Activated macrophages pre-incubated in vitro enhance rather than suppress mitogen-stimulated lymphocyte transformation.

It has previously been shown in mice that activated peritoneal macrophages, when first removed from the peritoneal cavity, suppress mitogen-stimulated lymphocyte transformation in vitro as measured by [3H]-thymidine incorporation. In this study, it is demonstrated that activated macrophages that have been incubated alone in vitro (pre-incubated) for greater than 8--12 h, lose the capacity to suppress and in fact markedly enhance mitogen-stimulated lymphocyte transformation. This enhancement of lymphocyte transformation by activated macrophages occurred over a wide range of macrophage concentrations and required viable macrophages. Supernatants collected from pre-incubated activated macrophages did not enhance lymphocyte transformation. Paradoxically, pre-incubated activated macrophages did not lose the capacity to inhibit multiplication of intracellular Toxoplasma gondii. Thus, one, but not necessarily all functional capacities of activated macrophages, is markedly altered by in vitro incubation.