No increased reactivity was noted among non-citrullinated control antigens including native fibrinogen and vimentin. Lung tissues were evaluated by quantifying: cellular accumulation in bronchoalveolar lavage, collagen levels, and histology. An antigen microarray was used to evaluate autoantibody generation in each condition. Results Female SKG mice developed arthritis and lung disease with increased prevalence and severity when compared to intact male mice. The absence of testosterone after orchiectomy led to increased arthritis, lung disease and autoantibody generation in orchiectomized male mice compared to intact male mice. Conclusions SKG mice represent an authentic sexually dimorphic mouse model of both the joint and lung COTI-2 disease seen in rheumatoid arthritis. Testosterone protects against the development of joint and lung disease in male SKG mice. Rheumatoid arthritis (RA) is usually a systemic disease with both articular and extra-articular disease manifestations that preferentially affects women (1). Because of the predominance of autoimmunity in women, the role of estrogen in immunity and autoimmunity has been analyzed much more extensively than that of testosterone. Estrogen has been shown to influence T- and B-cell maturation and to promote a Th2 CD4+ T-cell phenotype which can lead to increased antibody production by plasma cells (as examined by (2)). Male sex hormones have been shown to play an important role in immune SOCS2 regulation as well, and could contribute COTI-2 to the sex differences seen in RA. For example, testosterone inhibits the secretion of inflammatory cytokines such as TNF- and IFN- from stimulated human peripheral blood leukocytes (3). Macrophages from orchiectomized mice have higher cell surface expression of toll-like receptor 4 (TLR 4) rendering the mice more susceptible to endotoxic shock compared to intact counterparts (4). Cross-sectional individual analysis indicates that men with RA have a lower mean serum testosterone level compared to healthy men (5). Such cross-sectional studies are limited in that they cannot determine if low testosterone displays a primary risk factor for disease development, an effect of inflammation, or the disease process itself. Taken together, these and other studies suggest that testosterone may have an important immunoregulatory role in autoimmune diseases such as RA. Extra-articular disease manifestations, including pulmonary disease, are an important source of morbidity and mortality COTI-2 in patients with RA. Progressive rheumatoid arthritis-associated interstitial lung disease (RA-ILD) occurs in nearly 10% of RA patients and is associated with significantly reduced survival (6). Little is known about the mechanisms by which lung disease evolves in the context of RA and even less is known about the role of sex hormones in disease development. To address this issue, we analyzed the role of testicular-derived sex hormones around the development of arthritis, interstitial pneumonia and anti-citrullinated peptide antibodies (ACPA) in SKG mice. As we will show, female SKG mice develop arthritis and interstitial pneumonia with more rapid onset and with increased prevalence and severity compared to male SKG mice. Using a surgical orchiectomy approach, we prospectively investigated the effect of testosterone around the development of arthritis, interstitial lung disease and autoantibody formation. MATERIALS AND METHODS Animals SKG mice were maintained in a specific pathogen free environment in our animal colony. All experiments were approved by the National Jewish Institutional Animal Care and Use Committee. Surgical orchiectomy and measurement of testosterone Anesthetized and surgically prepared male mice (4C6 weeks) were placed in dorsal recumbency. 1C2 cm ventral midline incisions were made at the scrotum and the skin was retracted to expose the tunica. The COTI-2 tunica was pierced and the testes were pushed out one at a time. The testes were raised to expose the underlying blood vessels and tubules. The testes were removed using forceps to pull off each testicle; any minor bleeding was controlled by direct pressure using forceps. All deferential vessels and ducts.