The lipoprotein of the outer membrane of Escherichia coli is a B-cell mitogen in mice. Polyclonal activation of B lymphocytes was measured by an increase in thymidine uptake, by the development of plaque-forming cells against densely coupled trinitrophenylated sheep red cells, and by selectively increased rates of synthesis and secretion of leucine-labeled IgM. Murein-free and muropeptides-containing lipoprotein are effective in B-cell activation, while free murein is inactive. Removal of ester-linked fatty acids from the amino-terminal end of the lipoprotein by alkaline hydrolysis abolishes the mitogenicity of the lipoprotein. B lymphocytes from high responder (C3H/Tif and BALB/c nu/nu) or from low responder (C3H/HeJ) mice to the mitogen lipopolysaccharide (LPS) both respond well to the lipoprotein. Anti-immunoglobulin antibodies inhibit the mitogenic stimulation of B cells by lipoprotein. A complex of structures including the Ig-receptor molecules, the LPS receptor, and the lipoprotein receptor appear involved in the regulation of mitogenic stimulation of B cells to proliferation and differentiation to IgM-secreting cells.

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