The development of human functional Ig precursors into plasma cells expressing IgM, IgG, IgA, or IgE was compared. Purified human B cells were stimulated at limiting dilution with irradiated EL4 helper cells, IL-2, and IL-4. B cells proliferated exponentially until Day 8 of culture. Nondividing plasma cells of all isotypes were detectable in ELISPOT assays between Days 8 and 10 and secreted 1.8 ± 0.7 ng antibody per cell within 24 hr. This indicates that plasma cells of all isotypes, including IgE, bear a comparable potential to secrete antibody. It further shows that Ig switching does not delay the development into IgE plasma cells, despite that switching from IgM to IgE in vitro required 6 days of IL-4 action. The proliferation and Ig production by B cells readily declined after Days 8 and 10, respectively, and could not be prolonged by restimulating B cells with fresh helper cells and lymphokines in secondary cultures. This indicates that B cells have developed into nondividing, high rate Ig-secreting plasma cells within 9 days, and that they do not differentiate any further under the applied conditions. In contrast to IgM, IgG, and IgA committed B cells, IgE switched cells did not undergo clonal expansion, since the numbers of functional IgE precursors corresponded to the maximal numbers of IgE-secreting plasma cells, whereas the numbers of IgM-, IgG-, or IgA-secreting cells exceeded the number of functional precursors 15-fold. The results demonstrate that human B cells of all isotypes, including IgE, have the potential to secrete antibody at a comparably high rate, and that the IL-4-induced switch process does not delay the differentiation into plasma cells. © 1993 Academic Press. All rights reserved.
