In vitro activation of human suppressor cells: Relative quantitation and specificity of suppressor cells generated in primary and secondary mixed lymphocyte cultures

Human suppressor cells activated in primary and secondary mixed lymphocyte cultures (MLC) were quantitatively assessed for their ability to suppress the proliferative response of fresh responding lymphocytes to the original sensitizing (specific suppression) and third party, unrelated (non-specific suppression) alloantigens. Maximal levels of both specific and non-specific suppressor cell activities generated in primary MLC were found in the population of cells harvested on day 7 which corresponded to the peak of proliferation in bulk culture and decayed upon further culture reaching a nadir concomitant with the end of proliferation in primary bulk MLC. Maximal reactivation of the suppressor cells was accomplished by restimulation in secondary MLC with the original sensitizing alloantigens. Assessment of suppressor cell activity revealed that specific rechallenge with the original stimulating alloantigens in secondary MLC resulted in significant increases in both the levels of suppressor cell activity and in the specificity of action of the suppressor cell population as compared to suppressor cells harvested from primary MLC on day 7. Both primary and secondary MLC activated suppressor cells were capable of suppressing the induction of cytolytic lymphocytes in primary MLC. Suppression was not simply the result of altered kinetics of the fresh micro MLC response.