Based on previous findings that administration of anti-IL-17 prevented arthritis in Borrelia-vaccinated and -infected mice and induced production of CD4+CD25+ T lymphocytes with immunoregulatory function, we hypothesized that IL-35 would decrease the severity of arthritis by inhibiting the production of IL-17 in Borrelia-vaccinated and -infected mice. The gene discussed is IL17A; the disease is arthritic joint disease.