The role of monocyte factors in the differentiation of immunoglobulin secreting cells from human peripheral blood B cells

Supernatants from cultures of human mononuclear phagocytes (MΦ) were found to support pokeweed mitogen- (PWM) induced generation of immunoglobulin-secreting cells (ISC) in cultures of T cell-depleted, human peripheral blood B cells. The MΦ factor did not augment PWM-stimulated B cell proliferation. Moreover, MΦ factor itself was not a polyclonal B cell activator, in that no ISC were generated unless PWM was present in the B cell cultures. MΦ factor release, however, did not require PWM stimulation, but occurred spontaneously under standard conditions used to elicit the PWM response. The active factor found in MΦ culture supernatants originated from MΦ and not from the few contaminating T cells in the MΦ population. MΦ factor did not replace the requirement for T cells in this system, however, since factor activity was apparent only when B cell cultures contained a small number of T cells. Finally, MΦ factor augmented generation of both IgM and IgG ISC. These data emphasize the important role of MΦ in the differentiation of human B cells into ISC and indicate that at least some of the function of MΦ in this system is accomplished by secreted factors.