The constitutive culture supernatant (SN) of the macrophage tumor line P388D1 (P388 SN) and the concanavalin A (Con A)-induced culture supernatant of the T cell hybridoma FS6-14.13 (FS6 Con A SN) were shown to contain nonspecific factors capable of inducing increased Ia expression by normal resting B cells in a dose-dependent manner. In six consecutive experiments the relative increase in Ia expression induced by P388 SN was 4.9 +/- 0.9, with FS6 Con A SN 10.7 +/- 1.5, and with a combination of both preparations 13.0 +/- 1.7. This increase in Ia expression was observed to occur in virtually all the B cells, reaching maximum levels within 24 h of culture. The interleukin-induced increase in B cell Ia expression occurred in the absence of ancillary signals provided by ligand-receptor Ig cross-linking and despite the fact that virtually all the control B cells, cultured in the absence of factors, remained in G0. These results suggest that functional receptors for at least some interleukins are expressed on normal resting B cells and their effects can be manifest in the absence of additional activating signals. The increased Ia expression induced by the nonspecific factor preparations was shown to be correlated with enhanced antigen-presenting capacity by the B cells to T cell hybridomas. The nature of the interleukins responsible for these effects remains to be definitively determined, however, the activity of FS6 Con A SN was shown to correlate with B cell growth factor activity and increased B cell Ia expression was not observed using interleukin 2 (IL-2) or interferon-gamma, prepared by recombinant DNA technology.
A helper factor(s) distinct from interleukin 2 (IL-2) was shown to be present in the concanavalin A-stimulated supernatant of normal mouse spleen cells (normal Con A Sn). Spleen cells thoroughly depleted of T cells required both IL-2 and this factor to produce antibody-secreting cells in response to sheep erythrocytes, although in the presence of IL-2 and a few T cells the requirement for the factor was less apparent. The factor had an apparent approximately 40,000 mol wt. The factor was found in normal Con A Sn that had been depleted of IL-2 by absorption with IL-2-dependent T cells and was absent from Con A-stimulated supernatants of the IL-2-producing T cell hybridoma, FS6-14.13. These results indicate that multiple helper factors control the B cell response to antigen and that IL-2, in addition to its T cell growth promoting activity, plays a direct role in B cell responses.