The I Region Transplantation Antigens: Immunogenicity and Enhancement

SUMMARY The immunogenicity of I region transplantation antigens was studied using congenic and recombinant strains differing only at the I region. Three separate types of incompatibility were studied with antigenic differences arising from: either the IA, IB and IC subregions; or from the IA and IB subregions only; or from the IB and IC subregions of the H‐2 complex, (i) When the incompatibility included specificities arising from the IA and IB subregions, i.e. the whole I region or just IA and IB, there was early first graft rejection (due to an H‐2I incompatibility), anti‐Ia antibodies were formed after graft rejection, and substantial degrees of enhancement could be produced by the passive administration of antiserum. Second grafts were usually rejected in accelerated fashion when placed on mice who had just rejected a first graft. By contrast, grafts to hyperimmune mice were either not rejected or underwent delayed graft rejection. In general, the passive administration of antiserum did not lead to suppression of antibody formation after a skin graft, (ii) When the differences arose from IB and IC, first grafts were not rejected, but immunized mice rejected their grafts due to the H‐2IC derived incompatibility). The results demonstrate that the I region contains at least two H loci, one of strong and the other of weak immunogenicity and that high titre anti‐Ia antibody can be formed after a skin graft. The enhancement studies, taken together with other data, demonstrate the predominating influence of the I region in the production of enhancement for H‐2 incompatible skin grafts.