Evidence for the generation of specific T suppressor lymphocytes by Streptococcus intermedius

Crude extracellular products of the Streptococcus intermedius "CEP-Si" were able to strongly decrease the in vitro proliferation of stimulated human peripheral blood mononuclear cells (HPBMC) when evaluated by (3H)-thymidine and (3H)-leucine uptake. On the other hand, CEP-Si only slightly decreased this proliferation when HPBMC were either not stimulated or cultured in poor conditions or immature cells, i.e. thymocytes were used as target instead. Also in vivo CEP-Si was ineffective when target cells were not highly reactive. Both in vivo and in vitro, the effect of CEP-Si was proportional to the time of contact with the target cells. The dynamics of the effects of CEP-Si suggest the generation of "something" ("suppressor cells") which cause an abrupt drop in the values of (3H)-thymidine uptake rather than a progressive decrease of the values, which could indicate a loss effector or helper cells. On the other hand, CEP-Si suppressed the in vitro primary immunization of human HPBMC against SRBC and, furthermore, human HPBMC incubated with CEP-Si were able to suppress the primary immune response against SRBC of, CEP-Si untreated, HPBMC. In some instances of insufficient time of contact of CEP-Si with the target cells, an enhancement of the immune response was observed both in vivo and in vitro. Moreover, the histological pattern of the spleens of C57 BL/6 mice injected with semipurified products of CEP-Si were consistent with an adjuvant-like effect. Finally, the target HPBMC for the semipurified products of CEP-Si acquired the ultrastructural and antigenic characteristics of suppressor T lymphocytes.