MECHANISMS IN CD4 ANTIBODY-MEDIATED TRANSPLANTATION TOLERANCE - KINETICS OF INDUCTION, ANTIGEN DEPENDENCY AND ROLE OF REGULATORY T-CELLS

CBA/Ca mice may be made tolerant to minor histoincompatible B1O.BR ski n grafts by treatment with a short course of non-depleting anti-mouse CD4 and CD8 monoclonal antibodies (mAb), during the transplantation pe riod. We wished to determine when, in relation to antibody therapy, th e T cells became tolerant. This was investigated by a series of adopti ve transfer experiments in which mAb-treated cells were removed from t herapeutic antibody at defined times after skin grafting, and exposed to fresh antigen in the absence of further mAb treatment. We show here that T cells do not become fully tolerant until 5 weeks after skin gr afting. If antibody therapy is continued for the full 5 weeks, T cell tolerance can still be established, suggesting that antibody therapy d oes not prevent lymphocytes from registering the presence of antigen. Once the tolerant state is established, it is difficult to break that tolerance by lymphocyte infusions from normal donors. This ''resistanc e'' is mediated by T cells of the tolerant host. We show that the main tenance of both tolerance and ''resistance'' requires a continuous sup ply of antigen. When tolerant cells were ''parked'' in T cell-depleted mice, tolerance and ''resistance'' were eventually lost by 6 months. In contrast, ''parked'' cells exposed to fresh antigen at any time up to 4 months remained tolerant and ''resistant'' indefinitely. Finally, we wished to establish whether ''resistance'' was peculiar to this fo rm of peripheral tolerance, or whether it might also be present in tol erance considered to be classically central. We observed resistance to be greater in the mAb-treated peripherally tolerant group, but noted that some of the centrally tolerant animals also exhibited a level of resistance above that of T cell-ablated controls. This suggests that a tolerance mechanism whose role is only minor in central tolerance may have a major role in antibody-mediated peripheral tolerance.