The survival of fetal and bone marrow monocyte-derived alveolar macrophages is promoted by CD44 and its interaction with hyaluronan.

Alveolar macrophages maintain lung homeostasis by performing important roles in immunosurveillance and lung surfactant catabolism. They express high levels of CD44 and are one of the few macrophage populations that constitutively bind hyaluronan, a ligand for CD44 and component of pericellular and extracellular matrices. Using adoptive transfer experiments and a mouse model of inflammation, we found that alveolar macrophages are initially depleted after an inflammatory insult then rapidly self-renew and return to original numbers after the resolution phase. Monocytes recruited to an inflamed lung differentiate and contribute to the alveolar macrophage pool, but this occurs over a much slower time frame than alveolar macrophage self-renewal. CD44 expression on both fetal and bone marrow-derived alveolar macrophages promoted their survival and provided a competitive advantage over CD44-deficient alveolar macrophages at homeostasis and after inflammation. CD44-mediated hyaluronan binding was induced by the alveolar environment, and this interaction promoted alveolar macrophage survival both ex vivo and in vivo. Without CD44, alveolar macrophages lacked a hyaluronan coat, were more susceptible to death, and were present at lower numbers in the alveolar space. This demonstrates a new role for CD44 and hyaluronan in promoting alveolar macrophage survival.