Coexpression and functional cooperation of CTLA-4 and CD28 on activated T lymphocytes

Studies Using Antigen-Presenting Cells Lacking Expression of Both B7-1 (CD80) and B7-2 (CD86) Show Distinct Requirements for B7 Molecules During Priming Versus Restimulation of Th2 But Not Th1 Cytokine Production

The differentiation of CD4+ T cells into a Th1 vs Th2 phenotype profoundly influences the outcome of autoimmune and infectious diseases. B7 costimulation has been shown to affect the production of both Th1 and Th2 cytokines, depending on the system studied. There is, consequently, great interest in manipulating the B7 costimulatory signal for therapeutic purposes. To optimally manipulate this key immunoregulatory pathway, the contribution of B7 costimulation to cytokine production requires further clarification. We have compared the B7 requirement for cytokine production by naive vs previously activated T cells using DO11.10 TCR transgenic CD4+ T cells and splenic APCs from mice lacking B7 expression. Our data indicate that induction of IL-4 production and Th2 differentiation by naive T cells is highly dependent on B7 molecules, whereas IL-4 production by previously activated T cells is B7 independent. The predominant contribution of B7-mediated signals to Th1 cytokine production by both naive and primed T cells is upon IL-2 production (and expansion) rather than IFN-γ (effector cytokine) production. Thus, our studies demonstrate that the antigenic experience of a T cell at the time of B7 blockade may determine whether blockade predominantly affects T cell expansion, differentiation, or effector cytokine production. These differential effects of B7 costimulation on IL-2 vs IFN-γ production and on IL-4 production by naive vs primed T cells have important implications for understanding how B7:CD28/CTLA4 blockade can be effectively used to manipulate cytokine production in vivo.

Repeated additions of hyaluronan alters granulation tissue deposition in sponge implants in mice

The role for the metabolism of hyaluronic acid in the repair process is uncertain. Fetal dermal wounds do not heal by scarring and have sustained high levels of hyaluronic acid. In contrast, adult dermis is repaired by scarring and has less hyaluronic acid. Initially after injury, hyaluronic acid is elevated in both adult and fetal wounds, and although it remains elevated in fetal repair, it is rapidly degraded in adult wounds. The chronic addition of hyaluronic acid or hyaluronidase to polyvinyl alcohol sponge implants in adult mice was investigated in this study. Polyvinyl alcohol sponge implants containing a central reservoir were placed subcutaneously in the dorsum of adult male CD-1 mice. Mice were divided into three groups: a phosphate-buffered saline control, a 20 microgram hyaluronic acid treatment group, and a 10 U hyaluronidase treatment group. The central reservoir of each sponge implant received appropriate compound every 3 days for 2 weeks via transdermal injection and were then evaluated histologically. At 2 weeks, the cellular density and the quantity of granulation tissue deposition were the greatest in the hyaluronidase group and were lowest in the hyaluronic acid group. In addition, the organization of collagen fiber bundles was the most dense in the hyaluronidase group and least in the hyaluronic acid group. In a second experiment, polyvinyl alcohol sponge implants in mice received either phosphate-buffered saline solution or 20 microgram hyaluronic acid every 3 days for 1 week. On day 5, an aliquot of fluorescently tagged native collagen was injected into the sponges. Sponges were harvested at day 7, cryosections made, and the presence of autofluorescent collagen fibers assessed. The autofluorescent collagen fiber bundles in the phosphate-buffered saline solution group were organized in thick parallel bundles, whereas the collagen bundles from hyaluronic acid-treated implants were organized in fine lacelike structures. Chronic addition of hyaluronic acid appears to mimic the fetal dermal connective tissue matrix in which repair proceeds with diminished collagen deposition, organized in finer collagen fiber bundles in granulation tissue. On the other hand, the removal of hyaluronic acid by the chronic administration of hyaluronidase increases the amount of granulation tissue. Elevated levels of hyaluronic acid in granulation tissue appear to modulate the ability of resident fibroblasts to organize collagen fiber bundles.

B7-1, But Not CD28, Is Crucial for the Maintenance of the CD4+ T Cell Responses in Human Leprosy

We used human leprosy as a model to compare patterns of costimulatory molecule expression in respect to the clinical/immunologic spectrum of disease. We found that B7-1, B7-2, and CD28 transcripts dominated in tuberculoid leprosy patients, who have potent T cell responses to Mycobacterium leprae. In contrast, CTLA-4 was more strongly expressed in lesions from lepromatous patients, who manifest specific T cell anergy to the leprosy bacterium. T cell clones from tuberculoid lesions were CD4+CD28+ or CD4+CD28−, and T cell clones from lepromatous lesions were predominantly CD8+CD28−. The M. leprae-specific recall response of CD4+ T cell clones from tuberculoid lesions was blocked by anti-B7-1 mAb, but not by anti-B7-2 mAb or CTLA-Ig. However, anti-CD28 and anti-CTLA-4 mAbs did not block activation of clones from tuberculoid lesions, suggesting that B7-1 may utilize another costimulatory pathway. Peripheral blood T cell responses in the lepromatous form were strongly regulated by CD28 during T cell activation, in contrast to the tuberculoid form. Thus, B7-1 costimulation could play a role in maintaining a strong immune response to the pathogen.

Impaired Alloantigen-Mediated T Cell Apoptosis and Failure To Induce Long-Term Allograft Survival in IL-2-Deficient Mice

We examined whether IL-2 regulates alloimmune responses by studying allograft survival in wild-type (IL-2+/+) and IL-2 gene-knockout (IL-2−/−) mice. The acute rejection of vascularized, cardiac allografts and the generation of allospecific CTLs were not impaired in the absence of IL-2. In contrast, blocking the B7-CD28 T cell costimulation pathway with CTLA4Ig induced long-term allograft survival (>100 days) in IL-2+/+ recipients but failed to do so in IL-2−/− mice or in wild-type mice that had been treated with IL-2-neutralizing Ab around the time of transplantation. Allografts rejected by IL-2−/− recipients exhibited extensive mononuclear cell infiltrates despite CTLA4Ig administration. In vivo allostimulation in the absence of IL-2 led to exaggerated T lymphocyte proliferation and impaired apoptosis of activated T cells in untreated and CTLA4Ig-treated mice. These findings indicate that endogenous IL-2 is required for the induction of long-term allograft survival, and that IL-2 regulates alloimmune responses by preparing activated T lymphocytes for alloantigen-induced apoptosis.

Targeting the B7/CD28:CTLA-4 costimulatory system in CNS autoimmune disease

The B7/CD28:CTLA-4 costimulatory pathway plays a critical role in determining the fate of immune responses (activation vs. down-regulation) and is a highly promising therapeutic target for treating autoimmune diseases. In this review, we highlight the mechanisms by which this costimulatory pathway operates emphasizing the role of the different components in the pathogenesis of relapsing experimental autoimmune encephalomyelitis, a CD4 T cell-mediated autoimmune model of multiple sclerosis. The separate and distinct roles of B7-1, B7-2 and CTLA-4 in positive and negative regulation of autoimmune pathogenesis are considered and a working model is proposed.

Localization in situ of costimulatory molecules and cytokines in B-cell non-Hodgkin's lymphoma

Costimulatory molecules are essential in cognate interactions between T and B lymphocytes. To study the prerequisites of functional interactions between malignant B cells and intermingled T cells in B-cell non-Hodgkin's lymphomas (B-NHL), we examined the expression of CD40, CD80 and CD86 and their ligands CD40 ligand (CD40L, CD154), CD28 and CTLA4 (CD152) using immunohistochemistry and confocal laser scanning microscopy. Almost all mucosa-associated lymphoid tissue (MALT) NHL were positive for CD40 and CD80 and in nine out of 14 cases were positive for CD86. The majority of follicle centre cell lymphomas (FCCL) expressed CD40, but were heterogeneous in their expression of CD80 and CD86. Most diffuse large cell lymphomas (DLCL) were CD80+, but lacked expression of CD86. These patterns reflect the differences in phenotype of normal marginal-zone B cells (as counterparts of MALT NHL) and germinal centre cells (as counterparts of FCCL and DLCL). Counter-receptors on T cells were detectable in 13 of 14 MALT NHL, 12 of 16 FCCL but only occasionally in DLCL (three of 12 cases). A subgroup of FCCL was identified with T-cell expression of CD40L, CD28 and CTLA4 simultaneously with strong expression of CD40 and CD86 on the tumour B cells. These results indicate that MALT NHL and a subset of FCCL are most optimally equipped for functional interactions with T cells. This may be supported by the demonstration of cytokine production - mainly in T cells - in MALT NHL [interleukin-2 (IL-2), interferon-gamma (IFN-gamma), IL-10] and FCCL (IL-2, IFN-gamma) and to a lesser extent in DLCL.

Kinetics of Expression of Costimulatory Molecules and Their Ligands in Murine Relapsing Experimental Autoimmune Encephalomyelitis In Vivo

We studied the kinetics of expression of costimulatory molecules and cytokines in the central nervous system (CNS) in murine relapsing experimental autoimmune encephalomyelitis (EAE). During the natural course of EAE, B7-2 expression in the CNS correlated with clinical signs, while B7-1 was exclusively expressed during remissions. Interestingly, B7-1 was expressed on infiltrating mononuclear cells as well as neuronal cells in the CNS. In the periphery, B7-1 expression on APCs peaked with clinical disease but decreased on T cells. CD28 and CTLA4 molecules, the two known ligands for B7-1 and B7-2, had distinct expression patterns in the CNS; CD28 was highly expressed and correlated with B7-2 expression on APCs (macrophages/microglia as well as astrocytes) and with the clinical signs of EAE. CTLA4, on the other hand, was expressed by substantially fewer cells during the effector phase of disease and peaked during remission, which is consistent with the emerging role of this molecule in the termination of immune responses. The expression of CD40 and CD40L in the CNS was increased during clinical attacks. The expression of IL-12, IFN-γ, and TNF-α correlated with disease activity and severity, while TGF-β was the only factor that was up-regulated during the recovery phase. Interestingly, TGF-β was also expressed by neurons during remission. This is the first study demonstrating the kinetics of the in vivo expression of costimulatory molecules, their ligands, and cytokines in an autoimmune disease model characterized by remissions and relapses. Our data suggest that the targeting of costimulatory molecules to block an immune response must take into account the expression patterns in the target organ.

Requirement of CD28-CD86 costimulation for allergen-specific T cell proliferation and cytokine expression

BACKGROUND

Allergen-specific T lymphocytes biased to the production of type 2 cytokines play an important role in the pathophysiology of atopic disease. It is not known whether optimal activation of these T cells requires costimulation via interaction of B7 (CD86/CD80) with CD28.

METHODS

Peripheral blood mononuclear cells (PBMC), isolated from 10 house dust mite Dermatophagoides pteronyssinus (Der p)-allergic asthma patients and 10 non-allergic control individuals, were stimulated with house dust mite (Der p) and the control antigens Candida albicans (CA) and tetanus toxoid (TT). The role of costimulation in activation for proliferation and cytokine mRNA production of peripheral blood T cells was studied by blocking CD28, CTLA-4, and their ligands CD80 (B7-1) and CD86 (B7-2).

RESULTS

The proliferation and the production of type 1 and 2 cytokine mRNA by T cells in response to Der p as well as the control antigens TT and CA was inhibited by simultaneously masking CD80 and CD86 using CTLA4-Ig, a soluble form of CTLA-4. Notably, Der p-specific proliferation of T cells from Der p-allergic asthma patients and non-allergic controls were inhibited equally well. Additional experiments with MoAbs revealed that activation of these T cells was optimally inhibited by blocking the interaction of CD28 with CD86.

CONCLUSION

In vitro responses of allergen- and antigen-specific T cells of allergic patients and non-allergic control persons are equally dependent on costimulation via the CD28-CD86 pathway, suggesting that inhibition of this pathway may prevent complete activation of allergen-specific T cells in allergic individuals in vivo.

Negative regulation of T cell activation

T cell activation is negatively regulated by cytotoxic T lymphocyte antigen 4 (CTLA-4) and the killer cell inhibitory receptors. Endocytosis and signaling of CTLA-4 are regulated by tyrosine phosphorylation. While T cell activation is mediated by phosphorylation of immunoreceptor tyrosine-based activation motifs and tyrosine kinases, inhibitory signals are delivered by tyrosine phosphatases. Unresponsiveness is also induced by modulation of signaling components of the T cell receptor complex.

B7-CTLA4 interaction enhances both production of antitumor cytotoxic T lymphocytes and resistance to tumor challenge

Expression of B7-family costimulatory molecules CD80 (B7-1) and CD86 (B7-2) on tumor cells enhances host immunity. However, the role of the two B7 receptors, CD28 and CTLA4 (CD152), on T cells in antitumor immune response has not been clearly elucidated. Based on the effects of anti-CD28 and anti-CTLA4 mAbs on T cell response, it was proposed that CD28-B7 interaction promotes antitumor immunity, whereas B7-CTLA4 interaction down-regulates it. A critical test for the hypothesis is whether selective engagement of CTLA4 receptors by their natural ligands CD80 and CD86 enhances or reduces antitumor immunity. Here we used tumors expressing wild-type and mutant CD80, as well as mice with targeted mutation of CD28, to address this issue. We report that in syngeneic wild-type mice, B7W (W88>A), a CD80 mutant that has lost binding to CD28 but retained binding to CTLA4, can enhance the induction of antitumor cytotoxic T lymphocytes (CTL); B7Y (Y201>A), which binds neither CD28 nor CTLA4, fails to do so. Consistent with these observations, B7W-transfected J558 plasmocytoma and EL4 thymoma grow significantly more slowly than those transfected with either vector alone or with B7Y. Optimal tumor rejection requires wild-type CD80. Moreover, expression of a high level of CD80 on thymoma EL4 cells conveys immunity in mice with a targeted mutation of CD28 gene. Taken together, our results demonstrate that B7-CTLA4 interaction enhances production of antitumor CTL and resistance to tumor challenge and that optimal enhancement of antitumor immunity by CD80 requires its engagement of both CD28 and CTLA4.

Human CTLA-4 is expressed in situ on T lymphocytes in germinal centers, in cutaneous graft-versus-host disease, and in Hodgkin's disease

Cytotoxic T-lymphocyte-associated antigen-4 (CTLA-4, CD152) is a molecule expressed on in vitro activated T cells. CTLA-4 shares important sequence homology with CD28 and binds to the same ligands, CD80 (B7-1) and CD86 (B7-2). CTLA-4 probably functions as a negative regulator of T lymphocyte activation in the mouse, although this remains to be proven for human T lymphocytes. We have developed new monoclonal antibodies against human CTLA-4 and have investigated the in situ expression of CTLA-4 in a wide variety of normal and pathological human tissues expressing CD80 and CD86. As revealed in this study, CTLA-4 is expressed on thymocytes in thymic medulla, on a subset of CD4+ T lymphocytes in germinal centers of follicular hyperplasia, on T cells, mainly CD8+, infiltrating skin affected by graft-versus-host disease, and on T cells, mainly CD4+, infiltrating Hodgkin's disease lesions. In immunoelectron microscopy, CTLA-4 was found on the plasma membrane as well as in the hyaloplasm and cytoplasmic vesicles, in agreement with its pattern of expression on in vitro activated T cells. Interestingly, no or at most scarce expression of CTLA-4 was found in granulomatous lymph nodes, T-cell-mediated inflammatory diseases, or non-Hodgkin's lymphomas, regardless of their expression of CD80 or CD86. Thus, expression of CTLA-4 appears to be induced in selective pathological conditions in vivo. The pathways leading to selective induction of CTLA-4 and its role in the pathophysiology of these conditions need to be further investigated.

Why do we need new treatments for rheumatoid arthritis?

Rheumatoid arthritis is a chronic systemic autoimmune inflammatory disease characterized by progressive joint damage. The classical treatments of the disease such as myocrisin and sulphasalazine, are not always effective at controlling the disease. This has necessitated the development of novel agents for treating rheumatoid arthritis. Most of these drugs are biological in nature and are targeted at specific sites of the inflammatory cascade of reactions. A number of clinical trials have been conducted. The clinical effects that have been observed are transient, necessitating repeated treatments and the risk of vaccination effects. Many of these agents have to be administered parenterally, production costs are very high. Consequently, chemical entities which can be taken orally need to be developed. Since the immune system is very complex with pleiotropic cytokines and redundancy in some of the regulatory networks, it may therefore be necessary to use multiple agents targeted at different specific sites of the inflammatory cascade or that different agents could be given at different stages of the disease, to induce disease remission and maintain the response to therapy. Cytokines such as tumour necrosis factor (TNF) and interleukin 1 (IL-1) play important physiological roles in the host's defence systems against infections and malignancy. The chronic inhibition of these cytokines by targeted therapies may therefore lead to the development of side effects. Thus, carefully controlled long-term studies will be required to assess the safety of selective targeting of processes involved in inflammation. A more recent novel approach is to target hypoxic tissues with bioreductive agents. Thus, some of the established rheumatoid arthritis treatments could be linked to bioreductive agents and released in hypoxic tissues where inflammation is occurring. This review summarizes the important developments in the therapy of rheumatoid arthritis. There is no doubt that despite these developments we need to develop new and advanced treatment modalities for rheumatoid arthritis.

Immunoliposomes containing antibodies to costimulatory molecules as adjuvants for HIV subunit vaccines

Immunoliposomes containing monoclonal antibodies (MAbs) to the costimulatory molecules CD28 and CTLA4 and their counterreceptors B7-1 (CD80) and B7-2 (CD86) were evaluated for the ability to increase the immune response to recombinant envelope protein rgp120 of the MN strain of human immunodeficiency virus type 1 (HIV-1) during vaccination. MAbs were attached to rgp120-containing liposomes via a biotin-avidin-biotin bridge. Mice vaccinated with immunoliposomes were found to have a strong delayed-type hypersensitivity (DTH) response to the weakly immunogenic gp120 that was dependent on the presence of the MAbs. However, this vaccination protocol did not induce humoral immunity. The DTH response was not accompanied by increased production of interferon gamma (IFN-gamma) or interleukin 4 (IL-4), implying that the primary cellular interaction was between the immunoliposomes and cells of the reticuloendothelial system and not helper T (Th) cells. This strategy of incorporating antibodies to costimulatory molecules on the surface of antigen-containing particulates, such as liposomes or microspheres, can be used to increase DTH immune responses to protein or peptide vaccines.

Identification of conserved amino acids in murine B7-1IgV domain critical for CTLA4/CD28:B7 interaction by site-directed mutagenesis: a novel structural model of the binding site

The B7: CD28/CTLA4 interaction plays a major role in T cell responses. Immune intervention targeted at this interaction has demonstrated a vast potential in enhancing tumor immunity and blocking autoimmunity and transplant rejection. However, the structural basis for this interaction is unclear. While we and others have performed site-directed mutagenesis to define amino acids involved in binding CD28 and CTLA4, these residues are localized in different regions, and it is unlikely for all of them to be directly involved. In addition, the effect of the mutations on the overall conformation of B7 has not been systematically evaluated. In this study, we have carried out site-directed mutagenesis to define the amino acids within B7-1 IgV-like domain which participate B7:CD28/CTLA4 interaction. Four anti-B7-1 mAbs that recognize three independent antigenic epitopes on B7-1 were used to monitor the effect of mutations on the overall conformation of B7-1. Of the five mutations in the IgV domain that we have produced, D113 > A appears to interfere with cell surface expression and/or overall conformation of B7-1. while four others do not significantly affect the overall conformation and cell surface expression of B7-1. Among them, G115 > A and Y91 > A eliminated B7-1 binding to both CD28Ig and CTLA4Ig; our previously reported mutants L109 > A and W88 > A selectively affect the B7-1 binding to either CD28Ig or CTLA4Ig. Structural modeling of B7-1 based on the structure of immunoglobulin revealed that these four and other previously identified critical amino acids in both IgV- and IgC-like domains can form a localized structure.

Biochemical features of anergic T cells

T cell anergy is a functionally defined state of hyporesponsiveness in which T cells neither proliferate nor produce IL2 following subsequent TCR ligation. Recent biochemical data from in vitro studies suggest that anergic cells do not utilize all of the signaling pathways normally initiated by TCR triggering. These findings appear to hold true for T cells rendered anergic in vivo, as well; however, biochemical studies on clonal anergy in vivo have been limited by the inability to recover a homogeneous population of anergic T cells. Here we review progress on TCR mediated signaling pathways as well as the description of surface marker phenotypes specific to T cell anergy.

Cytotoxic T lymphocyte-associated antigen 4 (CTLA-4) regulates the unfolding of autoimmune diabetes

Evidence has been accumulating that shows that insulin-dependent diabetes is subject to immunoregulation. To determine whether cytotoxic T lymphocyte-associated antigen 4 (CTLA-4) is involved, we injected anti-CTLA-4 mAb into a TCR transgenic model of diabetes at different stages of disease. When injected into young mice, months before they would normally become diabetic, anti-CTLA-4 induced diabetes rapidly and essentially universally; this was not the result of a global activation of T lymphocytes, but did reflect a much more aggressive T cell infiltrate in the pancreatic islets. These effects were only observed if anti-CTLA-4 was injected during a narrow time window, before the initiation of insulitis. Thus, engagement of CTLA-4 at the time when potentially diabetogenic T cells are first activated is a pivotal event; if engagement is permitted, invasion of the islets occurs, but remains quite innocuous for months, if not, insulitis is much more aggressive, and diabetes quickly ensues.

Cutting Edge: CTLA-4 Ligation Delivers a Unique Signal to Resting Human CD4 T Cells That Inhibits Interleukin-2 Secretion but Allows Bcl-XL Induction

We have assessed the functional effects of a panel of CTLA-4 mAbs on resting human CD4+ T cells. Our results demonstrate that some CTLA-4 mAbs can inhibit proliferative responses of resting CD4+ cells and cell cycle transition from G0 to G1. The inhibitory effects of CTLA-4 were evident within 4 h, at a time when cell surface CTLA-4 expression remained undetectable. Other CTLA-4 mAbs had no detectable inhibitory effects, indicating that binding of Ab to CTLA-4 alone is not sufficient to mediate down-regulation of T cell responses. Interestingly, while IL-2 production was shut off, inhibitory anti-CTLA-4 mAbs permitted induction and expression of the cell survival gene bcl-XL. Consistent with this observation, cells remained viable and apoptosis was not detected after CTLA-4 ligation.

Normal Responsiveness of CTLA-4-Deficient Anti-Viral Cytotoxic T Cells

CTLA-4 has been proposed to negatively regulate immune responses, and mice deficient for CTLA-4 expression succumb to a lymphoproliferative disorder within a few weeks after birth. This study assessed the responsiveness of CTLA-4-deficient T cells expressing a class I-restricted TCR specific for lymphocytic choriomeningitis virus (LCMV). The kinetics of T cell proliferation were studied in vitro after stimulation of T cells with full and partial T cell agonists. No gross abnormalities in CTLA-4-deficient T cells could be detected. Using adoptive transfer experiments, T cell responses were also measured in vivo after infection with LCMV. Low dose infection with LCMV leads to strong expansion of specific T cells followed by a reduction in T cells that parallels the elimination of Ag. The kinetics of T cell expansion and elimination after low dose LCMV infection were not affected by the absence of CTLA-4. High dose infection of mice with LCMV leads to a transient expansion of T cells followed by T cell exhaustion, where all specific T cells are eliminated. T cell exhaustion also occurred in the absence of CTLA-4. Thus, surprisingly, the absence of CTLA-4 did not interfere with T cell activation, down-regulation of ongoing T cell responses after the elimination of Ag, or the exhaustion of T cell responses in the presence of excessive amounts of Ag.

Interaction of the cytoplasmic tail of CTLA-4 (CD152) with a clathrin-associated protein is negatively regulated by tyrosine phosphorylation

CTLA-4 (CD152), high-avidity receptor for CD80 and CD86, is a powerful regulator of T cell activation. While CTLA-4 functions at the cell surface, it is primarily localized in intracellular vesicles and cycles to the cell surface. The CTLA-4 cytoplasmic domain contains sequences that direct its intracellular localization and regulate its signaling. Here we demonstrate that effector molecules involved in receptor trafficking and signaling interact with distinct, but overlapping, sequences in the CTLA-4 cytoplasmic domain. Using the yeast two-hybrid method, we demonstrate association of the mu2 subunit of AP-2, the clathrin-associated complex found in plasma membrane-associated coated pits, with the cytoplasmic tail of CTLA-4, but not CD28. The mu1 subunit of AP-1, found in Golgi-associated coated pits, associated with neither CTLA-4 nor CD28. Sequences required for interaction of mu2 and CTLA-4 were localized to residues, 161TTGVY in CTLA-4; this sequence is N-terminal to, but overlaps with, a previously identified SH2 binding motif, 165YVKM, involved in CTLA-4 signaling. Mu2 interacted preferentially with CTLA-4 when residue 165Y was nonphosphorylated, whereas a PI3 kinase SH2 domain interacted preferentially when 165Y was phosphorylated. In co-transfection experiments, both tyrosine residues in the cytoplasmic tail of CTLA-4 (165Y and 182Y) were phosphorylated by the T lymphocyte-associated tyrosine kinase, p56lck. Thus, phosphorylation of CTLA-4 residue 165Y may reciprocally regulate signaling and trafficking of CTLA-4 by determining which effector molecules bind to its cytoplasmic tail.

Is CTLA-4 a negative regulator for T-cell activation?

Immune responses can be a two-sided sword: although lack of immunity can be fatal, so can over-activation of the immune system. The concept of homeostatic balance in immune regulation has consequently been at the core of immune research. In this context, Yang Liu takes a cautious view of the concept that CTLA-4 is a negative regulator of T-cell activation.

Immunosenescence and germinal center reaction

Dysfunction of the immune system in aged individuals includes at least two important factors: accumulation of immunocytes with reduced function and accumulation of lymphocyte clones with self-reactive potential. Coincidently, there is a profound reduction of the germinal center reaction in the aged. While this reduction is likely the result of age-associated impairment in lymphocyte function (e.g. diminished response to costimulus, altered lymphokine production etc.), the reduction of germinal centers may itself make an important contribution to further immunological dysfunction.

Cytotoxic T lymphocyte antigen 4 (CTLA-4) interferes with extracellular signal-regulated kinase (ERK) and Jun NH2-terminal kinase (JNK) activation, but does not affect phosphorylation of T cell receptor zeta and ZAP70

Cytotoxic T lymphocyte antigen 4 (CTLA-4) is an important regulator of T cell homeostasis. Ligation of this receptor leads to prominent downregulation of T cell proliferation, mainly as a consequence of interference with IL-2 production. We here report that CTLA-4 engagement strikingly selectively shuts off activation of downstream T cell receptor (TCR)/CD28 signaling events, i.e., activation of the microtubule-associated protein kinase (MAPKs) ERK and JNK. In sharp contrast, proximal TCR signaling events such as ZAP70 and TCR-zeta chain phosphorylation are not affected by CTLA-4 engagement on activated T cells. Since activation of the ERK and JNK kinases is required for stimulation of interleukin (IL)-2 transcription, these data provide a molecular explanation for the block in IL-2 production imposed by CTLA-4.

In vivo expression of the CTLA4 inhibitory receptor in malignant and reactive cells from human lymphomas

The CTLA4 receptor is a CD28 homologue which induces inhibitory effect on activated T-cells. Peripheral T-cells proliferate spontaneously in CTLA4-deficient mice. These results led to an analysis of CTLA4 expression in human lymphomas (n = 82) including Hodgkin's disease (HD) and non-Hodgkin's lymphomas (NHLs), using immunohistochemistry. CTLA4 was present in neoplastic cells from most (10/11) T-cell malignancies, except for anaplastic and lymphoblastic subtypes (0/4). Malignant B-cells from rare (3/55) B-NHLs (all of follicular subtype) were also CTLA4-positive. Other B-NHLs (52/55) were negative in malignant B-cells and occasionally positive in T-cells. Reactive small lymphocytes, but not Reed-Sternberg cells, from all (12/12) HD cases were strongly CTLA4-positive. The CTLA4 ligands CD80 and CD86 were simultaneously expressed in most CTLA4-negative lymphoma cases. CTLA4 is thus expressed either in the reactive or in the malignant cell populations, depending on the lymphoma subtype. These results provide new insights leading towards therapeutic strategies based either on enhancement of anti-tumour immunity by CTLA4 blockade in reactive lymphocytes or on triggering of a CTLA4-mediated inhibitory pathway in lymphoma cells.

Interaction of CTLA-4 with AP50, a clathrin-coated pit adaptor protein

CTLA-4 plays a critical role in regulating the immune response. It is mainly located in cytoplasmic vesicles and is expressed only transiently on the surface after T cell activation. In this study, we demonstrate that CTLA-4 is associated with AP50, the medium chain of the clathrin-associated coated pit adaptor protein complex AP2. In a yeast two-hybrid screen, three individual cDNA clones that encode mouse AP50 were isolated, all of which can interact specifically with the cytoplasmic domain of mouse CTLA-4, but not with the cytoplasmic domain of mouse CD28. We have shown that CTLA-4 can bind specifically to AP50 when CTLA-4 and AP50 are cotransfected into human 293T cells. A Y201 to F201 mutation in the YVKM intracellular localization motif of the CTLA-4 cytoplasmic domain significantly diminished its binding to AP50. We also found that AP50 bound to a CTLA-4 peptide containing unphosphorylated Y201 but not to a peptide containing phosphorylated Y201. Conversely, the p85 subunit of phosphatidylinositol 3-kinase and, to a lesser extent, protein tyrosine phosphatase SYP (SHP-2) and SHP (SHP-1) bind only to the CTLA-4 peptide containing phosphorylated Y201. Therefore, the phosphorylation status of Y201 in the CTLA-4 cytoplasmic domain determines the binding specificity of CTLA-4. These results suggest that AP50 and the coated pit adaptor complex AP2 may play an important role in regulating the intracellular trafficking and function of CTLA-4.

Apoptosis resistance of nonobese diabetic peripheral lymphocytes linked to the Idd5 diabetes susceptibility region

Defects in lymphocyte apoptosis may lead to autoimmune disorders and contribute to the pathogenesis of type 1 diabetes. Lymphocytes of nonobese diabetic (NOD) mice, an animal model of autoimmune diabetes, have been found resistant to various apoptosis signals, including the alkylating drug cyclophosphamide. Using an F2 intercross between the apoptosis-resistant NOD mouse and the apoptosis-susceptible C57BL/6 mouse, we define a major locus controlling the apoptosis-resistance phenotype and demonstrate its linkage (logarithm of odds score = 3.9) to a group of medial markers on chromosome 1. The newly defined gene cannot be dissociated from Ctla4 and Cd28 and in fact marks a 20-centimorgan region encompassing Idd5, a previously postulated diabetes susceptibility locus. Interestingly, we find that the CTLA-4 (cytotoxic T lymphocyte-associated antigen 4) and the CD28 costimulatory molecules are defectively expressed in NOD mice, suggesting that one or both of these molecules may be involved in the control of apoptosis resistance and, in turn, in diabetes susceptibility.

Protective immunity to nematode infection is induced by CTLA-4 blockade

The recent observation that neutralization or genetic deletion of the T lymphocyte receptor CTLA-4 allows enhanced T cell reactivity offers new opportunities for immunotherapy against infectious agents. We used a neutralizing antibody to block CTLA-4 interaction with its ligands CD80 and CD86 during infection of mice with the nematode, Nippostrongylus brasiliensis. CTLA-4 blockade greatly enhanced and accelerated the T cell immune response to N. brasiliensis, resulting in a profound reduction in adult worm numbers and early termination of parasite egg production. The ability of CTLA-4 blockade to accelerate primary immune responses to a protective level during an acute infection indicates its potential as an immunotherapeutic tool for dealing with infectious agents.

Co-stimulation in T cell responses

Antigen-specific T cell responses have primarily been considered in terms of activation signals delivered through the TCR and the co-stimulatory molecule CD28. In the past few years, studies have demonstrated the critical importance of inhibitory signals for regulating lymphocyte activation. CD28 and its homologue cytotoxic T lymphocyte antigen-4 (CTLA-4) share the same counter-receptors on antigen-presenting cells, but recent experiments have shown that CD28 and CTLA-4 have opposite effects on T cell activation. The mechanisms responsible for integrating these activation and inhibitory signals at the cellular and molecular levels are just beginning to be elucidated.

Construction and expression in the yeast Pichia pastoris of functionally active soluble forms of the human costimulatory molecules B7-1 and B7-2 and the B7 counter-receptor CTLA-4

We have generated soluble recombinant forms of the costimulatory molecules B7-1 and B7-2, and their counter-receptor CTLA-4 using a yeast Pichia pastoris expression system. Fragments comprising the extracellular domains of human B7-1, B7-2, and CTLA-4 molecules were expressed at high levels and could be purified from culture supernatants following a simple one-step purification protocol. The recombinant proteins retained their functionality and specific binding to their natural counterparts could be demonstrated by FACS analysis. In T cell proliferation assays costimulatory activity of immobilized B7-1 and B7-2 proteins in the presence of an anti-CD3 antibody was observed with the B7-1 protein being more potent than B7-2.

CTLA-4-B7 interaction is sufficient to costimulate T cell clonal expansion

T cell costimulation, particularly by the B7 family members B7-1 and B7-2, plays a critical role in regulating T cell-mediated immunity. Two molecules on T cells, CD28 and CTLA-4, are known to bind to B7. It has been suggested that CD28-B7 interaction promotes T cell response, whereas B7-CTLA-4 interaction downregulates T cell clonal expansion. However, the proposed responses of individual receptors to B7 have not been verified directly. Here, we report that B7-1 promotes clonal expansion of CD28-deficient T cells, and that the CD28-independent costimulatory activity is mediated by CTLA-4, as it is completely blocked by intact and Fab of anti-CTLA-4 mAb. In addition, a mutant B7-1 molecule, B7W88 >A, which has lost binding to CD28 but retained significant CTLA-4 binding activity, promotes T cell clonal expansion. Furthermore, while presence of CD28 enhances T cell response to B7-1, such response is also completely blocked by anti-CTLA-4 mAb. Taken together, our results demonstrate that B7-CTLA-4 interaction promotes T cell clonal expansion, and that optimal T cell response to B7 is achieved when both CD28 and CTLA-4 interact with B7. These results establish an important function of CTLA-4 in promoting T cell activation, and suggest an alternative interpretation of the function of CTLA-4 in T cell activation.

B lymphocytes as autoantigen-presenting cells in the amplification of autoimmunity

The exact role of B cells in antigen presentation to naive T cells in vivo is presently not known. Here, we demonstrate the ability of a B cell subset consisting of B7-2pos-B cells to prime autoreactive T cells in B cell-deficient mice. In contrast, B cell-deficient mice are unable to mount a similar initiation and expansion of the autoimmune response. The expression of the B7-2 costimulatory molecule as well as the specificity to a self-antigen, either murine cytochrome c or murine ribonucleoproteins (the target of autoimmunity in SLE), enabled B cells as antigen-presenting cells to induce naive lymph node T cells to proliferate and to express IFN-gamma, IL-4, IL-5, and IL-10 cytokine mRNAs. In contrast, neither adoptively transferred B7-2neg-B cells nor nonspecific B7-2pos-B cells were able to activate naive T cells. In addition, anti-B7-2 treatment prevented the in vivo expression of the IL-4, IL-5, and IFN-gamma cytokine mRNA responses. Our results suggest a major role of autoantigen-specific B7-2pos-B cells in breaking T cell tolerance to self-antigen.

T cell-mediated elimination of B7.2 transgenic B cells

Transgenic mice were generated to explore the effects on lymphoid development and immune function of constitutive expression of murine B7.2 on B and T cells. The number of B lymphocytes in primary and secondary lymphoid tissues is normal in B7.2 transgenic lines expressing low levels of B7.2 on B cells, but markedly reduced in transgenic lines expressing moderate to high levels of the transgene on B cells. This reduction is not due to an intrinsic abnormality of the transgenic B cells, but is rather the consequence of an elimination by an immune mechanism requiring the engagement of CD28 on T cells. Interestingly, during cognate antigen-specific interaction with T cells in vivo, B7.2 transgenic B cells are not eliminated, but proliferate and differentiate normally. Our findings suggest that, in the absence of high affinity ligand for the TCR, the CD28-B7.2 system participates in the regulation of B cell homeostasis.

CD80 (B7-1) binds both CD28 and CTLA-4 with a low affinity and very fast kinetics

The structurally related T cell surface molecules CD28 and CTLA-4 interact with cell surface ligands CD80 (B7-1) and CD86 (B7-2) on antigen-presenting cells (APC) and modulate T cell antigen recognition. Preliminary reports have suggested that CD80 binds CTLA-4 and CD28 with affinities (Kd values approximately 12 and approximately 200 nM, respectively) that are high when compared with other molecular interactions that contribute to T cell-APC recognition. In the present study, we use surface plasmon resonance to measure the affinity and kinetics of CD80 binding to CD28 and CTLA-4. At 37 degrees C, soluble recombinant CD80 bound to CTLA-4 and CD28 with Kd values of 0.42 and 4 microM, respectively. Kinetic analysis indicated that these low affinities were the result of very fast dissociation rate constants (k(off)); sCD80 dissociated from CD28 and CTLA-4 with k(off) values of > or = 1.6 and > or = 0.43 s-1, respectively. Such rapid binding kinetics have also been reported for the T cell adhesion molecule CD2 and may be necessary to accommodate-dynamic T cell-APC contacts and to facilitate scanning of APC for antigen.

A sensitive enzyme immunoassay for the quantitation of human CTLA4Ig fusion protein in mouse serum: pharmacokinetic application to optimizing cell line selection

A sensitive, accurate, and precise enzyme immunoassay (EIA) for the quantitation of human CTLA4Ig in mouse serum was validated. The EIA method employed a technique in which a monoclonal anti-CTLA4 antibody was adsorbed onto 96-well polystyrene microtiter plates and used to capture the CTLA4Ig in mouse serum samples. The captured CTLA4Ig was then detected using a goat anti-human IgGFc antiserum conjugated to the enzyme horseradish peroxidase. The validation included assessments of method accuracy and precision, range of reliable response, lower limit of quantitation (LLQ), inter-analyst robustness, storage stability in mouse serum and assay specificity. The results indicate that this validated assay is precise, accurate, and reproducible. This EIA has a range of reliable response in 10% mouse serum of 0.14-4.58 ng ml-1 resulting in a 100% serum equivalent curve of 1.4-45.8 ng ml-1. Assessment of individual standard curve variations indicated a reproducible response with R2 values of > or = 0.995. The LLQ was established at 1.4 ng ml-1. The accuracy and precision estimates, based on the quality control values, were within 3.8% and 5.2% respectively, for CTLA4Ig. Stability of CTLA4Ig was established in mouse serum for 5 days at both 4 degrees C and room temperature, for 2 months at -70 degrees C and through five freeze-thaw cycles. This validated assay was successfully employed in the assessment of pharmacokinetic characteristics of CTLA4Ig in mice and to aid in the selection of an optimal CTLA4Ig-producing cell line.

Accumulation of CTLA-4 expressing T lymphocytes in the germinal centres of human lymphoid tissues

CTLA-4, a coreceptor with sequence homology to CD28 is expressed on T cells after activation. Mice deficient for CTLA-4 die young from massive infiltration of many organs by activated T cells, which highlights the essential inhibitory role the coreceptor plays in the regulation of the immune response. To study the prevalence and distribution of CTLA-4 in situ immunohistological analyses were carried out on human tonsils and lymph nodes. Expression of CTLA-4 was restricted to alpha beta T cells, and CTLA-4+ B cells were not observed. In T-cell areas, 2-10% of T cells were positive for CTLA-4 with similar percentages in the CD4+ and CD8+ subpopulations. In the germinal centres (GC) the fraction of CTLA-4+ T cells was much higher (70-90%). This was due to frequent expression of CTLA-4 on the CD4+ helper subpopulation. GC CD8+ T cells were rare and mostly did not express the coreceptor. The CTLA-4+ T-cell fraction was also over-represented among intraepithelial tonsillar T cells. Cycling (Ki-67+) and apoptotic (TUNEL+) T cells were never positive for CTLA-4, while a subset of CD25+ cells did express the coreceptor. Since CTLA-4 is essential for the physiological limitation of the immune response, GC T cells, which are mostly CTLA-4 positive, might be important in this process.

CD80 costimulation is essential for the induction of airway eosinophilia

CD80 and CD86 (B7-1 and B7-2) are the ligands on antigen-presenting cells (APCs) which bind CD28 and deliver the costimulatory signals necessary for T cell activation. The reasons for the existence of two CD28 binding molecules are not well understood. We created a mutant version of CTLA4-Ig that could selectively bind CD80 and block CD28-CD80 interaction but leave CD28-CD86 binding intact. CD80 blockade prevented antigen-induced accumulation of eosinophils and lymphocytes in the lung of immunized mice, but did not block antigen induced systemic blood eosinophilia or IgE antibody production. No preferential expression of CD80 could be demonstrated on a population of lung APC consisting mainly of macrophages. These results indicate that CD80 costimulation is not necessary for the induction of Th2 immune responses but rather for the maintenance or amplification of lung inflammatory responses.

Induction of Th1 and Th2 CD4+ T cell responses: the alternative approaches

T helper lymphocytes can be divided into two distinct subsets of effector cells based on their functional capabilities and the profile of cytokines they produce. The Th1 subset of CD4+ T cells secretes cytokines usually associated with inflammation, such as IFN-gamma and TNF and induces cell-mediated immune responses. The Th2 subset produces cytokines such as IL-4 and IL-5 that help B cells to proliferate and differentiate and is associated with humoral-type immune responses. The selective differentiation of either subset is established during priming and can be significantly influenced by a variety of factors. One of these factors, the cytokine environment, has been put forward as the major variable influencing Th development and is already well reviewed by others. Instead, in the current review, we focus on some of the alternative approaches for skewing Th1/Th2 responses. Specifically, we discuss the effects on Th priming of (a) using altered peptide ligands as antigens, (b) varying the dose of antigen, and (c) altering costimulatory signals. The potential importance of each of these variables to influence immune responses to pathogens in vivo is discussed throughout.

Blockade of CD28/B7 co-stimulation by mCTLA4-Hgamma1 inhibits antigen-induced lung eosinophilia but not Th2 cell development or recruitment in the lung

We have studied the role of the CD28/B7 co-stimulatory pathway in the development of a Th2-type lung immune response. Mice injected two or three times intraperitoneally with ovalbumin in alum adjuvant and then re-exposed to the same antigen by intranasal (i.n.) inoculation show infiltration of the lung tissue and appearance in the broncho-alveolar lavage (BAL) fluid of significant numbers of eosinophils and lymphocytes, in a pattern which is reminiscent of asthmatic inflammation. The accumulation of eosinophils in the airways is completely dependent on interleukin (IL)-5 secretion by CD4+ T cells. We have used mice transgenic for a soluble form of murine CTLA-4 (mCTLA4-Hgamma1) which binds to B7 molecules on antigen-presenting cells, thereby preventing their interaction with T cell-expressed CD28. mCTLA4-Hgamma1-transgenic mice immunized intraperitoneally and challenged i.n. with ovalbumin failed to generate any eosinophil infiltration, suggesting that little or no IL-5 was secreted in the lungs of these mice. In contrast with the complete lack of eosinophils, the numbers and phenotypes of infiltrating lymphocytes were comparable in the lungs of mCTLA4-Hgamma1-transgenic and normal mice. Also, lung lymphocytes from immunized mCTLA4-Hgamma1-transgenic and normal mice could be shown to secrete comparable amounts of IL-4 and IL-5 when stimulated in culture in the absence of mCTLA4-Hgamma1. We conclude that mCTLA4-Hgamma1 can efficiently block the production of IL-5 during in vivo responses and inhibit eosinophil recruitment, but that it does not block the development of CD4+ T cells into Th2 cells with the potential to secrete IL-5.

Costimulation and autoimmunity

The past year has seen significant advances in our understanding of the role of the B7-CD28/CTLA-4 pathway in T cell activation and self-tolerance. Recent studies have demonstrated that CTLA-4 is a critical negative regulator of T cell activation and autoreactivity, revealing a previously unsuspected means by which costimulation is involved in the maintenance and breakdown of self-tolerance. Manipulation of this costimulatory pathway in animal models of autoimmunity has shown an important role for this pathway in both the initiation and progression of autoimmune diseases.

T helper cell activation and human retroviral pathogenesis

T helper (Th) cells are of central importance in regulating many critical immune effector mechanisms. The profile of cytokines produced by Th cells correlates with the type of effector cells induced during the immune response to foreign antigen. Th1 cells induce the cell-mediated immune response, while Th2 cells drive antibody production. Th cells are the preferential targets of human retroviruses. Infections with human T-cell leukemia virus (HTLV) or human immunodeficiency virus (HIV) result in the expansion of Th cells by the action of HTLV (adult T-cell leukemia) or the progressive loss of T cells by the action of HIV (AIDS). Both retrovirus infections impart a high-level activation state in the host immune cells as well as systemically. However, diverging responses to this activation state have contrasting effects on the Th-cell population. In HIV infection, Th-cell loss has been attributed to several mechanisms, including a selective elimination of cells by apoptosis. The induction of apoptosis in HIV infection is complex, with many different pathways able to induce cell death. In contrast, infection of Th cells with HTLV-1 affords the cell a protective advantage against apoptosis. This advantage may allow the cell to escape immune surveillance, providing the opportunity for the development of Th-cell cancer. In this review, we will discuss the impact of Th-cell activation and general immune activation on human retrovirus expression with a focus upon Th-cell function and the progression to disease.

Specific stabilization of the 4F7 molecule on dendritic cells by contact allergens

Our laboratory has recently developed the monoclonal antibody 4F7 which recognizes a molecule on dendritic cells in the dermis of mice that is upregulated after application of contact allergens in vivo. Furthermore, this antibody detects an antigen on dendritic cells in spleen, lymph nodes and colon. In order to study the influence of contact allergens on the surface expression of the 4F7 molecules on dendritic cells, FACScan analysis of splenic dendritic cells was carried out after in vitro application of contact allergens. Freshly isolated splenic dendritic cells were found to be positive for 4F7, 33D1, N418 (CD11c) and MHC class II. After overnight culture the expression of the dendritic cell-specific molecules 4F7 and 33D1 was decreased. This downregulation was not inhibited by the addition of the cytokines TNF-alpha or GM-CSF during in vitro culture. However, in vitro treatment of freshly isolated dendritic cells with the contact allergen 2,4-dinitrofluorobenzene prevented this downregulation of the 4F7 surface molecules. The same effect was observed after treatment with other contact allergens (1-chloro-2,4-dinitrobenzene or potassium dichromate). Treatment with the irritant substance sodium dodecyl sulphate, the lectins concanavalin and lipopolysaccharide or the phorbol ester PMA did not prevent the downregulation of 4F7 and 33D1. Moreover, the influence of contact allergens on the expression of the molecules 4F7 and 33D1 was not inhibited by the protein synthesis inhibitor cycloheximide. No effects of contact sensitizers were detectable on the expression of MHC class II molecules or the costimulatory molecules B7 and heat-stable antigen. Our results show a specific stabilizing effect of contact allergens on the dendritic cell-specific molecules 4F7 and 33D1 independent of de novo protein synthesis.

Covalent dimerization of CD28/CTLA-4 and oligomerization of CD80/CD86 regulate T cell costimulatory interactions

T lymphocyte receptors CD28 and CTLA-4 bind costimulatory molecules CD80 (B7-1) and CD86 (B7-2) on antigen-presenting cells and regulate T cell activation. While distinct functional roles have been ascribed to each of these molecules, little is known about how they interact. To better characterize these interactions, we have used surface plasmon resonance to perform equilibrium and kinetic binding analyses of extracellular fragments of CD28/CTLA-4/CD80/CD86. We show that CTLA-4 and CD28 binding are both characterized by rapid kinetic on-rates and rapid dissociation rates. Native disulfide-linked homodimers of CD28 and CTLA-4 bound with two kinetically distinct binding sites, one of high avidity and slow dissociation and one of low avidity and more rapid dissociation. Monomeric CTLA-4 bound only with low affinity and rapid dissociation. Therefore, covalent dimerization of CTLA-4 is required for its high avidity binding. Oligomerization of CD80/CD86 is also required for high avidity CTLA-4 binding since CTLA-4 bound with low avidity to monomeric CD86. This contrasts with the ability of CD80/CD86 on antigen-presenting cells to bind CTLA4Ig with high avidity and predicts their organization as oligomers or clusters that permit multivalent binding. Thus, covalent receptor dimerization and ligand oligomerization are two key features of the CD28/CTLA-4/CD80/CD86 receptor system that control ligand binding and may regulate signal transduction by controlling the duration of receptor occupancy.

CD28-CD80/CD86 interactions in testicular immunoregulation

The expression of two accessory molecules on antigen-presenting cells (APC), the CD80/ B7-1 and CD86/B7-2 antigens, was studied in the testis of normal and non-obese diabetic (NOD) mice. In addition, the effect of CD28 stimulation on suppression of lymphocytes by testicular products was investigated. The testes of 4-week old NOD mice or normal BALB/c mice and the testis of 17-21-week old BALB/c mice contained no CD80 or CD86 expressing cells. In contrast, CD80+ and CD86+ cells were present in the testis of 14-22-week old NOD mice. The CD80+ cells and most of the CD86+ cells were CD11b/CD18 negative. There were some CD11b/CD18+ cells that expressed CD86 weakly. The CD80+ and CD86+ cells were often located adjacent to the vessel walls where a leukocyte not expressing CD80 or CD86 had attached to the endothelium. Some CD80+ and CD86+ cells were present among the interstitial cells. The CD80 and CD86 antigens could not be observed in the same cells as judged from stainings in parallel sections. Stimulation of ConA- or anti-CD3 epsilon-primed peripheral blood or spleen lymphocytes with anti-CD28 was able significantly to antagonize the growth-inhibitory effect of the M(r) > 5 K fraction of testis extracts, but could not abolish it with increasing concentrations of testis extract. The results suggest that T lymphocytes can not be activated locally in the testis of BALB/c and young NOD mice because of the absence of the necessary CD28 ligands, CD80 and CD86, from the APCs and because of the suppression of T lymphocytes by the testicular products. In the testis of older diabetic NOD mice lymphocyte activation may occur because the testes of these mice contain CD80+; CD11b/CD18-, CD86+; CD11b/CD18+ and CD86+; CD11b/CD18- cells and therefore, CD28 co-stimulation, which can antagonize the suppressive effect of testis extract, may occur. The possibilities for clonal anergy in testicular immunoregulation are discussed.