Adhesion receptor CD11b/CD18 contributes to neutrophil diapedesis across the bovine blood-milk barrier

Neutrophil phagocytosis is the most important defense mechanism against bac terial invasion of the mammary gland. However, because of the low neutrophi l count in lacteal secretion, neutrophil recruitment from blood is essentia l to the defense of the mammary gland against bacteria. Neutrophils migrate from blood across endothelium into the extracellular matrix and then acros s mammary epithelium into the infected lumen. Mechanisms for chemoattractan t-stimulated neutrophil transmigration were investigated using monolayers o f primacy mammary endothelium, collagen matrices, and monolayers of primary mammary epithelium. Complement factor C5a induced a time-dependent neutrop hil migration across all three barriers. The relative magnitude of neutroph il diapedesis through thr three barriers was: collagen>endothelium>epitheli um. Neutrophil migration across mammary arterial endothelial cells was almo st completely dependent on CD18, the beta-chain of the beta(2) integrins, a nd to a lesser extent on CD11b, one of the alpha-chains of the beta(2) inte grins. Neutrophil migration across collagen was partially blocked by monocl onal antibodies to CD18. No inhibition was observed by monoclonal antibodie s to CD11b. Conversely, neutrophil diapedesis across mammary epithelial cel ls was dependent to a greater extent on CD11b. These results provide eviden ce for different CD11b/CD18-dependent mechanisms for neutrophil diapedesis across the various cell layers of the blood-milk barrier. (C) 2000 Elsevier Science B.V. All rights reserved.