Ultrastructural and serological studies on the resistance of activated B cells to the cytotoxic effects of anti-immunoglobulin serum. Patch and cap formation of surface immunoglobulin on mitotic B lymphocytes.

Previous studies have shown that rabbit anti-mouse immunoglobulin sera (anti-Ig) which kill non-activated B lymphocytes in the presence of complement, are incapable of doing so when the cells are activated by antigen or mitogen into mitosis. Results reported here indicate that the resistance is not dependent on either the source of antiserum or complement, or on the presence of a mitotic inhibitor, colcemid. Immunoperoxidase staining-electron microscopy techniques were applied to assess whether there was any conspicuous difference between unstimulated versus mitogen-stimulated, mitotic cells with respect to density or distribution of cell surface Ig. No such differences were found; furthermore, mitotic cells showed rapid classical 'patch and cap' formation of cell surface Ig when incubated with anti-Ig at room temperature, indicating the retention of fluid membrane dynamics by lymphocytes in this stage of the cell cycle. In contrast to this cytotoxic resistance, T or B lymphocytes in mitosis were found to be as sensitive, or more so, to lysis by various other antisera when compared to non-mitotic cells. Thus the resistance of mitotic B cells to the cytotoxic effects of anti-Ig serum seems unique and appears independent of any conspicuous quantitative or qualitative change in cell surface Ig.