain features more in line with the innate immune system, perhaps suggesting a more ancient origin compared to T cells and B cells.

Lineage specific growth factors

A significant breakthrough in the understanding of hematopoiesis came with the demonstration of lineage-specific growth factors (reviewed by Kaushansky, 2006). Examples of such factors include G-CSF, which pro-motes the granulocytic lineage; M-CSF, which leads to monocytic development, or erythropoietin resulting in erythrocyte generation. Acquisition of a particular recep-tor renders precursors responsive to a particular lineage-specific growth factor, thereby marking an important event in lineage determination. Thus, precursors of distinct lineages can be identified by the presence of specific growth factor receptors. For example, the eryth-ropoietin receptor marks erythroid precursors.

Developmental stages and pathways of NK cell maturation C h A P T e R 1

In the case of lymphoid progenitors, CD127 (IL-7R) has been used to define CLPs (Kondo et al., 1997), and IL-7 is necessary for murine T cell and B cell develop-ment (Akashi et al., 1999; Peschon et al., 1994). This supports the notion that IL-7 is a growth factor specific for CLPs. However, murine NK cells develop normally in the absence of IL-7 (He and Malek, 1996). Several cases of human severe combined immune deficiency (SCID), caused by a mutation in the IL-7R (CD127), have also been reported. These patients lack T cells, however, NK cells are present and functional (Giliani et al., 2005; Puel et al., 1998). Thus IL-7, a growth factor specific for the development of CLPs into T cells, is not required for NK cell development. In contrast, deficiency of the cytokine receptor common -chain (CD132) in both mice (Cao et al., 1995) and humans (Buckley et al., 1997) results in the lack of T cells, B cells, and NK cells. These observa-tions led to the conclusion that some cytokines that sig-nal through the common -chain (including IL-2, -4, -7, -9, -15 and -21) are required for NK cell development.

Early studies of human NK cell differentiation from hematopoietic precursors used IL-2 (Miller et al., 1992). Paradoxically, this cytokine is not abundant in the bone marrow (BM) microenvironment. This suggested that another growth factor present in the BM milieu is responsible for NK progenitor development and expan-sion. IL-15 is expressed by BM stroma and was a pos-sible candidate (Mrozek et al., 1996; Puzanov et al., 1996). The receptor for IL-15 shares and subunits with IL-2 receptor, explaining the redundancy between IL-2 and IL-15 in vitro. Murine studies identified IL-15 as an NK-specific growth factor since IL-15/ mice show a near absence of NK cells (Kennedy et al., 2000) Similarly, the deficiency of the IL-15 receptor -subunit (CD122, shared with IL-2R) also results in a profound decrease in NK cells (Gilmour et al., 2001; Lodolce et al., 1998) As well, mice lacking the subunit of the IL-15 receptor (15R/) have a reduction in NK cells due to the lack of IL-15 transpresentation (Kawamura et al., 2003; Lodolce et al., 1998), and IL-15 transpresentation (via IL-15R) supports human NK cell differentiation in a xenogeneic mouse model (Huntington et al., 2009). In summary, IL-15 has emerged as a requisite NK specific growth factor, although it is not entirely NK specific, as it also acts on CD8 T cells.

In line with this, CD122 (IL-15R) has been used to isolate NK precursors (Rosmaraki et al., 2001). Primitive, nonlineage specific growth factors, including stem cell factor (SCF), FLT-3L and IL-3, also influence NK cell development (Williams et al., 1997). These growth fac-tors act upon the early hematopoietic precursors, induc-ing IL-15R (CD122) expression, thereby conferring IL-15 responsiveness (Yu et al., 1998). In line with this, IL-15/ or 15R/ mice are nearly devoid of mature NK cells (above), but they do have NK precursors

(Vosshenrich et al., 2005). These findings suggest that at least one function of IL-15 is to provide survival signals to the developing NK cell. This has been con-firmed by the demonstration that NK cell develop-ment in CD122 (IL-15R)/ mice can be restored by enforced, constitutive expression of the anti-apoptotic survival factor Bcl-2 (Minagawa et al., 2002). The downstream signalling through the IL-15/IL-2 receptor involves activation of JAK1/JAK3 and STAT3/STAT5b (Imada et al., 1998; Waldmann and Tagaya, 1999). Defi-ciencies of JAK3 or STAT5b also result in severe impair-ment in NK cell development (Imada et al., 1998; Park et al., 1995).

Sites of NK development: the importance of the developmental environment

BM HSCs give rise to lymphoid precursors and, at cer-tain stages of development, these cells migrate to sites that facilitate terminal differentiation. Migration is an important factor in determining lineage fate. The envi-ronmental cues present at a particular site are required for the initiation of developmental programs. While it is well established that the thymus is the site for T cell development and B cells develop in the BM, we are just beginning to unravel the sites of NK cell development.

Initially, NK cell development was believed to occur exclusively in the BM (Kim et al., 2002). The critical role of BM for NK cell maturation in mice has been shown by using bone-seeking radioactive isotopes that injure the BM stroma, inducing a profound block in NK cell maturation (Mellen et al., 1982). However, recent studies show that NK cells also develop in human sec-ondary lymphoid tissue (Freud et al., 2005). Freud and coworkers have identified consecutive stages of NK cell development starting from CD34 precursors, resulting in functional CD56 NK cells in lymph nodes (Freud et al., 2006). The relative importance of lymph nodes in NK cell development has not been completely estab-lished. Other tissues, such as intestinal epithelial layer (villous and crypt regions), also contain NK precursors (Chinen et al., 2007; Lynch et al., 2006) and are likely sites of NK cell development. The CD34 hematopoi-etic precursors isolated from gut tissue frequently co-express CD56 and differentiated into NK cells upon short-term culture with IL-15.

The thymus also appears to be a potential site of NK cell development. Thymocytes up to the double negative 2 stage retain the capacity to give rise to NK cells (Schmitt et al., 2004; Spits et al., 1998). Vosshenrich and col-leagues (2006) have defined a thymus-dependent NK cell developmental pathway in mice. These thymic-derived

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NK cells expressed high levels of CD127 (IL-7R), the transcription factor GATA-3 and differed function-ally from the majority of murine (splenic) NK cells. This example (thymic NK cell development) underscores the importance of the particular environment (i.e. niche) in guiding the maturation of NK progenitors. Distinct niches likely provide unique combinations of developmental cues (discussed later) that shape NK cell function.

A peculiar population of NK cells is abundant in the decidua of the pregnant uterus, and the properties of these NK cells distinguish them from peripheral blood NK cells (Koopman et al., 2003; Yadi et al., 2008). These cells are characterized by a CD56bright pheno-type and a lack of cytotoxicity. It has not been clearly resolved whether uterine NK cells migrate from periph-eral blood or are derived from precursors that differ-entiate locally within the uterine environment (Ashkar et al., 2003; Keskin et al., 2007; van den Heuvel et al., 2005). The latter possibility would explain their unique characteristics. Thus, one of the emerging views is that NK cells can complete differentiation in various organs outside of the BM, and depending upon the site, these cells differ functionally. Elucidation of the factors present in a particular site and mechanistic explanations for how they impact NK development requires further study. Furthermore, how various sites of differentiation contribute to the heterogeneity of NK cell subsets is not well established.

Fate determining interactions with stroma

Receptor-ligand systems that direct cellular develop-ment and differentiation have been collectively referred to as ‘morphogens’. They are the means by which the environment shapes the fate of developing progenitors (Moore, 2004). Signalling systems, including Notch, Wnt and others, are highly conserved throughout evolu-tion (Pires-daSilva and Sommer, 2003). These systems are important in many aspects of embryogenesis and, thus, in the differentiation of multiple organ systems. Importantly, the actions of morphogens are develop-mental stage- and context-dependent. For example, triggering of the Notch receptor represses B cell differ-entiation and skews lymphoid precursors to T-cell line-age (Schmitt et al., 2004). However at later stages of B-cell differentiation, Notch signalling is required for terminal B-cell maturation (Santos et al., 2007).

The difficulty in studying the importance of a partic-ular morphogen in hematopoiesis is related to their vast redundancy, due to multiple homologues that mediate similar (or overlapping) functions. Thus, the elimina-tion of a single factor may be compensated for by other

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homologues. Furthermore, morphogens have major roles in early embryogenesis and therefore genetic manipula-tions often result in lethal defects, prohibiting the study of their role in NK development. Methods to circum-vent this include the creation of BM chimeras by trans-plantation of foetal liver hematopoietic precursors into irradiated wild type recipients. This method can be used if the mouse embryo with deleted genes survives beyond the initiation of hematopoiesis (approximately day 9 post conception). Alternative approaches include conditional knock-out strategies using tissue specific or drug-inducible promoters driving the CRE recombi-nase. In this way, a given gene can be eliminated from a selected tissue (or lineage) and/or at the desired time by administration of the promoter-inducing drug.

Notch signalling plays a critical role in direct-ing hematopoiesis and lymphocyte development (MacDonald et al., 2001; Maillard et al., 2003). In par-ticular, thymic stroma supports T cell differentiation by the expression of the Notch ligand, delta like ligand-1 (DLL1). Accordingly the murine BM stromal cell line OP9, engineered to express DLL1, efficiently promotes in vitro T-cell differentiation by providing continuous Notch engagement (Schmitt and Zuniga-Pflucker, 2002). Early Notch signalling induces the acquisition of CD7 (La Motte-Mohs et al., 2005) and CD161 by hematopoi-etic precursors. Both also mark NK precursors (Bennett et al., 1996; Miller et al., 1994) , perhaps suggesting a role for Notch in NK commitment. Additional evidence for Notch involvement in NK differentiation comes from studies on murine early progenitors with lymphoid and myeloid developmental potential (EPMLs), where Notch signalling favoured NK development (Rolink et al., 2006). While DLL1 is the most extensively inves-tigated ligand, Notch engagement by a different ligand, Jagged 2, also promotes in vitro NK differentiation from hematopoietic precursors (DeHart et al., 2005). We interpret these (and our unpublished studies) to show that signalling through Notch induces the development of common T-NK precursors. Upon continuous Notch engagement progenitors advance towards the T-cell line-age (Schmitt et al., 2004), whereas early Notch signal-ling appears to be sufficient for NK cell development. Importantly, Notch signalling is not absolutely required for NK cell differentiation (Radtke et al., 2000). Hemato-poietic precursors cultured with, but not without Notch triggering (De Smedt et al., 2007) expressed cytoplas-mic CD3. Subsequently, NK cells resembling Notch-dependent in vitro derivatives could be found in human cord blood but not in adult blood. While Notch signal-ling is not absolutely required for NK cell differentiation, it is mandatory for T-cell development (Radtke et al., 2000). Collectively, the key factors required for T-cell differentiation, IL-7 and Notch signalling, are both dis-pensable for NK development, questioning whether the


common T/NK precursor is the only pathway for NK cell development.

The Wnt signalling system represents another fam-ily of morphogens that influences NK cell development. Numerous Wnt proteins exist (18 identified members in mammals) and can interact with a receptor complex made up of the frizzled receptor and the LDL receptor- related protein (Staal et al., 2008). There are at least two intracellular Wnt signalling pathways, known as canonical and noncanonical. Canonical Wnt signalling leads to stabilization of -catenin, which activates the LEF (lymphocyte enhancer factor) and TCF (T-cell fac-tor, gene name tcf7) family of transcription factors. In the absence of LEF/TCF, neither T cells nor NK cells develop (Held et al., 2003). Gain-of-function and loss-of-function variants also demonstrate the importance of Wnt signalling in lymphopoiesis. In particular, a nonde-gradable, constitutively active -catenin imposed lym-phoid potential onto myeloid precursors (Baba et al., 2005). Despite the role of -catenin in Wnt signalling, conditional deletion of -catenin did not abolish lym-phocyte development. Such cells showed sustained LEF/TCF expression, suggesting the contribution of an alternative, -catenin independent pathway leading to LEF/TCF upregulation (Staal and Sen, 2008). Overall, these studies show that canonical WNT signalling via TCF/LEF is important in lymphopoiesis, including NK cell generation. In support of this, TCF expression was detected in NK cells, specifically in the CD56bright subset (Toor et al., 2001).

Another interaction recently shown to be involved in stroma-dependent NK cell differentiation is between GAS6 and protein S. These two highly homologous lig-ands, expressed on stroma, trigger the Tyro3/Axl/Mer protein tyrosine kinase receptors present on NK pre-cursors. In animals lacking all three receptors, NK cells were phenotypically and functionally impaired (Caraux et al., 2006). Lack of only one receptor of this family (i.e. Axl) had a modest effect on NK cell development. Subsequently, fibroblasts expressing recombinant Gas6 could be shown to support in vitro NK cell differen-tiation (Caraux et al., 2006). Interestingly, the down-stream signalling of Axl is reciprocally associated with IL-15 signalling (Hafizi and Dahlback, 2006) since Axl and IL-15R can heterodimerize. As a result, the IL-15 and GAS6-Axl pathways transactivate one another (Budagian et al., 2005). Indeed this association between IL-15 and Axl is operational in NK cell differentiation from human HPCs in vitro (Park et al., 2008). Perhaps the notion that GAS6–Axl interactions can transactivate the IL-15 pathway might provide the clue as to why IL-15-deficient mice have a residual population of NK cells (Vosshenrich et al., 2005).

Other receptor-ligand pairs with morphogenic func-tions in the differentiation of organ systems may also be

involved in hematopoiesis. Members of the TNF super-family of surface receptors are important in the devel-opment of lymphoid tissues (Mebius, 2003). Reciprocal interactions between hematopoietic progenitors and the nonhematopoietic stroma involve the expression of lymphotoxin- on progenitors and lymphotoxin--recep-tor on stroma. This interaction (between LT and LT-R) triggers IL-15 production by stroma, which in turn, supports NK cell development (Iizuka et al., 1999; Lian et al., 2004). Still other morphogenic receptors, including hedgehog, TGF—Smad, may also be involved in NK cell development; however, their roles have not been fully investigated.

Transcription factors involved in NK cell differentiation

Signalling through cell surface receptors results in a cas-cade of events that may ultimately lead to the activation of transcription factors. These DNA-binding proteins recognize consensus sequences in the promoter regions of target genes and influence gene transcription. The bal-ance of multiple transcription factors, often with oppos-ing functions, ultimately dictates whether initiation or repression of gene transcription occurs. Thus, depend-ing upon the stimuli, certain genes are transcribed, while others are repressed. In this way, differentiation is directed towards a particular lineage. Consequently, sev-eral branching points in hematopoiesis are regulated by the balance of opposing transcription factors, such as Id pro-teins vs. E proteins or PU.1 vs. GATA-1 (see Figure 1.1). These opposing transcription factors regulate the expres-sion of a number of genes, including receptors for lineage specific growth factors (discussed earlier). In this respect, PU.1 and C/EBP drive expression of receptors for myeloid growth factors (G-, M- and GM-CSF), whereas GATA-1 induces erythropoietin receptor expression (Zhang et al., 1996). This opposition controls myeloid vs. erythroid lineage choice at a molecular level.

Transcription factors that play a dominant role in guiding differentiation towards a particular lineage have been referred to as master regulators. Expression of a master regulator is absolutely required for progres-sion past a defined stage of differentiation. Precursors deficient in that factor are therefore unable to advance past a given checkpoint. PAX5 is an example of a mas-ter regulator of B-cell development. Cells deficient in PAX5 are unable to differentiate into mature B cells but retain myeloid, T cell and NK cell potential (Schaniel et al., 2002). Numerous transcription factors are involved in NK development and functional maturation. Mice lacking these factors have impaired NK cell generation. However, in most cases, the defect is not confined to

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PU.1CEBP-α

ID2ID3

Myeloid

HSC

Lymphoid

GATA-3LEF/TCF

GATA-3E2AGATA-3

T cell‘ThymicNK cell’

ImmatureNK cell

MatureNK cell

MITFMEF

SHP-1

Figure 1.1 l Transcription factors important for lineage choice at pertinent branching points in differentiation of hSCs towards the NK lineage. Key transcription factors control the myeloid vs. lymphoid choice, such that PU.1 (dose dependent) and CeBP favour myeloid lineage, whereas GATA-3 and LeF/TCF family members (downstream of Wnt) promote the lymphoid lineage. iD2 expression is required for NK cell development and antagonizes T cell fate promoted by e proteins (e2A, which is downstream of Notch). GATA-3 is necessary for both T cell and ‘thymic NK cell’ development. Other transcription factors, including MiTF, MeF and ShP-1, are required for functional NK cell maturation.

the NK lineage. Along these lines, no master regulator (equivalent to PAX5 for B cells) has been identified or agreed upon for the NK lineage.

Consistent with the dominant role for IL-15 in NK cell development, transcription factors that are either down-stream of the IL-15R or are involved in IL-15 production by stroma are important for NK develop-ment. Accordingly, transcription factor defects influenc-ing IL-15 signalling can be either intrinsic or extrinsic to the progenitor cell. Stat5a and Stat5b are transcription factors involved in signalling through the IL-2 and IL-15 receptor common chain (i.e. CD122). Deficiency in Stat5b resulted in a 50% reduction in splenic NK cells. These mice also showed a profound attenuation in NK cytotoxicity, which was not rescued by exogenous IL-2 or IL-15 (Imada et al., 1998). In parallel studies, Stat5a deficiency was associated with a marginal influ-ence on either the number of NK cells or their function. As an example of hematopoietic progenitor cell-extrinsic factors, IRF-1 (interferon-regulatory factor-1) is required for IL-15 expression. Accordingly, irf-1/ mice show an NK cell deficiency resembling IL-15/ animals (Ogasawara et al., 1998). Hematopoietic progenitors from these irf-1/ mice differentiate into NK cells with exogenous IL-15. Thus, the IRF-1 deficiency results in the impairment of the environmental support required for

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NK cell development. In contrast, irf-2/ mice appear to have a defect that is intrinsic to NK precursors, resulting in a selective loss of mature NK cells in periphery, while BM NK cells are relatively unimpaired (Taki et al., 2005).

As described previously, Notch signalling has been implicated in NK cell development. Downstream effec-tors of Notch include the helix loop helix (HLH) proteins from the E protein family, including E2A and E47. These transcription factors are involved in the development towards the T-cell lineage (Ikawa et al., 2006). Similar to Notch deficiency (MacDonald et al., 2001), E47 dele-tion abrogates T-cell development, but NK cells and myeloid cells develop normally (Ikawa et al., 2006). A different set of transcription factors, which nega-tively regulates E proteins, is the set of dominant nega-tive helix-loop-helix proteins, Id2 and Id3 (inhibitor of DNA binding 2 and 3). Both Id2 and Id3 are highly expressed by NK precursors. Upon further development, Id3 expression falls dramatically, while Id2 is sustained. These results support the involvement of Id2 in NK cell maturation. In fact, in the absence of Id2, NK cells fail to expand and mature, even though CD122DX5 NK precursors are present in BM (Yokota et al., 1999). The activity of Id proteins in NK cell development involves the dominant negative regulation of T-cell differen-tiation, in favour of NK cell development (Heemskerk


et al., 1997; Ikawa et al., 2001). The balance between Id2 and E2A dictates the ability of precursors to expand and terminally differentiate into NK cells. Moreover, the mat-urational defects seen in the absence of Id2 can be cor-rected by the deletion of E2A (Boos et al., 2007). Thus, Id transcription factors antagonize the Notch induced E proteins, diverting progenitors from the T-cell lineage towards the NK lineage (Fujimoto et al., 2007).

As described earlier, Wnt signalling results in activa-tion of the transcription factors TCF-1 and LEF-1. In the absence of both, mice displayed a profound reduc-tion in NK cells and T cells but not B cells (Held et al., 2003). The reduction in NK cells was seen both in the BM and spleen. Further studies showed that compared to LEF-1, TCF-1 played a more substantial role in NK cell differentiation and phenotypic maturation. Human studies confirmed that TCF-1 is acquired by HPCs as they differentiate into NK cells in vitro and that TCF-1 transcript and protein could be found in the CD56bright, fraction of PB NK cells (Toor et al., 2001).

The transcription factor PU.1, a member of the Ets family, is strongly associated with myeloid differen-tiation (Dahl and Simon, 2003). PU.1 functions as a dose- and stage-dependent regulator of lineage fate in hematopoiesis. At an early branching point in hemat-opoiesis, PU.1 and GATA-1 antagonize one another, facilitating myeloid or erythroid differentiation, respec-tively (Stopka et al., 2005). At later developmental stages, PU.1 represses NK cell and T cell specific genes, favouring alternative lineages, including myeloid and B cells (Kamath et al., 2008). In PU.1/ mice, NK cell differentiation is impaired, although not as severely as the T-cell and B-cell lineages (Colucci et al., 2001). Thus, the defect conferred by PU.1 deficiency is not NK specific. In contrast, a related transcription factor, Ets-1, is specifically required for NK development (Barton et al., 1998). In Ets-1/ mice, the NK cell numbers were reduced (threefold), and cytotoxicity was virtu-ally absent, whereas T cells, B cells and other blood lin-eages were quantitatively normal. As a result, affected mice were more susceptible to lymphoid tumours upon challenge with the RMA-S cell line. MEF (myeloid Elf-1 like), another member of the Ets family, is also important for NK cell development. MEF appears to be involved at the latter stages when NK cells gain cytotox-icity. Deletion of MEF resulted in greatly impaired NK cell killing, as well as IFN- production. Mechanistically, the lack of cytotoxicity reveals a role for MEF in regulat-ing perforin gene transcription (Lacorazza et al., 2002).

Similar effects on NK cell differentiation have been observed with C/EBP, a member of the leucine zipper family of transcription factors. These CCAAT/enhancer binding protein (C/EBP) transcription factors play a crit-ical role in myeloid vs. lymphoid lineage determination. C/EBP antagonizes the Tribbles homolog 2 (TRIB2)

protein, one of the downstream effectors of Notch sig-nalling (Wouters et al., 2007). Thus, C/EBP antago-nizes Notch signalling, favouring myeloid development. Another member of the C/EBP family, C/EBP plays a specific role in NK development. C/EBP/ mice showed nearly normal numbers of NK cells, albeit with diminished cytotoxicity and cytokine production capac-ity, suggesting a maturational defect resembling MEF-deficient animals (described earlier). Likewise, perforin expression was greatly diminished in splenocytes from C/EBP/ mice (Kaisho et al., 1999). Another tran-scription factor involved in NK cell functional matu-ration is MITF (microphthalmia transcription factor [MITF]), which similar to MEF, regulates perforin gene expression (Ito et al., 2001).

The family of distal-less (Dlx) homeobox proteins have been implicated in the development of multi-ple organ systems. This family contains multiple mem-bers, including Dlx 1 through Dlx 6. Dlx 3 is expressed preferentially at an immature stage of NK development (Sunwoo et al., 2008). The requirement for Dlx 3 in NK cell development has not been addressed since Dlx 3/ mice die at an early embryonic stage, prior to the onset of hematopoiesis. However, over-expression of either Dlx 1 or Dlx 2 resulted in an arrest of NK development at an immature stage. Quite interestingly, over-expression of Dlx 1 also leads to a profound defect in the develop-ment of T lymphocytes and B lymphocytes, in addition to NK cells. These findings may suggest that this factor is important at the CLP stage.

Another TF involved in functional NK cell maturation is T-bet (Townsend et al., 2004), a factor known to con-trol Th-1 lineage commitment and IFN- production by T cells (Szabo et al., 2000). T-bet/ mice show reduced numbers of mature NK cells in the periphery and an increase in phenotypically and functionally immature NK cells (Townsend et al., 2004). This defect results from a higher rate of NK cell apoptosis in the periphery (i.e. spleen). T-bet is upregulated through STAT1 sig-nalling, which, in turn, is triggered by IFN-. However, the requirement for STAT1 in NK cell maturation is less stringent than the requirement for T-bet, indicating that T-bet induced NK maturation can be STAT-1 independ-ent (Townsend et al., 2004).

In contrast to T-bet, which drives Th1 polarization, the transcription factor GATA-3 is critically involved in Th-2 polarization of T cells. T-bet and GATA-3 are thus, expressed in a mutually exclusive fashion by Th1 and Th2 polarized CD4 T cells, respectively (Ouyang et al., 1998). In the absence of GATA-3, T-cell differentia-tion is abrogated (Ting et al., 1996), whereas NK cells do develop. NK cells from GATA-3/ animals produce less IFN- and appear phenotypically immature (Samson et al., 2003). Thus, it appears that GATA-3, a factor critical for Th2 polarized CD4 T cells, is somewhat paradoxically

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required for IFN- production by NK cells. The impair-ment of NK cells from GATA-3/ mice likely reflects the importance of GATA-3 in the thymic pathway of NK cell maturation, discussed later (Vosshenrich et al., 2006).

Second messenger signalling in NK cell development

NK cells are regulated by surface receptors with acti-vating or inhibitory functions. The majority of activat-ing receptors use adapter proteins (including DAP10, DAP12, FcRI and CD3) to link surface receptors with intracellular signalling pathways, resulting in effec-tor functions. These signalling pathways involve second messengers, including Syk, ZAP70, Lck, PI3Kinases (phosphoinositide 3-kinases), SAP, Fyn, Vav, Grb-2, Phospholipase-C-1 and -2 (reviewed in Lanier, 2008).

The importance of the individual elements of activating receptor signalling pathways on NK cell development has been partially investigated. Animals deficient in DAP10 have normal numbers of NK cells. Since DAP10 serves as adapter protein for NKG2D, these NK cells had reduced NKG2D expression and function. Otherwise, NK cell activity was not diminished. Unexpectedly, DAP10/ mice showed enhanced resistance to skin carcinoma and did not have increased spontaneous tumour formation (Hyka-Nouspikel and Phillips, 2006). DAP10/ mice were also more resistant to melanoma challenge, thorough studies indicated that cell subsets other than NK cells (including NK-T cells and Tregs) were also involved in this effect. Similar to DAP10, deficiency of ZAP70 did not result in the impairment of NK cell development (Negishi et al., 1995). To the contrary, NK1.1CD3 cells were more numerous in ZAP70/ animals (Iwabuchi et al., 2001). The constitutional lack of other kinases, including Syk, or SAP/Fyn in mice did not block NK cell develop-ment either (Colucci et al., 2002; Turner et al., 2000).

PI3Ks are a family of kinases composed of multiple isoforms that encode both regulatory and catalytic subu-nits. The p110 isoform of the catalytic subunit of PI3K is required for NK cytokine secretion (IFN-, TNF- and GM-CSF), while the other tested isoform p110 was not. Neither of these individual isoforms were absolutely required for terminal NK maturation and/or acquisition of cytotoxicity (Kim et al., 2007; Tassi et al., 2007). However, in the absence of both isoforms, NK cell numbers and cyto-toxicity were greatly reduced. Studying a related protein, BCAP (B-cell adapter for phosphatidylinositol 3-kinase), it was noted that in BCAP/ mice the NK cells are overtly long-lived, mature and functionally active (MacFarlane et al., 2008). NK cells from these animals were more resist-ant to apoptosis, suggesting that BCAP mediated ITAM signalling (and activation of the Akt pathway) negatively impacts NK cell maturation and survival.

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The reductionist approach of studying individual proteins in NK cell development is feasible in rodents; however, the nonfully overlapping functions of individ-ual molecules may be a confounding factor when trans-lating these results to humans. Several genetic defects in humans are informative as to the role of these individ-ual proteins in NK development. For instance, DAP12 deficiency results in a rare syndrome known as Nasu–Hakola disease in which presents with impaired osteo-clast activity (bone cysts) and dementia. NK cells are normal and functional in these patients (Paloneva et al., 2000). Deficiency of another adapter molecule, CD3, was reported in a single human with TBNK severe combined immune deficiency (SCID) (Roberts et al., 2007). Curiously, the NK cell repertoire of this patient consisted of a peculiar population of CD56CD16 NK cells. Expansion of CD56CD16 cells has been previ-ously observed in other situations (following allogeneic hematopoietic cell transplantation, HIV infection and in cord blood). Such cells are thought to be either dysfunc-tional or immature. In this patient, virtually all NK cells were, CD56 and cytotoxic activity was diminished compared to healthy controls (Roberts et al., 2007). It is difficult to dissect whether this is due to a direct effect of CD3 deficiency on NK development or an indirect effect via severe impairment of T cells. T cells are the main source of IL-2, which next to IL-15, is the dominant survival factor for NK cells. The other clinical states characterized by an abundance of CD56CD16 cells are also associated with functional T cell deficiency (i.e. after HCT, in HIV infections or in newborns).

While activating receptor triggering leads to kinase activation, NK inhibitory receptors signal through phos-phatases. These phosphatases oppose kinase function. Phosphatases involved in NK cell signalling include SHP-1 (src homology region 2 domain-containing phosphatase 1; PTPN6), SHP-2 (src homology region 2 domain-containing phosphatase 1; PTPN11) and SHIP (SH2-containing inositol phosphatase; INPP5D) (Vely et al., 1997). SHP-1 deficient mice (motheaten-viable [me-v]) show impaired terminal NK cell maturation, characterized by reduced cytotoxicity and IFN- production (Clark et al., 1981; Lowin-Kropf et al., 2000). However, competitive trans-plantation into wild type hosts did not reveal intrinsic defects in NK cells from me-v mice (Kim et al., 2005). This was unexpected since inhibitory receptor signalling was important for NK licensing in this study. Perhaps in the absence of SHP-1, other phosphatases (i.e. SHP-2 and/or SHIP) may substitute, as the reverse occurs in SHIP deficient animals (Yusa and Campbell, 2003). The role of SHP-1 in the acquisition of NK function was also studied using a dominant negative variant of this molecule. NK cells expressing the dominant negative SHP-1 were defective at rejecting MHC deficient BM transplants but showed otherwise normal responsiveness


(Lowin-Kropf et al., 2000). SHP-2 deficient murine embryos did not survive to permit analysis of their NK cells. The role of SHIP in NK cell development and func-tion was also tested in two independent models, which showed that in the absence of SHIP, NK cells were more numerous with improved survival (Wang et al., 2002) and higher IFN- production (Parihar et al., 2005). Interestingly, SHIP/ NK cells also had limited expres-sion of MHC-specific inhibitory receptors, resulting in an inability to reject allogeneic BM grafts. Inhibitory recep-tor skewing may be related to SHP-1 substituting for SHIP (Wahle et al., 2007). The increased IFN- produc-tion by the SHIP/ NK cells provides insight into the functional dichotomy between human CD56bright and CD56dim NK cells, since the latter population expressed higher levels of SHIP when compared to CD56bright cells (Trotta et al., 2005). To further prove the point, that differential SHIP expression underlies the observed functional differences between CD56bright and CD56dim subsets, SHIP was overexpressed in CD56bright NK cells, resulting in a significant reduction in IFN- production.

The impact of activating and inhibitory surface recep-tor triggering on NK cell development has also been studied. Collectively, it appears that NK cells with com-petent inhibitory receptors have superior functionality. In contrast, a rare subset of NK cells that lack compe-tent MHC-specific inhibitory receptors was found to be nonfunctional. This has led to the concept of NK cell licensing—a requirement for inhibitory receptor signal-ling to attain full functionality (Kim et al., 2005; Lowin-Kropf and Held, 2000; Raulet et al., 2001). In contrast, exposure to ubiquitously expressed ligands triggering activating receptors resulted in a reduced capacity of mature NK cells (Fauriat et al., 2007; Sun and Lanier, 2008; Tripathy et al., 2008). This has fundamental importance for our understanding of the process of NK cell development and functional maturation. While the exact roles of particular inhibitory and activating recep-tors have not been dissected, the emerging picture is that this mechanism is in place to assure tolerance. Inhibitory receptor signalling capacitates NK cells, while activating signals appears to incapacitate them (Lowin-Kropf and Held, 2000; Raulet et al., 2001). This contrasts with the developmental requirement of T cells and B cells, which principally require activating signalling to successfully progress in development and maturation.

The NK cell ontogeny—lessons from evolution

Evolutionarily primitive species, up to the jawless ver-tebrates, rely solely on the innate immune system and lack both MHC and RAG gene families. The adaptive immune system, based on the activity of the RAG genes,

first appeared in jawed vertebrates. Thus, the hallmark of adaptive immunity is the expression of unique anti-gen recognition receptors generated by somatic recom-bination. These receptors, expressed by B cells (BCR) and T cells (TCR) confer specificity and memory. NK cells lack these highly variable receptors, and their acti-vation is controlled by the integrated signalling from numerous germline encoded receptors. Such a strategy is characteristic for cells of the nonadaptive or innate immune system.

It is difficult to establish whether NK cells predated the development of an adaptive immune system. For obvious reasons, phenotypic markers used to define mammalian (human) NK cells cannot be applied to invertebrates. One method of NK cell identification in invertebrates would be to search for cells with functional characteristics of NK cells (i.e. perforin mediated cyto-toxicity and IFN- production). It is interesting to note that immunocytes (i.e. macrophages) from a molluscan slug (Incilaria fruhstorferi) express perforin and can reject skin allografts. In line with this notion, the rejected tissue showed features of perforin-induced cell death (Furuta et al., 2006). Therefore, lack of lymphocytes in this invertebrate is potentially compensated for by immunocytes that mediate both perforin-dependent cytotoxicity and phagocytosis. In higher species (i.e. ver-tebrates), these functional characteristics are performed by separate types of cells. However, it has been recently documented that human dendritic cells (DCs) (which have phagocytic properties) can also acquire perforin and kill tumours (Stary et al., 2007).

NK cells use the ‘missing self ’ strategy of immune recognition to identify targets for elimination. This approach is reminiscent of the rules governing mating by fusion of sea sponge colonies of the Botryllus species (De Tomaso et al., 2005). Individual colonies select fusion-partners on the basis of sharing of histocompatability antigens. Fusion with another colony that is missing a ‘self ’ allele is prevented by a mechanism of rejection, likely immune in nature. Coincidentally, receptors with high homology to the mammalian NK receptors (CD94 and/or CD161) have been identified on hemocytes (i.e. blood cells) of Botryllus as well as a related sea sponge, Ciona intestinalis (Khalturin et al., 2003; Zucchetti et al., 2008). The CD94 homologues in Ciona intestina-lis have a similar function to the mammalian receptors since they inhibit hemocyte activation, thereby reducing phagocytosis (Zucchetti et al., 2008). Hence, inhibitory receptors with homology to mammalian NK-related lectin-like receptors are expressed on phagocytic cells in jawless vertebrates. Therefore, the missing-self strategy could have evolved primarily as a selection criterion for mating. Obviously, the use of this same strategy by NK cells could be an independent phenomenon, but it is dif-ficult to overlook the role of NK cells in the process of

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foetal implantation in mammals, which resembles the role of missing self recognition in implantation of marine invertebrate Botryllus schlosseri (Lightner et al., 2008). Moreover MHC, the target of NK recognition, has been implicated in partner selection in rodents and humans (Yamazaki and Beauchamp, 2007).

The evolution of NK cells as a cell type can also be considered from the perspective of NK-specific recep-tors. There are two main genomic clusters of receptors that encode proteins that regulate NK cell activation and inhibition. These are the leukocyte receptor clus-ter (LRC, chromosome 19q13.4) and NK gene complex (NKC, chromosome 12p13.1-2 in humans) (Trowsdale et al., 2001; Yokoyama and Plougastel, 2003). Encoded within the LRC are several gene families that share immunoglobulin-like structure and immune function. Included are the killer immunoglobulin-like receptors (KIR), as well as leukocyte Ig-like receptors or immu-noglobin-like transcripts (LILR or ILT) (Cella et al., 2000). Both KIR and ILT clusters are believed to be related, perhaps derived from a common ancestor (Volz et al., 2001). Interestingly, KIR are expressed by NK cells and rare subsets of T cells (Uhrberg et al., 2001). In contrast, ILT receptors are found predominantly on NK cells, monocytes and DCs (Cella et al., 2000).

The NKC encodes a number of lectin-like recep-tors. Similar to the LRC, the NKC contains NK specific genes that are intermingled with receptors found on other cell types. Some of these lectin-like receptors are found mainly on dendritic and myeloid cells (Dectin1, CLEC4A/DCIR, Lox1), while several are distributed predominantly on NK cells and a subset of T cells (such as CD69, CD94, NKG2D), and still others are expressed by both myeloid/DC and NK cells (LLT1, MICL). The phylogenetic relationships between different lectin-like receptors within the NKC delineated 28 lineages of orthologous genes. The phylogenetic and physical cluster-ing of NKC genes points to their origin by duplications, likely from a common precursor (Hao et al., 2006). As mentioned earlier, receptors encoded by LRC and NKC genes define functional activity of NK cells and are the means by which we classify these cells. The distribution of NK receptors argues that they can be placed evolu-tionarily between the myeloid and T cell lineages.

Lessons from embryogenesis

During foetal development, the first site of primitive hematopoiesis is the yolk sac. At a later time, it shifts to the AGM region (aorta–gonad–mesonephros) and continues in the foetal liver. Eventually, hematopoiesis is established in the BM. The contribution of the yolk sac to the intra-embryonic and definitive hematopoiesis is a matter of debate (Dzierzak and Speck, 2008; Tavian and

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Peault, 2005). More likely, these two regions (AGM and the yolk sac) are two independent sites of blood stem cell generation (Tavian and Peault, 2005; Yokota et al., 2006). Hematopoietic precursors from these two sites have been compared for their potential to generate dis-tinct blood lineages (Tavian et al., 2001). This was done using in vitro culture assays for B cell (MS-5 stromal cell line) and T cell (foetal thymus organ culture [FTOC] assay) differentiation. Yolk sac precursors (i.e. extra-embryonic) could generate primitive nucleated erythro-cytes, myeloid cells and NK cells, but lacked T cell and B cell generation potential. In contrast, progenitors from the AGM region (i.e. intra-embyronic hematopoietic cells) readily generated both B cells and T cells, as well as other lineages.

Thus NK cells, along with myeloid and erythroid cells, can be derived from yolk sac hematopoietic precursors, whereas T cells and B cells could not. Interestingly, the human embryonic stem cell line, H9, shows the same pattern of generating myeloid and NK cells but not T cells or B cells (Martin et al., 2008). In line with these findings, NK cells have been identified in human foetal liver as early as week 6 of gestation, whereas T cells are first observed in the foetal liver at 15–16 weeks (Phillips et al., 1992). These foetal NK cells had unconventional features, including the cytoplasmic expression of CD3 and CD3 subunits. However, they lacked membrane CD3 expression or TCR gene rearrangements, clearly distinguishing them from T cells. These findings dem-onstrate that NK development precedes that of T cells and B cells during embryogenesis. The presence of CD3 and CD3 subunits in the cytoplasm, may point to a common pathway of development for T cells and NK cells. Interestingly a proportion of cord blood NK cells also show cytoplasmic CD3, while this trait is not seen in adult PB NK cells. Studies of NK differentiation in vitro (De Smedt et al., 2007) show that cytoplasmic CD3 expression is induced by DLL1-Notch triggering, perhaps implicating Notch signalling in the ontogeny of foetal liver NK cells but not adult NK cells.

Lessons from NK cell immune reconstitution after hematopoietic cell transplantation

Immune reconstitution is a process of rebuilding the immune system from transplanted HSCs. The reap-pearance of hematopoietic lineages follows a reproduc-ible order, with monocytoid cells emerging first in the peripheral blood, followed by granulocytes and then NK cells. The recovery of NK cells significantly precedes T cells and B cells, with respect to both cell number and functional maturation (Storek et al., 2008). The tim-ing of NK cell reconstitution coincides with myeloid


recovery, similar to the pattern observed during embry-onic development (discussed earlier). This supports the notion that posttransplant immune reconstitution reca-pitulates ontogeny.

The first wave of NK cells found in peripheral blood, after transplant have distinct features (Jacobs et al., 1992). They are CD56 but are predominantly CD16 and KIR. The vast majority of these, early recovering NK cells express the CD94/NKG2A inhibitory receptor heterodimer as their sole MHC-specific inhibitory recep-tor (Cooley et al., 2005; Nguyen et al., 2005; Shilling et al., 2003). In many respects, the early recovering NK cells closely resemble the CD56bright subset of peripheral blood NK cells, whereas the fraction of cells correspond-ing to the CD56dimCD16 NK cell fraction increase and predominate at later times after transplant (Shilling et al., 2003). This orderly appearance of CD56bright and CD56dimCD16 subsets supports the model that CD56bright cells are recently differentiated and upon further maturation they assume the characteristics of CD56dimCD16 cells. However the confounding role of immunosuppressive agents (Cyclosporin A) cannot be ruled out, as the predominance of CD56bright NK cells could also reflect their relative resistance to Cyclosporin A (Wang et al., 2007). An important factor in the early posttransplant reconstitution of NK cells are high levels of IL-15 elicited by the pretransplant conditioning chem-otherapy and irradiation (Miller et al., 2005).

Stages of NK cell development

The differentiation of every type of cell can be seen as a network of phenotypic and epigenetic changes that ulti-mately leads to the mature cell type. Stages of devel-opment represent semistable nodes in this network, at which developing precursors accumulate before tra-versing to the next juncture (Warren and Rothenberg, 2003). Since developmental stages are mostly defined by surface phenotype, the scheme of NK development elucidated in mice is not easily applicable to humans. This is related to the fact that key phenotypic markers of humans NK cells do not apply to mice (i.e. CD56, CD16, and KIR).

Systematic analysis of phenotypic and functional characteristics of NK cells has led the Yokoyama labora-tory to propose five stages of murine NK development (Kim et al., 2002). The first stage is the NK precursor, marked by a CD122NK1.1 phenotype (Rosmaraki et al., 2001). These cells go on to acquire NK1.1 (NKRp1, CD161) and CD94/NKG2A at stage II, fol-lowed by Ly49 at stage III. Notably the acquisition of the two major types of MHC-specific inhibitory recep-tors (Veinotte et al., 2003) (CD94/NKG2A followed by Ly49) marks important steps in maturation, a pattern

also observed in humans (Grzywacz et al., 2006) (dis-cussed later). At the fourth stage, termed the ‘expan-sion stage’, NK cells undergo significant proliferation before reaching the final maturational stage, stage V, characterized by full cytotoxic ability and IFN- pro-duction. Stage V NK cells also acquire Mac-1 (CD11b) and CD43. More recently, cells at this final stage of maturation have been further subdivided according to the level of CD27 expression (Hayakawa and Smyth, 2006). Murine CD27high NK cells progress to a CD27low stage. In contrast to the CD27low subset, CD27high NK cells had lymph node migratory capacity and the abil-ity to interact with DCs. These CD27high NK cells also had higher cytotoxicity and produced more IFN- in response to IL-12 and/or IL-18 stimulation compared to the CD27low counterparts. In support of their relative immaturity, the CD27high NK cells uniquely expressed c-kit receptor (CD117) and IL-7R (CD127) and showed a lower proportion of Ly49 receptor expressing cells. How these CD27high NK cells are related to the thymic path-way of NK maturation (Vosshenrich et al., 2006), also marked by CD127 expression, is not known.

The stages of human NK differentiation did not emerge as a direct correlate of murine studies. Beginning with the NK precursor subset, as defined in mice (Rosmaraki et al., 2001), the available reagents for stain-ing of human CD122 (IL-2/IL-15R) do not allow for a clear discrimination of a distinct CD34CD122 sub-set of human hematopoietic precursor cells (Grzywacz, unpublished). Instead, CD7 has been used to distin-guish cells committed to the NK/T lineage (Miller et al., 1994). NK1.1 is an early marker of murine NK cells, which corresponds to human CD161. Human NK pre-cursors with a CD34CD161CD56 phenotype have been characterized in vitro and are also found in periph-eral blood and cord blood (Bennett et al., 1996). The loss of CD34 suggests that they have progressed in dif-ferentiation; however, these cells have not yet acquired cytotoxic ability. Culture with IL-2 promoted further NK differentiation, with the acquisition of CD56 and other NK receptors, cytotoxicity and IFN- production. Other investigators have also found that CD161 marks an early common NK/T precursor found in murine foe-tal thymus (Michie et al., 2000).

More recently the expression of CD45RA, along with integrin 47 has been used to identify a subset of human peripheral blood CD34dim hematopoietic precursors that preferentially differentiate into NK cells in vitro upon IL-15 stimulation (Freud et al., 2005). The presence of integrin 47 has previously been used to distinguish pro-genitor cells with gut and lymph node homing capacity (Yoshida et al., 2001). The Caligiuri group proposed that CD34CD45RAIntegrin7 cells home to lymph nodes and differentiate into NK cells at this site. This was based on the identification of lymph node resident cells at four

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consecutive stages of NK development (Freud et al., 2006). These include the pro-NK cell (CD34CD45RA Integrin7, stage 1), pre-NK cells (CD34CD117, stage 2), iNK (CD34CD117CD161CD94, stage 3) and CD117lowCD94 CD56bright NK cells (stage 4). The authors go on to hypothesize that CD56dim NK cells constitute the final, fifth stage of NK differentia-tion, that is completed outside of the LNs (Freud and Caligiuri, 2006). Notably, CD56 acquisition is not an essential criterion in this model; however, consecu-tive NK developmental stages are marked by increasing CD56 expression. Our own studies on NK cell differ-entiation in vitro show that CD34 cells cultured on a monolayer of murine foetal liver cells and in the pres-ence of cytokines (IL-3, IL-7, IL-15, FLT-3L and SCF) robustly develop into NK cells. Using CD56 as a marker of NK cells, we distinguished two CD56 subsets: immature NK cells (CD56CD117highCD94) that could give rise to a more mature, functional NK cells (CD56CD117lowCD94) (Grzywacz et al., 2006). The transition from the CD56CD117highCD94 to the CD56CD117lowCD94 stage was associated with the acquisition of activating receptors (NKp30, NKp46 and NKG2D), inhibitory receptors (CD94/NKG2A) and functionality (cytotoxicity and IFN- production). The immature NK cells did not express perforin or granzyme B but were CD161, which, as mentioned earlier, is an early marker of murine and human NK cells. Immature CD56CD117highCD94 cells could be found in UCB and thus, represent physiological rel-evant intermediates of NK maturation. Moreover, the CD56CD117highCD94→CD56CD117lowCD94 transition is equivalent to the stage 3→stage 4 progres-sion in lymph nodes (Freud et al., 2006).

The NK cells derived from in vitro culture rarely expressed KIR or CD16. Moreover, the acquisition of these two receptors was not coordinated at a single cell level, meaning that some developing NK cells acquired KIR but not CD16 (and vice versa) (Grzywacz, unpub-lished). This is somewhat unexpected considering that in vivo KIR expression is almost exclusive to the CD16 NK subset. Overall, in vitro derived NK cells as well as the NK cells developing in LN have features reminis-cent of the CD56bright subset. The work by Ferlazzo et al. (2004) documented the abundance of CD56bright NK cells in secondary lymphoid tissue. Isolation of these cells and further in vitro culture with IL-2 lead to perforin, KIR and CD16 acquisition. Thus far, the conditions required to advance the in vitro derived NK cells to a CD56dimCD16 stage are difficult to repro-duce. Chan et al. (2007) had the interesting observa-tion that CD56bright NK cells from the peripheral blood can develop into CD56dim cells upon interaction with synovial fibroblasts through a CD56:FGF-R1 interac-tion. It would be interesting to determine whether in

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vitro derived NK cells will follow a similar pattern. In another study transpresentation of IL-15 promoted acquisition of CD16 and KIR by CD56brightCD16 NK cells in mice engrafted with human hematopoietic sys-tem (Huntington et al., 2009).

As with murine NK cells, human peripheral blood NK cells can be divided on the basis of CD27 staining intensity. CD56bright and CD56dim human NK cells show CD27high and CD27low expression, respectively. This has led the Smyth group to propose that murine CD27high NK cells correspond to CD56bright cells in humans (Silva et al., 2008). In fact, the LN homing, DC interaction, and IFN- production of the murine CD27high NK cells are also properties of CD56bright NK cells (Fehniger et al., 2003). Moreover, murine CD27high cells recon-stitute early after BM transplantation, followed by the emergence of CD27low NK cells, reminiscent of the predominance of CD56bright cells early after transplant in humans. However, the CD27high subset in mice is more cytotoxic, as opposed to the relatively low cyto-toxicity of human CD56bright (CD27high) NK cells. Also, the expression of CD94/NKG2A, universally high on human CD56bright NK cells, does not distinguish murine CD27high and CD27low subsets. Despite these discrep-ancies, the dichotomy of NK cells subsets distinguished by CD27 expression can be observed in both mice and humans and appears to correspond reasonably well with a linear model of NK cell maturation.

Acquisition of inhibitory receptors during NK cell development

NK cells are restrained from auto-aggression by inhibi-tory receptors that are specific for MHC class I (HLA). Human MHC-specific inhibitory receptors belong to two structurally distinct families: (1) the lectin-like, CD94/NKG2A complex and (2) the immunoglobulin- like, KIR. Acquisition of CD94/NKG2A by NK pro-genitors marks an important step in NK development. This is because CD94/NKG2A expression is coor-dinated with attainment of functionality (activating receptor expression, cytotoxicity and IFN- production) (Grzywacz et al., 2006). Thus, the linking of inhibitory receptor expression with effector mechanisms appears to be a form of tolerance during NK development.

The order of inhibitory receptor acquisition during NK cell development appears not to be circumstantial. The ligand for CD94/NKG2A is HLA-E (Braud et al., 1998; Lee et al., 1998). This nonclassical HLA molecule has lim-ited polymorphism. The conserved sequence of HLA-E and its ubiquitous expression, assures that CD94/NKG2A will find its ligand on all healthy cells and tis-sues (Kaiser et al., 2005). Similarly, CD94 and NKG2A both have strikingly conserved sequences in the human


population, compared with other NK receptors (Shum et al., 2002). In contrast, different KIR receptors recog-nize only a selected set of HLA-C, B, or A molecules as their ligands (Parham, 2005). Thus, expression of a given KIR does not guarantee an effective inhibitory interac-tion with self-MHC. In this sense, it appears biologically justified that NK cells rely on the CD94/NKG2A as the first safety mechanism. In fact, all the CD56bright NK cells express high levels of CD94/NKG2A, perhaps cor-responding to their relatively recent developmental his-tory. This is not true for the CD56dimCD16 NK cells, where the majority of cells express at least one self- specific inhibitory receptor, but frequently it is a KIR and not CD94/NKG2A. If CD56dim NK cells are derived from CD56bright NK cells, then this developmental tran-sition would be associated with acquisition of KIR and the loss of CD94/NKG2A. Alternatively, the existence of such CD94/NKG2AKIR CD56dim cells might sup-port the existence of another developmental pathway (discussed later).

A peculiar subset of NK cells that lack self-specific inhibitory receptors has been identified in both mice (Fernandez et al., 2005) and humans (Anfossi et al., 2006; Cooley et al., 2007). Since licensing comes about through inhibitory interactions with self MHC (Kim

et al., 2005), these ‘not licensed’ NK cells have weak, if any, cytotoxicity despite expression of perforin and granzyme B. Upon in vitro culture with IL-15 they could acquire CD94/NKG2A and/or KIR expression (Cooley et al., 2007). Moreover, phenotypically most of the cells are CD56dimCD16 NK cells, making it difficult to fit these finding in the linear model of NK cell develop-ment, according to which NK cells are derived from CD56bright intermediates, highly positive for CD94/NKG2A (as described earlier). As well, the contribution of CD56CD117highCD94 developmental interme-diates (Grzywacz et al., 2006) to the inhibitory recep-tor negative subset has not been verified nor excluded. Collectively, the outstanding question is whether CD56dim NK cells, which do not express CD94/NKG2A or KIR are derived from CD56bright NK cells.

Linear and branching models of human NK cell development

The most commonly accepted model of NK cell develop-ment depicts this process as a linear scheme (Figure 1.2). The dominant population of human peripheral blood NK cells—the CD56dimCD16 subset—corresponds to the

CD56+

CD117high

CD94–

CD56dim

CD16+

CD56dim

CD16+

CD56–

CD16+CD56bright

KIR+

CD94+/–

KIR–

CD94–

?

?

CD94++

Figure 1.2 l The developmental relationship of NK cell subsets found in humans. The intermediate stage in NK cell development, characterized by CD56CD117highCD94 phenotype, can advance to the next developmental stage CD56brightCD117lowCD94 (Freud et al., 2006; Grzywacz et al., 2006). CD56bright cells found in peripheral blood and secondary lymphoid organs can acquire CD16 and KiR expression after progressing to CD56dimCD16 stage (Chan et al., 2007; Ferlazzo et al., 2004; huntington et al., 2009; Romagnani et al., 2007) (bottom, large arrow). however, the conversion in the opposite direction has also been demonstrated in certain conditions (Loza and Perussia, 2004; Mailliard et al., 2005) (top, smaller arrow). The CD56CD16 subset (Gaddy and Broxmeyer, 1997), which includes CD56CD16CD122 cells (harada et al., 2004) could potentially represent intermediate stage of alternative pathway developing directly into CD56dim NK cells, independent of a CD56bright NK cell stage. The subset of predominantly CD56dimCD16 NK cells, that lack self-hLA specific KiR or CD94/NKG2A has been characterized as potentially immature (Anfossi et al., 2006; Cooley et al., 2007). Their relationship to the CD56bright→CD56dim progression of NK cell development is not resolved. They could be derived from the immature CD56brightCD94/NKG2A subset or are possibly intermediates of an alternative NK differentiation pathway.

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final stage of maturation, whereas CD56bright NK cells are considered to be relatively immature and recently differentiated intermediates. Arguments in support of this notion have been presented previously, including: (1) the differentiation of CD34 progenitors first into CD56bright-like NK cells in vitro, as well as in lymph nodes, (2) the ability of CD56bright NK cells to acquire phenotypic properties of CD56dimCD16 NK cells upon interaction with tissue-resident (synovial) fibroblasts (Chan et al., 2007) and (3) CD56CD16 lymph node derived NK cells that acquire CD16 and/or KIR under the influence of IL-2 (Ferlazzo et al., 2004). Additional evidence supporting the previous assertion comes from studies on immune reconstitution after transplantation, where CD56bright NK cells precede the emergence of the CD56dimCD16 counterparts. It has also been shown that CD56bright NK cells have longer telomeres com-pared to CD56dim cells (Romagnani et al., 2007). Lastly, transpresentation of IL-15 induced acquisition of CD16 and KIR by CD56brightCD16 NK cells in a xenogeneic mouse model engrafted with human hematopoietic sys-tem (Huntington et al., 2009).

However, an opposing view is that CD56bright and CD56dimCD16 NK cells represent two distinct termi-nal differentiation states, in support of a branching model of NK cell development (Figure 1.2). Perhaps it would be premature to completely abandon this concept. The progression from a CD56bright to CD56dimCD16 cell is not necessarily unidirectional, since the opposite direc-tion (CD56dim to CD56bright) has also been documented. Others have proposed that CD56bright NK cells resemble CD56dimCD16 NK cells that have been activated with IL-12 (Loza and Perussia, 2004). Likewise, under the influence of IL-18, CD56dim NK cells acquire character-istics of CD56bright NK cells, including downregulation of CD16, acquisition of LN migratory molecules (CCR7), production of large amounts of IFN- and the ability to interact with DCs (Mailliard et al., 2005). Moreover, activation of NK cells by susceptible target cells leads to a loss of CD16 by metalloproteinase-mediated shed-ding (Grzywacz et al., 2007; Harrison et al., 1991). Thus, the linear model of NK cell development describ-ing CD56bright NK cells as being an immature intermedi-ate should be weighed against the alternative possibility that CD56bright and CD56dim NK cells represent deriva-tives of two different developmental pathways and are two different terminal differentiation states. The ability of one cell type to transition into another (Chan et al., 2007; Loza and Perussia, 2004) supports the notion that they correspond to distinct states of activation.

A number of NK cell subsets can be found in the peripheral blood that are difficult to understand how they arose from a linear model of NK cell development. For instance, a distinct population of CD56CD16 NK cells has been identified in healthy adult peripheral blood.

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These cells are more prevalent in cord blood (Gaddy and Broxmeyer, 1997), in HIV infection (Mavilio et al., 2005), and after BM transplantation. This subset gives rise to CD56dimCD16 NK cells under the influence of IL-2 (Gaddy and Broxmeyer, 1997) and has been pro-posed to represent an immature population of recently differentiated NK cells. In particular, a subpopulation of CD56CD16 cells coexpressing CD122 could very potently expand and differentiate into NK cells in cul-ture with feeder cell line derived from Wilms tumour (Harada et al., 2004). The same cell line promoted engraftment of CD56dim/CD16 NK population in xenogeneic mouse model of human NK development and strikingly CD56bright NK cells were not observed in this model (Harada et al., 2005). CD56CD16 NK cells could represent an intermediate stage in the CD56dim NK cell development, and it is conceivable that they are developmentally independent of CD56bright NK cells. An alternative possibility is that this NK subset is related to T cell deficiency or immaturity. Moreover the percentage of this fraction of NK cells correlates inversely with the percentage of CD3 T cells in a group of post-transplant patients (Grzywacz, unpublished). The expansion of CD16CD56 NK cells in T cell deficient individu-als may be due to insufficient T cell help (perhaps inad-equate IL-2 production). Moreover CD16CD56 NK cells could represent improperly matured NK cells, but they could also correspond to an ‘exhausted’ and/or ‘helpless’ NK subset. Notably all the NK cells found in a patient with immunodeficiency due to a CD3 muta-tion were CD16CD56. This is a remarkable observa-tion deserves further study to elucidate the ontongy of this subset. Reminiscent of the other instances where CD16CD56 NK cell expansions were observed, T cells were absent in this patient. In summary, CD16CD56 NK cells do not easily fit into the linear model of NK cell development. The existence of these cells supports the notion that distinct pathways of NK cell development exist and that they represent an intermediate stage of a pathway that gives rise to CD56dim NK cells. Similarly, the rare subset of NK cells that lack self-specific inhibi-tory receptors (Anfossi et al., 2006; Cooley et al., 2007) do not conform to the linear model of NK cell develop-ment. Phenotypically, the majority of those cells are CD56dimCD16, yet they do not express CD94/NKG2A or self-specific KIR. These cells are developmentally immature (Cooley et al., 2007) and it is hard to recon-cile how these cells could be derived from CD56bright NK cells, which are uniformly CD94/NKG2A.

Boundaries of NK cell lineage

Several recent reports have revealed the existence of cells that have sparked controversy regarding their assignment


to the NK lineage. As described earlier, thymus- derived NK cells (Veinotte et al., 2006; Vosshenrich et al., 2006) are a rare subset of murine NK cells which express CD127, high levels of GATA-3 and differ func-tionally from majority of splenic NK cells. The NK cell character of these cells has been called into question by the demonstration of frequent TCR / gene rearrange-ments, cytoplasmic expression of CD3 and/or TCR- (Stewart et al., 2007; Veinotte et al., 2006). Thus, the thymic NK cells may emerge as cells that have aborted T cell differentiation. The ability of thymocytes (at least up to the DN2 stage) to become NK cells or DCs is well established (Masuda et al., 2007; Shen et al., 2003). This lineage promiscuity is lost later in the devel-opment (DN3 stage), when thymocytes express TCR . The onset of TCR and rearrangement occurs at the DN2 stage, preceding TCR rearrangement (Livak et al., 1999). NK cells in humans and in mice commonly express germline TCR (Biondi et al., 1989; Stewart et al., 2007). Thus, there is a theoretical possibility that after the initiation of TCR or rearrangement thymo-cytes can abort T-cell differentiation in favour of NK or DC lineage. This possibility appears to be realized in the form of thymic NK cells, or at least in a fraction of NK cells that show TCR rearrangements. The argu-ment that TCR rearrangement draws the line between T cells and NK cells (or other lineages) is hard to refute (Lanier, 2007). However, such definition is not com-patible with practical designation of NK cell vs. T cell lineage, as it requires intracellular staining for detection of CD3 and/or TCR , as well as molecular testing for TCR rearrangement. Purifying viable NK cells accord-ing to such definitions becomes simply impossible, as all these techniques involve killing the cells. Thus, the novel subset of thymic NK cells revived the discussion on the distinction between NK cells and T cells that appeared to be solved long ago (Lanier, 2007). Before we use the initiation of the TCR gene rearrangement to distinguish NK and T cells, it must be amenable to test-ing without destruction of the cells. Otherwise such def-inition, although biologically correct, will remain mute.

Other subsets of cells that reside on the boundaries of the NK lineage are interferon producing killer DCs (IKDC) (Chan et al., 2006; Taieb et al., 2006). Such cells share the properties of both NK cells and plasma-cytoid DCs. (see Chapter 3) They are capable of both direct cytotoxicity and production of class I interferons. These functional features are considered to be unique to the two separate cell types (NK cells and plasmacy-toid DCs, respectively). More recently two independ-ent groups have questioned the existence of true IKDC, showing that distinct subsets produce IFN- and per-form cytotoxicity in this heterogeneous population

consisting mostly of activated NK cells (Blasius et al., 2007; Vosshenrich et al., 2007). Other studies on the mechanism of action of TLR7 agonists in the treatment of skin cancer have revealed that DCs can acquire cellu-lar cytotoxicity upon TLR triggering (Stary et al., 2007). Conversely, it has been also shown that NK cells can acquire efficient antigen presenting capacity upon stimu-lation (Hanna et al., 2004). Thus, DCs can acquire func-tional properties of NK cells and NK cells can acquire the function considered characteristic for DCs. Cells sharing properties of NK cells and DCs could reveal the plasticity and potential close developmental relationship of these cells (Spits and Lanier, 2007). Collectively, the controversies on the boundaries between NK cells and T cells as well as between NK cells and DCs underscore the complexity of developmental pathways and place NK cells between these two types of cells.

Summary

The development of multipotent hematopoietic cells into NK cells is a complex process. It is guided by envi-ronmental cues and intrinsic responsiveness of precursor cells to external signals. As hematopoietic progenitors progress in differentiation towards NK lineage two concomitant processes occur: (1) acquisition of NK specific gene expression pattern and (2) gradual loss of the ability to express genes characteristic for other line-ages. Transcription factors play a critical role in guiding lineage determination, and even though much progress has been made, we are still far from disentanglement of the network of transcription factors that lead to the development of NK cells. Interactions of hematopoi-etic progenitors with the environment provides growth factors and morphogenic signals that affect lineage fate and guide functional maturation via the triggering of inhibitory and/or activating receptors. The progression from multipotent hematopoietic precursors to mature NK cells can be described on the basis of stages of NK cell development. While most commonly visualized as a linear process, we cannot rule out a branching model of NK cell development at the present time. The devel-opment of any hematopoietic lineage can follow heter-ogenous pathways that lead to a common final point—a mature cell. As precursors traverse an individual devel-opmental pathway, they are exposed to distinct sets of transcription factors, differing in magnitude and time of exposure. These differences eventually affect the final phenotypic and functional features of terminally dif-ferentiated cell. Therefore the heterogeneity of NK cell subsets observed in both humans and mice may reflect distinct pathways of NK cell development

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Natural Killer Cells || Developmental stages and pathways of NK cell maturation