Distinct requirements for IL-6 in polyclonal and specific Ig production induced by microorganisms

The role of IL-6 in Ig production induced in the mouse by lactate dehydroge nase-elevating virus (LDV), Toxoplasma gondii or lipopolysaccharide (LIPS) was assessed. Following infection with LDV, a strong activator of B cells, an early and transient IL-6 production was observed, that originated predom inantly from macrophages. Whereas LDV-induced B lymphocyte proliferation ap peared independent of IL-6, mice deficient for this cytokine showed a marke d reduction in their total T-dependent IgG2a production when compared to th eir normal counterparts. By contrast, specific responses directed against e ither LDV or non-viral antigens administered at the time of infection were not decreased in the absence of IL-6. Similarly, polyclonal, but not anti-p arasite IgG2a production triggered by T gondii infection was strongly depen dent on the presence of IL-6. Finally, T-independent total IgG3 secretion t riggered by LIPS was also markedly reduced in IL-6-deficient mice. These re sults suggest that IL-6 plays a major role in T-dependent and T-independent polyclonal Ig production following B lymphocyte activation by viruses, and parasites, but not in specific antibody responses induced by the same micr oorganisms.