A high m.w. B cell growth factor (50,000 BCGF) prepared from lectin-activated human peripheral blood lymphocyte culture supernatants acts only on B cells preactivated by a first signal. This first signal can be delivered in vitro (with anti-mu antibody (Ab)) or in vivo. Upon costimulation with anti-mu Ab the 50,000 BCGF induces an early and transient proliferative response, whereas the response to interleukin 2 (IL-2) develops more progressively. To determine the respective targets of the 50,000 BCGF and of IL-2, B cells were activated with anti-mu Ab and separated according to the expression of the IL-2 receptor (cluster designation (CD)25 antigen). CD25+ B cells do not respond to the 50,000 BCGF and do not acquire this responsiveness after an additional culture with IL-2. CD25- B cells respond to the 50,000 BCGF and not to IL-2. However, when CD25- B cells are cultured for 3 days with the 50,000 BCGF they become responsive to IL-2. These results demonstrate a pathway of B cell activation based on the ordered and sequential action of anti-mu Ab, the 50,000 BCGF, and IL-2.

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