Pediatr Blood Cancer 2004;42:401–403

COMMENTARYLangerhans Cell Histiocytosis: A Pathologic Combination

of Oncogenesis and Immune Dysregulation{

For many of us within the world of histiocytoses, thenames Christian Nezelof and Francoise Basset are closelylinked to Langerhans cell histiocytosis (LCH), empha-sised by their excellent overview in 1998 entitled: LCHresearch; past, present, and future [1]. Nezelof, thefounding President of the international Histiocyte Societyand his close co-worker were instrumental in the nosologictranslation of histiocytosis X to LCH, when their classicmanuscript entitled: ‘‘histiocytosis X; histogenetic argu-ments for a Langerhans cell origin’’ appeared [2]. Thesimilarity of the histiocytosis X-cell and Langerhans cellwas postulated on the recognition of Birbeck granules inthe histiocytes by electron microscopy. Old soldiers neverdie, which is clear by reading the current manuscript byNezelof and Basset in this issue of Medical and PediatricOncology [3]. Although major advances in LCH havebeen put forward over the last decades, the ‘‘discussion’’as to whether LCH is a reactive disease (due to an externaltrigger of proliferation) or a neoplastic process (due to anintrinsic autonomous proliferation) is still on-going.

LCH might be the unique pathologic combination ofoncogenesis and chronic immune dysregulation. Ingeneral, neoplastic cells are known to elicit an inflamma-tory microenvironment on which they depend for theircontinuous growth and survival, in particular in pre-malignant states [4]. Neoplastic cells produce an array ofcytokines and chemokines some of which are mitogenicand/or chemoattractants for other cells like macrophages,granulocytes, and lymphocytes. In addition, infiltratinginflammatory cells secrete proteolytic enzymes, cytokinesand chemokines, which can be mitogenic for the neo-plastic cells. These factors potentiate proliferation andgrowth. In other words cytokine and chemokine balancesregulate neoplastic outcome. The balance is also criticalfor regulating the type and extent of inflammatoryinfiltrate that forms. Thus high levels of monocytes andeosinophils, in response to an altered balance of pro-versus anti-inflammatory cytokines cause cytotoxicity andtumour regression. We have shown very high levels of afair number of cytokines in LCH lesions creating a truecytokine storm [5]. We recently provided evidence ofchemokine imbalances as the basis for the blockage of thematuration of the CD1aþ LCH-cell in the lesion [6].

Nezelof and Basset hypothesise very elegantly thatLCH might be based on the failure of the immune systemto switch from an innate to an adaptive mode [3]. To keep

the discussion alive, here we will try to provide somestrength for the neoplastic point of view. In order to have ameaningful discussion about whether LCH is a neoplasticor reactive disease, a strict definition of the concept ofneoplasia is essential. As stated before [7], we defineneoplasia as a clonal proliferation of cells harbouringsomatic genetic abnormalities, which enable autonomousgrowth and phenotypic distinction from their physiologiccounterparts.

Although Nezelof and Basset state in point no. 2: ‘‘LCcells in LCH are morphologically, histochemocally,and functionally similar to normal LCs’’, the phenotypeof LCH cells differs substantially compared to normalLangerhans cells [8]. LCH cells are rounded, lackingprominent dendritic extensions, and typically have amoderate amount of homogeneous, pink, granular cyto-plasm, and distinct cell margins. The LCH cells reflect animmature, activated Langerhans type dendritic cell, asshown by us and others [6,9]. In the non-pulmonary casesof LCH, clonality of the lesional cells is undisputed asassessed by the HUMARA assay [10,11]. Substantialsupport for the idea that LCH is a genetically causeddisease is provided by twin studies, that show an 80 and33% concordance between monozygotic and dizygotictwins [12]. Moreover, familial clustering has beenobserved between siblings as well as vertical transmissionfrom generation to generation of persons with this disease[12]. By use of comparative genomic hybridisation (CGH)and loss of heterozygosity (LOH) experiments, concurrentDNA copy number changes were observed in LCH, as wellas loss of DNA sequences targeting chromosomes 1p, 5p,6q, 9, 16, 17, and 22q. The highest frequency of LOH wasfound on 1p region and on chromosome 7 [13].

The first observation of increased chromosomal break-age in patients with LCH [14] has now been confirmed andextended by several others [13,15]. This suggests a defect

——————{This manuscript was originally submitted to and accepted for

publication in Medical & Pediatric Oncology by its Editor-in-Chief,

Dr. G. D’Angio.

*Correspondence to: R. Maarten Egeler, Department of Pediatrics,

Division of Immunology, Hematology, Oncology, BMT and

Autoimmune Diseases, Leiden University Medical Center (LUMC),

PO Box 9600, 2300 RC Leiden, The Netherlands.

E-mail: RM.Egeler@LUMC.nl

Received 17 June 2003; Accepted 6 August 2003

� 2004 Wiley-Liss, Inc.DOI 10.1002/pbc.10464

in either DNA repair or replication pathways and providespotential candidate genes to be examined in LCH. Besideschromosomal instability, abnormal clones in LCH havebeen observed, showing a t(7;12)(q11.2;p13) transloca-tion as an example [14]. Furthermore the cell-cycleregulation of the Langerhans cell in LCH is severelydisrupted [16,17], probably as a result of a combination ofboth external signals (growth factors and cytokines), andintrinsic factors from as yet unidentified DNA changes.

The findings from contemporary cytogenetic studies[13,14] add to the earlier description of the existence ofLCH concurrent with the myelodysplastic syndrome inchildren [18]. Their cases studied also showed abnorm-alities of chromosomes 7 and 8. We at that time hadalready raised the question whether LCH should beconsidered a myeloid dendritic stem cell disorder, closelyrelated to the pediatric myelodysplastic disorders (MDS)[19]. The observation that there may be increasedchromosomal breakage in patients with LCH [13–15]suggests a defect in either replication pathways or DNArepair and provides potential candidate genes to beexamined in LCH. The possible link between LCH andMDS is of special interest. This is because a defect onchromosome 7 is a common finding in MDS. There areseveral familial syndromes with defective DNA repairon chromosome 7, such as Fanconi anaemia, multiplefamilial pediatric MDS cases, or severe congenitalneutropenia, which have an association to MDS and acutemyeloid leukaemia (AML). Interestingly, the leukaemia’sarising in these disorders frequently have a monosomy 7/(7q-) or chromosome 1 abnormality, the same abnormal-ities as shown by the recent CGH and LOH studies [13].Research has demonstrated that these chromosome 7alterations may be ‘‘secondary’’ to an underlying geneticcause or predisposition to the disease that maps to anotherhuman chromosome [20]. These results indicate thatthe deleted chromosomal segments may contain genesimportant in LCH initiation and progression.

As also stated by Nezelof and Basset [3], it is now wellrecognised that the pathogenesis of LCH is due to ablockage in the normal maturation pathway of the LCHcells. It has been shown that LCH cells from bone/chroniclesions are undoubtedly immature Langerhans-type den-dritic cells [9]. This is evidenced by their expression ofCD68 and CD14, their expression of intracellular MHCclass II and their negativity for CD86 and DC-LAMP.Furthermore, they have the same allostimulatory activityas immature normal DCs. Additional evidence for thismaturation block came from our own studies of chemo-kine receptor expression on the CD1aþ LCH cells [6,8].DCs express a distinct pattern of functional chemokinereceptors at different stages of their maturation [21,22].These homing receptors along with their correspondingligands play a critical role in positioning these cells indistinct sites to allow them to carry out their appropriate

function at a particular stage of their maturation. In ourstudies, we demonstrated that the lesional CD1aþ cells areindeed in an immature state as defined by their expressionof the chemokine receptor CCR6. Conversely, CCR7expression appeared to be absent on the lesional CD1aþcells. This chemokine receptor is indicative of DCmaturation in that it localises DC in lymphoid organs byresponding to CCR7 agonists. This is in keeping with thefact that LCH cells are hardly ever found in lymph nodesthat drain the lesional sites. Thus, despite the variousinflammatory stimuli present in LCH lesions, such asTNF-a, which should induce the maturation of the LC, theCD1a cells do not lose their expression of CCR6 and donot up-regulate CCR7 [6]. In contrast to our own findings,Fleming et al. recently reported coincident expression ofthe chemokine receptors CCR6 and CCR7 by pathologicLangerhans cells in LCH [23]. One possibility for thediscrepancy between our results could be in the type oflesion that was studied. It has been reported that in somecases in vivo, most notably in self-healing cutaneouslesions, a more mature phenotype can be observed. LCHcells in them appear to down-regulate CD14 and up-regulate CD86 and DC-LAMP [9]. It may be that in theselesions the LCH cells have overcome the maturationblockade and are, therefore, able to down-regulate CCR6and up-regulate CCR7 as they mature. This would in turnrelease these LCH cells from the local environment of theCCR6 ligand, CCL20/MIP-3a, and allow them to followthe normal lymphoid drainage pathways. This body ofwork has shed more light on the character of the LCH cells;however, it still leaves us with the question whether LCH isa neoplastic or a reactive disease. The cytokine andchemokine profiles nonetheless might explain the uniformdistribution in LCH as stated by Nezelof and Basset. Themultifocal and multicentric distribution of LCH of boneper se should not be seen as an argument against aneoplastic disorder (point no. 3). Other examples of non-uniform multifocal distributed bone neoplasms are multi-ple osteochondromas or enchondromatosis (Ollier dis-ease) [24,25], in that these are examples of multifocal bonedisease, without visceral organ involvement. At thismoment, however, there is no evidence to conclude thatorgan centred lesions of LCH are of different nature fromtheir bony counterparts. Geismann et al. recently showedthat CD1aþ LCH cells could differentiate toward matureDCs in response to CD40 triggering in vitro [9]. In vivo,however, we have shown abundant production of CD40 byCD1aþ LCH cells [26]. Thus, the reversion to normalbehaviour in vitro suggests that the lesional microenvir-onment controls the abnormal responses of LCH cells insitu and thus points towards the reactive nature of thedisease.

The nature of LCH is very relevant to possible futuretreatments. If the disease is reactive, then therapiesdesigned to block chemokine receptors may be feasible.

402 Egeler et al.

However, if there is a neoplastic component or the cause ofthe developmental arrest exists elsewhere in the body, thenchemokine receptor blockade will only serve to spread thedisease throughout the body. To design appropriatetreatments for LCH, it thus will be essential to addressthe question whether LCH is reactive or neoplastic; and ifreactive as suggested by Nezelof and Basset [3], then theprecise cause of the reactivity needs to be discovered. Onehas to realise that LCH, however, might be the uniqueexample of the combination of immune dysregulation andoncogenesis.

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R. Maarten Egeler, MD, PhD*

Nicola E. Annels, PhD

Department of Pediatric ImmunologyHematology, Oncology

Bone Marrow Transplantationand Autoimmune Diseases

Leiden University Medical Center (LUMC)Leiden, The Netherlands

Pancras C.W. Hogendoorn, MD, PhD

Department of PathologyLeiden University Medical Center (LUMC)

Leiden, The Netherlands

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