T cell derived IL-6 is differentially required for antigen-specific antibody secretion by primary and secondary B cells.

IL-6 (formerly PCTGF, HP-1, BSF-2, HGF, IFN-beta 2, 26 kDa) is a recently defined lymphokine demonstrating activity on multiple cell types, including hepatocytes, thymocytes, T cells, plasmacytomas, and B cells. The biologic effects of IL-6 on lymphocytes, particularly B cells, suggest this factor may be involved in the regulation of normal immune responses. Accordingly, we have investigated the role of IL-6 in Ag-specific responses of B cells from both naive and Ag-primed mice. When Ag-primed splenic T cells were used as a source of help, naive (primary) B cell responses specific for the hemagglutinin molecule of the influenza A virus (PR8) were fully inhibited by the addition of an anti-IL-6 antiserum, and are thus IL-6 dependent. In contrast, secondary B cell responses were essentially IL-6 independent, being unaffected by this antiserum even at concentrations 10-fold higher than required to completely inhibit primary responses. This differential IL-6 requirement was further investigated by using a panel of hemagglutinin molecule-specific Th clones. Consistent with the above findings, a Th1 clone secreting biologically active IL-6 enables antibody secretion by both primary and secondary B cells, whereas Th1 clones that do not produce IL-6 support secondary responses, but fail to help primary B cell responses unless exogenous IL-6 is added. These results provide the first instance of differential lymphokine requirements among primary vs secondary B cell responses, and suggest T cell-derived IL-6 plays a critical role during the regulation of humoral immune responses. Moreover, functionally distinct Th1 clones were identified that differed in IL-6 secretion and their corresponding ability to induce Ig secretion by primary and secondary B cells.