The induction of suppressor T cells by lipopolysaccharide in human peripheral blood lymphocyte cultures in the presence of fetal calf serum

Stimulation of human peripheral blood lymphocytes (PBL) by lipopolysaccharide (LPS) requires human serum and helper T cells. When PBL were cultured in medium containing fetal calf serum (FCS), not only did LPS-induced mitogenesis fail to occur but responses were significantly below control levels. We have investigated the mechanism by which LPS inhibits mitogenesis in cultures with FCS. Cell separation and reconstruction experiments showed that in the presence of FCS, LPS induced suppressor T cells which could suppress stimulation of PBL. In addition to suppressing stimulation by LPS, these suppressor T cells were also capable of suppressing mitogenic responses to FCS, phytohemagglutinin, and concanavalin A but not to pokeweed mitogen. The suppressor function was resistant to irradiation with 3000 rad. Under different culture conditions LPS may induce either helper or suppressor T cells which in turn may determine whether stimulation or inhibition of mitogenesis occurs.