Abstract
Costimulation blockade (CoB), specifically CD28/B7 inhibition with belatacept, is an emerging clinical replacement for calcineurin inhibitor-based immunosuppression in allotransplantation. However, there is accumulating evidence that belatacept incompletely controls alloreactive T cells that lose CD28 expression during terminal differentiation. We have recently shown that the CD2-specific fusion protein alefacept controls costimulation blockade-resistant allograft rejection in nonhuman primates. Here, we have investigated the relationship between human alloreactive T cells, costimulation blockade sensitivity and CD2 expression to determine whether these findings warrant potential clinical translation. Using polychromatic flow cytometry, we found that CD8(+) effector memory T cells are distinctly high CD2 and low CD28 expressors. Alloresponsive CD8(+) CD2(hi) CD28(-) T cells contained the highest proportion of cells with polyfunctional cytokine (IFNγ, TNF and IL-2) and cytotoxic effector molecule (CD107a and granzyme B) expression capability. Treatment with belatacept in vitro incompletely attenuated allospecific proliferation, but alefacept inhibited belatacept-resistant proliferation. These results suggest that highly alloreactive effector T cells exert their late stage functions without reliance on ongoing CD28/B7 costimulation. Their high CD2 expression increases their susceptibility to alefacept. These studies combined with in vivo nonhuman primate data provide a rationale for translation of an immunosuppression regimen pairing alefacept and belatacept to human renal transplantation.