IL4I1 and acute respiratory distress syndrome: While this study reveals that the therapeutic efficacy of MuSCs for ALI is attributed to their immunosuppressive effects on neutrophils via the IL4I1-TSG6 axis, it does not rule out that IL4I1-catalyzed tryptophan metabolites may also act, directly or indirectly, on macrophages to exert their immunomodulatory effects.