Given that GM-CSF is required for the development of monocyte-derived alveolar macrophages (48), we speculate that the decreased alveolar macrophage numbers in Cpa3Cre/+ and ST2−/− mice with COPD-like disease may be explained by a loss of ST2-mediated MC release of such factors. Here, CSF2 is linked to chronic obstructive pulmonary disease.