Release of colony-stimulating activity from thymus-derived lymphocytes

Colony-stimulating activity (CSA) is essential for in vitro differentiation of bone marrow cells into colonies of granulocytes and mononuclear cells. While blood monocytes and macrophages are a major source of CSA, recent studies have indicated that CSA may be produced by lymphocytes responding to immunologic stimulation. Lymphocytes, purified from spleens and thymuses of mice by glass wool columns, were incubated in CMRL-1066 medium with fetal calf serum in vitro. Lymphocytes from the thumus and spleen released CSA when cultured in vitro, with peak levels of CSA observed after 7 days of incubation. Stimulation of cultures with phytohemagglutinin, concanavalin A, or pokeweed mitogen resulted in a 2-5-fold increase in CSA release, with peak levels of CSA released after 4 days of incubation. Thymus-dependent lymphocytes were responsible for the release of CSA from unstimulated and mitogen-stimulated cultures, since the incubation of these cultures with rabbit anti-mouse T cell sera abolished their ability to release CSA. Anti-mouse B cell sera had no effect on the ability of lymphocyte cultures to release CSA. These studies suggest that thymocytes and thymus-derived lymphocytes can release CSA in vitro and may be responsible for the increase in CSA observed in certain immunologic reactions.