ANTI-CD4 MONOCLONAL ANTIBODY-INDUCED ALLOGRAFT TOLERANCE IN RATS DESPITE PERSISTENCE OF DONOR-REACTIVE T-CELLS

Although CD4-targeted therapy abrogates acute rejection and may induce permanent graft acceptance in rodents, little is known about the mech anisms of longterm graft survival in these models. Recently, we have s hown that treatment with a nondepleting anti-CD4 monoclonal antibody ( mAb) (RIB-5/2) induces longterm survival of renal, heart, and skin all ografts in strong major histocompatibility complex I/II incompatible r at strains. Here, we demonstrate that the development of major histoco mpatibility complex-specific and tissue-nonspecific tolerance rather t han graft adaptation is responsible for long-term anti-CD4 mAb-induced transplant survival, Donor-specific but not third-party heart and pan creatic islet grafts were accepted permanently without adjunctive ther apy in long-term kidney allograft recipients, and infusion of naive or alloimmune splenocytes failed to break the tolerant state. Interestin gly, alloreactive T cells were not depleted in these long-term survivo rs, as ex vivo donor-specific mixed lymphocyte reaction was largely un affected. The reverse transcriptase-polymerase chain reaction analyses of long-term renal allografts before and after donor-specific antigen challenge revealed no changes in CD3 mRNA level, but showed up-regula tion of CD25, interleukin (IL) 2, interferon (IFN) gamma, IL-4, and IL -10 mRNA in the early phase, suggesting the presence of alloreactive T cells in tolerant rats, At later time points, the expression of IFN-g amma declined rapidly, whereas IL-4 persisted, resulting in a reversal of IFN-gamma/IL-4 ratio. Our data demonstrate the stability of anti-C D4 mAb-induced tolerance despite persistence of alloreactive T cells, suggesting the role of active tolerance-maintaining mechanisms. The T helper (Th) 1/Th2 shift may be involved in this regulatory process, as anti-CD4 mAb prevents acute graft-deteriorating rejection by effectiv ely blocking Th1 responses, and well-functioning grafts may tolerize t hemselves by inducing regulatory cells.