Human B lymphocyte colony responses. I. General characteristics and modulation by monocytes.

The present study investigated the ability of human B cell-enriched subpopulations to focally proliferate and form colonies in semisolid cultures after stimulation with staph protein A (SpA). After 6 days of incubation, cultures of B-enriched populations exhibited distinct colonies, the number being dependent on the concentration of SpA and the cell density. Optimal colony responses were 1.6 x 10(3) per 1 x 10(6) B lymphocytes, and greater than 83% of the colony-forming cells expressed surface immunoglobulin (sIg). The depletion of adherent monocytes from the B cell-enriched preparations decreased the colony responses approximately 3-fold compared with the nondepleted B cell populations. Adding optimal numbers of adherent monocytes to the monocyte-depleted B cells restored the colony responses; however, less augmentation was observed in single-layer co-cultures containing greater than optimal numbers of monocytes. Identical experiments in double-layer semisolid cultures revealed that relatively greater numbers of monocytes were required to enhance B cell colony responses. Thus, progressively higher ratios of monocytes to B cells resulted in increasing numbers of colonies and failed to demonstrate the diminished colony responses observed in the single-layer system. These studies demonstrate that human B cells form distinct colonies when activated by SpA and that normal adherent monocytes modulate the magnitude of colony responses. Although monocytes predominately enhance B cell clonal differentiation, the evidence presented also suggests that, to a lesser extent, soluble inhibitory materials are elaborated.