Effects of T cell frequency and graft size on transplant outcome in mice.

The features that determine whether graft-reactive T lymphocytes develop into effector cells capable of mediating organ destruction are not well understood. To investigate potential factors involved in this process, we first confirmed that female recipient mice acutely rejected minor Ag-disparate male skin, but not heart transplants. Despite this difference in outcome, heart and skin transplantation induced antidonor T cell responses of similar magnitude, specificity, and cytokine profile. The heart-graft-primed T cells transiently infiltrated the graft and ultimately induced the development of chronic transplant vasculopathy. Increasing the frequency of donor-reactive T cells by presensitization or by using TCR (CD8+ antimale)-transgenic recipients did not mediate acute rejection but accelerated the pace and severity of the vasculopathy. Surprisingly, decreasing the tissue mass of the donor heart by 50% resulted in acute rejection of these smaller grafts without increasing the frequency of antidonor effector T cells in the recipients. In complementary studies, placement of one or two male skin grafts on a single recipient did not affect the frequency or cytokine profile of the induced antimale T cell repertoire. Nonetheless, the recipients of single grafts acutely rejected the transplanted skin while the recipients of two skin grafts did not. These results provide new insight into the pathogenesis of transplant vasculopathy and provide an explanation for the difference in outcome between murine skin and heart transplants by highlighting the novel concept that the efficiency of transplant-reactive T cell immunity is heavily influenced by the tissue burden it encounters at the effector stage.