Demonstration that human B cells respond differently to interleukin 2 and B cell differentiation factor based on their stages of maturation

The mechanisms whereby interleukin 2 (IL 2), interferon-γ (IFN-γ) and B cell differentiation factor (BCDF) alone or in combination modulate human B cell differentiation are currently under intensive study. To dissect out the effects of individual lymphokines contained in mixed lymphocyte reaction culture supernatants (MLR-CS) on B cell differentiation, we employed pure factors that possessed the same activity as factors contained in MLR-CS (IL 2: 50 U/ml, IFN-γ: 7 U/ml, BCDF-Nal: 5 pM/ml, BCDF-YA2: 12.5% v/v) singly and in combination to human B cells. By activating purified human B cells with Staphylococcus aureus Cowan I (SAC) for 3 days, separating B blast cells by the Percoll centrifugation method, and then either using these B blast cells as B cells in the earlier stage after SAC-activation, or further culturing these B blast cells for 4 more days without any stimuli and using these B cells as B cells in the later stage after SAC-activation, we could define two different populations of cells. Disparity in the populations could be demonstrated by the observation that B cells in the earlier stage were 81.2% Tac-antigen⁺, 23.2% B₂⁺, 68.9% transferrin receptor⁺, and 90.5% HLA-DR⁺, whereas B cells in the later stage were observed to be less positive for each surface antigen: 36.1% Tac-Ag⁺, 8.3% B₂⁺, 45.3% transferrin receptor⁺, and 58.7% HLA-DR⁺. By adding each factor to both B cell fractions, we also demonstrated functional differences in the two populations. B cells in the earlier stage of activation only differentiated in response to IL 2 or IL 2 + IFN-γ but not to BCDF, which was in contrast to B cells in the later stage that did not differentiate in response to IL 2 but did differentiate to BCDF. However, B cells in both stages proliferated in response to IL 2 but not to BCDF. Finally, we separated B cells in the later stage into two populations by the Percoll discontinuous gradient centrifugation. Lower density (larger) B cells were observed to proliferate but not to differentiate in response to IL 2, whereas higher density (smaller) B cells were observed to differentiate in response to BCDF. Therefore, we conclude that activated B cells initially become large and gain Tac-Ag and differentiate in response to IL 2 alone as well as the combination of IL 2 and IFN-γ, whereas later in the more mature stage they become smaller again and differentiate into Ig-secreting cells only in response to BCDF.