RhoH is required to maintain the integrin LFA-1in a nonadhesive state on lymphocytes

Lisa K Cherry1,3, Xiaoyu Li2, Pascale Schwab1, Bing Lim2,3 & Lloyd B Klickstein1

Lymphocyte function–associated antigen 1 (LFA-1) is relatively nonadhesive on resting lymphocytes; however, the mechanisms

underlying changes in its adhesiveness are poorly understood. In this study, we generated a Jurkat T cell clone, J+hi1.14, that

contained low amounts of mRNA for RhoH, a leukocyte-specific inhibitory Rho family member. J+hi1.14 cells expressed

constitutively adhesive LFA-1 and the cells bound spontaneously to intracellular adhesion molecules 1, 2 and 3. Reconstitution

of RhoH mRNA expression in J+hi1.14 cells reverted the adhesion phenotype to that of wild-type. We obtained similar results

using RNA interference in peripheral blood lymphocytes. These data demonstrate that RhoH is required for maintenance of

lymphocyte LFA-1 in a nonadhesive state.

Lymphocyte function–associated antigen 1 (LFA-1) is a b2 integrinfound on all leukocytes1. LFA-1 is expressed most highly on lympho-cytes and is important for leukocyte migration, antigen presentationand cellular cytotoxicity1. Counter-receptors for LFA-1 include inter-cellular adhesion molecule 1 (ICAM-1; ref. 2), ICAM-2 (ref. 3) andICAM-3 (ref. 4) as well as junctional adhesion molecule A5. Cellularadhesion via LFA-1 is mediated mainly by the reversible acquisition ofan adhesive state6–10, hence LFA-1 is found in a nonadhesive state onresting cells and becomes adhesive when the cells are subjected to anyof a variety of stimuli11–16. The adhesive state of LFA-1 may representboth conformational changes in the molecule to increase affinity6 aswell as clustering of integrin on the cell surface to augment avidity7.The dynamic regulation of LFA-1-mediated adhesion is poorly under-stood; however, it is believed that maintenance of the low-avidity stateof LFA-1 is an active process, as LFA-1 is generally constitutivelyadhesive when the protein is expressed on cells other than leukocytes17

and when purified18. The GFFKR amino acid sequence found in thecytoplasmic domain of all integrin a-subunits is required for main-tenance of a low adhesive state, but how the GFFKR sequenceparticipates in adhesion regulation remains unknown19. Furthermore,the GFFKR sequence does not govern adhesion specificity, as allintegrin a-subunits contain this sequence and it is well establishedthat integrins may be selectively activated20. In an effort to develop asystematic approach to the regulation of integrin-mediated adhesion,we undertook a genome-wide scan using retrovirus insertion muta-genesis followed by selection for cells bearing constitutively adhesiveLFA-1. This approach has the potential to identify many of theregulatory elements in a complex signal-transduction pathway,in this case the regulation of LFA-1-mediated adhesion, without

requiring any assumptions regarding mechanism. Using this strategy,we identified RhoH, an inhibitory Rho family member21 that isexpressed exclusively in leukocytes22, as an important element forthe maintenance of the low adhesive state of LFA-1 on resting T cells.

RESULTS

A mutation in RHOH

We developed a genomics strategy of retrovirus insertion mutagenesisto identify important regulatory elements of LFA-1 adhesion. Thisapproach did not require any assumptions regarding the specificregulatory mechanism(s) involved. We prepared the PU3.1 retrovirus,which contains intact long terminal repeat promoter and enhancersequences, as the mutagen to allow activation or inactivation of genetranscription. We infected wild-type Jn.9 Jurkat cells (1.7 � 109 total)by culturing them together with the GP+envAm12/PU3.1 producercells so that 8% of the input cells were infected, which yielded onaverage 1.3 provirus insertion events per cell (data not shown). Afterfour cycles of selection by panning on plates coated with ICAM-1linked to the Fc fragment (ICAM-1–Fc), a spontaneously adherentpopulation of cells was evident, and we cloned the cells for furtheranalysis. We screened clones for those that were spontaneouslyadherent to ICAM-1 (Fig. 1), and Southern blot analysis with aretrovirus-specific probe to the gene encoding neomycin resistance(neor) identified three unique cell clones in the population ofapproximately 40 clones analyzed. Two of the clones each containedtwo proviral insertions and one contained a single insertion (Fig. 1).PCR amplification and determination of the sequences adjacent to theprovirus insertions followed by comparison with the Human GenomeProject database localized the insertion sites. The Jurkat clone

Published online 8 August 2004; doi:10.1038/ni1103

1Division of Rheumatology, Immunology and Allergy, Department of Medicine, Brigham & Women’s Hospital, Smith Building room 650, 1 Jimmy Fund Way, Boston,Massachusetts 02115, USA. 2Department of Medicine, Beth Israel Deaconess Medical Center, Harvard Institutes of Medicine, 77 Avenue Louis Pasteur, Boston,Massachusetts 02215, USA. 3Present addresses: Genzyme Corporation, 1 Mountain Road, Framington, Massachusetts 01701, USA (L.K.C.) and Genome Institute ofSingapore, 60 Biopolis Street, Singapore 138672 (B.L.). Correspondence should be addressed to L.B.K (lklickstein@rics.bwh.Harvard.edu).

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J+hi1.14 contained two proviral insertions. The first insertion waslocalized to chromosome 12q24 in a ‘hypothetical’ gene with homo-logies to a mitochondrial ATPase subunit and to chondroitin 4sulfotransferase. Only two expressed sequence tags have been reportedin the entire ‘hypothetical’ gene and none from a lymphocyte source,so this insertion seemed unlikely to be relevant to LFA-1-mediatedadhesion. The second insertion was in RHOH on chromosome 4p13,just downstream from the first exon and in the opposite orientation(Fig. 2). Southern blot analysis after restriction enzyme digestion ofgenomic DNA purified from the Jurkat clones Jn.9 and J+hi1.14 usingan exon 1A RHOH probe confirmed the location of the proviralinsertion in RHOH (Fig. 2). RhoH is a leukocyte-specific, inhibitoryRho family member that is expressed most abundantly in lympho-cytes. The cellular expression of RhoH parallels that of LFA-1, so weselected the J+hi1.14 clone and RhoH for further study.

Quantitative RNA hybridization analysis of mRNA from theJ+hi1.14 clone with a full-length RHOH cDNA probe and comparisonwith the parental wild-type Jurkat clone Jn.9 showed that the steady-state mRNA abundance was less than half that of the wild-type cells(Fig. 3). Sequence analysis of cDNA clones corresponding to theremaining RhoH mRNA present in the J+hi1.14 cells showeda sequence identical to that published22. A 50–80% decrease inRhoH mRNA abundance has been established to be sufficient forfunctional effects21.

LFA-1 is constitutively adhesive on J+hi1.14

Blocking antibody studies confirmed that the J+hi1.14 cells werespontaneously adherent to the ICAM-1–Fc in an LFA-1- and ICAM-1-dependent way (Fig. 4a). There was no binding of the cells touncoated wells or to wells coated with a recombinant IgG1 Fcfragment prepared and purified from Chinese hamster ovary cells ina way identical to that used to prepare ICAM-1–Fc. The cells were alsospontaneously adherent to the other LFA-1 counter-receptors, ICAM-2 and ICAM-3, as expected if the cellular LFA-1 were indeed in an

adhesive state (Fig. 4b). J+hi1.14 cells also constitutively adhered tothe 40-kilodalton chymotryptic fragment of fibronectin, to an a4b1-specific ligand and to recombinant soluble ICAM-1, which effectivelyruled out the possibility of any substantial contribution from ‘cryptic’Fc receptors (data not shown). Although the J+hi1.14 cells expressedless cell surface LFA-1 than did the wild-type clone Jn.9, the J+hi1.14cells had higher expression of epitopes for the activation-reporterantibodies KIM127 and mAb24 than did wild-type cells (Fig. 4c),similar to that in Jurkat cells treated with the chemokine CXCL4(SDF-1) and analyzed after 3 min (Fig. 4d).

To confirm that the adherence of J+hi1.14 cells to ICAM-1 requiredLFA-1, we treated the J+hi1.14 cells with ethylmethanesulfonate andsubjected them to ‘immunopanning’ on the TS1/22 monoclonal anti-body (mAb) to LFA-1 as described23. After several rounds of selection,a nonadherent population was evident, and we cloned a derivative ofJ+hi1.14, called J+hi1.14-b2.3, that lacked cell surface LFA-1 (Fig. 5a).Adhesion assay of the J+hi1.14-b2.3 cells to purified ICAM-1–Fcshowed complete loss of constitutive and inducible binding, compar-able to that seen with a previously described LFA-1-deficient Jurkatclone, J-b2.7 (ref. 23; Fig. 5b). Thus, we conclude that all observedadhesion of J+hi1.14 to ICAM-1–Fc was mediated by LFA-1.

Inhibition of LFA-1 adhesion by RhoH

If the provirus insertion event in RHOH, the associated decrease insteady-state mRNA expression and the presumed decreased RhoH

kb

23

9.4

6.6

4.4

2.3

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J-β 2

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.35

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.14

a b

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1.35

J+hi

1.10

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Jn.9 J+hi1.14

EcoRV

23

1A 1B 2RHOH

PRhoH1 kb

LTR LTR

EcoRV EcoRV

neor

PLTR

EcoRV EcoRV

Figure 2 One of the two provirus insertions in Jurkat clone J+hi1.14 is in

RHOH. Southern blot analysis of Jn.9 and J+hi1.14 genomic DNA digested

with EcoRV. The provirus carries an EcoRV site in each flanking long

terminal repeat (LTR; left). Black boxes indicate the three RHOH exons(identified below each box). PLTR and PRhoH indicate the promoters for the

retrovirus and RHOH respectively. Hybridization used an exon 1A–specific

probe. Arrowhead indicates position of the disrupted RHOH allele (right).

Left margin, position of 23-kb marker.

Jn.9

J+hi1

.14

J+hi1

.19

2.2-kb RhoH

28S

18S

Figure 3 Blot hybridization analysis of Jurkat clone RNA. Hybridization of a

full-length RHOH cDNA probe to the 2.2-kb band corresponding to the RhoH

mRNA. Bottom, ethidium staining of the gel before transfer shows the 28S

and 18S rRNA subunits, demonstrating comparable loading of the lanes.

Figure 1 Isolation of constitutively adherent Jurkat clones. (a) Analysis of

Jurkat clones by standard ICAM-1 adhesion assay to confirm constitutive

adhesiveness. Vertical axis, percent of input cells adherent after washing,

determined by fluorescence measurements. Error bars indicate standard

deviation of quadruplicate measurements. J-b2.7, negative control;

J+hi1.19, positive control. Jn.9 is the wild-type, parental Jurkat clone.

Clones J+hi1.10, J+hi1.14 and J+hi1.35 are constitutively adhesive for

ICAM-1. (b) Genomic DNA from various clones in a was digested withHindIII and analyzed by Southern blot with a probe specific for neo r carried

by the PU3.1 retrovirus. kb, kilobases.

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inhibitory activity caused the J+hi1.14 cells to express constitutivelyadhesive LFA-1, then reconstitution of the J+hi1.14 cells with func-tional RhoH should cause reversion of the adhesion phenotype to thatof the Jn.9 wild-type cells. We tested this hypothesis in two indepen-dent experiments. In the first, we fused J+hi1.14 cells with thewild-type clone Jn.9, a source of RhoH mRNA, and analyzed thestable, pooled hybrids in the ICAM-1 adhesion assay. The Jn.9 plusJ+hi1.14 hybrid, Jn/1.14, had expression of LFA-1 equivalent to that ofthe parental cells (data not shown) and regained the constitutive, lowadhesion of the wild-type cells (Fig. 6). Furthermore, fusion of theJ+hi1.14 cells with J-lo1.3 (ref. 23), a Jurkat clone that bears adominant negative inhibitor of LFA-1-mediated adhesion, yieldedhybrids, Jlo/1.14 cells, with the J-lo1.3 phenotype (Fig. 7). Theseexperiments also established that the LFA-1 on J+hi1.14 was capableof adopting a ‘low adhesiveness’ conformation. A mutation in Itgalitself (encoding the LFA-1 a-subunit) could not account for the

constitutively adhesive phenotype of J+hi1.14, as an Itgal mutationthat led to a constitutively adhesive LFA-1 protein should yield adominant adhesion phenotype.

To determine definitively whether decreased expression of RhoHcaused LFA-1 to become constitutively adhesive, we transfected theJ+hi1.14 clone, the wild-type Jn.9 clone and an independently derivedconstitutively adhesive clone, J+hi1.19, with a hemagglutinin-taggedRhoH cDNA construct and a green fluorescent protein cDNA in aretrovirus vector. We selected pooled populations of transfectedcells by flow sorting and expanded them in culture. Immunoblotanalysis with antibody to hemagglutinin tag (anti–hemagglutinintag) confirmed similar expression of recombinant protein inthe transfectants (Fig. 7a). An adhesion assay of the pooledRhoH transfectants demonstrated that the constitutively adhesivephenotype of the J+hi1.14 cells had reverted to that of wild-type byreconstitution of RhoH mRNA expression. In contrast, the adhesion

Figure 4 The adhesion of J+hi1.14 cells to

ICAM-1–Fc is dependent on LFA-1. (a) Antibody-

mediated inhibition of cell adhesion to ICAM-1–

Fc–coated wells. Wild-type Jn.9, the nonadhesive

clone J-lo1.3 and J+hi1.14 were analyzed with

the standard ICAM-1 adhesion assay. The cells

and/or wells were preincubated with antibody for

30 min at 24 1C before the start of the assay. B,adhesion-blocking mAb; N, mAb to the same

molecule and of the same isotype but that does

not block adhesion (control). For the LFA-1 a-

subunit (Anti-aL), the blocking mAb was TS1/22

and the nonblocking mAb was TS2/4.

For the LFA-1 b-subunit (Anti-b2), the blocking

mAb was TS1/18 and the nonblocking mAb was

CBR LFA1/7. The ICAM-1 blocking mAbs were

RR1/1 (left) and R6.5 (right). The adhesion

assay was repeated five times; a representative

experiment is shown. P o 0.01, mean values of

unstimulated adhesion of J+hi1.14 versus that

of Jn.9 or J+hi1.14 treated with adhesion-

blocking mAb (Student’s t-test). (b) Binding of

J+hi1.14 cells to ICAM-2 and ICAM-3. Adhesion

assays and data presentation are described in

Methods; however, plates were coated with ICAM-

2–Fc or ICAM-3–Fc instead of ICAM-1–Fc. The

adhesion assay was repeated three times; arepresentative experiment is presented.

(c) Activation epitope expression on J+hi1.14 cells. Cells were stained with an activation-reporter mAb (above plots) at 37 1C. Histograms represent cell

number versus fluorescence intensity. Inset numbers in histograms indicate percentage fluorescence of each antibody normalized to staining with TS1/22.

Vertical dashed lines used for comparison of histograms. (d) Activation epitope expression on Jn.9 cells after treatment of the cells with CXCL4. TS1/22,

positive control for LFA-1; X63, negative control antibody. Histogram shows cell number versus fluorescence intensity. Dotted lines indicate LFA-1 bearing

the KIM127 activation epitope with and without treatment of the cells with CXCL4.

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Jn.9J+hi1.14J-lo1.3

None Anti-β2 Anti-αL Anti-ICAM-1

B BN N 0

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PMA (50 ng/ml)

Unstimulated

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ICAM-2 ICAM-3ba

103102101 104

X63

KIM127

TS1/22

Relative fluorescence

KIM127 + CXCL4

200

0

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0

HBSS TS1/22 KIM127 mAb24

Jn.9

J+hi1.14

104103102101100103102101100103102101100103102101

100% 4.8% 2%

100% 6% 7.5%

0%

0%

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J+hi1.14J-β2.7 1.14-β2.3Jn.9

b

Jn.9

J+hi1.14

J-β2.7

2/1A4.1 (neg)

TS1/18(anti-β2)

Cel

l num

ber

Relative fluorescence

200

200

200

0

a TS2/16(anti-β1)

104103103 102102 101101103102101103102101100

TS2/4(anti-αL)

200

1.14-β2.3

Unstimulated

Figure 5 An LFA-1-deficient clone derived from

J+hi1.14 fails to bind to ICAM-1. (a) Surface

expression of integrins by flow cytometry. Jn.9

and J-b2.7 are wild-type and LFA-1-deficient

Jurkat cells, respectively (positive and negative

controls, respectively, for LFA-1 expression);

J+hi1.14-b2.3 is an LFA-1-deficient subclone of

J+hi1.14; 2/1A4.1 is a CD16 mAb (negative

control); TS1/18 and TS2/4 are LFA-1 b-subunitand a-subunit mAbs, respectively; TS2/16 is a

CD29 (b1 integrin) mAb (staining control).

(b) Assay of adhesion to ICAM-1–Fc. The

experiment was done twice; a representative

experiment is presented. P o 0.001, mean

values for adhesion of J+hi1.14 versus that of

J+hi1.14-b2.3 (Student’s t-test).

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to ICAM-1 of the J+hi1.19 clone, which bears a distinct mutation, wasunaffected (Fig. 7b).

To confirm that the inhibitory function of RhoH noted in JurkatT cells was physiologically relevant, we prepared human peripheralblood leukocytes by Ficoll density gradient centrifugation andcultured them for 5–7 d in the presence of phytohemagglutinin. Weinfected these cells, as well as control Jurkat T cell clones, with alentivirus vector24 that directed expression of an interfering RNA

specific for RhoH. After an additional 2 d of culture, we testedthe cells for adhesion to recombinant soluble ICAM-1 using apublished assay25. Downregulation of RhoH mRNA by 50–90%resulted in constitutive adhesion of the wild-type Jurkat cells andthe lymphocytes to purified ICAM-1 (Fig. 8). We concludethat expression of a threshold amount of functional RhoH wasrequired for maintenance of the default low adhesive state ofLFA-1 in Jurkat T cells and in peripheral blood lymphocytes treatedwith phytohemagglutinin.

DISCUSSION

Here, we prepared a library of Jurkat T cells bearing on average 1.3retroviral insertions per cell and subjected it to selection for adysregulated LFA-1-mediated adhesion phenotype such that the cellswere highly adhesive to ICAM-1 in the absence of any cellularstimulation. In one of the three cell clones obtained, we identifiedRhoH as being an essential element of the mechanism whereby LFA-1is maintained in a nonadhesive state on resting lymphocytes.

RhoH acts by inhibiting the effects of other Rho family GTPases.The activation of transcription factor NF-kB by Rac1, cdc42 andRhoA, for example, is suppressed by RhoH. Similarly, activation of thep38 MAP kinase by Rac1 and cdc42 is strongly suppressed by RhoH.Thus, RhoH seems to be a pivotal Rho family member that canmodulate the effects of other small GTP-binding proteins21. Severalstudies in lymphocytes have indicated involvement of RhoA as aregulator of LFA-1-mediated adhesion, although its effects are com-plex. RhoA is involved in cellular de-adhesion26, and distinct regionsof RhoA are involved in triggering both LFA-1 affinity modulationand avidity changes in response to treatment of cells with thechemokine CCL21 (ref. 27). Rac1 and H-ras are involved in integ-rin-dependent migration in monocytes28, but there are little data tosupport the idea of direct involvement of these proteins in theactivation of lymphocyte LFA-1. Cytohesin 1 has been reported toregulate LFA-1-mediated adhesion in lymphocytes and to interactwith ADP ribosylation factor small GTPases29; however, it is unclear ifthis activity is governed by RhoH.

0

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Jn.9

Mn2+ (1 mM)

Unstimulated

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100

J+hi1.14 J-lo1.3 Jn/1.14 Jlo/1.14

Figure 6 The adhesion phenotype of J+hi1.14 is reverted by cell fusion.

J+hi1.14 cells were fused with the wild-type clone Jn.9 or with the J-lo1.3

clone, which bears a dominant inhibitor of LFA-1-mediated adhesion. Pooled

hybrids Jn/1.14 and Jlo/1.14 were tested for adhesion to ICAM-1–Fc. Error

bars indicate standard deviation. The adhesion assay was repeated tree

times; a representative experiment is presented. P o 0.01, mean values of

unstimulated adhesion of J+hi1.14 versus that of Jn/1.14 or Jlo/1.14

(Student’s t-test).

20

40

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80

Jn.9 PHAblasts

J-β2.7

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J+hi1.140

Figure 8 RhoH RNA interference in human peripheral blood T cells leads toa constitutively adherent phenotype. Wild-type Jn.9 Jurkat cells, J+hi1.14

cells, the negative control J-b2.7 cells and human peripheral blood

lymphocytes (PHA blasts) treated with phytohemagglutinin were subjected to

lentivirus-mediated transfection. Open bars, cells infected with virus

prepared from the PLL3.7 vector only; filled bars, cells infected with virus

prepared from the PLL-hu5RhoH plasmid that directs the expression of a

small interfering RNA specific for RhoH; shaded bars, uninfected cells

stimulated with PMA (50 ng/ml), included as controls for the adhesion

assay. After 2–3 d in culture, cells were tested for adhesion to purified,

recombinant ICAM-1. The experiment was repeated twice; a representative

assay is presented. P o 0.01, mean value of the unstimulated adhesion of

PHA blasts transfected with the control virus versus PHA blasts transfected

with the virus producing a small interfering RNA specific for RhoH

(Student’s t-test).

0

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Jn.9 J+hi1.19

J+hi1.14

PMA (50 ng/ml)

No stimulation

Jn.9 J+hi1.19

J+hi1.14

Vector transfection RhoH cDNA

Con

trol

a b

J+hi

1.14

J+hi

1.19

Jn.9

Figure 7 Transfection with RhoH cDNA reverts the J+hi1.14 phenotype

to that of wild-type cells. Wild-type Jn.9 cells, J+hi1.14 cells and the

independently derived, constitutively adhesive J+hi1.19 cells weretransfected with a retrovirus vector that directs expression of hemagglutinin-

tagged RhoH and green fluorescent protein via a bicistronic transcript.

(a) Analysis of lysates of stably transfected cells by immunoblot with anti-

hemagglutinin tag. Arrow indicates hemagglutinin-RhoH. (b) Adhesion assay

of cells transfected with a vector control or the retrovirus encoding RhoH

and green fluorescent protein. The experiment was repeated twice; a

representative assay is presented. P o 0.01, mean value of unstimulated

adhesion of J+hi1.14 versus that of hemagglutinin-RhoH–transfected

J+hi1.14 cells (Student’s t-test).

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Rap1 is a small GTPase reported to regulate both LFA-1 and a4b1

fibronectin receptor–mediated adhesion in lymphocytes. Overexpres-sion of a Rap1GAP, SPA-1, or expression of a dominant negativemutant of Rap1 leads to decreased basal and phorbol 12-myristate13-acetate (PMA)–triggered LFA-1-mediated adhesion to ICAM-1(refs. 26,30). Consistent with the possibility of involvement of Rap1in the regulation of cell adhesion, expression in lymphoid cells of aconstitutively activated form of Rap1 led to augmented adhesionmediated by LFA-1 and a4b1 (refs. 31,32). Furthermore, Rap1 isactivated over the same time course during which cell adhesiondevelops in lymphocytes treated with the chemokines CCL21 orCXCL4 (ref. 32), although de-adhesion may be regulated indepen-dently of Rap1 (ref. 32). We tested the activity of a4b1 in J+hi1.14 cellsand found that the integrin was constitutively adhesive, comparedwith basal a4b1-mediated adhesion in wild-type Jn.9 cells (data notshown). Thus, the integrins that show increased adhesiveness in theJ+hi1.14 cells are the same as those that become more adhesive whenRap1 is activated. The hypothesis most consistent with available datais that RhoH may inhibit the activity of Rap1, thus rendering LFA-1relatively nonadhesive on resting leukocytes, which are the only cellsthat express RhoH21,22.

Unlike conventional Rho family GTPases, RhoH is GTPasedeficient and therefore remains in a GTP-bound, constitutivelyactivated state21. Consequently, RhoH activity is not dynamicallyregulated by the cycle of GDP-GTP exchange but perhaps only overmuch longer intervals by ‘titration’ of the amount of mRNA expres-sion and presumably the corresponding amount of protein. We foundan increase in LFA-1-mediated adhesion in Jurkat T cells and inperipheral blood lymphocytes after a decrease in steady-state RhoHmRNA; however, an increase in steady-state RhoH mRNA by trans-fection of wild-type Jn.9 cells with the RhoH cDNA did not affectLFA-1-mediated adhesion, which suggested that the effects ofRhoH are maximal in resting cells. The physiological signals thatmodulate RhoH mRNA and protein expression, if any, remain to bedetermined, although phorbol ester treatment of Jurkat cells andtreatment of purified T cells with anti-CD3, both of which triggerLFA-1-mediated adhesion, decrease steady state expression of RhoHmRNA21. Given the broad inhibitory effects of RhoH on manylymphocyte processes, including integrin-mediated adhesion, and itslack of GTPase activity, we hypothesize that RhoH may be the ‘default’,initial inhibitory signal for shutting off activation pathways in restingleukocytes. Signaling pathways could be activated selectively byappropriate signals that enter downstream of RhoH, for example,from a chemokine receptor to Rap1, thus overriding the inhibitoryeffects of RhoH.

METHODSInsertional mutagenesis. The PU3.1 retrovirus was prepared by replacement of

the pol and env genes in wild-type Moloney retrovirus with a neor expression

cassette from the U3Hygro retrovirus33 (E. Ruley, Emory University, Atlanta,

Georgia). PU3.1 has an intact long terminal repeat promoter and enhancer and

is able to alter gene expression by cis activation of adjacent genes or by

interference with splicing or transcription34. Amphotrophic producer cells

GP+ envAm12 (A. Bank, Columbia University, New York, New York) were

cultured and transfected as described35. A clone was selected, GP+ envAm12/

PU3.1, that produced 5 � 105 colony-forming units/ml of PU3.1 retrovirus in

the conditioned media. Cloned wild-type Jurkat cells, Jn.9 (ref. 23), were used

as the starting population to be infected. Jn.9 cells at a density of 4 � 105 cells/

ml were cultured together for 24 h with GP+ envAm12/PU3.1 cells at 30%

confluence in 20 T175 flasks with 8 mg/ml of polybrene. After infection, Jurkat

cells were washed off the adherent producer cells and were serially passaged for

removal of any contaminating producer cells by adherence. The library of

infected Jurkat cells was amplified by culture in roller bottles, then was divided

into aliquots and stored in liquid nitrogen.

Selection for Jurkat cells bearing constitutively active LFA-1. The selection

strategy entailed applying the unstimulated library of Jn.9 cells bearing provirus

insertions onto 10-cm plates (LabTek, Fisher Scientific) coated with purified

ICAM-1–Fc23 at a concentration of 0.8 mg/ml in 50 mM Tris, pH 9.0. The

concentration of ICAM-1–Fc chosen was such that the wild-type Jn.9 Jurkat

cells did not bind in the absence of stimulation, but bound when treated with

50 ng/ml of PMA. Cells were allowed to adhere to the coated dishes at 37 1C for

35 min, after which nonadherent cells were gently washed away. After four

cycles of this selection, a spontaneously adherent population of cells was

evident, and these were cloned and analyzed.

Analysis of provirus insertion sites. Southern blot analysis of genomic DNA

purified from Jurkat clones and digested with HindIII used a 637-bp probe

specific for neor, prepared by amplification with PCR using the Neo3.24

(5¢-TCAGAAGAACTCGTCAAGAAGGCG-3¢) and Neo5.25 (5¢-CTGAAT-

GAACTGCAGGACGAGGCAG-3¢) primers. The number of hybridizing frag-

ments in each lane was considered to represent the number of provirus

insertion events for the clone. Genomic sequence adjacent to the provirus

insertion was amplified by PCR36 (Clontech) with retroviral long terminal

repeat primers LT5.28 (5¢-CCAATAAACCCTCTTGCAGTTGCATCCG-3¢) and

LT6.27 (5¢-GGTCTCCTCTGAGTGATTGACTACCCG-3¢). Amplified DNA

fragments were subjected to automated nucleotide sequencing with the

LT7.18 primer (5¢-CGTCAGCGGGGGTCTTTC-3¢). The genomic location of

the provirus insertion was determined by BLAST analysis of the Human

Genome Database (http://www.ncbi.nlm.nih.gov/genome/seq/HsBlast.html)

and was confirmed by Southern blot analysis with a unique probe specific

for a gene adjacent to the insertion. For analysis of RHOH, we obtained a

1,409-bp cDNA (ID 302591) through the IMAGE consortium37.

ICAM adhesion assay. The adhesion assay with wells coated Fc fusion proteins

of ICAM-1, ICAM-2 or ICAM-3 was done as described23 and every sample was

assayed in quadruplicate. Adherence is reported as the percent of input cells

bound and was calculated as follows: [(washed fluorescence – background) /

(total fluorescence – background)] � 100. For adhesion assays of peripheral

blood lymphocytes, purified soluble ICAM-1 was substituted for the ICAM-1–

Fc fusion protein because a substantial fraction of the cells expressed Fc

receptors and adhered to the Fc region independently of LFA-1.

Cell lines, cell clones, cell fusion experiments and flow cytometry. Jn.9 is a

wild-type Jurkat T cell clone23. J-b2.7 is a Jurkat mutant that lacks cell surface

LFA-1 (ref. 25) and was included in some assays as a negative control. J-lo1.3 is

a Jurkat mutant bearing a dominant negative inhibitor of LFA-1-mediated

adhesion23 and was also included as a negative control. J+hi1.19 is a

constitutively adhesive clone that was derived independently from wild-type

Jurkat after radiation mutagenesis (data not shown). Cell fusion experiments

and flow cytometry were done as described23, the activation-reporter mAb

KIM127 and mAb24 were used at 37 1C and all other steps were done on ice.

Flow cytometry of the hybrid cell pool with propidium iodide staining

confirmed a DNA content greater than 2N, as expected for a mixture of

hybrid cells.

Monoclonal antibodies. The hybridoma clones that produce mAbs TS1/22

(anti-CD11a), TS1/18 (anti-CD18), TS2/4 (anti-CD11a), TS2/16 (anti-CD29)

and P3X63 (negative control IgG1) were obtained from American Type Culture

Collection. The hybridoma producing mAb 2/1A4.1 (anti-CD16) has been

described38. The hybridoma producing mAb YZ1 (ref. 39; anti-CD35) was

obtained originally from R. Jack (Triad Pharmaceuticals, San Diego,

California). The mAb CBR LFA1/7 (ref. 40; anti-CD18) was provided by

T. Springer (CBR Institute, Boston, Massachusetts) and the ICAM-1 (CD54)

mAbs RR1/1 and R6.5 (ref. 41) were gifts from R. Rothlein (Boehringer-

Ingelheim, Ridgefield, Connecticut). The activation-reporter mAbs to CD18

included mAb24 (ref. 42), a gift from N. Hogg (Imperial Cancer Research

Fund, London, UK) and KIM127 (ref. 43), a gift from M. Robinson (Celltech,

Slough, UK). Purified mAb was prepared from conditioned media of each

hybridoma by protein A affinity chromatography.

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Expression of hemagglutinin-tagged RhoH. The Kozak consensus translation

initiation sequence44 and a hemagglutinin epitope tag were introduced into the

N-terminal region of the human wild-type human RHOH cDNA with a PCR-

based technique and the following primers: 5¢-CTCTCTCGAGAGCGCCTTG-

TAGAAGCGCGTATGGCTTC-3¢ and 5¢-ATGAATTCTTAGAAGATCTTG-

CACTCATTGATGGAGAAGA-3¢. The PCR fragment was digested with

XhoI and EcoRI (sites underlined above) and was subcloned into the XhoI

and EcoRI sites of the bicistronic retroviral expression vector, MIG-w, which

contained an internal ribosomal entry site–enhanced green fluorescent protein

expression cassette (provided by L. van Parijs, Massachusetts Institute of

Technology, Cambridge, Massachusetts) to yield the plasmid PLL-hu5RhoH.

A retroviral stock was produced by transient transfection of 293T cells with the

retroviral plasmid together with the packaging plasmids as described45. At 48 h

after transfection, retroviral supernatant was collected, filter-sterilized (0.45 mm

filter pore size) and stored at �80 1C. Viral titers of about 1 � 106 colony-

forming units/ml were used to infect Jurkat cells in the presence of polybrene

(8 mg/ml). After 72 h, green fluorescent protein–positive cells were isolated by

flow sorting and the pooled green fluorescent protein–positive cells were used

for adhesion assays.

Immunoblot and nucleic acid blot hybridization analysis. Cells transfected

with the retrovirus that directs expression of hemagglutinin-RhoH were lysed

in 20 mM Tris-HCl, pH 7.5, 100 mM NaCl, 1 mM EDTA, 1% Nonidet-P40,

1 mM dithiothreitol, 1 mM phenylmethylsulfonylfluoride, 5 mg/ml of leupeptin

and 5% glycerol. Whole-cell lysates (30 mg) were resolved by 10% SDS-PAGE

and the proteins were transferred to polyvinyldifluoride membranes. Blocked

membranes were probed for 1 h at 25 1C with a 1:1,000 dilution of

anti-hemagglutinin (sc-7392; Santa Cruz Biotech), followed by horseradish

peroxidase–conjugated secondary antibody, and immunoreactive bands

were visualized with the enhanced chemiluminescence system (Amersham

Pharmacia Biotech). RNA hybridization and Southern blots were done by

hybridization of 32P-labeled probe fragments after transfer of nucleic acids to

nitrocellulose membranes with only minor modifications of the original

published protocols46,47.

RNA interference. A lentivirus-based system was used as described24. The

sequence 5¢-AAGGCTTGGGCCGCTTTTGTTTT-3¢ from the 5¢ untranslated

region of RhoH mRNA, base pairs 147–169 (inclusive) from GenBank

accession number Z35227.1, was subcloned into the PLL3.7 vector, and

conditioned media obtained after transient transfection of 293T cells was used

to transduce Jurkat clones and human lymphocytes. RT-PCR detected

a decrease in steady-state mRNA abundance of 50–90%, depending on

the experiment.

ACKNOWLEDGMENTSWe thank X. Bu for technical assistance. Supported by National Institutes ofHealth (F32AR08632 to L.K.C., R01DK047636 to B.L. and R01AR47243 toL.B.K.), the Leukemia and Lymphoma Society (B.L.), the Lymphoma ResearchFoundation (X.L.) and the Arthritis Foundation (P.S.).

COMPETING INTERESTS STATEMENTThe authors declare that they have no competing financial interests.

Received 11 March; accepted 24 June 2004

Published online at http://www.nature.com/natureimmunology/

1. Springer, T.A. Adhesion receptors of the immune system. Nature 346, 425–434(1990).

2. Marlin, S.D. & Springer, T.A. Purified intercellular adhesion molecule-1 (ICAM-1)is a ligand for lymphocyte function-associated antigen 1 (LFA-1). Cell 5, 813–819(1987).

3. Staunton, D.E., Dustin, M.L. & Springer, T.A. Functional cloning of ICAM-2,a cell adhesion ligand for LFA-1 homologous to ICAM-1. Nature 339, 61–64(1989).

4. de Fougerolles, A.R., Stacker, S.A., Schwarting, R. & Springer, T.A. Characterizationof ICAM-2 and evidence for a third counter-receptor for LFA-1. J. Exp. Med. 174,253–267 (1991).

5. Ostermann, G., Weber, K.S., Zernecke, A., Schroder, A. & Weber, C. JAM-1 is a ligandof the b2 integrin LFA-1 involved in transendothelial migration of leukocytes. Nat.Immunol. 3, 151–158 (2002).

6. Tagaki, J. & Springer, T.A. Integrin activation and structural rearrangement. Immunol.Rev. 186, 141–163 (2002).

7. Hogg, N., Henderson, R., Leitinger, B., Porter, A. & Stanley, P. Mechanisms contri-buting to the activity of integrins on leukocytes. Immunol. Rev. 186, 164–171(2002).

8. Liddington, R.C. & Ginsberg, M.H. Integrin activation takes shape. J. Cell Biol. 158,833–839 (2002).

9. Arnaout, M.A. Integrin structure: new twists and turns in dynamic cell adhesion.Immunol. Rev. 186, 125–140 (2002).

10. Hynes, R.O. Integrins: Bidirectional, allosteric signaling machines. Cell 110, 673–687(2002).

11. Rothlein, R. & Springer, T.A. The requirement for lymphocyte function-associatedantigen 1 in homotypic leukocyte adhesion stimulated by phorbol ester. J. Exp. Med.163, 1132–1149 (1986).

12. Chatila, T.A., Geha, R.S. & Arnaout, M.A. Constitutive and stimulus-induced phosphor-ylation of CD11/CD18 leukocyte adhesion molecules. J. Cell. Biol. 10, 3435–3444(1989).

13. van Kooyk, Y., Weder, P., Heije, K. & Figdor, C.G. Extracellular Ca2 + modu-lates leukocyte function-associated antigen-1 cell surface distribution on T lympho-cytes and consequently affects cell adhesion. J. Cell Biol. 124, 1061–1070(1994).

14. Dustin, M.L. & Springer, T.A. T-cell receptor cross-linking transiently stimulatesadhesiveness through LFA-1. Nature 341, 619–624 (1989).

15. Szabo, M.C., Butcher, E.C., McIntyre, B.W., Schall, T.J. & Bacon, K.B. RANTESstimulation of T lymphocyte adhesion and activation: role for LFA-1 and ICAM-3.Eur. J. Immunol. 27, 1061–1068 (1997).

16. Weber, K.S., Klickstein, L.B., Weber, P.C. & Weber, C. Chemokine-induced monocytetransmigration requires cdc42-mediated cytoskeletal changes. Eur. J. Immunol. 28,2245–2251 (1998).

17. Larson, R.S., Hibbs, M.L. & Springer, T.A. The leukocyte integrin LFA-1 reconstitutedby cDNA transfection in a nonhematopoietic cell line is functionally active and nottransiently regulated. Cell Regul. 1, 359–367 (1990).

18. Johnston, S.C., Dustin, M.L., Hibbs, M.L. & Springer, T.A. On the species specificityof the interaction of LFA-1 with intercellular adhesion molecules. J. Immunol. 145,1181–1187 (1990).

19. O’Toole, T.E. et al. Integrin cytoplasmic domains mediate inside-out signal transduc-tion. J. Cell Biol. 124, 1047–1059 (1994).

20. Weber, C., Alon, R., Moser, B. & Springer, T.A. Sequential regulation of a4b1 and a5b1

integrin avidity by CC chemokines in monocytes: implications for transendothelialchemotaxis. J. Cell Biol. 134, 1063–1073 (1996).

21. Li, X. et al. The hematopoiesis-specific GTP-binding protein RhoH is GTPase deficientand modulates activities of other Rho GTPases by an inhibitory function. Mol. Cell.Biol. 22, 1158–1171 (2002).

22. Dallery, E. et al. TTF, a gene encoding a novel small G protein, fuses to the lymphoma-associated LAZ3 gene by t(3,4) chromosomal translocation. Oncogene. 10, 2171–2178 (1995).

23. Cherry, L.K., Weber, K.S.C. & Klickstein, L.B. A dominant Jurkat T cell mutation thatinhibits LFA-1-mediated cell adhesion is associated with increased cell growth.J. Immunol. 167, 6171–6179 (2001).

24. Rubinson, D.A. et al. A lentivirus-based system to functionally silence genes in primarymammalian cells, stem cells and transgenic mice by RNA interference. Nat. Genet.33, 401–406 (2003).

25. Weber, K.S.C., York, M.R., Springer, T.A. & Klickstein, L.B. Characterization oflymphocyte function-associated antigen 1 (LFA-1) deficient T cell lines. The aL andb2 subunits are interdependent for cell surface expression. J. Immunol. 158, 273–279(1997).

26. Liu, L., Schwartz, B.R., Lin, N., Winn, R.K. & Harlan, J.M. Requirement forRho A kinase activation in leukocyte de-adhesion. J. Immunol. 169, 2330–2336(2002).

27. Giagulli, C. et al. RhoA and zeta PKC control distinct modalities of LFA-1 activation bychemokines: critical role of LFA-1 affinity triggering in lymphocyte in vivo homing.Immunity. 20, 25–35 (2004).

28. Weber, K.S. et al. Dual role of H-Ras in regulation of lymphocyte function antigen-1activity by stromal cell-derived factor-1a: implications for leukocyte transmigration.Mol. Biol. Cell. 12, 3074–3086 (2001).

29. Geiger, C. et al. Cytohesin-1 regulates b-2 integrin-mediated adhesion throughboth ARF-GEF function and interaction with LFA-1. EMBO J. 19, 2525–2536(2000).

30. Liu, L. et al. The GTPase Rap1 regulates phorbol 12-myristate 13-acetate-stimulatedbut not ligand-induced b1 integrin-dependent leukocyte adhesion. J. Biol. Chem. 277,40893–40900 (2002).

31. Sebzda, E., Bracke, M., Tugal, T., Hogg, N. & Cantrell, D.A. Rap1A positively regulatesT cells via integrin activation rather than inhibiting lymphocyte signaling. Nat.Immunol. 3, 251–258 (2002).

32. Shimonaka, M. et al. Rap1 translates chemokine signals to integrin activation, cellpolarization, and motility across vascular endothelium under flow. J. Cell Biol. 161,417–427 (2003).

33. Chang, W., Hubbard, S.C., Friedel, C. & Ruley, H.E. Enrichment of insertionalmutants following retrovirus gene trap selection. Virology 193, 737–747(1993).

34. Soriano, P., Cone, R.D., Mulligan, R.C. & Jaenisch, R. Tissue-specific and ectopicexpression of genes introduced into trangenic mice by retroviruses. Science 234,1409–1413 (1986).

966 VOLUME 5 NUMBER 9 SEPTEMBER 2004 NATURE IMMUNOLOGY

A R T I C L E S©

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tp://

ww

w.n

atur

e.co

m/n

atur

eim

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olog

y

35. Markowitz, D., Goff, S. & Bank, A. Construction and use of a safe and efficientamphotropic packaging cell line. Virology 167, 400–406 (1988).

36. Siebert, P.D., Chenchik, A., Kellogg, D.E., Lukyanov, K.A. & Lukyanov, S.A. Animproved PCR method for walking in uncloned genomic DNA. Nucleic Acids Res.23, 1087–1088 (1995).

37. Lennon, G., Auffray, C., Polymeropoulos, M. & Soares, M.B. The I.M.A.G.E. consortium:an integrated molecular analysis of genomes and their expression. Genomics. 33,15–52 (1996).

38. Otabor, I. et al. A role for lipid rafts in C1q-triggered O2- generation by human

neutrophils. Mol. Immunol. 41, 185–190 (2004).39. Changelian, P.S., Jack, R.M., Collins, L.A. & Fearon, D.T. PMA induces the ligand-

independent internalization of CR1 on human neutrophils. J. Immunol. 134,1851–1858 (1985).

40. Petruzzelli, L., Maduzia, L. & Springer, T.A. Activation of lymphocyte function-associated molecule-1 (CD11a/CD18) and Mac-1 (CD11b/CD18) mimicked by anantibody directed against CD18. J. Immunol. 155, 854–866 (1995).

41. Rothlein, R., Dustin, M.L., Marlin, S.D. & Springer, T.A. A human intercellular adhesionmolecule (ICAM-1) distinct from LFA-1. J. Immunol. 137, 1270–1274 (1986).

42. Landis, R.C., Bennett, R.I. & Hogg, N. A novel LFA-1 activation epitope maps to the Idomain. J. Cell Biol. 120, 1519–1527 (1993).

43. Andrew, D. et al. KIM185 a monoclonal antibody to CD18 which induces a change inthe conformation of CD18 and promotes both LFA-1- and CR3-dependent adhesion.Eur. J. Immunol. 23, 2217–2222 (1993).

44. Kozak, M. Point mutations define a sequence flanking the AUG initiator codon thatmodulates translation by eukaryotic ribosomes. Cell 44, 283–292 (1986).

45. Ory, D.S., Neugeboren, B.A. & Mulligan, R.C. A stable human-derived packaging cellline for production of high titer retrovirus/vesicular stomatitis virus G pseudotypes.Proc. Natl. Acad. Sci. USA 93, 11400–11406 (1996).

46. Southern, E.M. Detection of specific sequences among DNA fragments separated bygel electrophoresis. J. Mol. Biol. 98, 503–517 (1975).

47. Thomas, P.S. Hybridization of denatured RNA and small DNA fragments transferredto nitrocellulose. Proc. Natl. Acad. Sci. USA 77, 5201–5205 (1980).

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