ICOS protects against mortality from acute lung injury through activation of IL-5⁺ ILC2s

Idiopathic pulmonary fibrosis (IPF) is a progressive lung disease causing irreversible lung scarring and loss of pulmonary function. IPF Patients suffer from a high rate of pulmonary infections and acute exacerbations of disease that further contribute to pulmonary decline. Low expression of the inducible T-cell costimulatory molecule (ICOS) in peripheral blood mononuclear cells predicts decreased survival of IPF patients, but the mechanisms by which ICOS protects are unclear. Using a model of bleomycin-induced lung injury and fibrosis, we now demonstrate that ICOS expression enhances survival from lung injury rather than regulating fibrogenesis. Of ICOS-expressing cells, type 2 innate lymphocytes (ILC2s) are the first to respond to bleomycin-induced injury, and this expansion is ICOS dependent. Interestingly, a similar decrease in ICOS⁺ ILCs was found in lung tissue from IPF patients. Interleukin (IL)-5, produced primarily by ILC2s, was significantly reduced after lung injury in ICOS^-/- mice, and strikingly, treatment with IL-5 protected both ICOS^-/- and wild-type mice from mortality. These results imply that low ICOS expression and decreased lung ILC2s in IPF patients may contribute to poor recovery from infections and acute exacerbation and that IL-5 treatment may be a novel therapeutic strategy to overcome these defects and protect against lung injury.