Different reactivity of activated human B cells to B-cell growth factor and interleukin 2 in the costimulation assay with anti-IgM antibody and in the preactivation assay with Staphylococcus bacteria.

The two main assay systems which have been developed for the study of lymphokine-mediated human B-cell proliferation, i.e., the costimulation assay with anti-mu antibody and the preactivation assay with Staphylococcus aureus Cowan I (SAC) bacteria, were compared. Purified interleukin 2 (IL-2), obtained by the recombinant DNA technology (r-IL-2), enhanced the proliferative response of anti-mu-stimulated human B cells in the costimulation assay with anti-mu antibody and maintained the B-cell proliferation induced by preactivation with SAC bacteria. Although the majority of T-cell clones, established from normal peripheral blood T lymphocytes, showed production of both IL-2 and B-cell growth factor (BCGF) following phytohemagglutinin (PHA)-stimulation, some T-cell clones were found whose supernatants (PHA-SN), apparently free of IL-2, manifested strong BCGF activity in the costimulation assay with anti-mu antibody. However, the same clonal, IL-2-free, T-cell SN displayed no BCGF activity in the preactivation assay with SAC bacteria. When B cells were activated for 3 days with anti-mu antibody, followed by the addition of r-IL-2 or clonal T-cell SN containing BCGF for an additional 3 days, r-IL-2 showed the ability to maintain B-cell proliferation, whereas clonal SN containing BCGF had virtually no effect. These data indicate that the costimulation assay with anti-mu antibody explores the reactivity of normal human B cells to both BCGF and IL-2, whereas the preactivation assay with SAC bacteria, due to a shorter reactivity to BCGF of activated human B cells, essentially represents a probe for the study of IL-2-promoted B-cell proliferation.