Immune Cell-Derived C3a and C5a Costimulate Human T Cell Alloimmunity

Abstract

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.

Monitoring and Cell-Specific Deletion of C5aR1 Using a Novel Floxed GFP-C5aR1 Reporter Knock-in Mouse

“…Taken together, our data strongly argue against C5aR1 expression in naive and activated murine CD4 + T cells. This finding is in contrast to results observed in the human system, in which C5aR1 expression was demonstrated in T cells (76). Based on our findings, we suggest a model by which C5a regulates CD4 + T cell responses in the mouse exclusively at the APC level.…”

Molecules Great and Small

“…Confirmatory human experiments published in 2013 show that C3a and C5a are generated during in vitro cultures of human T cells responding to allogeneic dendritic cells (DCs) (27). Both partners express the receptors for C3a and C5a (124)(125)(126)(127), and C3aR-and C5aR-antagonists inhibit human T-cell proliferation, whereas recombinant C3a/C5a promotes alloreactive human CD4 1 T-cell expansion.…”

“…C3aR/ C5aR signaling is also required for T-cell homeostasis, because T cells deficient in both receptors spontaneously undergo accelerated cell death in vitro and in vivo (24). The observations derived from murine models also apply to human T cells (27). Building on these findings, a 2013 publication showed that resting human CD4 1 T cells contain C3 in granules that is rapidly cleaved by cathepsin-L to C3a and secreted after CD3 ligation.…”

Production of complement components by cells of the immune system

“…Complement has been considered more recently to be involved in both the homeostasis and the effector functions of the T cell. In cognate T cell and APC interactions the locally produced C3a and C5a are able to bind to their respective receptor on the T cell membrane, which subsequently stimulates the effector functions and maintains the viability of the T cell [100]. The T cell itself can produce complement components upon activation via T cell receptor (TCR) and CD28, which results in induced protein expression of C3, C5, FB and FD [101][102][103][104].…”