Involvement of IL-6, apart from its role in immunity, in mediating a chronic response during experimental arthritis

Interleukin-6 (IL-6) is highly produced during arthritis but its exact function is still unknown. In this study we examined if IL-6, apart from its role in immunity, was involved in the local inflammatory response in experimental arthritis. IL-6 deficient (IL-6(-/-)) and wild-type mice were first compared in the antigen-induced arthritis model. IL-6 deficiency resulted in a mild, transient inflammation whereas wild-type mice developed a chronic, destructive synovitis. Wild-type mice immunized with one-tenth of the normal antigen dose still developed chronic arthritis despite low antibody levels, excluding reduced humoral immunity in IL-6(-/-) mice as a crucial phenomenon. In addition, passive immune-complex-induced arthritis did not differ between wild-type and IL-6(-/-) mice. Another option is reduced levels of Th1 cells in IL-6(-/-) mice. However, transfer of antigen-specific wild-type lymph node cells to IL-6(-/-) mice enhanced acute joint inflammation and increased cartilage damage but still could not sustain chronic inflammation, suggesting involvement of nonimmune elements of IL-6 activity in chronicity. In line with this, nonimmunologically mediated zymosan-induced arthritis developed similarly in the first week, but only wild-type mice developed chronic synovitis. These results indicate an important role for IL-6 in propagation of joint inflammation, potentially independent of its role in immunity.