CD28-B7 BLOCKADE AFTER ALLOANTIGENIC CHALLENGE IN-VIVO INHIBITS TH1 CYTOKINES BUT SPARES TH2

Blocking the CD28-B7 T cell costimulatory pathway with the fusion prot ein CTLA4Ig inhibits alloimmune responses in vitro and in vivo and ind uces tolerance to cardiac allografts in mice and rats, but the mechani sms mediating the tolerant state in vivo are unknown. Here, we report the effects and potential mechanisms of CTLA4Ig in the rat renal allog raft model. LEW rats were nephrectomized and received renal allografts from major histocompatibility complex-incompatible WF rats. While all untreated and control immunoglobulin (Ig)-treated animals acutely rej ected their allografts and died, 86% of rats that received a single in jection of CTLA4Ig on day 2 after transplantation had prolonged surviv al (>60-100 days) with preserved renal function. By contrast, only 29% of animals that received CTLA4Ig on the day of engraftment had prolon ged survival. Long-term survivors (>100 days) exhibited donor-specific tolerance, accepting donor-matched WF but acutely rejecting third-par ty BN cardiac allografts. Immunohistological analysis of grafts sample d at 1 week after transplantation showed that both control and CTLA4Ig -treated animals had mononuclear cell infiltrates, with a higher perce ntage of CD4(+) cells in the CTLA4Ig-treated group. However, while thi s was associated with vasculitis and tubulitis in control grafts, ther e was no evidence of tissue injury in CTLA4Ig-treated animals. The imm une response leading to graft rejection in control animals was charact erized by expression of the T helper (Th) type 1 cytokines interleukin (IL)-2 and interferon-gamma. In contrast, the persistent CD4(+) infil trate without graft rejection in CTLA4Ig-treated animals was associate d with increased staining for the Th2-related cytokines IL-4 and IL-10 . Furthermore, grafts from CTLA4Ig-treated animals had marked upregula tion of intragraft staining for IgG1, but not IgG2a or IgG2b. Administ ration of rIL-2 to CTLA4Ig-treated animals restored allograft rejectio n in 50% of animals tested. These results confirm that blockade of the CD28-B7 pathway after alloantigenic challenge induces donor-specific acceptance of vascularized organ allografts, and indicates in this mod el that CTLA4Ig inhibits Th1 but spares Th2 cytokines in vivo.