Ceramide impairs apoptotic cells phagocytosis (efferocytosis) by alveolar macrophages

Abstract

Pulmonary emphysema is characterized by a loss of alveolar cells via apoptosis along with an inflammatory response, which may be explained by decreased efferocytosis of apoptotic cells. We studied the role of ceramides, pro‐apoptotic sphingolipids markedly increased in emphysema lungs, in the regulation of alveolar macrophage (AM) efferocytosis. Engulfment experiments were performed with rat AM obtained via bronchoalveolar lavage. Labeled apoptotic Jurkat cells or carboxylated beads were used as targets. AM were treated with various ceramide species and phagocytosis was quantified by flow cytometry. Ceramide treatment (10µM Cer 6:0, 2‐18h) significantly impaired AM efferocytosis (33% and 55% inhibition at 2h and 18h, respectively) in a dose‐dependent manner, without inducing AM apoptosis. Interestingly, ceramide treatment had no effect on monocyte‐derived macrophage efferocytosis or on AM Fc‐mediated phagocytosis. The mechanism of ceramide‐induced efferocytosis impairment was independent of the RhoA ‐ Rho kinase pathway, but required endogenous new ceramide synthesis, primarily via the neutral sphingomyelinase pathway. Since there were increased levels of ceramide in AM after exposure to cigarette smoke extract, we conclude that ceramides may be the molecular mediators of the inhibitory effect of cigarette smoke on AM efferocytosis in emphysema. AHA 0826103G (DP) and RO1HL077328 (IP).