Alloreactive T cells: expression of HLA-D antigens, stimulation of autologous MLR, and possible immunoregulatory function.

Primed in MLC with allogeneic stimulators T cells acquire the capacity of expressing HLA-D and DR antigens and of stimulating the MLC response of autologous lymphocytes. When primed T cells from HTCs are used as stimulators, a bimodal distribution of responses with clear-cut "typing responses" and no significant "back stimulation" is observed. This pattern many be due to the expansion during priming of a population of HLD-D restricted suppressors since irradiated primed T cells inhibit the MLC responsiveness of HLA-A "compatible" lymphocytes. The development and size of such a population is not dependent, however, on the strength of the antigenic stimulus used for priming since no differences were seen between the pattern of reactions induced by T cells primed against HLA-D identical or HLA-D different cells. Primed OKT4+ and OKT8+ T cells share the capacity of expressing Ia antigens and of inducing "HLA-D restricted suppression." We suspected that a similar phenomenon accounted for the behavior of two HLA-D heterozygous cells as if they were HTCs. Although no suppression was found, the fact that these cells typed for their "silent" antigen when tested as responders, yet failed to express it when tested as stimulators, supports the theory that different genes control the MLC-responding and -stimulating capacities.