Engagement of the PD-1 immunoinhibitory receptor by a novel B7 family member leads to negative regulation of lymphocyte activation

PD-1 is an immunoinhibitory receptor expressed by activated T cells, B cells, and myeloid cells. Mice deficient in PD-1 exhibit a breakdown of peripheral tolerance and demonstrate multiple autoimmune features. We report here that the ligand of PD-1 (PD-L1) is a member of the B7 gene family. Engagement of PD-1 by PD-L1 leads to the inhibition of T cell receptor-mediated lymphocyte proliferation and cytokine secretion. In addition, PD-1 signaling can inhibit at least suboptimal levels of CD28-mediated costimulation. PD-L1 is expressed by antigen-presenting cells, including human peripheral blood monocytes stimulated with interferon gamma, and activated human and murine dendritic cells. In addition, PD-L1 is expressed in nonlymphoid tissues such as heart and lung. The relative levels of inhibitory PD-L1 and costimulatory B7-1/B7-2 signals on antigen-presenting cells may determine the extent of T cell activation and consequently the threshold between tolerance and autoimmunity. PD-L1 expression on nonlymphoid tissues and its potential interaction with PD-1 may subsequently determine the extent of immune responses at sites of inflammation.

Expression of MHC class II-associated invariant chain (Ii;CD74) in thymic epithelial neoplasms

Thymic epithelial cells express major histocompatibility complex (MHC) class II and are involved in T-cell ontogeny. In these cells, MHC class II-associated invariant chain (CD74) is involved in antigen presentation during T-cell selection. We studied a range of thymic epithelial neoplasms for CD74 expression by neoplastic epithelial cells to determine whether such expression correlates with MHC class II expression and tumor type. Sixty-four thymic epithelial neoplasms (27 cases of benign thymoma, 20 cases of invasive thymoma, and 17 cases of true thymic carcinoma) were studied for neoplastic epithelial cell expression of CD74 and MHC class II molecules by immunohistochemical staining of paraffin-embedded tissue. Neoplastic epithelial cells in 88% of thymic carcinomas (15/17), 70% of invasive thymomas (14/20), but only 33% of benign thymomas (9/27) were immunoreactive for CD74. A subset of CD74-positive neoplasms was positive for MHC class II as well, with higher relative rates of dual positivity in more aggressive neoplasms. In addition, specific histologic subtypes of thymic epithelial neoplasms displayed differing patterns of CD74 positivity. Based on these findings, CD74 and MHC class II are useful markers for the classification of thymic epithelial neoplasms.

p27kip1 functions as an anergy factor inhibiting interleukin 2 transcription and clonal expansion of alloreactive human and mouse helper T lymphocytes

Although recent in vitro studies have begun to decipher the molecular events that characterize the anergic state, their in vivo biologic relevance and potential clinical importance remain unclear. Here, using anergic human T-cell clones and tolerant alloreactive mouse T cells that do not induce graft-versus-host disease, we show that p27kip1 cyclin-dependent kinase inhibitor is an essential regulator responsible for the blockade of clonal expansion of anergic T cells in vitro and in vivo. Moreover, in anergic cells, p27kip1 associates with the c-Jun co-activator JAB1, resulting in defective transactivation of AP-1 and interleukin 2 transcription. Therefore, pharmacological agents that upregulate the expression of or prevent the degradation of p27kip1 during antigen recognition should be part of new therapeutic strategies to induce antigen-specific T-cell unresponsiveness.

Ox40-ligand has a critical costimulatory role in dendritic cell:T cell interactions

The tumor necrosis factor family molecule Ox40-ligand (Ox40L) has been identified as a potential costimulatory molecule and also has been implicated in T cell homing and B cell activation. To ascertain the essential functions of Ox40L, we generated and characterized Ox40L-deficient mice. Mice lacking Ox40L exhibit an impaired contact hypersensitivity response, a dendritic cell-dependent T cell-mediated response, due to defects in T cell priming and cytokine production. In contrast, Ox40L-deficient mice do not have defects in T cell homing or humoral immune responses. In vitro, Ox40L-deficient dendritic cells are defective in costimulating T cell cytokine production. Thus, Ox40L has a critical costimulatory function in vitro and in vivo for dendritic cell:T cell interactions.

PTPROt: an alternatively spliced and developmentally regulated B-lymphoid phosphatase that promotes G0/G1 arrest

Protein tyrosine phosphatases (PTP) regulate the proliferation, differentiation, and viability of lymphocytes by modulating their signaling pathways. By using the differential display assay, we have cloned a putative receptor-type PTP, which is predominantly expressed in B-lymphoid tissues (lymph nodes and spleen). This PTP, termed PTPROt (truncated), is a tissue-specific alternatively-spliced form of a human epithelial PTP, PTPRO (PTPU2/GLEPP1). Whereas the epithelial PTPRO includes an approximately 800-amino acid extracellular domain, the major (3 kb) PTPROt cDNA predicts a unique 5' untranslated region and truncated (8 amino acids) extracellular domain with a conserved transmembrane region and single catalytic domain. PTPROt cDNAs encode functional approximately 47-kD and approximately 43-kD PTPs, which are most abundant in normal naive quiescent B cells and decreased or absent in germinal center B cells and germinal center-derived diffuse large B-cell lymphomas. Because PTPROt was predominantly expressed in naive quiescent B cells, the enzyme's effects on cell-cycle progression were examined. When multiple stable PTPROt sense, antisense, and vector only B-cell transfectants were grown in reduced serum and synchronized with nocodazole, PTPROt sense clones exhibited markedly increased G0/G1 arrest. Taken together, these data implicate PTPROt in the growth control of specific B-cell subpopulations.

Regulation of human intestinal intraepithelial lymphocyte cytolytic function by biliary glycoprotein (CD66a)

Human small intestinal intraepithelial lymphocytes (iIEL) are a unique population of CD8alphabeta+ TCR-alphabeta+ but CD28- T lymphocytes that may function in intestinal epithelial cell immunosurveillance. In an attempt to define novel cell surface molecules involved in iIEL function, we raised several mAbs against activated iIELs derived from the small intestine that recognized an Ag on activated, but not resting, iIELs. Using expression cloning and binding studies with Fc fusion proteins and transfectants, the cognate Ag of these mAbs was identified as the N domain of biliary glycoprotein (CD66a), a carcinoembryonic Ag-related molecule that contains an immune receptor tyrosine-based inhibitory motif. Functionally, these mAbs inhibited the anti-CD3-directed and lymphokine-activated killer activity of the P815 cell line by iIELs derived from the human small intestine. These studies indicate that the expression of biliary glycoprotein on activated human iIELs and, potentially, other mucosal T lymphocytes is involved in the down-regulation of cytolytic function.

B7-2 expressed on EL4 lymphoma suppresses antitumor immunity by an interleukin 4-dependent mechanism

For T cells to become functionally activated they require at least two signals. The B7 costimulatory molecules B7-1 and B7-2 provide the "second signal" pivotal for T cell activation. In this report, we studied the relative roles of B7-1 and B7-2 molecules in the induction of antitumor immunity to the T cell thymoma, EL4. We generated EL4 tumor cells that expressed B7-1, B7-2, and B7-1+B7-2 by transfecting murine cDNAs. Our results demonstrate that EL4-B7-1 cells are completely rejected in syngeneic mice. Unlike EL4-B7-1 cells, we find that EL4-B7-2 cells are not rejected but progressively grow in the mice. A B7-1- and B7-2-EL4 double transfectant was generated by introducing B7-2 cDNA into the EL4-B7-1 tumor line that regressed in vivo. The EL4-B7-1+B7-2 double transfectant was not rejected when implanted into syngeneic mice but progressively grew to produce tumors. The double transfectant EL4 cells could costimulate T cell proliferation that could be blocked by anti-B7-1 antibodies, anti-B7-2 antibodies, or hCTLA4 immunoglobulin, showing that the B7-1 and B7-2 molecules expressed on the EL4 cells were functional. In vivo, treatment of mice implanted with double-transfected EL4 cells with anti-B7-2 monoclonal antibody resulted in tumor rejection. Furthermore, the EL4-B7-2 and EL4-B7-1+B7-2 cells, but not the wild-type EL4 cells, were rejected in interleukin 4 (IL-4) knockout mice. Our data suggests that B7-2 expressed on some T cell tumors inhibits development of antitumor immunity, and IL-4 appears to play a critical role in abrogation of the antitumor immune response.

Changes in the strength of co-stimulation through the B7/CD28 pathway alter functional T cell responses to altered peptide ligands

T cells require a TCR and a co-stimulatory signal for activation. We have examined the effect of the strength of TCR and co-stimulatory signals on proliferation and production of cytokines by differentiated T cell clones. The TCR signal was varied using antigen dose and altered peptide ligands. The co-stimulatory signal was varied by using as antigen-presenting cells, Chinese hamster ovary cell transfectants that express different levels of the B7-1 molecule with similar levels of MHC class II. Our results show that the level of co-stimulation has a profound effect on the response to an antigen, and that a strong co-stimulatory signal can convert a weak agonist into a full agonist and an agonist into a superagonist. Antigenicity is not absolute but a function of the strengths of the TCR and co-stimulatory signals. Increasing the strength of co-stimulation can lower antigen concentration required for maximal proliferative responses by T cell clones by 5 log. These results show that the level of expression of co-stimulatory molecules will profoundly regulate T cell clonal expansion and effector functions.

CD48-deficient mice have a pronounced defect in CD4(+) T cell activation

We have generated mice deficient in the expression of the lymphocyte cell surface antigen CD48 (Blast-1, BCM1, sgp-60) by gene targeting in embryonic stem cells. Mice homozygous for the CD48 mutation (CD48(-/-) mice) are severely impaired in CD4(+) T cell activation. Proliferative responses to mitogens, anti-CD3 mAb, and alloantigen are all reduced. Experiments in which T cells and antigen-presenting cells from either wild-type or CD48(-/-) mice were cocultured reveal that CD48 is important on both T cells and antigen-presenting cells. The most dramatic impairment was observed in experiments in which highly purified T cells were stimulated through the T cell receptor in the presence of the phorbol ester, phorbol 12-myristate 13-acetate. The results of these experiments raise the possibility that CD48 plays a role in signaling through the T cell receptor.

Cutting edge: MHC class I triggering by a novel cell surface ligand costimulates proliferation of activated human T cells

BY55 is a human cell surface molecule whose expression is restricted to NK cells, a subset of circulating CD8+ T lymphocytes, and all intestinal intraepithelial T lymphocytes. Here, we report that BY55 is a novel NK receptor showing broad specificity for both classical and nonclassical MHC class I molecules, and that optimal binding requires a prior aggregation of MHC class I complexes. Using BY55 transfectants, we have identified functional consequences of MHC class I/ligand interactions for the class I-bearing cell. The triggering of MHC class I molecules on human T cell clones by BY55 delivered a potent proliferative signal in the presence of soluble CD3 mAb. The costimulatory signal provided by MHC class I ligation was only seen in activated, and not resting, peripheral blood T cells. This observation represents an additional and/or alternative pathway to CD28 costimulation and may be of particular relevance in memory T cells lacking CD28, such as intestinal intraepithelial T lymphocytes, which are CD28- but BY55+.

The murine Cd48 gene: allelic polymorphism in the IgV-like region

The murine CD48 molecule is a member of the immunoglobulin superfamily which regulates the activation of T lymphocytes. prior cloning experiments using mRNA from two different mouse strains had yielded discrepant sequences within the IgV-like domain of murine CD48. To resolve this issue, we have directly sequenced genomic DNA of 10 laboratory strains and two inbred strains of wild origin. The results of our analysis reveal an allelic polymorphism within the IgV-like domain of murine CD48.

Cloning of BY55, a novel Ig superfamily member expressed on NK cells, CTL, and intestinal intraepithelial lymphocytes

Expression of the BY55 protein has been shown to be tightly associated with NK and CD8+ T lymphocytes with cytolytic effector activity. To determine the function of this protein, we molecularly cloned BY55 cDNA. The cDNA sequence predicts a cysteine-rich, glycosylphosphatidylinositol-anchored protein of 181 amino acids with a single Ig-like domain weakly homologous to killer inhibitory receptors. Reduction and carboxyamidomethylation of immunoprecipitated BY55 gave a band of 27 kDa, whereas reduction alone led to an 80-kDa species, suggesting that BY55 is a tightly disulfide-linked multimer. RNA blot analysis revealed BY55 mRNAs of 1.5 and 1.6 kb whose expression was highly restricted to NK and T cells. BY55 was expressed on the CD56dim, CD16+ subset of NK cells, which have high cytolytic activity, but was not expressed and was not induced on the CD56bright, CD16-subset of NK cells, a subset with high proliferative, but low cytolytic, capacity. In human tissues, BY55 mRNA was expressed only in spleen, PBL, and small intestine (in gut lymphocytes). BY55 was expressed on all intestinal intraepithelial lymphocytes, which were predominantly CD3+TCRalpha/beta+CD4-CD8+CD11b+CD28-CD45RO+C D56-CD101+CD103+ (alphaEbeta7 integrin). In addition, BY55 was expressed on most CD8+CD28- peripheral blood T cells. These phenotypic relationships suggest that CD8+CD28+ precursor CTL may terminally differentiate into CD8+CD28-BY55+ effector CTL and that some of the peripheral blood CD8+CD28- subset may represent recirculation from mucosal epithelial immune sites.

The BB1 monoclonal antibody recognizes both cell surface CD74 (MHC class II-associated invariant chain) as well as B7-1 (CD80), resolving the question regarding a third CD28/CTLA-4 counterreceptor

The identification of all CD28/CTLA-4 counterreceptors is critical to our understanding of this pivotal pathway of T cell activation. Clouding our understanding has been the reported discrepancies in expression and function of the B7-1 (CD80) molecule based upon the use of the BB1 vs other anti-B7-1 mAbs. To resolve this issue, we have cloned a BB1-binding molecule from the BB1+B7-1(-) NALM-6 pre-B cell line. Here, we demonstrate that this BB1-binding molecule is identical to the cell surface form of CD74 (MHC class II-associated invariant chain). CD74-transfected cells bound the BB1 mAb but not other anti-CD80 mAbs, CD28-Ig, or CTLA4Ig. Absorption and blocking experiments confirmed the reactivity of BB1 mAb with CD74. A region of weak homology was identified between CD74 and the region of B7-1 encoding the BB1 epitope. Therefore, the BB1 mAb binds to a protein distinct from B7-1, and this epitope is also present on the B7-1 protein. Many of the puzzling observations in the literature concerning the expression of human B7-1 are resolved by an understanding that BB1 staining is the summation of CD74 plus B7-1 expression. This observation requires the field to reconsider studies using BB1 mAb in the analysis of CD80 expression and function.

Attractin (DPPT-L), a member of the CUB family of cell adhesion and guidance proteins, is secreted by activated human T lymphocytes and modulates immune cell interactions

Attractin is a normal human serum glycoprotein of 175 kDa that is rapidly expressed on activated T cells and released extracellularly after 48-72 hr. We have cloned attractin and find that, as in its natural serum form, it mediates the spreading of monocytes that become the focus for the clustering of nonproliferating T lymphocytes. There are two mRNA species with hematopoietic tissue-specific expression that code for a 134-kDa protein with a putative serine protease catalytic serine, four EGF-like motifs, a CUB domain, a C type lectin domain, and a domain homologous with the ligand-binding region of the common gamma cytokine chain. Except for the latter two domains, the overall structure shares high homology with the Caenorhabditis elegans F33C8.1 protein, suggesting that attractin has evolved new domains and functions in parallel with the development of cell-mediated immunity.

The leukocyte semaphorin CD100 is expressed in most T-cell, but few B-cell, non-Hodgkin's lymphomas

The 150-kd transmembrane protein CD100 is the first semaphorin protein shown to be expressed in lymphoid tissue. CD100 is present in the interfollicular T cell zones and is also expressed by B cells in the germinal centers of secondary lymphoid follicles, but not in the mantle zones. The CD100 molecule was recently cloned, and CD100 transfectants were shown to induce homotypic aggregation of human B cells and improve their viability in vitro, suggesting that CD100 may play a role in lymphocyte aggregation and germinal center formation. We studied the expression of CD100 in 138 clinical cases representing a range of lymphoproliferative disorders, to determine whether this molecule is expressed in these neoplastic processes. In general, we found CD100 expression to be common in peripheral T-cell non-Hodgkin's lymphomas but rare in B-cell non-Hodgkin's lymphomas. CD100 expression was not detectable in low-grade B-cell non-Hodgkin's lymphomas, including cases of small lymphocytic lymphoma (18 cases), marginal zone lymphoma (10 cases), and mantle cell lymphoma (10 cases), as might be expected for these neoplasms that are not of follicular center cell origin. Surprisingly, we found that the vast majority of follicular lymphomas (37 of 40 cases) as well as diffuse large-cell lymphomas of B-cell type (35 cases) did not express CD100. The neoplastic cells in 3 of 11 cases of predominantly large-cell-type follicular lymphoma did express CD100. In contrast, all five cases of high-grade, small non-cleaved (Burkitt-like) B-cell lymphoma were immunoreactive for CD100 expression, as were 18 of 20 cases (90%) of malignant T cell neoplasms. Northern blot analysis of CD100 expression correlated with immunohistochemical findings. Absence of expression of CD100 by neoplastic follicular center B cells is a common feature in follicular lymphomas, but expression of CD100 by T cells is maintained in T-cell lymphoproliferative disorders.

Expansion of autoreactive T cells in multiple sclerosis is independent of exogenous B7 costimulation

Multiple sclerosis (MS) is an inflammatory disease of the myelinated central nervous system that is postulated to be induced by myelin-reactive CD4 T cells. T cell activation requires an antigen-specific signal through the TCR and a costimulatory signal, which can be mediated by B7-1 or B7-2 engagement of CD28. To directly examine the activation state of myelin-reactive T cells in MS, the costimulation requirements necessary to activate myelin basic protein (MBP) or tetanus toxoid (TT)-reactive CD4 T cells were compared between normal controls and MS patients. Peripheral blood T cells were stimulated with Chinese hamster ovary (CHO) cells transfected either with DRB1*1501/DRA0101 chains (t-DR2) alone, or in combination with, B7-1 or B7-2. In the absence of costimulation, T cells from normal subjects stimulated with the recall antigen TT p830-843 were induced to expand and proliferate, but stimulation with MBP p85-99 did not have this effect. In marked contrast, T cells from patients with MS stimulated with MBP p85-99 in the absence of B7-1 or B7-2 signals expanded and proliferated. Thus, MBP-reactive CD4 T cells in patients with MS are costimulation independent and have been previously activated in vivo. These experiments provide further direct evidence for a role of activated MBP-specific CD4 T cells in the pathogenesis of MS.

In vivo expression of B7-1 and B7-2 by follicular lymphoma cells can prevent induction of T-cell anergy but is insufficient to induce significant T-cell proliferation

Expression of B7 family costimulatory molecules on B cells defines their capacity to function as antigen presenting cells (APCs). B cells that do not express B7 costimulatory molecules induce T-cell tolerance. Therefore, the expression of B7 costimulatory molecules on malignant B cells might be critical for their recognition by anti-tumor-specific T cells. Here we show that virtually all germinal center (GC)-derived B-cell lymphomas including follicular lymphoma (FL) and diffuse large cell lymphoma, but not mantle cell lymphoma or small lymphocytic lymphomas (SLL/CLL), express B7-1 (CD80) and B7-2 (CD86) on their cell surface in situ, although at extremely low levels. Despite their expression of low levels of B7-1 and B7-2, FL cells could not induce significant allogeneic T-cell proliferation. However, B7 costimulatory molecules on FL appeared to be functional because they were capable of increasing T-cell proliferation of preactivated T cells in a secondary allogeneic mixed lymphocyte response. Moreover, low B7 expression was sufficient to prevent the induction of alloantigen-specific anergy in vitro. Therefore, we postulate that whereas low-level expression of B7 is not sufficient to initiate a productive antilymphoma T-cell response, it might be sufficient to prevent T-cell tolerance in vivo.

Expression of a hypoglycosylated form of CD86 (B7-2) on human T cells with altered binding properties to CD28 and CTLA-4

CD80 (B7-1) and CD86 (B7-2) on APC provide a major costimulatory signal through interactions with CD28 on T cells. Absent from resting human T cells, CD86 is up-regulated early upon T cell activation, whereas CD80 expression appears later. Whereas T cell expression of CD80 has been implicated in costimulation, the functional significance of CD86 expression on T cells is unclear. We now demonstrate that CD86 expressed on human CD4+ T cell clones does not provide a costimulatory signal for other CD4+ T cell clones. Binding studies using CD28-Ig and CTLA-4-Ig fusion proteins demonstrate that CD86 expressed on T cells has significantly reduced binding affinity for CTLA-4 and no detectable binding to CD28. Biochemical analysis demonstrates that post-translational modifications of CD86 in human T cells are different from those of CD86-transfected Chinese hamster ovary cells or EBV-transformed B cells, in that T cells express a hypoglycosylated form of CD86 on the surface membrane. Thus, our results suggest that while CD86 is expressed on a number of different cell types, its costimulatory function and affinity for its ligands may be regulated by cell type-specific post-translational modifications.

Expression of a hypoglycosylated form of CD86 (B7-2) on human T cells with altered binding properties to CD28 and CTLA-4

CD80 (B7-1) and CD86 (B7-2) on APC provide a major costimulatory signal through interactions with CD28 on T cells. Absent from resting human T cells, CD86 is up-regulated early upon T cell activation, whereas CD80 expression appears later. Whereas T cell expression of CD80 has been implicated in costimulation, the functional significance of CD86 expression on T cells is unclear. We now demonstrate that CD86 expressed on human CD4+ T cell clones does not provide a costimulatory signal for other CD4+ T cell clones. Binding studies using CD28-Ig and CTLA-4-Ig fusion proteins demonstrate that CD86 expressed on T cells has significantly reduced binding affinity for CTLA-4 and no detectable binding to CD28. Biochemical analysis demonstrates that post-translational modifications of CD86 in human T cells are different from those of CD86-transfected Chinese hamster ovary cells or EBV-transformed B cells, in that T cells express a hypoglycosylated form of CD86 on the surface membrane. Thus, our results suggest that while CD86 is expressed on a number of different cell types, its costimulatory function and affinity for its ligands may be regulated by cell type-specific post-translational modifications.

CD101 is expressed by skin dendritic cells. Role in T-lymphocyte activation

CD101 was first described in our laboratory using two different monoclonal antibodies, BA27 and BB27, recognizing a 140-kDa disulfide-bonded homodimeric polypeptide on a small subset of circulating T lymphocytes and on most activated T cells in vitro. Further, it has been reported that most intestinal mucosal T lymphocytes expressed CD101. The gene coding for the CD101 antigen has been cloned and found to be identical to the gene coding for the recently described V7 antigen, corresponding to a type I trans-membrane protein with seven immunoglobulin-like loops in its extracellular domain. To define surface proteins that are involved in skin dendritic cell (DC) localization or function, we looked for the expression of CD1O1 on skin DC migrating from human skin explants. The majority of these DC had a phenotype of Langerhans cell (LC)-like mature DC, i.e., HLA-DR+ CD1a+ CD1c+ CD11a+ CD11c+ CD40+ CD50+ CD54+ CD58+ CD80+ CD83+ CD86+. We found that CD101 was expressed by a major subset of these HLA-DR+ CD1a+ CD1c+ LC-like skin DC. Next, we studied the effect of anti-CD101 monoclonal antibodies on primary allogeneic and on soluble antigen-specific mixed skin DC-lymphocyte reactions. We showed that two different monoclonal antibodies, BB27 and V7.1, inhibited the T-lymphocyte proliferative responses and that the inhibitory effect was overcome by high doses of exogenous IL-2. As both DC and T lymphocytes expressed CD101 molecules, we determined that the inhibitory effect was achieved both at the responder T-cell level and at the DC level. Thus, CD101 which is expressed on a subset of circulating T lymphocytes, has also been found on a subpopulation of LC-like DC. This molecule plays a major role in the activation of T cells by skin DC.

Maintenance of human T cell anergy: blocking of IL-2 gene transcription by activated Rap1

In the absence of costimulation, T cells activated through their antigen receptor become unresponsive (anergic) and do not transcribe the gene encoding interleukin-2 (IL-2) when restimulated with antigen. Anergic alloantigen-specific human T cells contained phosphorylated Cbl that coimmunoprecipitated with Fyn. The adapter protein CrkL was associated with both phosphorylated Cbl and the guanidine nucleotide-releasing factor C3G, which catalyzes guanosine triphosphate (GTP) exchange on Rap1. Active Rap1 (GTP-bound form) was present in anergic cells. Forced expression of low amounts of Rap1-GTP in Jurkat T cells recapitulated the anergic defect and blocked T cell antigen receptor (TCR)- and CD28-mediated IL-2 gene transcription. Therefore, Rap1 functions as a negative regulator of TCR-mediated IL-2 gene transcription and may be responsible for the specific defect in IL-2 production in T cell anergy.

Induction of T cell clonal anergy results in resistance, whereas CD28-mediated costimulation primes for susceptibility to Fas- and Bax-mediated programmed cell death

Since TCR-mediated stimulation induces T cells to become sensitive to Fas-mediated activation-induced cell death (Fas-AICD), we examined whether anergized and CD28-costimulated T cell clones were equally sensitive to Fas-AICD. Here, we show that TCR signal in the presence or absence of CD28 costimulation induced equivalent expression of Fas and Fas ligand. Although anergized cells expressed Fas and Fas ligand, they were resistant to Fas-AICD. Induction of anergy resulted in up-regulation and persistent expression of moderate amounts of bcl-xL and bax and absence of induction of bad. In contrast, CD28-costimulated cells that also expressed Fas and Fas ligand were initially resistant to Fas-AICD but became susceptible after 72 h of culture. Although Fas-mediated apoptosis was the major mechanism of AICD, the IL-1beta-converting enzyme-like protease inhibitor zVAD-FMK totally abrogated DNA fragmentation but not cell death, suggesting that additional Fas-independent apoptotic mechanisms were also operative. Resistance to apoptotic cell death was temporally associated with a dramatic increase of bcl-xL and the presence of bcl-xL:bax heterodimers. Subsequent sensitivity to AICD was associated with down-regulation of bcl-xL, induction of bad, and the displacement of bax from bcl-xL:bax heterodimers. Although induced following CD28 costimulation, bcl-2 did not protect against AICD. Therefore, besides its role in promotion of viability, prevention of anergy, and clonal expansion, CD28 costimulation also has a central role in the induction of subsequent AICD by up-regulating apoptotic mediators.

Induction of T cell clonal anergy results in resistance, whereas CD28-mediated costimulation primes for susceptibility to Fas- and Bax-mediated programmed cell death

Since TCR-mediated stimulation induces T cells to become sensitive to Fas-mediated activation-induced cell death (Fas-AICD), we examined whether anergized and CD28-costimulated T cell clones were equally sensitive to Fas-AICD. Here, we show that TCR signal in the presence or absence of CD28 costimulation induced equivalent expression of Fas and Fas ligand. Although anergized cells expressed Fas and Fas ligand, they were resistant to Fas-AICD. Induction of anergy resulted in up-regulation and persistent expression of moderate amounts of bcl-xL and bax and absence of induction of bad. In contrast, CD28-costimulated cells that also expressed Fas and Fas ligand were initially resistant to Fas-AICD but became susceptible after 72 h of culture. Although Fas-mediated apoptosis was the major mechanism of AICD, the IL-1beta-converting enzyme-like protease inhibitor zVAD-FMK totally abrogated DNA fragmentation but not cell death, suggesting that additional Fas-independent apoptotic mechanisms were also operative. Resistance to apoptotic cell death was temporally associated with a dramatic increase of bcl-xL and the presence of bcl-xL:bax heterodimers. Subsequent sensitivity to AICD was associated with down-regulation of bcl-xL, induction of bad, and the displacement of bax from bcl-xL:bax heterodimers. Although induced following CD28 costimulation, bcl-2 did not protect against AICD. Therefore, besides its role in promotion of viability, prevention of anergy, and clonal expansion, CD28 costimulation also has a central role in the induction of subsequent AICD by up-regulating apoptotic mediators.

The human semaphorin-like leukocyte cell surface molecule CD100 associates with a serine kinase activity

CD100 is a 150-kDa homodimeric glycoprotein broadly expressed on the surface of human hematopoietic cells. CD100 has been recently identified as the first lymphoid gene that belongs to the semaphorin gene family. Semaphorins function as chemorepellent molecules in the nervous system, but the function of CD100 remains poorly understood. In lymphoid cells, it has been suggested to play a role in homotypic cell adhesion and in T cell activation. We demonstrate that in T cells and natural killer cells a serine kinase activity is immunoprecipitated with CD100. Distinct epitopes of CD100 have been defined with specific monoclonal antibodies, mediating opposite effects at the functional level, especially in T cells. The kinase activity is retained only with an antibody against a particular epitope of CD100. Additionally, a fusion protein containing the cytoplasmic domain of the molecule retains the kinase activity in cellular lysates, and CD100 itself is presumably a favorite substrate of the kinase. These findings suggest that a serine kinase pathway may participate in the different functional effects triggered through the distinct epitopes of CD100 and is likely involved in the biological effects of this semaphorin-like leukocyte cell surface molecule.