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

Neutrophil phagocytosis is the most important defense mechanism against bacterial invasion of the mammary gland. However, because of the low neutrophil count in lacteal secretion, neutrophil recruitment from blood is essential to the defense of the mammary gland against bacteria. Neutrophils migrate from blood across endothelium into the extracellular matrix and then across mammary epithelium into the infected lumen. Mechanisms for chemoattractant-stimulated neutrophil transmigration were investigated using monolayers of primary mammary endothelium, collagen matrices, and monolayers of primary mammary epithelium. Complement factor C5a induced a time-dependent neutrophil migration across all three barriers. The relative magnitude of neutrophil diapedesis through the three barriers was: collagen>endothelium>epithelium. Neutrophil migration across mammary arterial endothelial cells was almost completely dependent on CD18, the β-chain of the β2 integrins, and to a lesser extent on CD11b, one of the α-chains of the β2 integrins. Neutrophil migration across collagen was partially blocked by monoclonal antibodies to CD18. No inhibition was observed by monoclonal antibodies to CD11b. Conversely, neutrophil diapedesis across mammary epithelial cells was dependent to a greater extent on CD11b. These results provide evidence for different CD11b/CD18-dependent mechanisms for neutrophil diapedesis across the various cell layers of the blood–milk barrier.