Previous studies have shown that Listeria monocytogenes-immune T cells, adoptively transferred into normal mice with killed Listeria organisms, induced peritoneal exudates rich in Ia-positive macrophages. We show now that culture fluids generated by Listeria-immune exudate cells and Listeria contain an activity that elicits Ia-rich exudates when injected intraperitoneally. The factor that recruits Ia-positive macrophages must be injected several times during a 2-d period for optimal demonstration of its activity. The induction of the factor is immunologically specific and requires Ia-positive macrophages, primed T lymphocytes, and antigen challenge. The factor is a nondialyzable protein and is not genetically restricted in its activity. The macrophages in the exudates induced by the factor bear Fc receptors, take up latex, synthesize I-A, but bear few C3 receptors. We have thus identified an immune mediator capable of controlling the Ia phenotype of the exudate macrophages.

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