Cord blood B cell differentiation. Synergistic effect of pokeweed mitogen and Staphylococcus aureus on in vitro differentiation of B cells from human neonates.

B lymphocyte differentiation into immunoglobulin secreting cells is a process depending on the presence of functionally mature B lymphocytes, monocytes and regulatory T lymphocytes. Cord blood B lymphocytes present in isolated cord blood mononuclear cell (MNC) preparations are normally unable to differentiate into immunoglobulin secreting plaque forming cells (PFC) when cultured in the presence of pokeweed mitogen (PWM) alone or killed Staphylococcus aureus Cowan 1 (SAC1) alone. However, each one of these activators induces PFC formation by B lymphocytes in adult MNC cultures. In the present study we show that these two activators can act synergistically to induce a significant in vitro PFC response in cord blood MNC's. The synergism of PWM and SAC1 exhibits a requirement for a specific sequence of addition in order to induce a positive response in neonatal cells. If both activators are not added simultaneously at the initiation of culture, only the initial addition of SAC1 followed by PWM will result in increased PFC production. The action of PWM and SAC1 on cord blood MNC can each be replaced by conditioned media. Supernatants from monocyte cultures containing soluble factors such as interleukin-1 (IL-1) can substitute for the activity of SAC1 while supernatants containing soluble T-cell factors (interleukin-2[IL-2], T cell replacing factor (TRF), B cell differentiation factor (BCDF), etc) can replace PWM in the cord blood MNC cultures. The results suggest that the synergistic effect of these two activators overcomes a partial immaturity or an excessive suppressor activity of human cord blood MNC.