AbstractObjectiveInterleukin‐4 (IL‐4) is an antiinflammatory cytokine that inhibits the onset and severity of proteoglycan‐induced arthritis (PGIA). To distinguish the role of IL‐4 in the innate immune response versus the adaptive immune response, we generated mice with a specific deletion of the IL‐4 receptor α‐chain (IL‐4Rα) in macrophages and neutrophils.MethodsTo obtain mice in which IL‐4Rα is deleted in macrophages and neutrophils, we intercrossed mice carrying a loxP‐flanked (floxed) IL‐4Rα allele and Cre recombinase expressed under control of the regulatory region for the lysozyme M gene (LysMcre mice) with conditional IL‐4Rαflox/flox mice and then mated them to complete IL‐4Rα−/− mice to obtain hemizygous LysMcreIL‐4Rαflox/− mice. LysMcre‐negative IL‐4Rαflox/− mice (IL‐4Rαflox/− mice) were used as control mice. PGIA was induced by immunization with human PG in adjuvant. The onset, incidence, and severity of arthritis were monitored over time. Levels of proinflammatory cytokines were measured in the sera of PG‐immunized mice, and cytokine and chemokine transcripts were measured in joints.ResultsThe severity of PGIA was exacerbated in IL‐4Rα−/− and LysMcreIL‐4Rαflox/− mice in comparison with control (IL‐4Rαflox/−) mice. The increase in arthritis susceptibility in IL‐4Rα−/− and LysMcreIL‐4Rαflox/− mice correlated with elevated serum levels of the proinflammatory cytokines IL‐1β and IL‐6 and with elevated cytokine (IL‐1β and IL‐6) and chemokine (macrophage inflammatory protein 1α [MIP‐1α] and MIP‐2) transcripts from joints. However, arthritis susceptibility did not correlate with IL‐2 or interferon‐γ (IFNγ) concentrations or with PG‐specific antibody IgG2a isotype, since levels of IL‐2, IFNγ, or PG‐specific antibody IgG2a isotype in control (IL‐4Rαflox/−) and LysMcreIL‐4Rαflox/− mice were reduced in comparison with those in IL‐4Rα−/− mice.ConclusionThese findings indicate that IL‐4 functions as a major antiinflammatory cytokine in PGIA by governing the activity of macrophages/neutrophils and less so by controlling T cell activity and autoantibody isotype expression.