Graft-Derived IL-6 Amplifies Proliferation and Survival of Effector T Cells That Drive Alloimmune-Mediated Vascular Rejection

Background. IL-6 is an inflammatory cytokine that controls effector T cell responses but the mechanisms by which it controls allogeneic immune responses and vascular rejection that leads to transplant arteriosclerosis (TA) are poorly understood. Methods. We have examined the mechanism by which IL-6 contributes to the pathogenesis of vascular rejection and TA using a murine aortic interposition model of vascular rejection. Results. The absence of IL-6 production from artery graft cells reduced the development of vascular rejection and arteriosclerotic thickening. There was no apparent effect of donor-derived IL-6 on endothelial cell integrity or on the intimal accumulation of smooth muscle cells, macrophages, and anti-donor antibodies. However, reduced vascular pathology in IL-6(-/-)artery grafts was accompanied by a significant reduction in the accumulation of CD4 and CD8 T cells. Further, the absence of graft-derived IL-6 resulted in a significant decrease in the activation and proliferation of alloreactive CD4 and CD8 T cells after transplantation as well as in a marked increase in cell death of effector T cells. Alloreactive effector Tcells that expanded in the absence of IL-6 were also more susceptible to Fas-mediated activation-induced cell death in vitro. Finally, systemic neutralization of IL-6R did not reduce arteriosclerotic thickening but reduced endothelial integrity in allograft arteries, indicating differential effects of specific elimination of IL-6 in graft cells and systemic IL-6 neutralization. Conclusions. Donor-derived IL-6 amplifies the expansion of allogeneic T cell responses that cause vascular rejection and TA by increasing T cell proliferation and preventing Fas-mediated T cell death.