CD44 Is Required to Resolve Lung Inflammation

Abstract

The hallmarks of lung inflammation and repair are the influx of inflammatory cells and release of mediators with subsequent deposition of extracellular matrix. Inflammatory cells, particularly neutrophils, are recruited to the lung and undergo apoptosis. Removal of apoptotic neutrophils has been suggested to be an important component of the resolution of lung inflammation, but minimal data have been provided demonstrating the importance of this process in vivo. CD44 is an abundant glycoprotein on the macrophage cell surface and is the major receptor of the extracellular matrix glycosaminoglycan hyaluronan. In order to assess the role of hyaluronan-CD44 interactions in an in vivo model of lung inflammation and repair, we instilled intratracheal bleomycin into CD44-deficient (knockout) and wild-type control mice. CD44 knockout mice develop normally. We observed a striking phenotype following bleomycin treatment. There is a marked difference in survival. Seventy-five percent of the CD44 knockout mice died by day 17 vs 10% of the control mice. Histologic evaluation of the lungs demonstrates intense inflammation in the CD44 knockout mice at day 14 compared to the control mice. There is a sevenfold to 10-fold increase in hyaluronan content in the lungs of the wild-type mice that peaks at day 7 to 9 and is then cleared by day 21. In the CD44 knockout mice, the hyaluronan content remains elevated at day 14. In addition, there is persistent expression of inflammatory genes such as macrophage inflammatory protein-2 in the CD44 knockout mice. The inflammatory cells are largely neutrophils. Using an immunohistochemical approach (Apo-Tag), we examined the frequency of apoptotic cells in control mice and CD44-deficient mice at day 7 and day 14. There is a >10-fold difference in staining of apoptotic neutrophils with persistence of the apoptotic neutrophils in the CD44 knockout mice at day 14. We also found a 200-fold increase in active transforming growth factor-β in the BAL of CD44 knockout vs control mice. Collectively, these data suggest that CD44 may have a critical role in resolving lung inflammation by mediating the removal of apoptotic neutrophils and the release of active transforming growth factor-β.