Host T cells resist graft-versus-host disease mediated by donor leukocyte infusions

Delayed lymphocyte infusions (DLIs) are used to treat relapse occurring post bone marrow transplantation (BMT) and to increase the donor chimerism in recipients receiving nonmyeloablative conditioning. As compared with donor lymphocytes given early post-BMT, DLIs are associated with a reduced risk of graft-vs-host disease (GVHD). The mechanism(s) responsible for such resistance have remained incompletely defined. We now have observed that host T cells present 3 wk after lethal total body irradiation, at the time of DLI, contribute to DLI-GVHD resistance. The infusion of donor splenocytes on day 0, a time when host bone marrow (BM)-derived T cells are absent, results in greater expansion than later post-BMT when host and donor BM-derived T cells coexist. Selective depletion of host T cells with anti-Thy1 allelic mAb increased the GVHD risk of DLI, indicating that a Thy1(+) host T cell regulated DLI-GVHD lethality. The conditions by which host T cells are required for optimal DLI resistance were determined. Recipients unable to express CD28 or 4-1BB were as susceptible to DLI-GVHD as anti-Thy1 allelic mAb-treated recipients, indicating that CD28 and 4-1BB are critical to DLI-GVHD resistance. Recipients deficient in both perforin and Fas ligand but not individually were highly susceptible to DLI-GVHD. Recipients that cannot produce IFN-gamma were more susceptible to DLI-GVHD, whereas those deficient in IL-12 or p55 TNFRI were not. Collectively, these data indicate that host T cells, which are capable of generating antidonor CTL effector cells, are responsible for the impaired ability of DLI to induce GVHD. These same mechanisms may limit the efficacy of DLI in cancer therapy under some conditions.     

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.     

Blockade of CTLA-4 signals inhibits Th2-mediated murine chronic graft-versus-host disease by an enhanced expansion of regulatory CD8+ T cells

CTLA-4 (CD152) is thought to be a negative regulator of T cell activation. Little is known about the function of CTLA-4 in Th2-type immune responses. We have investigated the effect of initial treatment with anti-CTLA-4 mAb on murine chronic graft-vs-host disease. Transfer of parental BALB/c splenocytes into C57BL/6 x BALB/c F1 mice induced serum IgE production, IL-4 expression by donor CD4+ T cells, and host allo-Ag-specific IgG1 production at 6-9 wk after transfer. Treatment with anti-CTLA-4 mAb for the initial 2 wk significantly reduced IgE and IgG1 production and IL-4 expression. Analysis of the splenic phenotype revealed the enhancement of donor T cell expansion, especially within the CD8 subset, and the elimination of host cells early after anti-CTLA-4 mAb treatment. This treatment did not affect early IFN-gamma expression by CD4+ and CD8+ T cells and anti-host cytolytic activity. Thus, blockade of CTLA-4 greatly enhanced CD8+ T cell expansion, and this may result in the regulation of consequent Th2-mediated humoral immune responses. These findings suggest a new approach for regulating IgE-mediated allergic immune responses by blockade of CTLA-4 during a critical period of Ag sensitization.     

CTLA-4 engagement and regulatory CD4+CD25+ T cells independently control CD8+-mediated responses under costimulation blockade

Blockade of costimulatory signals is a promising therapeutic target to prevent allograft rejection. In this study, we sought to characterize to what extent CTLA-4 engagement contributes to the development of transplantation tolerance under the cover of CD40/CD40L and CD28/CD86 blockade. In vitro, we found that inhibition of the primary alloresponse and induction of alloantigen hyporesponsiveness by costimulation blockade was abrogated by anti-CTLA-4 mAb. In addition, regulatory CD4(+)CD25(+) T cells (T(REG)) were confirmed to play a critical role in the induction of hyporesponsiveness by anti-CD40L and anti-CD86 mAb. Our data indicated that CTLA-4 engagement is not required for activation or suppressor function of T(REG). Instead, in the absence of either CTLA-4 signaling or T(REG), CD8(+) T cell division was enhanced, whereas the inhibition of CD4(+) T cell division by costimulation blockade remained largely unaffected. In vivo, the administration of additional anti-CTLA-4 mAb abrogated anti-CD40L- and anti-CD86 mAb-induced cardiac allograft survival. Correspondingly, rejection was accompanied by enhanced allograft infiltration of CD8(+) cells. We conclude that CTLA-4 signaling and T(REG) independently cooperate in the inhibition of CD8(+) T cell expansion under costimulation blockade.     

Analysis of the role of negative T cell costimulatory pathways in CD4 and CD8 T cell-mediated alloimmune responses in vivo

Negative costimulatory signals mediated via cell surface molecules such as CTLA-4 and programmed death 1 (PD-1) play a critical role in down-modulating immune responses and maintaining peripheral tolerance. However, their role in alloimmune responses remains unclear. This study examined the role of these inhibitory pathways in regulating CD28-dependent and CD28-independent CD4 and CD8 alloreactive T cells in vivo. CTLA-4 blockade accelerated graft rejection in C57BL/6 wild-type recipients and in a proportion of CD4(-/-) but not CD8(-/-) recipients of BALB/c hearts. The same treatment led to prompt rejection in CD28(-/-) and a smaller proportion of CD4(-/-)CD28(-/-) mice with no effect in CD8(-/-)CD28(-/-) recipients. These results indicate that the CTLA-4:B7 pathway provides a negative signal to alloreactive CD8(+) T cells, particularly in the presence of CD28 costimulation. In contrast, PD-1 blockade led to accelerated rejection of heart allografts only in CD28(-/-) and CD8(-/-)CD28(-/-) recipients. Interestingly, PD-1 ligand (PD-L1) blockade led to accelerated rejection in wild-type mice and in all recipients lacking CD28 costimulation. This effect was accompanied by expansion of IFN-gamma-producing alloreactive T cells and enhanced generation of effector T cells in rejecting allograft recipients. Thus, the PD-1:PD-L1 pathway down-regulates alloreactive CD4 T cells, particularly in the absence of CD28 costimulation. The differential effects of PD-1 vs PD-L1 blockade support the possible existence of a new receptor other than PD-1 for negative signaling through PD-L1. Furthermore, PD-1:PD-L1 pathway can regulate alloimmune responses independent of an intact CD28/CTLA-4:B7 pathway. Harnessing physiological mechanisms that regulate alloimmunity should lead to development of novel strategies to induce durable and reproducible transplantation tolerance.     

CD28-independent costimulation of T cells in alloimmune responses.

T cell costimulation by B7 molecules plays an important role in the regulation of alloimmune responses. Although both B7-1 and B7-2 bind CD28 and CTLA-4 on T cells, the role of B7-1 and B7-2 signaling through CTLA-4 in regulating alloimmune responses is incompletely understood. To address this question, we transplanted CD28-deficient mice with fully allogeneic vascularized cardiac allografts and studied the effect of selective blockade of B7-1 or B7-2. These mice reject their grafts by a mechanism that involves both CD4(+) and CD8(+) T cells. Blockade of CTLA-4 or B7-1 significantly accelerated graft rejection. In contrast, B7-2 blockade significantly prolonged allograft survival and, unexpectedly, reversed the acceleration of graft rejection caused by CTLA-4 blockade. Furthermore, B7-2 blockade prolonged graft survival in recipients that were both CD28 and CTLA-4 deficient. Our data indicate that B7-1 is the dominant ligand for CTLA-4-mediated down-regulation of alloimmune responses in vivo and suggest that B7-2 has an additional receptor other than CD28 and CTLA-4 to provide a positive costimulatory signal for T cells.     

A retro-inverso peptide mimic of CD28 encompassing the MYPPPY motif adopts a polyproline type II helix and inhibits encephalitogenic T cells in vitro.

Complete activation of T cells requires two signals: an Ag-specific signal delivered via the TCR by the peptide-MHC complex and a second costimulatory signal largely provided by B7:CD28/CTLA-4 interactions. Previous studies have shown that B7 blockade can either ameliorate experimental autoimmune encephalomyelitis by interfering with CD28 signaling or exacerbate the disease by concomitant blockade of CTLA-4 interaction. Therefore, we developed a functional CD28 mimic to selectively block B7:CD28 interactions. The design, synthesis, and structural and functional properties of the CD28 free peptide, the end group-blocked CD28 peptide, and its retro-inverso isomer are shown. The synthetic T cell-costimulatory receptor peptides fold into a polyproline type II helical structure commonly seen in regions of globular proteins involved in transient protein-protein interactions. The binding determinants of CD28 can be transferred onto a short peptide mimic of its ligand-binding region. The CD28 peptide mimics effectively block the expansion of encephalitogenic T cells in vitro suggesting the potential usefulness of the peptides for the treatment of autoimmune disease conditions requiring down-regulation of T cell responses.     

Spontaneous colitis occurrence in transgenic mice with altered B7-mediated costimulation

The B7 costimulatory molecules govern many aspects of T cell immune responses by interacting with CD28 for costimulation, but also with CTLA-4 for immune suppression. Although blockade of CTLA-4 with Ab in humans undergoing cancer immune therapy has led to some cases of inflammatory bowel disease, spontaneous animal models of colitis that depend upon modulation of B7 interactions have not been previously described. In this study, we demonstrate that mice expressing a soluble B7-2 Ig Fc chimeric protein spontaneously develop colitis that is dependent on CD28-mediated costimulation of CD4(+) T cells. We show that the chimeric protein has mixed agonistic/antagonist properties, and that it acts in part by blocking the cell intrinsic effects on T cell activation of engagement of CTLA-4. Disease occurred in transgenic mice that lack expression of the endogenous B7 molecules (B7 double knock-out mice), because of the relatively weak costimulatory delivered by the chimeric protein. Surprisingly, colitis was more severe in this context, which was associated with the decreased number of Foxp3(+) regulatory T cells in transgenic B7 double knock-out mice. This model provides an important tool for examining how B7 molecules and their effects on CTLA-4 modulate T cell function and the development of inflammatory diseases.     

Induction of CTL responses by simultaneous administration of liposomal peptide vaccine with anti-CD40 and anti-CTLA-4 mAb

Activation of APC via CD40-CD40 ligand pathway induces up-regulation of costimulatory molecules such as B7 and production of IL-12. Interaction between B7 on APC and CD28 on naive T cells is necessary for priming the T cells. On the other hand, interaction between B7 on APC and CTLA-4 on activated T cells transduces a negative regulatory signal to the activated T cells. In the present study, we attempted to generate tumor-specific CTL by s.c. administration of antigenic peptides encapsulated in multilamellar liposomes (liposomal peptide vaccine) with anti-CD40 mAb and/or anti-CTLA-4 mAb. Liposomal OVA257-264 and anti-CD40 mAb or anti-CTLA-4 mAb were administrated to C57BL/6 mice and the splenocytes were cocultured with OVA257-264 for 4 days. The splenic CD8+ T cells showed a significant cytotoxicity against EL4 cells transfected with cDNA of OVA. In addition, administration of both anti-CD40 and anti-CTLA-4 mAb enhanced the CTL responses. Considerable CTL responses were induced in MHC class II deficient mice by the same procedure. This finding indicated that CTL responses could be generated even in the absence of Th cells. When BALB/c mice were immunized with pRL1a peptide that are tumor-associated Ag of RLmale symbol1 leukemia cells using the same procedure, significant CTL responses were induced and prolonged survival of the BALB/c mice was observed following RLmale symbol1 inoculation. These results demonstrate that anti-CD40 mAb and anti-CTLA-4 mAb function as immunomodulators and may be applicable to specific cancer immunotherapy with antitumor peptide vaccine.     

B7-independent inhibition of T cells by CTLA-4

CTLA-4 is an inhibitory molecule that regulates T cell expansion and differentiation. CTLA-4 binding to B7-1/B7-2 is believed to be crucial for its inhibitory signal both by competing for CD28 binding to the same ligands and aggregating CTLA-4 to deliver negative signals. In this study, we demonstrate that B7 binding is not essential for CTLA-4 activity. CTLA-4 knockout T cells are hyperresponsive compared with wild-type T cells in B7-free settings. Expression of a B7-nonbinding CTLA-4 mutant inhibited T cell proliferation, cytokine production, and TCR-mediated ERK activation in otherwise CTLA-4-deficient T cells. Finally, transgenic expression of the ligand-nonbinding CTLA-4 mutant delayed the lethal lymphoproliferation observed in CTLA-4-deficient mice. These results suggest that ligand binding is not essential for the CTLA-4 function and supports an essential role for CTLA-4 signaling during T cell activation.     

An important role of CD80/CD86-CTLA-4 signaling during photocarcinogenesis in mice

Although previous studies have shown that altered B7 costimulation plays a critical role in UV irradiation-induced regulation of immunity, the individual roles of the B7 receptors (CD28 and CTLA-4) or the B7 family members (CD80 and CD86) have not been explored. Thus, we investigated CTLA-4 signaling during photocarcinogenesis of chronically UV-B-exposed mice using an antagonistic anti-CTLA-4 Ab. Anti-CTLA-4-treated mice developed significantly fewer UV-induced tumors. Moreover, anti-CTLA-4 treatment induced long-lasting protective immunity because progressively growing UV tumors inoculated into anti-CTLA-4- and UV-treated mice that had not developed tumors were rejected. Next, we used mice deficient for CD80, CD86, or both in photocarcinogenesis studies to assess the relative contributions of these CTLA-4 ligands. Double-deficient mice showed significantly reduced UV-induced skin tumor development, whereas CD86(-/-) mice produced skin cancer earlier compared with CD80(-/-) and control mice. The growth of UV-induced tumors appears to be controlled by UV-induced suppressor T cells, because CD80(-/-)/CD86(-/-) mice had strongly reduced numbers of UV-induced CD4(+)CD25(+) suppressor T cells. In vitro, CTLA-4 blockade inhibited the suppressor activity of UV-induced CD4(+)CD25(+) T cells, suggesting that reduced photocarcinogenesis might be due to decreased numbers or function of suppressor T cells. Together, these data indicate that blocking CD80/86-CTLA-4 signaling induced immune protection against the development of UV-induced skin tumors. Furthermore, CD86-mediated costimulation appears to play a more critical role in the protection against photocarcinogenesis than CD80.     

Cutting edge: a crucial role for B7-CD28 in transmitting T help from APC to CTL

Although APC activation via CD40-CD40L signaling plays a critical role in enabling CD4(+) T cells to provide the "help" necessary for cross-priming of naive CTL, it is unclear how this makes the APC competent for priming. We have investigated the roles of B7-1/B7-2 and their receptors [corrected] CD28/CTLA-4 in cross-priming of CD4-dependent CTL in vivo. We find that both CD28 and B7-1/B7-2 are required for CD40-activated APC to cross-prime CTL, and that priming by CD40-activated APC was prevented by blockade of CD28. Conversely, augmenting CD28 signals with an agonistic Ab bypassed the requirement for CD4(+) T help or CD40 activation. Interestingly, blockade of the negative regulatory B7 receptor CTLA-4 failed to prime CTL in the absence of T help. These results support a model in which activation-induced up-regulation of B7 molecules on APC leads to increased CD28 signaling and a commitment to cross-priming of CD4-dependent CTL.     

Analysis of the cellular mechanism of antitumor responses and autoimmunity in patients treated with CTLA-4 blockade

We have demonstrated previously that the administration of CTLA-4 blockade has mediated objective cancer regression and autoimmunity in patients with metastatic melanoma. To explore the mechanism of these in vivo effects, we have studied the changes in lymphocyte phenotype and function in patients receiving anti-CTLA-4 Ab (MDX-010). Patients with stage IV melanoma or renal cell cancer were treated every 3 wk with an anti-CTLA-4 Ab with or without peptide immunization. Pheresis samples were analyzed using flow cytometry to determine lymphocyte cell surface markers. Gene expression analyses and proliferation assays were conducted on purified T cell subsets. Anti-CTLA-4 Ab did not inhibit the suppressive activity of CD4+CD25+ cells in vitro or in vivo. In addition, there was no decrease in the expression of CD4+CD25+ cells in whole PBMC, nor a decrease in Foxp3 gene expression in the CD4+ or CD4+CD25+ purified cell populations posttreatment. The percentage of CD4+, CD8+, CD4+CD25+, and CD4+CD25- T cells in PBMC expressing the activation marker HLA-DR increased following anti-CTLA-4 Ab administration. Therefore, our results suggest that the antitumor effects of CTLA-4 blockade are due to increased T cell activation rather than inhibition or depletion of T regulatory cells.     

Blockade of B7-H1 on macrophages suppresses CD4+ T cell proliferation by augmenting IFN-gamma-induced nitric oxide production

PD-1 is an immunoinhibitory receptor that belongs to the CD28/CTLA-4 family. B7-H1 (PD-L1) and B7-DC (PD-L2), which belong to the B7 family, have been identified as ligands for PD-1. Paradoxically, it has been reported that both B7-H1 and B7-DC co-stimulate or inhibit T cell proliferation and cytokine production. To determine the role of B7-H1 and B7-DC in T cell-APC interactions, we examined the contribution of B7-H1 and B7-DC to CD4+ T cell activation by B cells, dendritic cells, and macrophages using anti-B7-H1, anti-B7-DC, and anti-PD-1 blocking mAbs. Anti-B7-H1 mAb and its Fab markedly inhibited the proliferation of anti-CD3-stimulated naive CD4+ T cells, but enhanced IL-2 and IFN-gamma production in the presence of macrophages. The inhibition of T cell proliferation by anti-B7-H1 mAb was abolished by neutralizing anti-IFN-gamma mAb. Coculture of CD4+ T cells and macrophages from IFN-gamma-deficient or wild-type mice showed that CD4+ T cell-derived IFN-gamma was mainly responsible for the inhibition of CD4+ T cell proliferation. Anti-B7-H1 mAb induced IFN-gamma-mediated production of NO by macrophages, and inducible NO synthase inhibitors abrogated the inhibition of CD4+ T cell proliferation by anti-B7-H1 mAb. These results indicated that the inhibition of T cell proliferation by anti-B7-H1 mAb was due to enhanced IFN-gamma production, which augmented NO production by macrophages, suggesting a critical role for B7-H1 on macrophages in regulating IFN-gamma production by naive CD4+ T cells and, hence, NO production by macrophages.     

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.     

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.     

CTLA-4+CD8+ T cells that encounter B7-2+ iris pigment epithelial cells express their own B7-2 to achieve global suppression of T cell activation

Pigment epithelial (PE) cells cultured from the eye possess the novel property of suppressing TCR-dependent activation of T cells in vitro. Iris PE (IPE) cells accomplish this suppression by a direct cell contact mechanism in which B7-2 expressed by the PE cells interacts with CTLA-4 on responding T cells. Because CTLA-4 expression is constitutively expressed on a very small proportion of naive splenic T cells and since exposure of splenic T cells to IPE leads to global T cell suppression, we have inquired into the mechanism by which suppression is achieved. Using splenic T cells and IPE from donor mice with disrupted genes for CD80 (B7-1), CD86 (B7-2), CTLA-4, and/or CD28, we report that B7-2(+) IPE in the presence of anti-CD3 supported selectively the activation of CTLA-4(+) CD8(+) T cells that express their own B7-2 and secrete enhanced amounts of active TGFbeta. By contrast, activation of CTLA-4-negative T cells, especially CD4(+) cells, in these cultures was profoundly suppressed. Because global suppression of T cell activation in these cultures was obtained only when both IPE and T cells possessed B7-2 genes and expressed the costimulators as surface molecules, we propose that T cells activated in the presence of parenchymal cells from the eye (an immune privileged site) express B7-2 in a manner that equips them to suppress bystander T cells. Thus, B7-2 expression on T cells participates in their eventual ability to function as regulators in vitro.     

A novel alloantigen-specific CD8+PD1+ regulatory T cell induced by ICOS-B7h blockade in vivo

Delayed ICOS-B7h signal blockade promotes significant prolongation of cardiac allograft survival in wild-type but not in CD8-deficient C57BL/6 recipients of fully MHC-mismatched BALB/c heart allografts, suggesting the possible generation of CD8(+) regulatory T cells in vivo. We now show that the administration of a blocking anti-ICOS mAb results in the generation of regulatory CD8(+) T cells. These cells can transfer protection and prolong the survival of donor-specific BALB/c, but not third party C3H, heart grafts in CD8-deficient C57BL/6 recipients. This is unique to ICOS-B7h blockade, because B7 blockade by CTLA4-Ig prolongs graft survival in CD8-deficient mice and does not result in the generation of regulatory CD8(+) T cells. Those cells localize to the graft, produce both IFN-gamma and IL-4 after allostimulation in vitro, prohibit the expansion of alloreactive CD4(+) T cells, and appear to mediate a Th2 switch of recipient CD4(+) T cells after adoptive transfer in vivo. Finally, these cells are not confined to the CD28-negative population but express programmed death 1, a molecule required for their regulatory function in vivo. CD8(+)PD1(+) T cells suppress alloreactive CD4(+) T cells but do not inhibit the functions by alloreactive CD8(+) T cells in vitro. These results describe a novel allospecific regulatory CD8(+)PD1(+) T cell induced by ICOS-B7h blockade in vivo.     

A role for the B7-1/B7-2:CD28/CTLA-4 pathway during negative selection

Although costimulation plays an important role in activating naive T cells, its role in negative selection is controversial. By following thymocyte deletion induced by endogenous superantigens in mice lacking B7-1 and/or B7-2, we have identified a role for both B7-1 and B7-2 in negative selection. Studies using CD28-deficient and CD28/CTLA-4-double-deficient mice have revealed that either CD28 or another as yet undefined coreceptor can mediate these B7-dependent signals that promote negative selection. Finally, CTLA-4 delivers signals that inhibit selection, suggesting that CTLA-4 and CD28 have opposing functions in thymic development. Combined, the data demonstrate that B7-1/B7-2-dependent signals help shape the T cell repertoire.     

Expression and contribution of B7-1 (CD80) and B7-2 (CD86) in the early immune response to Leishmania major infection.

CD28 interactions promote T cell responses, and whether B7-1 or B7-2 is utilized may influence Th cell subset development. CD28 blockade by CTLA-4Ig treatment or by targeted gene disruption has yielded different conclusions regarding the role of CD28 in the development of Th1 and Th2 cells following Leishmania major infection. In this study, we demonstrate that B7-mediated costimulation is required for the development of the early immune response following infection of resistant or susceptible mice. In contrast, CD28-/- BALB/c mice infected with L. major produce cytokines comparable to those of infected wild-type mice. Treatment of CD28-/- mice with CTLA-4Ig did not diminish this response, suggesting that a B7-independent pathway(s) contributes to the early immune response in these mice. In conventional BALB/c or C3H mice, B7-2 functions as the dominant costimulatory molecule in the initiation of early T cell activation following L. major infection, leading to IL-4 or IFN-gamma production, respectively. The preferential interaction of B7-2 with its ligand(s) in the induction of these responses correlates with its constitutive expression relative to that of B7-1. However, B7-1 can equally mediate costimulation for the production of either IL-4 or IFN-gamma when expressed at high levels. Thus, in leishmaniasis, costimulation involving B7-1 or B7-2 can result in the production of either Th1 or Th2 cytokines, rather than a preferential induction of one type of response.