CCR2 and pulmonary alveolar proteinosis: Histologic and cytologic observations in the CCR2-deficient patients suggest a mechanism accounting for the development of PAP: a small alveolar macrophage population unable to clear surfactant at a rate commensurate with ongoing production.3 The reduced numbers of alveolar macrophages in lung tissues and BAL are consistent with impaired recruitment of CCR2-deficient monocytes to the alveoli.