The kinetics of lymphocyte stimulation by mitogens in solid cultures were used to study the role of cell interactions in these responses. An absolute requirement for cell interactions, possibly through soluble mediators, was found in lymphocyte responses to calcium ionophore A23187, phytohemagglutinin HA17 and tetradecanoylphorbol acetate. Accessory leukocytes were adherent to plastic surfaces and did form E rosettes. Autologous erythrocytes were also capable of providing essential cooperative signals. 2-Mercaptoethanol, in contrast, only provided nonessential cooperative signals for lymphocyte stimulation in solid cultures.

Generation of effector cells, the expression of cell mediated lympholysis (CML) and the reactivity of mononuclear cells in solid cultures, were explored in patients with Systemic Lupus Erythematosus (SLE). While proliferative responses to alloantigens were comparable to the controls, a significant decrease in CML capacity was found in SLE T lymphocytes; further, T cells stimulated with a T cell mitogen in solid cultures showed a diminished proliferative response to soluble factors. Functional cell interaction defects rather than intrinsic T cell abnormalities may be operating in SLE.

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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 or 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.