Emerging evidence indicates that complement provides costimulatory signals for murine T cells but whether complement impacts human T cells remains unclear. We observed production of complement activation products C3a and C5a during in vitro cultures of human T cells responding to allogeneic dendritic cells (DC). Both partners expressed the receptors for C3a (C3aR) and C5a (C5aR), and C3aR- and C5aR-antagonists inhibited T cell proliferation. Recombinant C3a/C5a promoted CD4+ T cell expansion, bypassed the inhibitory effects of CTLA4-Ig, and induced AKT phosphorylation, the latter biochemically linking C3aR/C5aR to known T cell signaling pathways. Lowering DC C3a/C5a production by siRNA knockdown of DC C3 reduced T cell alloresponses. Conversely downregulating DC expression of the complement regulatory protein decay accelerating factor increased immune cell C3a/C5a and augmented T cell proliferation, identifying antigen presenting cells as the dominant complement source. Pharmacological C5aR blockade reduced graft versus host disease (GVHD) scores, prolonged survival, and inhibited T cell responses in NOD scid γcnull mouse recipients of human PBMCs, verifying that the mechanisms apply in vivo. Together our findings unequivocally document that immune cell-derived complement impacts human T cell immunity and provide the foundation for future studies targeting C3aR/C5aR as treatments of GVHD and organ transplant rejection in humans.

While activation of serum complement mediates antibody-initiated vascular allograft injury, increasing evidence indicates that complement also functions as a modulator of alloreactive T cells. We tested whether blockade of complement activation at the C5 convertase step affects T cell-mediated cardiac allograft rejection in mice. The anti-C5 mAb BB5.1, which prevents the formation of C5a and C5b, synergized with sub-therapeutic doses of CTLA4Ig to significantly prolong the survival of C57BL/6 heart grafts that were transplanted into naive Balb/c recipients. Anti-C5 mAb treatment limited the induction of donor-specific IFNγ-producing T cell alloimmunity without inducing Th2 or Th17 immunity in vivo and inhibited primed T cells from responding to donor antigens in secondary mixed lymphocyte responses. Additional administration of anti-C5 mAb to the donor prior to graft harvest further prolonged graft survival and concomitantly reduced both the in vivo trafficking of primed T cells into the transplanted allograft and decreased expression of T cell chemoattractant chemokines within the graft. Together these results support the novel concept that C5 blockade can inhibit T cell-mediated allograft rejection through multiple mechanisms, and suggest that C5 blockade may constitute a viable strategy to prevent and/or treat T cell-mediated allograft rejection in humans.

Background Alloreactive memory T cells prevent costimulatory blockade-induced heart graft survival in mice, but whether and how preexisting autoreactive T cells affect solid organ transplants under these conditions is unknown. Methods We tested the impact of preexisting cardiac myosin (CM)-specific immunity on murine heart transplant recipients treated with donor specific transfusion (DST) plus anti-CD154 mAb MR1. Results Pre-immunization with CM but not control ovalbumin abrogated the graft prolonging effects of DST/MR1, whether administered 2 weeks or >6 weeks prior to transplantation. Adoptive transfer of spleen cells from CM-immunized mice into naïve recipients had similar effects. CM-specific immunity did not cross-react with donor antigens and pre-immunization with CM had no impact on the survival or histology of DST/MR1 treated syngeneic heart grafts, the latter indicating that persistent autoimmunity is insufficient to cause rejection in the context of costimulatory blockade. We observed that the CM-pre-immunized mice produced higher frequencies of donor-reactive T cells with higher ratios of CD8+/CD4+Foxp3+ cells, suggesting that the autoreactive memory T cells provide help for activation of alloreactive T cells despite the costimulatory blockade. Conclusions These mechanistic insights linking auto- and alloimmunity in a model of murine heart transplantation have clinical relevance to the known association between autoimmunity and an elevated risk of acute and chronic heart transplant injury in humans.