Human B-cell-stimulatory-factor production by both T4+ and T8+ lymphocytes

The T-cell subsets responsible for the production of human B-cell-stimulatory factor (BSF) have been identified. Peripheral blood mononuclear cells (PBMNC), E-rosette-forming cells, and isolated T4+/T8+ subpopulations were cultured for 5 days with and without phytohemagglutinin (PHA) stimulation; the supernatants were then assayed for BSF. BSF activity was detected by a costimulation assay using either Staphylococcus aureus Cowan I (SAC) or goat anti-human IgM as a B-cell comitogen. Supernatants from all four PHA-stimulated cell preparations were found to exhibit BSF activity. However, supernatants from the unseparated PBMNC showed a kinetic pattern for BSF production different from that of the E-rosette-enriched T-cell population in that the BSF activity of the former reached a maximum at Day 2, followed by a rapid decrease, whereas BSF production by the latter reached a plateau at Days 4-5. Although BSF activity was observed in supernatants from both T4+- and T8+-stimulated T-cell subsets, supernatants from T8+ cells contained 50% less activity than supernatants from T4+ cells. Supernatants from the unstimulated fractions and the mitogen-containing medium control did not exhibit BSF activity. These results indicate that both the helper and the suppressor cell fractions, i.e., T4+ and T8+ lymphocytes, are responsible for the production of BSF. However, it is unclear whether the BSF activities detected in both T-cell subsets are mediated by the same or different molecular entities.