Absence of recipient C3aR1 signaling limits expansion and differentiation of alloreactive CD8⁺ T cell immunity and prolongs murine cardiac allograft survival

Activation, differentiation, and expansion of alloreactive CD8⁺ T cells, the dominant effectors that mediate murine heart allograft rejection, requires allorecognition, costimulation, and cytokine-initiated signals. While previous work showed that alloreactive CD4⁺ T cell immunity entails immune cell-produced and locally activated complement, whether and how C3a receptor 1 (C3aR1) signaling impacts transplant outcomes and the mechanisms linking C3aR1 to alloreactive CD8⁺ T cell activation/expansion remain unclear. Herein we show that recipient C3aR1 deficiency or pharmacological C3aR1 blockade synergizes with tacrolimus to significantly prolong allograft survival versus tacrolimus-treated controls (median survival time 21 vs. 14 days, P <.05). Recipient C3aR1-deficiency reduced the frequencies of posttransplant, donor-reactive CD8⁺ T cells twofold. Reciprocal adoptive transfers of naive WT or C3ar1^−/− CD8⁺ T cells into syngeneic WT or C3ar1^−/− allograft recipients showed that T cell–expressed C3aR1 induces CD8⁺ T proliferation, mTOR activation and transcription factor T-bet expression. Host C3aR1 indirectly facilitates alloreactive CD8⁺ T cell proliferation/expansion by amplifying antigen presenting cell costimulatory molecule expression and innate cytokine production. In addition to expanding mechanistic insight, our findings identify C3aR1 as a testable therapeutic target for future studies aimed at improving human transplant outcomes.