I-region-coded products expressed on both macrophages and thymus epithelium influence T-cell activities.

I-region-coded products expressed on the surface of both macrophages (Mph) and thymus epithelium were shown to restrict T-cell activities at the level of antigen recognition and determination of the T-cell repertoire. Thus keyhole limpet haemocyanin (KLH) -pulsed Mph could elicit a delayed-type hypersensitivity response in a KLH-sensitized mouse only when the Mph and the sensitized mouse were compatible at the I-A region of the major histocompatibility complex. Also, experiments with thymus-grafted chimaeras showed that I-region compatibility between the thymus of the cell donor and the reticuloendothelial system of the recipient was essential for successful transfer of the DTH reaction. Thus, F1 T cells derived from stem cells differentiating in a P1 thymus graft, where F1 parents (P1 and P2) differed only at defined regions of the H-2 complex, could always transfer sensitivity to P1 mice. However, successful transfer to P2 mice was possible only if P1 and P2 were H-21-compatible.