Activated T cells transcribe an alternatively spliced mRNA encoding a soluble form of Qa-2 antigen

The immunology of the macaque placenta: A detailed analysis and critical comparison with the human placenta

The cynomolgus monkey is increasingly considered in toxicological research as the most appropriate model for humans due to the species' close physiological contiguity, including reproductive physiology. Here, literature on the cynomolgus monkey placenta is reviewed in regards to its similarity to the human placenta and particularly for its immunological role, which is not entirely mirrored in humans. Pertinent original data are included in this article. The cynomolgus monkey placenta is evaluated based on three aspects: first, morphological development; second, the spatial and temporal appearance of maternal and fetal immune cells and certain immune cell products of the innate and adaptive immune systems; and third, the expression of relevant immune tolerance-related molecules including the homologs of anti-human leucocyte antigen, indoleamine 2,3-dioxygenase, FAS/FAS-L, annexin II, and progesterone. Parameters relevant to the immunological role of the placenta are evaluated from the immunologically immature stage of gestational day (GD) 50 until more mature stages close to birth. Selected comparisons are drawn with human and other laboratory animal placentas. In conclusion, the cynomolgus monkey placenta has a high degree of morphological and physiological similarity to the human placenta. However, there are differences in the topographical distribution of cell types and immune tolerance-related molecules. Three basic features are recognized: (1) the immunological capacity of the placenta changes throughout the lifetime of the organ; (2) these immunological changes include multiple parameters such as morphological adaptations, cell type involvement, and changes in immune-relevant molecule expression; and (3) the immune systems of two genetically disparate individuals (mother and child) are functionally intertwined at the maternal-fetal interface.

New Insights Into the Role of Qa-2 and HLA-G Non-classical MHC-I Complexes in Malignancy

It is well established that the immune system can identify and destroy neoplastic transformed cells in a process known as immunosurveillance. Most studies have focused on the classical major histocompatibility complex (MHC) class Ia molecules, which are known to play an important role on the presentation of tumor antigens to the immune system in order to activate a response against tumor cells. However, a larger family of non-classical MHC class Ib-related molecules has received less attention. In this mini-review, we discuss the role of class Ib murine Qa-2 and its proposed human HLA-G homolog on immunosurveillance during embryogenesis and cancer. Whereas, both HLA-G and Qa-2 are involved in immune tolerance in pregnancy, the current evidence suggests that they play opposite roles in cancer. HLA-G appears to promote tumor progression while Qa-2 acts as a tumor suppressor awaking the immune system to reject tumor cells, as suggested by studies on different cancer cell models, such as melanoma, lymphoma, lung carcinoma, and our own results in mammary carcinoma.

Suppression effect on IFN-γ of adipose tissue-derived mesenchymal stem cells isolated from β2-microglobulin-deficient mice

Administration of bone marrow-derived mesenchymal stem cells (MSCs) is a possible treatment for graft-versus-host disease (GVHD) following allogeneic hematopoietic stem cell transplantation and other inflammatory conditions. To address the mechanism of immunosuppression by MSCs, in particular those derived from adipose tissue (AMSCs), AMSCs were isolated from three different mouse strains, and the suppressive capacity of the AMSCs thus obtained to suppress interferon (IFN)-γ generation in mixed lymphocyte reaction cultures serving as an in vitro model of GVHD were assessed. It was revealed that the AMSCs had a potent capacity to suppress IFN-γ production regardless of their strain of origin and that such suppression was not associated with production of interleukin-10. In addition, the results demonstrated that β2-microglobulin (β2m)-deficient AMSCs from β2m^-/- mice were also potent suppressor cells, verifying the fact that the mechanism underlying the suppression by AMSCs is independent of major histocompatibility complex (MHC) class I expression or MHC compatibility. As AMSCs appear to have immunosuppressive properties, AMSCs may be a useful source of biological suppressor cells for the control of GVHD in humans.

The role of T cell immunoglobulin mucin domains 1 and 4 in a herpes simplex virus-induced Behçet's disease mouse model

The T cell immunoglobulin mucin (TIM) proteins regulate T cell activation and tolerance. TIM-1 plays an important role in the regulation of immune responses and the development of autoimmune diseases. TIM-4 is a natural ligand of TIM-1, and the interaction of TIM-1 and TIM-4 is involved in the regulation of T helper (Th) cell responses and modulation of the Th1/Th2 cytokine balance. Behçet's disease (BD) is a chronic, multisystemic inflammatory disorder with arthritic, intestinal, mucocutaneous, ocular, vascular, and central nervous system involvement. Tim-1 expression was lower in a herpes simplex virus-induced BD mouse model compared to that in asymptomatic BD normal (BDN) mice. Tim-4 expression was higher in BD mice than that in BDN mice. In this study, we investigated the Tim expression in a BD mouse model with BD-like symptoms. Tim-1 and Tim-4 expression was regulated by an expression vector or siRNA injected into the BD mouse model. The Tim-1 vector injected into BD mice resulted in changes in BD-like symptoms and decreased the severity score. Treatment with Tim-4 siRNA also improved BD-like symptoms and decreased the severity score accompanied by upregulation of regulatory T cells. We showed that regulating Tim-1 or Tim-4 affected BD-like symptoms in mice.

CCL21 attenuates HSV-induced inflammation through up-regulation of CD8+ memory cells

CCR7 and its ligand, CCL21, are known to establish microenvironments for the initiation of immune responses in secondary lymphoid tissue. It has also been reported that CCR7 ligand gene-deleted mice have defects in lymphocyte homing. In addition, the injection of the CCR7 ligand was shown to induce the expression of memory T cells. In this study, we analyzed the expression of CCR7 and its ligand in HSV-induced Behçet's disease (BD)-like inflammation of mice. Additionally, plasmids containing the CCR7 ligand CCL19 or CCL21, pcDNA3.1-CCL19 or pcDNA3.1-CCL21, respectively, were injected into symptomatic mice, and changes in the population of memory T cells were determined. After administration of pcDNA3.1-CCL21, the frequencies of CD8+CD44+, CD8+CD62L- memory T cells were significantly up-regulated and the symptoms were not deteriorated when compared to the control vector injected group. Specifically, the difference in frequencies of CCR7+ peripheral blood mononuclear cells between active BD patients and inactive BD patients was similar to that of HSV-induced BD-like mice. These results suggest that CCR7, its ligand, and CD8+ memory cells are correlated with the regulation of BD symptoms.

The role of Qa-2, the functional homolog of HLA-G, in a Behcet's disease-like mouse model induced by the herpes virus simplex

Background

It has been suggested that the HLA-G molecule is a genetic risk factor for Behcet's disease (BD). In this study, we evaluated the level of Qa-2, a murine nonclassical class I MHC molecule and possible functional homolog of HLA-G, to determine if it was associated with various symptoms of BD-like mice. In addition, we investigated siRNA (small interfering RNA) treatment to determine if it inhibited Qa-2 expression, thereby changing the symptoms of mice.

Methods

RNA interference (RNAi) and vector transfection were employed to manipulate gene expression in vivo in mice. siRNA (small interfering RNA) or Qa-2 expression vector was applied to inhibit or up-regulate Qa-2 expression, respectively.

Results

The Qa-2 levels in granulocytes were lower in BD-like mice than in normal controls. The silencing of Qa-2 by intravenous injection of siRNA (500 nmol/mouse, 4 times at 3-day intervals) specifically reduced the Qa-2 levels and worsened the BD-like symptoms.

Conclusions

Silencing Qa-2 by injecting siRNA into mice resulted in deterioration of symptoms in BD-like mice.

Isolation of major histocompatibility complex Class I genes from the tammar wallaby (Macropus eugenii)

The major histocompatibility complex (MHC) plays an essential role in the adaptive immune system of vertebrates through antigen recognition. Although MHC genes are found in all vertebrates, the MHC region is dynamic and has changed throughout vertebrate evolution, making it an important tool for comparative genomics. Marsupials occupy an important position in mammalian phylogeny, yet the MHC of few marsupials has been studied in detail. We report the isolation and analysis of expressed MHC Class I genes from the tammar wallaby, a model marsupial used extensively for the study of mammalian reproduction, genetics, and immunology. We determined that there are at least 11 Class I loci in the tammar genome and isolated six expressed Class I sequences from spleen and testes cDNA libraries, representing at least four loci. Two of the Class I sequences contain substitutions at sites known to be important for antigen binding, perhaps impacting their ability to bind peptides, or the types of peptide to which they bind. Phylogenetic analysis of tammar wallaby Class I sequences and other mammalian Class I sequences suggests that some tammar wallaby and red-necked wallaby loci evolved from common ancestral genes.

Haplotype characterization of transcribed ovine major histocompatibility complex (MHC) class I genes

The ovine major histocompatibility complex (MHC) remains poorly characterized compared with those of other livestock species. Molecular genetic analysis of the bovine MHC has revealed considerable haplotype and allelic diversity that earlier serological analysis had not detected. To develop cellular and molecular tools to support development of vaccines against intracellular pathogens of sheep, we have undertaken a molecular genetic analysis of four distinct ovine MHC haplotypes carried by two heterozygous Blackface rams. We have identified 12 novel class I transcripts and used a class I sequence-specific genotyping system to assign each of these transcripts to individual haplotypes. Using a combination of phylogenetic analysis, haplotype and transcript expression data, we identified at least four distinct polymorphic class I MHC loci, three of which appear together in a number of combinations in individual haplotypes. The haplotypes were further characterized at the highly polymorphic Ovar-DRB1 locus, allowing selection of the progeny of the two founder rams for the establishment of an MHC-defined resource population.

Evidence that HLA-G is the functional homolog of mouse Qa-2, the Ped gene product

Qa-2, a murine class Ib major histocompatibility complex (MHC) molecule, is a possible functional homolog of human leukocyte antigen G (HLA-G). Both molecules have been implicated in immunoregulation and embryonic development and both occur in membrane-bound and soluble isoforms that arise by alternative splicing. Soluble splice variants have been implicated in the reproductive functions of HLA-G. While soluble variants of Qa-2 have been previously detected in T lymphocytes, we now demonstrate the presence of mRNA for one of the two known soluble forms of Qa-2 in eight-cell embryos and in blastocysts. Qa-2 is glycosylphosphatidylinositol (GPI) linked in the outer leaflet of the cell membrane and is found in lipid raft microdomains where other raft-associated proteins transduce signals into the cell. In contrast, HLA-G has a truncated six amino acid cytoplasmic tail. By fluorescence co-localization in JEG-3 cells, using fluorescent cholera toxin beta subunit (a lipid raft marker) and anti-HLA-G antibody, we have demonstrated that membrane-bound HLA-G also localizes to lipid rafts, consistent with functional homology between the two molecules. Finally, our experiments in which we have purified Qa-2 and transferred it via a process known as protein painting to Qa-2 negative cells represent a model for potential therapy involving HLA-G.

Listing, location, binding motifs, and expression of nonclassical class I and related genes and molecules

The tables presented in this appendix list nonclassical class I or related genes/molecules arranged by the chromosomal region where they are encoded. This includes genes that fall into the Ib region of the murine major histocompatibility complex (MHC) which includes H2-Q, -T, and -M, as well as CD1, which lies outside the MHC region. A final table includes genes/molecules that are encoded in diverse regions. They are included in this section because they are either class I related in that their heavy chain is related to classical class I and/or they are associated with ion given is for the C57BL/6 (B6) strain unless otherwise noted.

A soluble isoform of the rhesus monkey nonclassical MHC class I molecule Mamu-AG is expressed in the placenta and the testis

The nonclassical MHC class I locus HLA-G is expressed primarily in the placenta, although other sites of expression have been noted in normal and pathological situations. In addition, soluble HLA-G isoforms have been detected in the serum of pregnant and nonpregnant women as well as men. The rhesus monkey placenta expresses a novel nonclassical MHC class I molecule Mamu-AG, which has features remarkably similar to those of HLA-G. We determined that the rhesus placenta expresses Mamu-AG mRNA (Mamu-AG5), retaining intron 4 as previously noted in HLA-G5. Immunostaining experiments with Ab 16G1 against the soluble HLA-G5 intron 4 peptide demonstrated that an immunoreactive protein(s) was present in the syncytiotrophoblasts of the chorionic villi of the rhesus placenta, within villous cytotrophoblasts, and occasionally within cells of the villous stroma. The Mamu-AG5 mRNA was readily detected in rhesus testis (although not in ejaculated sperm). Whereas an Ab against membrane-bound Mamu-AG stained few cells, primarily in the interstitium of the testis, there was consistent immunostaining for Mamu-AG5 in cells within the seminiferous tubules, which was corroborated by localization of Mamu-AG mRNA by in situ hybridization. While primary spermatocytes were negative, Sertoli cells, spermatocytes, and spermatids were consistently positive for 16G1 immunostaining. The specific recognition of the soluble Mamu-AG isoform was confirmed by Western blotting of Mamu-AG5 expressed in heterologous cells. The results demonstrate that a soluble nonclassical MHC class I molecule is expressed in the rhesus monkey placenta and testis, and confirm and extend the unique homology between HLA-G and the rhesus nonclassical molecule Mamu-AG.

The nonclassical major histocompatibility complex molecule Qa-2 protects tumor cells from NK cell- and lymphokine-activated killer cell-mediated cytolysis

The cytotoxic activity of NK cells is regulated by class I MHC proteins. Although much has been learned about NK recognition of class I autologous targets, the mechanisms of NK self-tolerance are poorly understood. To examine the role of a nonpolymorphic, ubiquitously expressed class Ib Ag, Q9, we expressed it on class I-deficient and NK-sensitive B78H1 melanoma. Presence of this Qa-2 family member on tumor cells partially protected targets from lysis by bulk lymphokine-activated killer (LAK) cells. H-2K(b)-expressing B78H1 targets also reduced LAK cell activity, while H-2D(b) offered no protection. Importantly, blocking with F(ab')(2) specific for Q9 or removal of this GPI-attached molecule by phospholipase C cleavage restored killing to the level of vector-transfected cells. Experiments with LAK cells derived from H2(b) SCID and B6 mice established that NK1.1(+)TCR(-) NK and NK1.1(+)TCR(+) LAK cells were the prevalent cytolytic populations inhibitable by Q9. Treatment of mice with poly(I:C) also resulted in generation of Q9-regulated splenic cytotoxicity. LAK cells from different mouse strains responded to Q9, suggesting that the protective effect of this molecule is not detectably influenced by Ly49 polymorphisms or the presence/absence of Q9 in NK-harboring hosts. We propose that Q9 expressed on melanoma cells serves as a ligand for yet unidentified NK inhibitory receptor(s) expressed on NK1.1(+) NK/T cells.

The short forms of HLA-G are unlikely to play a role in pregnancy because they are not expressed at the cell surface

HLA-G is a nonclassical class I MHC molecule of unknown function expressed on human invasive trophoblast. In trophoblast cells, HLA-G mRNA is alternatively spliced into a variety of forms which are predicted to encode a full length membrane-bound form, three short membrane-bound isoforms and two soluble isoforms. The aim of this study was to determine which of these protein isoforms are translated, which are expressed on the cell surface and which are secreted. Artificial cDNAs encoding the isoforms were generated by PCR mutagenesis, ligated to an epitope tag and transfected into a human cell line capable of expressing MHC class I. Protein products of appropriate sizes were detected in cells transfected with cDNAs encoding all membrane-bound forms, but surface biotinylation studies indicated that only full length membrane-bound HLA-G was present at the cell surface. Full length HLA-G was also detected by surface antibody binding and flow cytometry. Soluble HLA-G1 was detected in cells transfected with the appropriate cDNA only after treatment with monensin, which inhibits transport of glycoproteins through the Golgi apparatus. These results suggest that full length HLA-G, but not short HLA-G isoforms can be expressed on the surface of human cells and that soluble HLA-G is rapidly secreted. Thus, it is likely that the full length membrane-bound and soluble forms of HLA-G are the only biologically active forms to which the mother is exposed.

The murine liver-specific nonclassical MHC class I molecule Q10 binds a classical peptide repertoire

The biological properties of the nonclassical class I MHC molecules secreted into blood and tissue fluids are not currently understood. To address this issue, we studied the murine Q10 molecule, one of the most abundant, soluble class Ib molecules. Mass spectrometry analyses of hybrid Q10 polypeptides revealed that alpha1alpha2 domains of Q10 associate with 8-9 long peptides similar to the classical class I MHC ligands. Several of the sequenced peptides matched intracellularly synthesized murine proteins. This finding and the observation that the Q10 hybrid assembly is TAP2-dependent supports the notion that Q10 groove is loaded by the classical class I Ag presentation pathway. Peptides eluted from Q10 displayed a binding motif typical of H-2K, D, and L ligands. They carried conserved residues at P2 (Gly), P6 (Leu), and Pomega (Phe/Leu). The role of these residues as anchors/auxiliary anchors was confirmed by Ala substitution experiments. The Q10 peptide repertoire was heterogeneous, with 75% of the groove occupied by a multitude of diverse peptides; however, 25% of the molecules bound a single peptide identical to a region of a TCR V beta-chain. Since this peptide did not display enhanced binding affinity for Q10 nor does its origin and sequence suggest that it is functionally significant, we propose that the nonclassical class I groove of Q10 resembles H-2K, D, and L grooves more than the highly specialized clefts of nonclassical class I Ags such as Qa-1, HLA-E, and M3.

Expression of a nonclassical MHC class Ib molecule in the eye

BACKGROUND

MHC class Ia molecules are absent, or expressed at low levels, on cells lining the anterior chamber of the eye, an immune-privileged site. Although the scarcity of class Ia MHC antigens may protect cells from T cell-mediated tissue injury, it may also render them vulnerable to natural killer cell-mediated cytolysis. There is growing evidence that MHC class Ib molecules share similar functions to class Ia. In this study, we examine the expression and distribution of Qa-2, one of the best-characterized murine MHC class Ib molecules in the eye.

METHODS

The transcription of Qa-2 mRNA in whole eye and eye-derived cells was analyzed by sensitive and specific RNase protection and reverse transcription-polymerase chain reaction assays. Immunohistochemistry, flow cytometry, and ELISA were used to determine whether Qa-2 was expressed as cell surface proteins. Expression levels of Qa-2 were monitored in resting cells and cells stimulated with interferon-gamma.

RESULTS

Expression of membrane-bound and soluble Qa-2 isoforms was detected in various tissues of the eye, including cell subsets lining the anterior chamber. Immunohistological staining revealed Qa-2 expression on corneal epithelium as well as endothelium, iris ciliary bodies, and retina. These observations were confirmed by analysis of cultured, eye-derived cells. Qa-2 expression was inducible by interferon-gamma. Qa-2 was not detected in lens cells.

CONCLUSIONS

The results demonstrate that membrane-bound and soluble MHC class Ib molecules are expressed by eye cells. Expression of Qa-2 in the corneal endothelium and other substructures lining the anterior chamber suggests that this class Ib protein may contribute to the immune-privileged status of the anterior chamber.

Molecular analysis of six dog leukocyte antigen class I sequences including three complete genes, two truncated genes and one full-length processed gene

We have isolated six distinct dog leukocyte antigen (DLA) class I sequences. An additional functional nonclassical class I gene, DLA-79, was characterized previously. This brings the number of isolated canine class I sequences to seven. These seven loci account for nearly all class I sequences detected in genomic DNA by Southern blotting. With approximately seven members, the class I gene family in dog is considerably less complex than in either human or mouse. Three of the six sequences described in this article contain complete class I genes. These genes have the typical arrangement of exons and introns, and their predicted protein sequences have the features expected of expressed class I molecules. Two sequences contain truncated genes. These genes, having only partial class I homology and disruptive mutations, are clearly nonfunctional. The remaining locus contains a full-length processed gene which is unique among characterized class I loci. Sequence comparisons were performed to examine evolutionary relationships among the family members. Two sequences, DLA-64 (complete) and -53 (truncated), appear to be chimeras presumably formed by interlocus recombination. Using the reverse transcription-polymerase chain reaction, mRNA originating from each of the three complete genes was identified in canine peripheral blood leukocytes. DLA-79 specific transcripts were identified previously. Thus, each of the four complete canine class I genes is transcribed in vivo.

BoLA class I charge heterogeneity reflects the expression of more than two loci

Internationally recognized allo-antisera in lymphocyte microcytotoxicity assays are thought to detect allelic products of a single highly polymorphic class I locus. A recent report suggested that two bovine lymphocyte antigen (BoLA) class I loci are expressed at the protein level. However, 1D-IEF analysis of BoLA class I molecules reveals multi-band patterns which cannot be reconciled with the reported number of loci. The aim of this study was to investigate the origins of the charge diversity of BoLA class I molecules observed using 1D-IEF. BoLA class I molecules appear to be glycosylated at a single N-linked position with a complex type carbohydrate moiety which has up to three terminal sialic acid residues. Class I molecules immunoprecipitated from resting bovine PBL are not phosphorylated. Neither modification is responsible for the observed charge heterogeneity. Peptide mapping reveals that different BoLA charge variants have distinct digestion patterns. Furthermore, a number of different polypeptides are associated with each serological specificity. These polypeptides appear to be encoded by different loci which exist in linkage disequilibrium. The number of charge variants with different peptide maps indicates that the BoLA system has a minimum of three class I loci expressed at the protein level.

Antigen presentation by non-classical class I molecules

Non-classical class I genes are no longer clearly distinguished from classical ones in mammals, and they are found also in fishes, frogs and chickens. They contribute to immune responses against pathogens. Given the number and diversity of class Ib products, their various tissue distribution patterns, and the wide range of peptides they bind, new functions are to be expected.

Characterization of T cell proliferative responses induced by anti-Qa-2 monoclonal antibodies

The MHC class I Qa-2 Ag are attached to the cell surface by a glycanphosphatidylinositol (GPI) anchor. Crosslinking of Qa-2 and several other cell surface Ag attached by the GPI linkage has been shown to lead to cell activation. We have developed 10 new anti-Qa-2 mAb and characterized their capacity to induce proliferation of spleen cells. In the absence of anti-Ig-mediated crosslinking, none of the mAbs alone could induce activation. However, mAb 23.1 which reacts with the alpha 3 domain of Qa-2, when combined with most of the other mAbs (alpha 1, alpha 2 domain reactive), activated cells in the absence of anti-Ig crosslinking. The mAb pair 23.1 plus 24.16 was the most proficient and induced proliferation in the absence of any exogenous second signals. Responses were greatly enhanced and equivalent to those seen with anti-CD3 by the addition of phorbol myristate acetate (PMA). Ionomycin, rIL-2, or rIL-4 also potentiated anti-Qa-2 responses but less efficiently than PMA. Significant strain variation in the magnitude Qa-2-mediated proliferative responses was observed correlating with the levels of Qa-2 expressed on the cell surface. Crosslinking of Qa-2 molecules by the mAb combinations was required because monovalent Fab fragments failed to activate cells. F(ab')2 fragments of mAb 23.1 plus 24.16 induced vigorous proliferation indicating that accessory cell presentation of the mAb via Fc receptors was not required. Immobilized (plate bound) anti-Qa-2 mAb induced proliferation suggesting that the Qa-2 pathway may be distinct from that of other GPI molecules such as Thy-1 and Ly-6. Populations enriched for T cells (approximately 95%) responded as well as whole spleen cells, whereas B lymphocytes failed to proliferate to anti-Qa-2. Both CD4+ and CD8+ cells were activated following crosslinking of Qa-2. Finally, T cell activation mediated by Qa-2 induced elevation of [Ca2+]i, IL-2R expression, and the release of IL-2. These data demonstrate that crosslinking of Qa-2 on T lymphocytes represents a potent pathway for inducing cell activation.

Signals delivered via the Qa-2 molecule can synergize with limiting anti-CD3-induced signals to cause T lymphocyte activation

Qa-2 is a glycolipid anchored, MHC encoded class I molecule expressed at high levels on all murine peripheral T lymphocytes. Anti-Qa-2 antibodies have previously been found to stimulate T cells to proliferate in the presence of crosslinking antibody and PMA. We have examined the effect of anti-Qa-2 antibodies on T cells stimulated with a suboptimal concentration of immobilized anti-CD3. When anti-Qa-2 antibodies were co-immobilized with limiting anti-CD3, in the absence of PMA, a clear augmentation of T cell proliferation was seen. Interestingly, the co-stimulatory anti-Qa-2 antibodies could be directed against epitopes mapped to either the alpha 3 or the alpha 1/alpha 2 Qa-2 domains. As was the case with activation induced by soluble/crosslinked anti-Qa-2 antibodies plus PMA, CD8+ T cells were less able to be costimulated with anti-Qa-2 antibodies than CD4+ cells. Surprisingly, Ca2+ mobilization was only seen when two anti-Qa-2 antibodies reactive to separate structural domains were co-crosslinked on the surface of Indo-1 loaded T cells with a suboptimal concentration of anti-CD3. Collectively these results raise questions regarding the mechanism of Qa-2 mediated signaling and its potential role in T cell activation.