Interleukin-4 regulates proteoglycan-induced arthritis by specifically suppressing the innate immune response

Objective. Interleukin-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 a-chain (IL-4R alpha) in macrophages and neutrophils. Methods. To obtain mice in which IL-4R alpha is deleted in macrophages and neutrophils, we inter-crossed mice carrying a loxP-flanked (floxed) IL-4R alpha allele and Cre recombinase expressed under control of the regulatory region for the lysozyme M gene (LysM(cre) mice) with conditional IL-4R alpha(flox/flox) mice and then mated them to complete IL-4R alpha(-/-) mice to obtain hemizygous LysM(cre)IL-4R alpha(flox/-) mice. LysM(cre)-negative IL-4R alpha(flox/-) mice (1L-4R alpha(flox/-) 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 and chemokine transcripts were measured in joints. Results. The severity of PGIA was exacerbated in IL-4R alpha(-/-) and LysM(cre)IL-4R alpha(flox/-) mice in comparison with control (IL-4R alpha(flow/-)) mice. The increase in arthritis susceptibility in IL-4Ra-/- and LysM... IL-4R alpha(flow/-) mice correlated with elevated serum levels of the proinflammatory cytokines IL-1 beta and IL-6 and with elevated cytokine JL-1 beta and IL-6) and chemokine (macrophage inflammatory protein la [MIP-1 alpha] and MIP-2) transcripts from joints. However, arthritis susceptibility did not correlate with IL-2 or interferon-gamma (IFN-gamma) concentrations or with PG-speciric antibody IgG2a isotype, since levels of IL-2, IFN gamma, or PG-specific antibody IgG2a isotype in control (1L-4R alpha(flox/-)) and LysM(cre)IL-4R alpha(flox/-) mice were reduced in comparison with those in IL-4Ra(-/-) mice. Conclusion. These 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.