CD80 (B7-1) and CD86 (B7-2) are functionally equivalent in the initiation and maintenance of CD4+ T-cell proliferation after activation with suboptimal doses of PHA

Effective activation of T cells requires engagement of two separate T-cell receptors. The antigen-specific T-cell receptor (TCR) binds foreign peptide antigen-MHC complexes, and the CD28 receptor binds to the B7 (CD80/CD86) costimulatory molecules expressed on the surface of antigen-presenting cells (APC). The simultaneous triggering of these T-cell surface receptors with their specific ligands results in an activation of this cell. In contrast, CTLA-4 (CD152) is a distinct T-cell receptor that, upon binding to B7 molecules, sends an inhibitory signal to T cell activation. Many in vitro and in vivo studies demonstrated that both CD80 and CD86 ligands have an identical role in the activation of T cells. Recently, functions of B7 costimulatory molecules in vivo have been investigated in B7-1 and/or B7-2 knockout mice, and the authors concluded that CD86 could be more important for initiating T-cell responses, while CD80 could be more significant for maintaining these immune responses. In this study, we directly compared the role of CD80 and CD86 in initiating and maintaining proliferation of resting CD4(+) T cells in an in vitro mode system that allowed to provide the first signal-to-effector cells through the use of suboptimal doses of PHA and the second costimulatory signal through cells expressing CD80 or CD86, but not any other costimulatory molecules. Using this experimental system we demonstrate that the CD80 and CD86 molecules can substitute for each other in the initial activation of resting CD4(+) T cells and in the maintenance of their proliferative response.     

Multiple layers of CD80/86-dependent costimulatory activity regulate primary, memory, and secondary lymphocytic choriomeningitis virus-specific T cell immunity

The lymphocytic choriomeningitis virus (LCMV) system constitutes one of the most widely used models for the study of infectious disease and the regulation of virus-specific T cell immunity. However, with respect to the activity of costimulatory and associated regulatory pathways, LCMV-specific T cell responses have long been regarded as relatively independent and thus distinct from the regulation of T cell immunity directed against many other viral pathogens. Here, we have reevaluated the contribution of CD28-CD80/86 costimulation in the LCMV system by use of CD80/86-deficient mice, and our results demonstrate that a disruption of CD28-CD80/86 signaling compromises the magnitude, phenotype, and/or functionality of LCMV-specific CD8(+) and/or CD4(+) T cell populations in all stages of the T cell response. Notably, a profound inhibition of secondary T cell immunity in LCMV-immune CD80/86-deficient mice emerged as a composite of both defective memory T cell development and a specific requirement for CD80 but not CD86 in the recall response, while a related experimental scenario of CD28-dependent yet CD80/86-independent secondary CD8(+) T cell immunity suggests the existence of a CD28 ligand other than CD80/86. Furthermore, we provide evidence that regulatory T cells (T(REG)s), the homeostasis of which is altered in CD80/86(-/-) mice, contribute to restrained LCMV-specific CD8(+) T cell responses in the presence of CD80/86. Our observations can therefore provide a more coherent perspective on CD28-CD80/86 costimulation in antiviral T cell immunity that positions the LCMV system within a shared context of multiple defects that virus-specific T cells acquire in the absence of CD28-CD80/86 costimulation.     

Mouse inducible costimulatory molecule (ICOS) expression is enhanced by CD28 costimulation and regulates differentiation of CD4+ T cells

The inducible costimulatory (ICOS) molecule is expressed by activated T cells and has homology to CD28 and CD152. ICOS binds B7h, a molecule expressed by APC with homology to CD80 and CD86. To investigate regulation of ICOS expression and its role in Th responses we developed anti-mouse ICOS mAbs and ICOS-Ig fusion protein. Little ICOS is expressed by freshly isolated mouse T cells, but ICOS is rapidly up-regulated on most CD4(+) and CD8(+) T cells following stimulation of the TCR. Strikingly, ICOS up-regulation is significantly reduced in the absence of CD80 and CD86 and can be restored by CD28 stimulation, suggesting that CD28-CD80/CD86 interactions may optimize ICOS expression. Interestingly, TCR-transgenic T cells differentiated into Th2 expressed significantly more ICOS than cells differentiated into Th1. We used two methods to investigate the role of ICOS in activation of CD4(+) T cells. First, CD4(+) cells were stimulated with beads coated with anti-CD3 and either B7h-Ig fusion protein or control Ig fusion protein. ICOS stimulation enhanced proliferation of CD4(+) cells and production of IFN-gamma, IL-4, and IL-10, but not IL-2. Second, TCR-transgenic CD4(+) T cells were stimulated with peptide and APC in the presence of ICOS-Ig or control Ig. When the ICOS:B7h interaction was blocked by ICOS-Ig, CD4(+) T cells produced more IFN-gamma and less IL-4 and IL-10 than CD4(+) cells differentiated with control Ig. These results demonstrate that ICOS stimulation is important in T cell activation and that ICOS may have a particularly important role in development of Th2 cells.     

The inducible costimulator plays the major costimulatory role in humoral immune responses in the absence of CD28

CD28 plays crucial costimulatory roles in T cell proliferation, cytokine production, and germinal center response. Mice that are deficient in the inducible costimulator (ICOS) also have defects in cytokine production and germinal center response. Because the full induction of ICOS in activated T cells depends on CD28 signal, the T cell costimulatory capacity of ICOS in the absence of CD28 has remained unclear. We have clarified this issue by comparing humoral immune responses in wild-type, CD28 knockout (CD28 KO), and CD28-ICOS double-knockout (DKO) mice. DKO mice had profound defects in Ab responses against environmental Ags, T-dependent protein Ags, and vesicular stomatitis virus that extended far beyond those observed in CD28 KO mice. However, DKO mice mounted normal Ab responses against a T-independent Ag, indicating that B cell function itself was normal. Restimulated CD4(+) DKO T cells that had been primed in vivo showed decreased proliferation and reduced IL-4 and IL-10 production compared with restimulated CD4(+) T cells from CD28 KO mice. Thus, in the absence of CD28, ICOS assumes the major T cell costimulatory role for humoral immune responses. Importantly, CD28-mediated ICOS up-regulation is not essential for ICOS function in vivo.     

Suppression of experimental autoimmune encephalomyelitis using peptide mimics of CD28

The B7:CD28/CTLA-4 costimulatory pathway plays a critical role in regulating the immune response and thus provides an ideal target for therapeutic manipulation of autoimmune disease. Previous studies have shown that blockade of CD28 signaling by mAbs can both prevent and exacerbate experimental autoimmune encephalomyelitis (EAE). In this study, we have designed two CD28 peptide mimics that selectively block B7:CD28 interactions. By surface plasmon resonance, both the end group-blocked CD28 peptide (EL-CD28) and its retro-inverso isomer (RI-CD28) compete effectively with the extracellular domain of CD28 for binding to B7-1. Both the CD28 peptide mimics inhibited expansion of encephalitogenic T cells in vitro. A single administration of EL-CD28 or RI-CD28 peptide significantly reduced disease severity in EAE. Importantly, we show that either CD28 peptide mimic administered during acute disease dramatically improved clinical signs of EAE, suppressing ongoing disease. The ratio of CD80:CD86 expression was significantly lower on CD4(+) and F4/80(+) spleen cells in CD28 peptide-treated mice. Peripheral deletion of Ag-specific CD4(+) T cells occurs following in vivo blockade of CD28 with synthetic CD28 peptides.     

Expression of CD80/86 on B cells is essential for autoreactive T cell activation and the development of arthritis

Depletion of B cells in rheumatoid arthritis is therapeutically efficacious. Yet, the mechanism by which B cells participate in the inflammatory process is unclear. We previously demonstrated that Ag-specific B cells have two important functions in the development of arthritis in a murine model of rheumatoid arthritis, proteoglycan (PG)-induced arthritis (PGIA). PG-specific B cells function as autoantibody-producing cells and as APCs that activate PG-specific T cells. Moreover, the costimulatory molecule CD86 is up-regulated on PG-specific B cells in response to stimulation with PG. To address the requirement for CD80/CD86 expression on B cells in the development of PGIA, we generated mixed bone marrow chimeras in which CD80/CD86 is specifically deleted on B cells and not on other APC populations. Chimeras with a specific deficiency in CD80/CD86 expression on B cells are resistant to the induction of PGIA. The concentration of PG-specific autoantibody is similar in mice sufficient or deficient for CD80/86-expressing B cells, which indicates that resistance to PGIA is not due to the suppression of PG-specific autoantibody production. CD80/86-deficient B cells failed to effectively activate PG-specific autoreactive T cells as indicated by the failure of T cells from PG-immunized CD80/86-deficient B cell chimeras to transfer arthritis into SCID mice. In vitro secondary recall responses to PG are also dependent on CD80/86-expressing B cells. These results demonstrate that a CD80/86:CD28 costimulatory interaction between B cells and T cells is required for autoreactive T cell activation and the induction of arthritis but not for B cell autoantibody production.     

Differential requirements for expression of CD80/86 and CD40 on B cells for T-dependent antibody responses in vivo

The CD80/86-CD28 and CD40-CD40 ligand costimulatory pathways are essential for Th cell-dependent B cell responses that generate high-affinity, class-switched Ab in vivo. Disruption of either costimulatory pathway results in defective in vivo humoral immune responses, but it remains unclear to what extent this is due to deficient activation of Th cells and/or of B cells. To address this issue, we generated mixed chimeras in which CD80/86- or CD40-deficient bone marrow-derived cells coexist with wild-type (WT) cells, thereby providing the functional T cell help and accessory cell functions required for fully competent B cell responses. We were then able to assess the requirement for CD80/86 or CD40 expression on B cells producing class-switched Ig in response to a T-dependent Ag. In CD80/86 WT plus CD80/86 double-knockout mixed chimeras, both WT- and CD80/86-deficient B cells produced IgG1 and IgE responses, indicating that direct signaling by CD80/86 is not essential for efficient B cell activation. In marked contrast, only WT IgG1 and IgE responses were detected in the chimeras containing CD40-deficient cells, demonstrating that CD40 expression on B cells is essential for class switching by those B cells. Thus, while disrupting either the CD80/86-CD28 or the CD40-CD40 ligand costimulatory pathway abrogates T-dependent B cell immune responses, the two pathways are nonredundant and mediated by distinct mechanisms.     

IL-2 upregulates CD86 expression on human CD4(+) and CD8(+) T cells

The glycoprotein CD86 is an important costimulatory molecule that has been shown to be predominantly expressed on APCs, such as dendritic cells, macrophages, and B cells. More recently, CD86 was also detected on T cells in specific pathological conditions. The mechanisms of how CD86 might be induced and its functional role in T cells are not well understood. In the present study, we showed that treatment with IL-2 markedly upregulated CD86, but not CD80, in human CD4(+) and CD8(+) T cells. This upregulation occurred in the absence of bystander cells, and isolated naive CD4(+) or CD8(+) T cells exhibited different time-dependent CD86-expression patterns in response to IL-2. Upregulation of CD86 on activated T cells was reduced by Abs that block IL-2 and IL-2Rα (CD25), indicating a receptor-mediated mechanism. IL-2-dependent CD86 upregulation was blocked by pharmacological inhibitors of the NFAT and mammalian target of rapamycin pathways and was largely reduced by simultaneous exposure to IFN-α. Importantly, a marked increase in CD86 on T cells was also observed in vivo in IL-2-treated patients. In conclusion, IL-2 upregulates CD86 expression on human CD4(+) and CD8(+) T cells via a receptor-dependent mechanism that involves the NFAT and mammalian target of rapamycin pathways.     

B7-1 overexpression by thymic epithelial cells results in transient and long-lasting effects on thymocytes and peripheral T helper cells but does not result in immunodeficiency.

The B7-1 (CD80) molecule provides costimulatory function for the activation of T helper lymphocytes upon encounter with antigen. To investigate the role of this molecule in thymocyte maturation, we have generated transgenic (Tg) mice in which CD80 expression is driven by the keratin 14 promoter (K14). This overexpression of CD80 resulted in the loss of detectable cell surface CD28 expression on thymocytes and a significant reduction in both the surface T cell receptor expression and the ratio of CD4(+) to CD8(+) single-positive thymocytes in Tg animals compared to nontransgenic (non-Tg) controls. While many of these defects were transient, the significant decrease in CD4(+) versus CD8(+) T cell ratio persisted peripherally. Peripheral T cells from these Tg mice were found to be significantly hyporesponsive to T cell mitogens and in mixed leukocyte reaction, effects that our data indicate are due to reduced IL-2 production by Tg T cells upon activation. Despite these functional defects, immunization with both complex and simple protein antigens produced no differences in the proliferative or humoral responses to these antigens between Tg and non-Tg groups. These data indicate that thymic CD80 signaling results in the deletion of significant numbers of CD4(+) T cells but does not culminate in antigen-specific immunodeficiency.     

CD28, TNF receptor,and IL-12 are critical for CD4-independent cross-priming of therapeutic antitumor CD8+ T cells

Previously, we have shown that priming of therapeutic CD8(+) T cells in tumor vaccine-draining lymph nodes of mice vaccinated with GM-CSF secreting B16BL6 melanoma cells occurs independent of CD4 T cell help. In this study, we examined the contribution of the major costimulatory molecules, CD40 ligand (CD40L), CD80, and CD86, in the priming of CD8(+) T cells. Priming of therapeutic CD8(+) T cells by a GM-CSF-transduced tumor vaccine did not require CD40 and CD40L interactions, as therapeutic T cells could be generated from mice injected with anti-CD40L Ab and from CD40L knockout mice. However, costimulation via either CD80 or CD86 was required, as therapeutic T cells could be generated from mice injected with either anti-CD80 or anti-CD86 Ab alone, but administration of both Abs completely inhibited the priming of therapeutic T cells. Blocking experiments also identified that priming of therapeutic T cells in MHC class II-deficient mice required TNFR and IL-12 signaling, but signaling through CD40, lymphotoxin-betaR, or receptor activator of NF-kappaB was not essential. Thus, cross-priming of therapeutic CD8(+) T cells by a tumor vaccine transduced with GM-CSF requires TNFR, IL-12, and CD28 signaling.     

A critical role of costimulation during intrathymic development of invariant NK T cells

CD1d-restricted Valpha14(+) invariant NK T (iNKT) cells are a specialized alphabeta T cell subset that regulates both innate and adaptive immunity. Although costimulatory molecules are required for the activation of conventional T cells and for the development of Foxp3(+) T cells, their role in iNKT cell regulation is unclear. Here we report that mice deficient in CD80/CD86 and/or B7h exhibit severe defects in thymic iNKT cell maturation, associated with largely reduced iNKT cell number in the thymus and the periphery. We show that costimulation is necessary for the optimal expansion of postselected NK1.1(-) immature iNKT cells in the thymus and for the proper expression of the maturation markers T-bet and CD122. Surprisingly, costimulatory molecules on both hemopoietic and nonhematopoietic cells are required for iNKT cell development. Our results thus demonstrate a previously unknown function of costimulation in the intrathymic development of iNKT cells, distinct from that of conventional T cells and regulatory T cells.     

Established T cell-driven germinal center B cell proliferation is independent of CD28 signaling but is tightly regulated through CTLA-4

CD4 T cell activation is positively (CD28) and negatively (CTLA-4) regulated by the costimulatory ligands CD80 and CD86. A central question is how the balance between these two opposing forces is controlled as T cells differentiate. We have previously shown that CD28 signaling is absolutely required to prime naive CD4 T cells to differentiate into effectors that provide help for germinal centers and class-switched Ab responses. In this study, we show that the requirement for CD28 signaling is transient and effector CD4 T cells do not require CD28 signals to sustain their function. The CD28 independence of effector T cells within germinal centers suggested that a key function for CD80/CD86 under these circumstances might be to provide negative regulatory signals via the CD28 homologue CTLA-4. By examining germinal center responses in mice where the ability to signal through T cell CTLA-4 was compromised, we provide data that supports a critical role for CTLA-4 in down-regulating T cell help for germinal center B cells.     

Role of CD28/CD80-86 and CD40/CD154 costimulatory interactions in host defense to primary herpes simplex virus infection

Dependence of the primary antiviral immune response on costimulatory interactions between CD28/CD80-86 and between CD40/CD154 (CD40 ligand) has been correlated with the extent of viral replication in two models of systemic infection, lymphocytic choriomeningitis virus and vesicular stomatitis virus. To determine the role of these costimulatory interactions in the context of an acute cytolytic, but locally replicating viral infection, herpes simplex virus (HSV) infection was assessed in mice that had the CD28/CD80-86 or CD40/CD154 interactions disrupted either genetically or with blocking reagents (CTLA4Ig and MR1, respectively). CTLA4Ig treatment greatly reduced paralysis-free survival during primary acute HSV infection. This reflected an almost total ablation of the anti-HSV CD4(+) and CD8(+) T-cell responses due to anergy and reduced cell numbers, respectively. Disruption of CD40/CD154 interactions impaired survival, but the effect was less severe than that observed in CTLA4Ig-treated mice, with reductions observed in the CD4(+) T-cell but not CD8(+) T-cell responses. These two costimulatory pathways functioned in part independently, since disruption of both further impaired survival. The dependence on these costimulatory interactions for the control of primary HSV infection may represent a more widespread paradigm for nonsystemic viruses, which have restricted sites of replication and which employ immunoevasive measures.     

Differential incorporation of CD45, CD80 (B7-1), CD86 (B7-2), and major histocompatibility complex class I and II molecules into human immunodeficiency virus type 1 virions and microvesicles: implications for viral pathogenesis and immune regulation

Human immunodeficiency virus (HIV) infection results in a functional impairment of CD4(+) T cells long before a quantitative decline in circulating CD4(+) T cells is evident. The mechanism(s) responsible for this functional unresponsiveness and eventual depletion of CD4(+) T cells remains unclear. Both direct effects of cytopathic infection of CD4(+) cells and indirect effects in which uninfected "bystander" cells are functionally compromised or killed have been implicated as contributing to the immunopathogenesis of HIV infection. Because T-cell receptor engagement of major histocompatibility complex (MHC) molecules in the absence of costimulation mediated via CD28 binding to CD80 (B7-1) or CD86 (B7-2) can lead to anergy or apoptosis, we determined whether HIV type 1 (HIV-1) virions incorporated MHC class I (MHC-I), MHC-II, CD80, or CD86. Microvesicles produced from matched uninfected cells were also evaluated. HIV infection increased MHC-II expression on T- and B-cell lines, macrophages, and peripheral blood mononclear cells (PBMC) but did not significantly alter the expression of CD80 or CD86. HIV virions derived from all MHC-II-positive cell types incorporated high levels of MHC-II, and both virions and microvesicles preferentially incorporated CD86 compared to CD80. CD45, expressed at high levels on cells, was identified as a protein present at high levels on microvesicles but was not detected on HIV-1 virions. Virion-associated, host cell-derived molecules impacted the ability of noninfectious HIV virions to trigger death in freshly isolated PBMC. These results demonstrate the preferential incorporation or exclusion of host cell proteins by budding HIV-1 virions and suggest that host cell proteins present on HIV-1 virions may contribute to the overall pathogenesis of HIV-1 infection.     

CD80 and CD86 control antiviral CD8+ T-cell function and immune surveillance of murine gammaherpesvirus 68

The interactions between CD80 and CD86 on antigen-presenting cells and CD28 on T cells serve as an important costimulatory signal in the activation of T cells. Although the simplistic two-signal hypothesis has been challenged in recent years by the identification of different costimulators, this classical pathway has been shown to significantly impact antiviral humoral and cellular immune responses. How the CD80/CD86-CD28 pathway affects the control of chronic or latent infections has been less well characterized. In this study, we investigated its role in antiviral immune responses against murine gammaherpesvirus 68 (MHV-68) and immune surveillance using CD80/CD86(-/-) mice. In the absence of CD80/CD86, primary antiviral CD8(+) T-cell responses and the induction of neutralizing antibodies were severely impaired. During long-term immune surveillance, the virus-specific CD8(+) T cells were impaired in IFN-gamma production and secondary expansion and exhibited an altered phenotype. Surprisingly, a low level of viral reactivation in the lung was observed, and this effect was independent of CD28 and CTLA-4. Thus, CD80 and CD86, signaling through CD28 and possibly another unidentified receptor, are required for optimal immune surveillance and antiviral immune responses to murine gammaherpesvirus.     

B7 interactions with CD28 and CTLA-4 control tolerance or induction of mucosal inflammation in chronic experimental colitis

CD28-B7 interaction plays a critical costimulatory role in inducing T cell activation, while CTLA-4-B7 interaction provides a negative signal that is essential in immune homeostasis. Transfer of CD45RB(high)CD4(+) T cells from syngeneic mice induces transmural colon inflammation in SCID recipients. This adoptive transfer model was used to investigate the contribution of B7-CD28/CTLA-4 interactions to the control of intestinal inflammation. CD45RB(high)CD4(+) cells from CD28(-/-) mice failed to induce mucosal inflammation in SCID recipients. Administration of anti-B7.1 (but not anti-B7.2) after transfer of wild-type CD45RB(high)CD4(+) cells also prevented wasting disease with colitis, abrogated leukocyte infiltration, and reduced production of proinflammatory cytokines IL-2 and IFN-gamma by lamina propria CD4(+) cells. In contrast, anti-CTLA-4 treatment led to deterioration of disease, to more severe inflammation, and to enhanced production of proinflammatory cytokines. Of note, CD25(+)CD4(+) cells from CD28(-/-) mice similar to those from the wild-type mice were efficient to prevent intestinal mucosal inflammation induced by the wild-type CD45RB(high) cells. The inhibitory functions of these regulatory T cells were effectively blocked by anti-CTLA-4. These data show that the B7-CD28 costimulatory pathway is required for induction of effector T cells and for intestinal mucosal inflammation, while the regulatory T cells function in a CD28-independent way. CTLA-4 signaling plays a key role in maintaining mucosal lymphocyte tolerance, most likely by activating the regulatory T cells.     

Effects of specific immunotherapy on the B7 family of costimulatory molecules in allergic inflammation

The effect of allergen-specific immunotherapy (IT) on Ag presentation and T lymphocyte stimulation was evaluated by verifying the expression of costimulatory molecules in allergic patients. Thus, CD28 and CTLA-4, B7, and B7-H molecules on immune cells, as well as cytokine production, were analyzed in and out of the pollination period in 30 patients allergic to Betulaceae that had or had nor undergone specific IT. Results showed that IT attenuated the increase in the percentage of CD28(+)CD4 T cells and the decrease in the percentage of CTLA-4(+)CD4(+) T cells seen in untreated individuals. CD19(+)/CD80, CD19(+)/CD86(+), and CD14(+)/CD80(+) APCs were significantly augmented during pollination in unvaccinated individuals. B7-H1-expressing monocytes (CD14(+)) and B lymphocytes (CD19) as well as CD14 and CD19 B7-H1(+)/IL-10(+) APC were augmented in Betulaceae Ag-stimulated cell cultures of vaccinated patients independently of pollination, and were further increased in these individuals during pollination. As a result, the IL-10-IFN-gamma ratio in CD4(+), CD14(+), and CD19(+) cells increased in vaccinated patients, but decreased in unvaccinated individuals during pollination. These data clarify the cellular and molecular basis underlying the recent observation that peripheral expansion of IL-10-producing cells is associated with successful IT. B7-H1 could be an optimal target for IT of allergic diseases using mAbs.     

B Cell Responses to HIV Antigen Are a Potent Correlate of Viremia in HIV-1 Infection and Improve with PD-1 Blockade

Infection with Human Immunodeficiency Virus Type 1 (HIV-1) induces defects of both cellular and humoral immune responses. Impaired CD4+ T cell help and B cell dysfunction may partially explain the low frequency of broadly neutralizing antibodies in HIV-infected individuals. To understand the extent of B cell dysfunction during HIV infection, we assessed the level of B cell activation at baseline and after stimulation with a variety of antigens. Increased levels of viremia were associated with higher baseline expression of the activation marker CD86 on B cells and with decreased ability of B cells to increase expression of CD86 after in vitro stimulation with inactivated HIV-1. In a series of cell isolation experiments B cell responses to antigen were enhanced in the presence of autologous CD4+ T cells. HIV infected individuals had a higher frequency of PD-1 expression on B cells compared to HIV- subjects and PD-1 blockade improved B cell responsiveness to HIV antigen, suggesting that inhibitory molecule expression during HIV-1 infection may contribute to some of the observed B cell defects. Our findings demonstrate that during chronic HIV infection, B cells are activated and lose full capacity to respond to antigen, but suppression of inhibitory pressures as well as a robust CD4+ T cell response may help preserve B cell function.     

Selective requirement for CD40-CD154 in drug-induced type 1 versus type 2 responses to trinitrophenyl-ovalbumin

CD154 is transiently expressed by activated T cells and interacts with CD40 on B cells, dendritic cells, macrophages, and monocytes. This costimulatory receptor-ligand couple seems decisive in Ag-driven immune responses but may be differentially involved in type 1 vs type 2 responses. We studied the importance of CD40-CD154 in both responses using the reporter Ag popliteal lymph node assay in which selectively acting drugs generate clearly polarized type 1 (streptozotocin) or type 2 (D-penicillamine, diphenylhydantoin) responses to a constant coinjected Ag in the same mouse strain. Treatment of mice with anti-CD154 reduced characteristic immunological parameters in type 2 responses (B and CD4(+) T cell proliferation, IgG1 and IgE Abs, and IL-4 secretion) and only slightly affected the type 1 response (small decrease in IFN-gamma production, influx of CD11c(+) and F4/80(+) cells, and prevention of architectural disruption of the lymph node, but no effect on IgG2a Ab and TNF-alpha secretion or B and CD4(+) T cell proliferation). The findings indicate that the CD40-CD154 costimulatory interaction is a prerequisite in drug-induced type 2 responses and is only marginally involved in type 1 responses. The observed expression patterns of CD80 and CD86 on different APC (B cells in type 2 and dendritic cells in type 1) may be responsible for this discrepancy.     

Incomplete activation of CD4 T cells by antigen-presenting transitional immature B cells: implications for peripheral B and T cell responsiveness

B cells leave the bone marrow as transitional B cells. Transitional B cells represent a target of negative selection and peripheral tolerance, both of which are abrogated in vitro by mediators of T cell help. In vitro, transitional and mature B cells differ in their responses to B cell receptor ligation. Whereas mature B cells up-regulate the T cell costimulatory molecule CD86 (B7.2) and are activated, transitional B cells do not and undergo apoptosis. The ability of transitional B cells to process and present Ag to CD4 T cells and to elicit protective signals in the absence of CD86 up-regulation was investigated. We report that transitional B cells can process and present Ag as peptide:MHC class II complexes. However, their ability to activate T cells and elicit help signals from CD4-expressing Th cells was compromised compared with mature B cells, unless exogenous T cell costimulation was provided. A stringent requirement for CD28 costimulation was not evident in interactions between transitional B cells and preactivated CD4-expressing T cells, indicating that T cells involved in vivo in an ongoing immune response might rescue Ag-specific transitional B cells from negative selection. These data suggest that during an immune response, immature B cells may be able to sustain the responses of preactivated CD4(+) T cells, while being unable to initiate activation of naive T cells. Furthermore, the ability of preactivated, but not naive T cells to provide survival signals to B cell receptor-engaged transitional immature B cells argues that these B cells may be directed toward activation rather than negative selection when encountering Ag in the context of a pre-existing immune response.