The innate immune system is activated after injury and helps repair damaged tissue. By studying mice with skeletal muscle injury, Ajay Chawla and his colleagues now provide a road map for how innate immune cells contribute to muscle repair (Cell, 153, 376–388). Eosinophils, which turn out to be the crucial immune cell type in this process, produce the cytokine interleukin-4 (IL-4), which acts not directly on muscle cells but rather on fibro/adipocyte progenitor cells (FAPs) present in the regenerating muscle fibers.

IL-4–activated FAPs repair muscle in a number of ways. IL-4 stimulates FAP proliferation and the production of factors that enhance the differentiation of muscle progenitor cells. IL-4 also inhibits FAP differentiation into adipocytes and decreases fatty degeneration of injured muscle. Lastly, the researchers found that FAPs can efficiently phagocytose necrotic cells and that IL-4 signaling in FAPs is needed for the clearance of necrotic debris in injured muscle, despite the large numbers of macrophages present.