Regulatory mechanisms in the mixed lymphocyte reaction: evidence for the requirement of two T cells to interact in the generation of effective suppression.

The cellular requirements for suppression of a mixed lymphocyte reaction (MLR) by in vivo alloantigen-activated spleen cells were investigated. We found that the suppression was mitomycin-resistant at days 3 and 5 after antigen injection. By contrast, it was radiation-sensitive on day 3 but not on day 5. Cell fractionation studies using a plate separation technique monitored by flow microfluorometry were performed. We found that neither purified Lyt-2- cells nor purified Lyt-2+ cells independently could suppress an MLR. By contrast, the reconstituted T population was still suppressive. It also was found that culture supernatant factors from allogeneically in vitro restimulated Lyt-2- cells could suppress an MLR if Lyt-2+ cells were present in the responding population. These data obtained by positive selection and reconstitution show that at least two surface immunoglobulin-negative cells interact for the generation of effective MLR suppression. The possibility that we are dealing with a suppressor-inducer, suppressor-effector system is discussed.