Graft rejection in a congenic panel of rats with defined immune response genes for MHC class I antigens. I. Rejection of and priming to the RT1Aa antigen.

Allograft rejection in the rat has been shown to be under stringent immune response (Ir) gene control using major histocompatibility complex recombinant animals as donors. Presentation of an isolated class I antigenic difference to high responder recipients results in rapid graft rejection, but low responders fail to reject. This striking qualitative difference is also seen in some liver grafting experiments in which the donor presents a full MHC haplotype and minor antigen mismatch to the responders. Grafts of other organs, however, do not discriminate qualitatively between high and low responders when a full haplotype mismatch exists. We have used the canonical high and low-responder animals, (PVG X PVG-RT1u)F1 and PVG to examine whether any qualitative difference in responsiveness can be detected against the a haplotype using a variety of organ grafts. We have confirmed a qualitative difference between high and low responders using PVG.R1 donors presenting an isolated class I (Aa) difference. Rapid rejection by high responders contrasted with complete failure to reject by the low responders. No difference in rejection tempo was found when a full a haplotype mismatch was introduced. This could have reflected vigorous responses to I and C region differences, because rapid rejection through these regions was demonstrated using the PVG.r1 (AaIcCc) and PVG.r8 (AaIuCu) recombinants. The feeble immunogenicity of the Aa antigen for PVG animals was revealed by priming and cross-priming experiments showing not only that r1 failed to prime for subsequent r1 graft rejection, but that the Aa antigen presented in concert with Ia and Ca also failed to prime. An unexpected result was that the Aa antigen of r1 actually suppressed responsiveness, especially when delivered by a heart graft. This suppression not only extended to subsequent r1 grafts (for example, skin rafts) but also to subsequent grafts of a tissue. The mechanism of this suppression remains unclear but preliminary experiments argue in favor of enhancement rather than active suppression.