Didier A. Mandelbrot

Blockade of CTLA-4 on CD4+CD25+ regulatory T cells abrogates their function in vivo.

Naturally occurring CD4+ regulatory T cells (TR) that express CD25 and the transcription factor FoxP3 play a key role in immune homeostasis, preventing immune pathological responses to self and foreign Ags. CTLA-4 is expressed by a high percentage of these cells, and is often considered as a marker for TR in experimental and clinical analysis. However, it has not yet been proven that CTLA-4 has a direct role in TR function. In this study, using a T cell-mediated colitis model, we demonstrate that anti-CTLA-4 mAb treatment inhibits TR function in vivo via direct effects on CTLA-4-expressing TR, and not via hyperactivation of colitogenic effector T cells. Although anti-CTLA-4 mAb treatment completely inhibits TR function, it does not reduce TR numbers or their homing to the GALT, suggesting the Ab mediates its function by blockade of a signal required for TR activity. In contrast to the striking effect of the Ab, CTLA-4-deficient mice can produce functional TR, suggesting that under some circumstances other immune regulatory mechanisms, including the production of IL-10, are able to compensate for the loss of the CTLA-4-mediated pathway. This study provides direct evidence that CTLA-4 has a specific, nonredundant role in the function of normal TR. This role has to be taken into account when targeting CTLA-4 for therapeutic purposes, as such a strategy will not only boost effector T cell responses, but might also break TR-mediated self-tolerance.

CTLA-4 and T cell activation

The past year has seen significant advances in our understanding of the role of cytotoxic T lymphocyte antigen 4 (CTLA-4) in regulating T cell activation and tolerance. Recent studies indicate that CTLA-4 not only counterbalances CD28 signals but also can inhibit T cell responses independently of CD28. Recent work has also revealed a role for CTLA-4 in regulating Th1/Th2 differentiation. Manipulation of CTLA-4 in animal models of autoimmunity has shown that CTLA-4 regulates both the initiation and the progression of autoimmune diseases.

CTLA‐4 regulates cell cycle progression during a primary immune response

Engagement of CTLA‐4 is critical for inhibiting T cell immune responses. Recent studies have shown that CTLA‐4 plays a key role in regulating peripheral T cell tolerance. It has been suggestedthat one mechanism by which CTLA‐4 performs this function is by regulating cell cycle progression. Here, we investigate in depth the role of CTLA‐4 in regulating cell cycle progression in naive T cells by comparing the immune responses in the absence or presence of CTLA‐4. In the absence of CLTA‐4, T cells exhibit marked increases in T cell proliferation, IL‐2 mRNA and protein secretion, and cells cycling in the S and G2‐M phase. Analyses of cyclins, cyclin‐dependent kinases, and cell cycle inhibitors involved in the transition from the G1 to S phase reveal that cell cycle progression is prolonged in the absence of CTLA‐4. This is due to the early exit from the G1 phase, entry into the S phase, and prolonged S phase period. Re‐expression of the cell cycle inhibitor p27kip1 is delayed in the absence of CTLA‐4. These studies demonstrate that the B7u2009:u2009CTLA‐4 pathway exerts its major effects on T cell immune responses via regulation of the cell cycle.

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.

CD70 Signaling Is Critical for CD28-Independent CD8+ T Cell-Mediated Alloimmune Responses In Vivo

The inability to reproducibly induce robust and durable transplant tolerance using CD28-B7 pathway blockade is in part related to the persistence of alloreactive effector/memory CD8+ T cells that are less dependent on this pathway for their cellular activation. We studied the role of the novel T cell costimulatory pathway, CD27-CD70, in alloimmunity in the presence and absence of CD28-B7 signaling. CD70 blockade prolonged survival of fully mismatched vascularized cardiac allografts in wild-type murine recipients, and in CD28-deficient mice induced long-term survival while significantly preventing the development of chronic allograft vasculopathy. CD70 blockade had little effect on CD4+ T cell function but prevented CD8+ T cell-mediated rejection, inhibited the proliferation and activation of effector CD8+ T cells, and diminished the expansion of effector and memory CD8+ T cells in vivo. Thus, the CD27-CD70 pathway is critical for CD28-independent effector/memory CD8+ alloreactive T cell activation in vivo. These novel findings have important implications for the development of transplantation tolerance-inducing strategies in primates and humans, in which CD8+ T cell depletion is currently mandatory.

B7-dependent T-cell costimulation in mice lacking CD28 and CTLA4

To examine whether B7 costimulation can be mediated by a molecule on T cells that is neither CD28 nor CTLA4, we generated mice lacking both of these receptors. CD28/CTLA4(-/-) mice resemble CD28(-/-) mice in having decreased expression of T-cell activation markers in vivo and decreased T-cell proliferation in vitro, as compared with wild-type mice. Using multiple approaches, we find B7-dependent costimulation in CD28/CTLA4(-/-) mice. The proliferation of CD28/CTLA4(-/-) T cells is inhibited by CTLA4-Ig and by the use of antigen-presenting cells lacking both B7-1 and B7-2. CD28/CTLA4(-/-) T-cell proliferation is increased by exposure to Chinese hamster ovary cells transfected with B7-1 or B7-2. Finally, administration of CTLA4-Ig to CD28/CTLA4(-/-) cardiac allograft recipients significantly prolongs graft survival. These data support the existence of an additional receptor for B7 molecules that is neither CD28 nor CTLA4.

Cutting Edge: Transplantation Tolerance through Enhanced CTLA-4 Expression

Knockout and blocking studies have shown a critical role for CTLA-4 in peripheral tolerance, however, it is unknown whether augmenting CTLA-4 expression actually promotes tolerance. Here we demonstrate a specific and requisite role for CTLA-4 and its up-regulation in tolerance through anti-CD45RB. First, long-term murine islet allograft survival induced by anti-CD45RB is prevented by CTLA4-Ig, which interferes with B7:CTLA-4 interactions. Second, anti-CD45RB is ineffective in recipients lacking CTLA-4, B7-1, and B7-2. In contrast, CTLA4-Ig, which targets B7 on allogeneic cells, promotes long-term engraftment in these mice. Moreover, anti-CD45RB was effective in B7-deficient controls expressing CTLA-4. Finally, in wild-type mice, CTLA-4 expression returned to baseline 17 days after receiving anti-CD45RB, and was refractory to further increase. Transplantation and anti-CD45RB therapy at this time could neither augment CTLA-4 nor prolong engraftment. These data demonstrate a specific role for CTLA-4 in anti-CD45RB-mediated tolerance and indicate that CTLA-4 up-regulation can directly promote allograft survival.

Cutting Edge: CTLA-4 (CD152) Differentially Regulates Mitogen-Activated Protein Kinases (Extracellular Signal-Regulated Kinase and c-Jun N-Terminal Kinase) in CD4+ T Cells from Receptor/Ligand-Deficient Mice

Although CTLA-4 (CD152) has potent inhibitory effects on T cell function, the signaling events affected by this coreceptor remain to be fully defined. Mitogen-activated protein kinases (MAPK) extracellular signal-regulated kinase (ERK) and c-Jun N-terminal kinase (JNK) act as crucial regulators of multiple aspects of cell function. Ab ligation studies have reported an inhibitory effect of CTLA-4 on TCR-induced ERK and JNK activation. In this study, we have re-examined the specificity of CTLA-4 inhibition of MAPKs by using natural ligand with ex vivo-purified CD4+ T cells deficient in CD80 and CD86 (double knockout), or CTLA-4, CD80, and CD86 (triple knockout). Under these conditions, CTLA-4 ligation was found to up-regulate and sustain JNK activation, while inhibiting ERK activity. At the same time, JNK activation could not account for CTLA-4 induction of TGF-β production. Our findings demonstrate that CTLA-4 cosignaling is more complex than previously appreciated, with an ability to differentially regulate members of the MAPK family in T cells.

Rejection of Mouse Cardiac Allografts by Costimulation in trans

The activation of T cells by B7 costimulation in trans has been demonstrated in vitro, but the in vivo relevance is unknown. To study costimulation in trans of CD4+ T cells in vivo, we performed cardiac transplants from B7-1/B7-2-deficient mice to recipients that do not express MHC class II molecules on peripheral APCs, but do have functional CD4+ T cells (II−/4+ mice). This model restricts the B7-dependent activation of CD4+ T cells to costimulation in trans and excludes any contribution from indirect Ag presentation. We find that II−/4+ recipients reject B7-deficient grafts as rapidly as wild-type grafts, suggesting that costimulation in trans can mediate rejection as potently as costimulation in cis. Treatment of II−/4+ recipients of B7-deficient grafts with depleting Abs to CD4 or CD8 demonstrates that indirect Ag presentation to CD8+ cells does not significantly contribute to rejection. This is the first demonstration that costimulation in trans can mediate an immune response in vivo and has important therapeutic implications.

Association of B7-1 Co-Stimulation with the Development of Graft Arterial Disease Studies Using Mice Lacking B7-1, B7-2, or B7-1/B7-2

To investigate the roles of B7-1 and/or B7-2 co-stimulatory molecule in the development of graft arterial disease (GAD), major histocompatibility complex (MHC) class II-mismatched allograft hearts were transplanted into wild-type, B7-1(-/-), B7-2(-/-), or B7-1/B7-2(-/-) recipient mice. Grafts were explanted at 4 or 8 weeks and used for histological and immunohistochemical analyses, RNase protection assay, and flow cytometry of graft infiltrating cells. Grafts in wild-type recipients showed macrophage, recipient MHC class II, and B7 molecule co-localization by immunohistochemistry to GAD lesions. Flow cytometry revealed that CD11b(+) and MHC class II(+) graft infiltrating cells expressed B7-1 more than B7-2, whereas B7-2 expression was predominant in CD11b(-) cells at 4 and 8 weeks. GAD was significantly attenuated in the allografts in B7-1(-/-) and B7-1/B7-2(-/-) but not in B7-2(-/-) recipients compared to wild-type hosts. Interferon-gamma mRNA levels were comparable in all graft combinations, whereas interleukin-4 mRNA levels decreased in grafts in B7-2 deficient hosts, but did not correlate with GAD attenuation. The findings indicate distinct roles for B7-1 and B7-2 co-stimulatory molecules in the development of GAD, potentially because of differential expression of B7-1 and B7-2 molecules on distinct stimulator and/or effector cell populations.