Interleukin-2 effects on human B cells activated in vivo.

While activated B cells, as well as T cells, can express functional interleukin-2 receptors (IL-2R), the physiologic role of IL-2 in B-cell responses is still in doubt. Accordingly, we have examined the role of recombinant IL-2 (rIL-2) in the proliferative response, IL-2R expression, and the terminal differentiation of tonsillar B cells in comparative studies with T cells from the same source. For these analyses of physiologically activated lymphocytes, the B and T cells were first separated, then divided into subpopulations enriched for either resting or preactivated cells on the basis of relative cell density or activation antigen expression. As expected, the relatively low-density fraction of T cells was enriched for IL-2R-positive (Tac+) cells and displayed vigorous proliferative responses to IL-2 along with heightened Tac antigen expression. Moreover, limiting dilution analysis revealed that growth of these activated T cells could be perpetuated with the addition of IL-2. By comparison, relatively few tonsillar B cells expressed the Tac antigen. The addition of rIL-2 to cultured B cells of relatively low density resulted in an increase in Tac+ cells but only a minimal proliferative response. The subpopulation of tonsillar B cells expressing the Bac-1 activation antigen contained most of the IL-2 inducible Tac+ B cells, and rIL-2 induced an efficient but transient proliferative response by these activated B cells. The rIL-2-induced Tac+ B cells were noted to be relatively large. A fraction of these Tac+ cells, but not the Tac- cells, produced and secreted immunoglobulins. Incubation with rIL-2 enhanced the Ig secretion, and the anti-Tac antibody blocked this enhancement. Time-course analysis revealed that rIL-2 induced transient Tac expression, whereas mature plasma cells in 6-day cultures no longer expressed detectable Tac antigen. In conclusion, these observations suggest that IL-2 transiently upregulates expression of IL-2R, via which it induces the terminal growth and differentiation of activated B cells into plasma cells.