TOLERANCE INDUCTION TO CULTURED ISLET ALLOGRAFTS .2. THE STATUS OF ANTIDONOR REACTIVITY IN TOLERANT ANIMALS

Murine islet tissue cultured in 95% O-2 to eliminate/inactivate donor antigen-presenting cells can function indefinitely and induce a state of tolerance in nonimmunosuppressed, allogeneic recipients. Such cultured grafts represent a model of antigen presentation in which antigen (signal 1) is presented without the delivery of appropriate costimulatory activity (signal 2) necessary for T cell activation. As T cell inactivation has been proposed to result from this form of antigen presentation, we determined whether the tolerance generated in response to such cultured grafts was due to a passive (clonal deletion/inactivation) mechanism. We have shown that, although tolerant in vivo, animals bearing long-term cultured islet allografts are donorreactive in vitro as assessed by (1) CTL precursor frequency, (2) antidonor proliferative and cytotoxic responses, and (3) lymphokine production (IL-2, IL-3, TNF, and IFN-gamma). In addition, tolerance does not appear to be tissue (islet)-specific in that primed, donor-reactive T cells from tolerant animals react to islet cells in vitro and are capable of destroying donor-type islet grafts in vivo. Thus, the notion that ''signal 1'' antigen presentation, as represented by cultured islet allografts, leads to the clonal deletion or inactivation (anergy) of donor-reactive T cells is not supported by these results. Since this form of tolerance does not appear to be an intrinsic property of the donor-specific lymphocyte, these results are more consistent with a model of active regulatory tolerance in vivo.