Direct and cooperative mechanisms of lymphocyte triggering in liquid and solid cultures

The role of cell interactions in lymphocyte stimulation was analyzed by studying the kinetics of lymphocyte proliferation at different cell concentrations, and also by a lymphocyte microculture technique in solid medium. An absolute requirement for cell interactions was found in lymphocyte responses to concanavalin A, pokeweed mitogen, sodium periodate, purified protein derivative from Mycobacterium tuberculosis, and zinc chloride. No requirement for cell interactions was found in lymphocyte responses to calcium ionophore A23187. The existence of lymphocyte subpopulations with different requirements for cell interactions was observed in lymphocyte responses to phytohemagglutinin P, phytohemagglutinin HA 17, tetradecanoyl-phorbolacetate, antiserum to MOLT-4 lymphoblasts, antiserum to B411-4 lymphoblasts, antiserum to human embryo lung fibroblasts, and antiserum to HeLa cells infected with Herpes simplex virus. Lymphocyte responses to phytohemagglutinin P were potentiated by incorporation into the solid cultures of red blood cells of their membrane preparations suggesting that membrane-membrane interactions, either directly, or through soluble mediators are likely to be the basis of cell cooperation in this system. In solid cultures, phytohemagglutinin P, phytohemagglutinin P plus red blood cells, phytohemagglutinin HA 17, tetradecanoyl-phorbol-acetate and antiserum to MOLT-4 lymphoblasts were found to stimulate mainly thymus-dependent lymphocytes, whereas antiserum to Hela cells infected with Herpes simplex virus stimulated mainly non-thymus-dependent lymphocytes. Antiserum to B411-4 lymphoblasts stimulated both thymus-dependent and non-thymus dependent lymphocytes.