Interplay between BMP4 and IL-7 in human intrathymic precursor cells

Bone morphogenetic proteins (BMPs) play a pivotal role during vertebrate embryogenesis and organogenesis, and have also been described to function in regulating cell fate and determination in self-renewing tissues in adults. Recent results have demonstrated that the different components of the BMP2/4 signaling pathway are expressed in the human thymus. In this study, we provide evidence that BMP4 and IL-7 interplay is important in the maintenance of the human thymic progenitor population. Intrathymic CD34⁺ cells express BMP receptors (BMPRIA, BMPRIB, ActRIA, BMPRII), signal transduction molecules (Smad1, 5, 8 and 4), and produce BMP4. Neutralization of endogenous BMP4 by treatment with the antagonist Noggin reduces thymic precursor cell survival, and the addition of exogenous BMP4 decreases their proliferation. The treatment of chimeric human-mouse fetal thymus organ cultures with BMP4 inhibits cell expansion, arrests thymocyte differentiation, and leads to the accumulation of human CD34⁺ precursor cells. This effect is mainly attributed to the ability of BMP4 to counteract the IL-7-induced proliferation and differentiation of CD34⁺ cells. BMP4 down-regulates in the precursor cell population the expression of CD127 and inhibits the IL-7-dependent STAT5 phosphorylation. In addition, BMP signaling is promoted by IL-7. Our results also demonstrate that in thymic progenitors BMPs act downstream of Sonic Hedgehog, previously described to function as a maintenance factor for human intrathymic CD34⁺ precursor cells.     

Differentiation patterns of CD4/CD8 thymocyte subsets in cocultures of fetal thymus and lymphohemopoietic cells from c-fos transgenic and normal mice.

This study examined the involvement of c-fos protooncogene in thymocyte development from lymphohemopoietic T cell progenitors, within the thymic microenvironment. We first analyzed the thymocytes developing in vitro in the fetal thymus from the c-fos transgenic mice and found a high proportion of CD4+ single positive (SP) cells. We then seeded either fetal liver or bone marrow (BM) cells from normal donors onto lymphocyte-depleted fetal thymus explants of c-fos transgenic mice. The results showed an increased proportion of mature CD4+ SP and decreased CD4+CD8+ double positive (DP) cells. A similar pattern of CD4/CD8 thymocyte subsets was observed when either thymus or BM cells from c-fos transgenic mice developed within a normal thymic stroma. The kinetics of thymocyte development in organ culture (from Days 3 to 11) suggested that the SP cells obtained under these conditions may have bypassed the CD4+CD8+ DP phase. It appears that the altered pattern of thymocyte development manifested in adult c-fos transgenic mice can be induced by the early embryonic thymic stroma, and may also involve cells in the lymphohemopoietic tissues.     

Bone morphogenetic protein 2/4 signaling regulates early thymocyte differentiation

Bone morphogenetic protein (BMP)2 and BMP4 are involved in the development of many tissues. In this study, we show that BMP2/4 signaling is involved in thymocyte development. Our data suggest that termination of BMP2/4 signaling is necessary for differentiation of CD44(+)CD25(-)CD4(-)CD8(-) double negative (DN) cells along the T cell lineage. BMP2 and BMP4 are produced by the thymic stroma and the requisite BMP receptor molecules (BMPR-1A, BMPR-1B, BMPR-II), and signal transduction molecules (Smad-1, -5, -8, and -4) are expressed by DN thymocytes. BMP4 inhibits thymocyte proliferation, enhances thymocyte survival, and arrests thymocyte differentiation at the CD44(+)CD25(-) DN stage, before T cell lineage commitment. Neutralization of endogenous BMP2 and BMP4 by treatment with the antagonist Noggin promotes and accelerates thymocyte differentiation, increasing the expression of CD2 and the proportion of CD44(-)CD25(-) DN cells and CD4(+)CD8(+) double-positive cells. Our study suggests that the BMP2/4 pathway may function in thymic homeostasis by regulating T cell lineage commitment and differentiation.     

Gads-/- mice reveal functionally distinct subsets of TCRbeta+ CD4-CD8- double-negative thymocytes

TCRbeta expression in CD4(-)CD8(-) double-negative (DN) thymocytes induces signaling pathways that promote survival and proliferation, as well as differentiation into CD4(+)CD8(+) double-positive thymocytes. The signaling pathways that regulate survival, proliferation, and differentiation remain unclear. We used Gads-deficient mice to investigate the signaling pathways that regulate these cell fates. During this investigation, we focused on TCRbeta(+) DN thymocytes and found that there are at least three functionally distinct subsets of TCRbeta(+) DN thymocytes: TCRbeta(+) DN3E, TCRbeta(+) DN3L, and TCRbeta(+) DN4. Survival and proliferation of TCRbeta(+) DN3E were independent of Gads, but survival and proliferation of TCRbeta(+) DN3L cells were Gads dependent. Likewise, expression of Bcl-2 in TCRbeta(+) DN3E cells was Gads independent, but Gads was necessary for Bcl-2 expression in TCRbeta(+) DN3L cells. Bcl-2 expression was not dependent on Gads in TCRbeta(+) DN4 cells, but proliferation of TCRbeta(+) DN4 cells was Gads dependent. Gads was not required for the differentiation of DN thymocytes into DP thymocytes. In fact, Gads(-/-) DN3E cells differentiated into DP thymocytes more readily than wild-type cells. We conclude that signaling pathways required to initiate TCRbeta-induced survival and proliferation are distinct from the pathways that maintain survival and proliferation. Furthermore, signaling pathways that promote survival and proliferation may slow differentiation.     

Reduced generation but efficient TCR beta-chain selection of CD4+8+ double-positive thymocytes in mice with compromised CD3 complex signaling

Maturation to the CD4+8+ double-positive (DP) stage of thymocyte development is restricted to cells that have passed TCRbeta selection, an important checkpoint at which immature CD4-8- double-negative (DN) cells that express TCRbeta polypeptide chains are selected for further maturation. The generation of DP thymocytes following TCRbeta selection is dependent on cellular survival, differentiation, and proliferation, and the entire process appears to be mediated by the pre-TCR/CD3 complex. In this study, we investigate the signaling requirements for TCRbeta selection using mice single deficient and double deficient for CD3zeta/eta and/or p56lck. While the numbers of DP cells are strongly reduced in the single-deficient mice, a further drastic reduction in the generation of DP thymocytes is seen in the double-deficient mice. The poor generation of DP cells in the mutant mice is primarily due to an impaired ability of CD25+ DN thymocytes to proliferate following expression of a TCRbeta-chain. Nevertheless, the residual DP cells in all mutant mice are strictly selected for expression of TCRbeta polypeptide chains. DN thymocytes of mutant mice expressed TCRbeta and CD3epsilon at the cell surface and contained mRNA for pre-Talpha, but not for clonotypic TCRalpha-chains, together suggesting that TCRbeta selection is mediated by pre-TCR signaling in all cases. The data suggest differential requirements of pre-TCR signaling for cell survival on the one hand, and for the proliferative burst associated with TCRbeta selection on the other.     

p53 prevents maturation of T cell development to the immature CD4-CD8+ stage in Bcl11b-/- mice

Signaling pathways such as the pre-TCR and Wnt pathways regulate alpha/beta T cell differentiation in thymus. Mice lacking an essential component of the pre-TCR exhibit arrest at the (CD4(-)CD8(-)) (CD44(-)CD25(+)) stage (DN3) of thymocyte development, and introduction of p53 deficiency into those mice abrogates this arrest, resulting in transition to the (CD4(+)CD8(+)) double-positive (DP) stage. This paper examines the effect of inactivation of p53 on thymocyte development in Bcl11b(-/-) mice that exhibit arrest at the DN3 or (CD4(-)CD8(+)) immature single-positive (ISP) stage. No DP thymocytes were detected in thymocytes of adoptive transfer experiments in scid mice that were derived from p53(-/-)Bcl11b(-/-) precursors but ISP thymocytes increased in the proportion and in the cell number approximately three times higher than those from Bcl11b(-/-) precursors. Consistently, the level of apoptosis decreased to the level of wild-type precursors. These results suggest that inactivation of p53 is sufficient for DN3 thymocytes to differentiate into the ISP, but not to DP, stage of thymocyte development in Bcl11b(-/-) mice. This provides evidence for a novel p53-mediated checkpoint that regulates the transition from the DN3 to ISP stage of thymocyte development.     

Subtle defects in pre-TCR signaling in the absence of the Tec kinase Itk

alphabeta T cell development in the thymus is dependent on signaling through the TCR. The first of these signals is mediated by the pre-TCR, which is responsible for promoting pre-T cell proliferation and the differentiation of CD4(-)8(-)3(-) (DN) thymocytes into CD4(+)8(+)3(+) (DP) cells. In many cases, T cell signaling proteins known to be essential for TCR signaling in mature T cells are also required for pre-TCR signaling in DN thymocytes. Therefore, it came as a surprise to discover that mice lacking the Tec kinases Itk and Rlk, enzymes required for efficient activation of phospholipase C-gamma1 in mature T cells, showed no obvious defects in pre-TCR-dependent selection events in the thymus. In this report, we demonstrate that DN thymocytes lacking Itk, or Itk and Rlk, are impaired in their ability to generate normal numbers of DP thymocytes, especially when placed in direct competition with WT DN thymocytes. We also show that Itk is required for maximal pre-TCR signaling in DN thymocytes. These data demonstrate that the Tec kinases Itk and Rlk are involved in, but are not essential for, pre-TCR signaling in the thymus, suggesting that there is an alternative mechanism for activating phospholipase C-gamma1 in DN thymocytes that is not operating in DP thymocytes and mature T cells.     

The Catalytic Activity of the Mitogen-Activated Protein Kinase Extracellular Signal-Regulated Kinase 3 Is Required To Sustain CD4+ CD8+ Thymocyte Survival

Extracellular signal-regulated kinase 3 (ERK3) is an atypical member of the mitogen-activated protein kinase (MAPK) family whose function is largely unknown. Given the central role of MAPKs in T cell development, we hypothesized that ERK3 may regulate thymocyte development. Here we have shown that ERK3 deficiency leads to a 50% reduction in CD4⁺ CD8⁺ (DP) thymocyte number. Analysis of hematopoietic chimeras revealed that the reduction in DP thymocytes is intrinsic to hematopoietic cells. We found that early thymic progenitors seed the Erk3^−/− thymus and can properly differentiate and proliferate to generate DP thymocytes. However, ERK3 deficiency results in a decrease in the DP thymocyte half-life, associated with a higher level of apoptosis. As a consequence, ERK3-deficient DP thymocytes are impaired in their ability to make successful secondary T cell receptor alpha (TCRα) gene rearrangement. Introduction of an already rearranged TCR transgene restores thymic cell number. We further show that knock-in of a catalytically inactive allele of Erk3 fails to rescue the loss of DP thymocytes. Our results uncover a unique role for ERK3, dependent on its kinase activity, during T cell development and show that this atypical MAPK is essential to sustain DP survival during RAG-mediated rearrangements.     

Soluble factor-mediated differentiation of CD4+CD8+ thymocytes to single positives in vitro

Although the thymic microenvironment provides the necessary elements for T-cell differentiation, the precise role of individual components remains to be determined. In this paper, attempts were made to address the possibility that CD4 or CD8 single-positive (SP) thymocytes could be developed from immature CD4+CD8+ (double-positive; DP) thymocytes in a suspension culture in the presence of soluble factors. We observed that IL-4 and IFN-gamma weakly induced DP cells to differentiate to CD4 cells, but not to CD8. In contrast, IL-2 weakly induced differentiation to CD8. Interestingly, Con A sup strongly induced differentiation to CD8 SP from the purified DP thymocytes prepared from C57BL/6 or LCMV TCRtg mice. In particular, it was found that thymocyte culture with Con A sup generated CD69+DP cells, and the CD69+DP differentiated to CD8 SP under the suspension culture with soluble factors. Thus, Con A sup or combinations of IL-2, IL-4 and IL-7 strongly induced differentiation of CD69+DP to CD8 SP, whereas individual cytokines did not. These results suggest that soluble factors like cytokines play an important role in the generation of SP thymocytes in the absence of thymic stromal cells, at least from a distinctive subpopulation like CD69+DP thymocytes, and perhaps from those of broader range when in conjunction with TCR/MHC interaction.     

Requirement for sustained MAPK signaling in both CD4 and CD8 lineage commitment: a threshold model

Although there is general agreement that the RAS/MAPK signaling pathway is required for positive selection of CD4 T cells in the thymus, the role of this pathway in CD8 lineage commitment remains controversial. We show here that the differentiation of isolated cultured thymocytes to the CD8 as well as CD4 T cell lineage is sensitive to MEK inhibition and that both CD4 and CD8 thymocyte differentiation requires sustained MEK signaling. However, CD4 lineage commitment is promoted by a stronger stimulus for longer duration than required for CD8 lineage commitment. Interestingly, CD4 lineage commitment is not irreversibly set even after 10 h of signaling, well past early changes in gene expression. These findings are presented in the context of a model of lineage commitment in which a default pathway of CD8 lineage commitment is altered to CD4 commitment if the thymocyte achieves a threshold level of active MAPK within a certain time frame.     

Preferential effects of leptin on CD4 T cells in central and peripheral immune system are critically linked to the expression of leptin receptor

Leptin can enhance thymopoiesis and modulate the T-cell immune response. However, it remains controversial whether these effects correlate with the expression of leptin receptor, ObR. We herein addressed this issue by using in vivo animal models and in vitro culture systems. Leptin treatment in both ob/ob mice and normal young mice induced increases of CD4 SP thymocytes in thymus and CD4 T cells in the periphery. Interestingly, expression of the long form ObR was significantly restricted to DN, DP and CD4 SP, but not CD8 SP thymocytes. Moreover, in the reaggregated DP thymocyte cultures with leptin plus TSCs, leptin profoundly induced differentiation of CD4 SP but not CD8 SP thymocytes, suggesting that the effects of leptin on thymocyte differentiation might be closely related to the expression of leptin receptor in developing thymocytes. Surprisingly, ObR expression was markedly higher in peripheral CD4 T cells than that in CD8 T cells. Furthermore, leptin treatment with or without IL-2 and PHA had preferential effects on cell proliferation of CD4 T cells compared to that of CD8 T cells. Collectively, these data provide evidence that the effects of leptin on differentiation and proliferation of CD4 T cells might be closely related to the expression of leptin receptor.     

RasGRP1, but Not RasGRP3, Is Required for Efficient Thymic β-Selection and ERK Activation Downstream of CXCR4

T cell development is a highly dynamic process that is driven by interactions between developing thymocytes and the thymic microenvironment. Upon entering the thymus, the earliest thymic progenitors, called CD4⁻CD8⁻ ‘double negative’ (DN) thymocytes, pass through a checkpoint termed “β-selection” before maturing into CD4⁺CD8⁺ ‘double positive’ (DP) thymocytes. β-selection is an important developmental checkpoint during thymopoiesis where developing DN thymocytes that successfully express the pre-T cell receptor (TCR) undergo extensive proliferation and differentiation towards the DP stage. Signals transduced through the pre-TCR, chemokine receptor CXCR4 and Notch are thought to drive β-selection. Additionally, it has long been known that ERK is activated during β-selection; however the pathways regulating ERK activation remain unknown. Here, we performed a detailed analysis of the β-selection events in mice lacking RasGRP1, RasGRP3 and RasGRP1 and 3. We report that RasGRP1 KO and RasGRP1/3 DKO deficient thymi show a partial developmental block at the early DN3 stage of development. Furthermore, DN3 thymocytes from RasGRP1 and RasGRP1/3 double knock-out thymi show significantly reduced proliferation, despite expression of the TCRβ chain. As a result of impaired β-selection, the pool of TCRβ⁺ DN4 is significantly diminished, resulting in inefficient DN to DP development. Also, we report that RasGRP1 is required for ERK activation downstream of CXCR4 signaling, which we hypothesize represents a potential mechanism of RasGRP1 regulation of β-selection. Our results demonstrate that RasGRP1 is an important regulator of proliferation and differentiation at the β-selection checkpoint and functions downstream of CXCR4 to activate the Ras/MAPK pathway.     

Reduced thymocyte development in sonic hedgehog knockout embryos

The Hedgehog family of secreted intercellular signaling molecules are regulators of patterning and organogenesis during animal development. In this study we provide genetic evidence that Sonic Hedgehog (Shh) has a role in the control of murine T cell development. Analysis of Shh(-/-) mouse embryos revealed that Shh regulates fetal thymus cellularity and thymocyte differentiation. Shh is necessary for expansion of CD4(-)CD8(-) double-negative (DN) thymocytes and for efficient transition from the earliest CD44(+)CD25(-) DN population to the subsequent CD44(+)CD25(+) DN population and from DN to CD4(+)CD8(+) double-positive cells.     

T cell development in PU.1-deficient mice.

These studies address the role of PU.1 in T cell development through the analysis of PU.1-/- mice. We show that the majority of PU.1-/- thymocytes are blocked in differentiation prior to T cell commitment, and contain a population of thymocyte progenitors with the cell surface phenotype of CD44+, HSAbright, c-kitint, Thy-1-, CD25-, Sca-1-, CD4-, and CD8-. These cells correspond in both number and cell surface phenotype with uncommitted thymocyte progenitors found in wild-type fetal thymus. RT-PCR analysis demonstrated that PU.1 is normally expressed in this early progenitor population, but is down-regulated during T cell commitment. Rare PU.1-/- thymi, however, contained small numbers of thymocytes expressing markers of T cell commitment. Furthermore, almost 40% of PU.1-/- thymi placed in fetal thymic organ culture are capable of T cell development. Mature PU. 1-/- thymocytes generated during organ culture proliferated and produced IL-2 in response to stimulation through the TCR. These data demonstrate that PU.1 is not absolutely required for T cell development, but does play a role in efficient commitment and/or early differentiation of most T progenitors.     

Asynchronism of thymocyte development in vivo and in vitro

Although fetal thymus organ culture (FTOC) has become widely used to investigate T-cell development, the differences between thymocyte development in vivo and in vitro (in FTOC) remain largely unknown. In this study, the viability and numbers of thymocytes recovered from embryonic thymus lobes in different gestation days (gd) mice or from 15 day embryonic thymus lobes cultured for different days in FTOC system were evaluated. The expression of CD3, CD4, CD8, CD95 ligand (CD95L), and CD69 on thymocytes were analyzed by FACS. The results showed that thymocytes, either in vivo or in vitro, could differentiate from double negative (DN) cells to double positive (DP) cells and to single positive (SP) cells. But the number of total thymocytes and the percentage of DP cells in vitro were less than that in vivo, and the expression of CD95L and CD69 on thymocytes in vitro was higher than that in vivo. Our results suggested that although thymocyte development in vitro could recapitulate thymic development in vivo, the proliferation of thymocytes in vitro was less intensive than that in vivo; the differentiation of thymocytes in vitro was delayed compared with that in vivo; and the apoptosis and activation of thymocytes in vitro were higher than that in vivo. In conclusion, FTOC is a useful system for the study of T cell differentiation, but it is necessary to interpret the results from in vitro studies carefully since the thymocyte development in vitro is asynchronous from that in vivo.     

Selective support of a mouse thymus epithelial cell line （MTEC1） to the viability and proliferation of CD4＋CD8—,CD4^—CD8^—,and CD4^＋CD8^＋thymocytes in vitro

The MTEC1 cell line,established in our laboratory,is a normal epithelial cell line derived from thymus medulla of Balb/c mice and these cells constituteively produce multiple cytokines.The selection of thymic microenvironment on developing T cells was investigated in an in vitro system.Unseparated fresh thymocytes from Balb/c mice were cocultured with MTEC1 cells or/and MTEC1-SN,then,the viability,proliferation and phenotypes of cultured thymocytes were assessed.Without any exogenous stimulus,both MTEC1 cells and MTEC1-SN were able to maintain the viability of thymocytes,while only the MTEC1 cells,not the MTEC1-SN,could directly activate thymocytes to exhibit moderate proliferation,indicating that the proliferative signal is delivered through cell surface interatcions of MTEC1 cells and thymocytes.Phenotype analysis on FACS of viable thymocytes after coculture revealed that MTEC1 cells preferentially activate the subsets of CD4^ CD8^-,CD4^ CD^8 and CD^4- CD^8- thymocytes;whereas MTEC1-SN preferentially maintained the viability of CD4^ CD^8- and CD4^-CD8^ thymocyte subsets.For the Con A-activated thymocytes.both MTEC1 cells and MTEC1-SN provided accessory signal(s) to significantly increase the number of viable cells and to markedly enhance the proliferation of thymocytes with virtually equal potency,phenotyped as CD4^ CD8^-,CD4^-CD8^ ,and CD^4-CD8^-subests,In summary,MTEC1 cells displayed Selection of thymic epithelial cells on thymocyte subsets. selective support to the different thymocyte subsets,and the selectivity is dependent on the status of thymocytes.     

Distinct BMI-1 and EZH2 expression patterns in thymocytes and mature T cells suggest a role for Polycomb genes in human T cell differentiation

BMI-1 and EZH2 Polycomb-group (PcG) proteins belong to two distinct protein complexes involved in the regulation of hematopoiesis. Using unique PcG-specific antisera and triple immunofluorescence, we found that mature resting peripheral T cells expressed BMI-1, whereas dividing blasts were EZH2(+). By contrast, subcapsular immature double-negative (DN) (CD4(-)/CD8(-)) T cells in the thymus coexpressed BMI-1 and EZH2 or were BMI-1 single positive. Their descendants, double-positive (DP; CD4(+)/CD8(+)) cortical thymocytes, expressed EZH2 without BMI-1. Most EZH2(+) DN and DP thymocytes were dividing, while DN BMI-1(+)/EZH2(-) thymocytes were resting and proliferation was occasionally noted in DN BMI-1(+)/EZH2(+) cells. Maturation of DP cortical thymocytes to single-positive (CD4(+)/CD8(-) or CD8(+)/CD4(-)) medullar thymocytes correlated with decreased detectability of EZH2 and continued relative absence of BMI-1. Our data show that BMI-1 and EZH2 expression in mature peripheral T cells is mutually exclusive and linked to proliferation status, and that this pattern is not yet established in thymocytes of the cortex and medulla. T cell stage-specific PcG expression profiles suggest that PcG genes contribute to regulation of T cell differentiation. They probably reflect stabilization of cell type-specific gene expression and irreversibility of lineage choice. The difference in PcG expression between medullar thymocytes and mature interfollicular T cells indicates that additional maturation processes occur after thymocyte transportation from the thymus.     

The BMP Pathway Participates in Human Naive CD4+ T Cell Activation and Homeostasis

Bone Morphogenetic Proteins (BMPs) form a group of secreted factors that belongs to the TGF-β superfamily. Among different roles in a number of immune cell types, BMPs are known to regulate T cell development within the thymus, although the role of BMP signaling in human mature T cells remains elusive. In this study, we demonstrate that canonical BMP signaling is necessary during two critical events that regulate the size and function of human naive CD4⁺ T cell population: activation and homeostasis. Upon stimulation via TCR, naive CD4⁺ T cells upregulate the expression of BMP ligands triggering canonical BMP signaling in CD25⁺ cells. Blockade of BMP signaling severely impairs CD4⁺ T cell proliferation after activation mainly through regulation of IL-2, since the addition of this cytokine recuperates normal T cell expansion after inhibition of BMP signaling. Similarly, activation of canonical BMP pathway is required for both the maintenance of cell survival and the homeostatic proliferation induced by IL-7, a key factor for T cell homeostasis. Moreover, upregulation of two critical receptors for T cell homeostasis, CXCR4 and CCR9, triggered by IL-7 is also abrogated in the absence of BMP signaling. Collectively, we describe important roles of the canonical BMP signaling in human naive CD4⁺ T cell activation and homeostasis that could be valuable for clinical application.     

Inefficient cell spreading and cytoskeletal polarization by CD4+CD8+ thymocytes: regulation by the thymic environment

CD4(+)CD8(+) double-positive (DP) thymocytes express a lower level of surface TCR than do mature T cells or single-positive (SP) thymocytes. Regulation of the TCR on DP thymocytes appears to result from intrathymic signaling, as in vitro culture of these cells results in spontaneous TCR up-regulation. In this study, we examined cell spreading and cytoskeletal polarization responses that have been shown to occur in response to TCR engagement in mature T cells. Using DP thymocytes stimulated on lipid bilayers or nontransgenic thymocytes added to anti-CD3-coated surfaces, we found that cell spreading and polarization of the microtubule organizing center and the actin cytoskeleton were inefficient in freshly isolated DP thymocytes, but were dramatically enhanced after overnight culture. SP (CD4(+)) thymocytes showed efficient responses to TCR engagement, suggesting that releasing DP thymocytes from the thymic environment mimics some aspects of positive selection. The poor translation of a TCR signal to cytoskeletal responses could limit the ability of DP thymocytes to form stable contacts with APCs and may thereby regulate thymocyte selection during T cell development.