Neutrophil migration across intestinal epithelium: evidence for a role of CD44 in regulating detachment of migrating cells from the luminal surface

The migration of polymorphonuclear leukocytes (PMNs) across the intestinal epithelium is a histopathological hallmark of many mucosal inflammatory diseases including inflammatory bowel disease. The terminal transmigration step is the detachment of PMNs from the apical surface of the epithelium and their subsequent release into the intestinal lumen. The current study sought to identify epithelial proteins involved in the regulation of PMN migration across intestinal epithelium at the stage at which PMNs reach the apical epithelial surface. A panel of Abs reactive with IFN-γ-stimulated T84 intestinal epithelial cells was generated. Screening efforts identified one mAb, GM35, that prevented PMN detachment from the apical epithelial surface. Microsequencing studies identified the GM35 Ag as human CD44. Transfection studies confirmed this result by demonstrating the loss of the functional activity of the GM35 mAb following attenuation of epithelial CD44 protein expression. Immunoblotting and immunofluorescence revealed the GM35 Ag to be an apically expressed v6 variant exon-containing form of human CD44 (CD44v6). ELISA analysis demonstrated the release of soluble CD44v6 by T84 cells during PMN transepithelial migration. In addition, the observed release of CD44v6 was blocked by GM35 treatment, supporting a connection between CD44v6 release and PMN detachment. Increased expression of CD44v6 and the GM35 Ag was detected in inflamed ulcerative colitis tissue. This study demonstrates that epithelial-expressed CD44v6 plays a role in PMN clearance during inflammatory episodes through regulation of the terminal detachment of PMNs from the apical epithelial surface into the lumen of the intestine.