Defense of the bovine mammary gland by polymorphonuclear neutrophil leukocytes

The primary phagocytic cells of the bovine mammary gland, polymorphonuclear neutrophil leukocytes (PMN), and macrophages, comprise the first line of defense against invading pathogens. In the normal healthy mammary gland, macrophages predominate and act as sentinels to invading mastitis-causing pathogens. Once invaders are detected, macrophages, and possibly mammary epithelial cells, release chemoattractants that direct migration of PMN into the area. In the mammary gland, protection is only effective if rapid influx of PMN from the circulation and subsequent phagocytosis and killing of bacteria occur. The second line of defense against infection consists of a network of memory cells and immunoglobulins that interact with the first line of defense. To minimize mammary tissue damage caused by bacterial toxins and oxidative products released by PMN, elimination of invading bacteria must proceed quickly. Therefore, the inflammatory response needs to be regulated. Hormones, metabolites, and acute phase proteins act to influence the outcome of mastitis, especially around parturition. The number of circulating PMN in cows during early lactation is highly heritable and closely related to susceptibility to clinical mastitis at this time. Advances in molecular biology are making available the tools, techniques, and products to study and modulate host-pathogen interactions. For example, the cloning and expression of proteins such as recombinant bovine soluble (rbos) CD (cluster of differentiation) 14 antigens, may provide ways of minimizing damaging effects of endotoxin during acute coliform mastitis. Soluble CD14 binds and neutralizes lipopolysacharide (LPS) and causes local recruitment of PMN after binding of CD14-LPS complexes to mammary epithelial cells. Development of transgenic animals that express rbosCD14 in their milk could prevent infection by Gram-negative pathogens.