Cells and mediators which participate in immunoglobulin synthesis by human mononuclear cells. II. The mechanism of null cell participation in immunoglobulin synthesis and secretion by B cells.

Immunoglobulins were synthesized and secreted by human B cells cultured with T cells with receptors for FcM (TM) helper cells, monocytes, null cells and PWM for 7 days. Immunoglobulin synthesis did not take place if the null cells were omitted from the cultures irrespective of the duration of the culture period. Null cells incorporated into the cultures at only 25% of their optimal concentration did not affect immunoglobulin synthesis markedly by the cultured B cells. However, the number of B cells in the culture could not be diluted without an accompanying marked reduction in immunoglobulin synthesis. The B cells synthesized and secreted significant quantities of immunoglobulin even when the null cells were added as late as day 6 of the 7-day culture whereas no or very little immunoglobulin was synthesized if the B cells were not present from the beginning of the 7-day culture. It was demonstrated that cultured null cells do not transform into B cells and do not attain their immunoglobulin-synthesizing function. Furthermore, cultured B cells do not transform into null cells and do not attain their helper function. The null cells can also be distinguished from the B cells on the basis of cell-surface markers, receptors, and blastogenic responsiveness to phytomitogens. It is concluded that (i) the human circulating B cells require the null cells, in addition to the TM cells, monocytes and PWM, in culture in order to synthesize and secrete immunoglobulin; (ii) the null cell signal that stimulates immunoglobulin synthesis and secretion by the B cells is probably the last signal following the TM helper cell, monocyte and PWM signals received by the B cells; and (iii) the null cells and the B cells constitute distinct lineages of cells.