Inducible costimulator ligand regulates bleomycin‐induced lung and skin fibrosis in a mouse model independently of the inducible costimulator/inducible costimulator ligand pathway

Objective Systemic sclerosis is a connective tissue disease characterized by fibrosis of the skin and internal organs, including the lungs. Inducible costimulator (ICOS), which is expressed on activated T cells, and its ligand ICOSL, which is expressed on antigen‐presenting cells, have been considered a single receptor–ligand pair. Although the ICOS/ICOSL pathway is known to play various roles in adaptive immunity, its roles in innate immunity and tissue fibrosis remain unknown. Methods We assessed the roles of ICOS and ICOSL in tissue fibrosis by administering bleomycin intratracheally or intradermally into mice deficient in ICOS and/or ICOSL. Tissue fibrosis was evaluated by histologic or biochemical examination. Results ICOS deficiency attenuated the lung and skin fibrosis, whereas ICOSL deficiency aggravated it. Mice deficient in both ICOS and ICOSL exhibited accelerated fibrosis, reflecting a dominant role of ICOSL over ICOS in this model. Interestingly, ICOSL expression on macrophages and B cells derived from bronchoalveolar lavage fluid was significantly elevated in ICOS‐deficient mice as compared with wild‐type mice during this process. Thus, the levels of ICOSL expression on B cells and macrophages were inversely associated with the severity of tissue fibrosis. Conclusion Our results indicate that ICOSL expression on antigen‐presenting cells plays a previously unknown regulatory role during the development of bleomycin‐induced tissue fibrosis that is independent of the ICOS/ICOSL pathway. Further studies will be needed to clarify the roles of ICOS and ICOSL in the development of systemic sclerosis.