Low-dose chymotrypsin treatment inhibits neutrophil migration into sites of inflammation in vivo: effects on Mac-1 and MEL-14 adhesion protein expression and function.

Antibody blocking studies in the mouse suggest that the MEL-14 antigen is involved in neutrophil-endothelial cell interactions and may be important in neutrophil extravasation to sites of inflammation in vivo. We recently showed that chemotactic factor activation causes a rapid (within minutes) shedding of a large fragment of the MEL-14 antigen from the surface of neutrophils. We report here that chymotrypsin, at low doses (0.1 units/1 x 10(6) cells), but not trypsin, elastase, or collagenase, causes an activation-independent rapid loss (greater than 90%) of the MEL-14 antigen from the surface of murine neutrophils. Under the same treatment conditions chymotrypsin has no effect on the expression of four other neutrophil surface antigens, including the Mac-1 adhesion protein. Chymotrypsin treatment has no effect on neutrophil adhesion to plastic, migration to C5a, regulation of the Mac-1 antigen, but causes a greater than 95% reduction in neutrophil binding to high endothelial venules (HEV) in peripheral lymph nodes measured in the ex vivo frozen section HEV binding assay. The level of inhibition of neutrophil adhesion to HEV was comparable to that seen with the MEL-14 antibody. This experimental system allows us for the first time to specifically examine the consequences of removing the MEL-14 antigen from the surface of neutrophils on function in vivo. We show that treatment with chymotrypsin blocks greater than 85% of the ability of neutrophils injected back into the animal to home to the inflamed peritoneum. In similar in vivo experiments the MEL-14 antibody blocks neutrophil homing by 60-70%. These results further support the importance of the MEL-14 antigen in neutrophil extravasation in vivo and indicate that chymotrypsin could be useful in examining the molecular mechanisms involved in extravasation of leukocytes into a variety of diverse tissue sites of inflammation.