Murine splenic lymphocytes proliferate in response to supernatant material derived from Fc fragment-pulsed splenic adherent cells. The stimulatory supernatant results from the interaction of Fc fragments with adherent cells or adherent cell supernate. Isolation of the stimulatory material in the supernate by Sephadex chromatography revealed that the mitogenic component was a cleavage product of Fc with a mol wt of approximately 14,000. The spleen cell type responsible for the generation of mitogenic Fc subfragments appears to be a macrophage. Unstimulated macrophages release an active supernate without being exposed to Fc fragments. The supernate of unstimulated macrophages apparently contain an enzyme which is capable of cleaving Fc fragments into the 14,000-mol wt mitogenic molecules. The spleen cell population induced to proliferate in response to the adherent cell supernate is present in T-cell depleted and Sephadex G-10 filtered cell preparations. Depletion of cells bearing immunoglobulin on their surfaces results in a reduced proliferative response to the mitogenic supernatant material indicating that it is probably a B cell.

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