Neutrophil leukocytes and inflammation of the bovine mammary gland

Abstract

Observations on the production, distribution, structure, and function of neutrophils is briefly reviewed and related to the role of neutrophils in inflammation of the bovine mammary gland. The mature neutrophil is an end stage cell possessing an armament of two distinct types of granules containing a variety of enzymatic and non-enzymatic constituents. Neutrophils are produced in the bone marrow by an orderly process of mitosis and maturation. The developing neutrophils form a morphologic continuum with functional compartmentalization into mitotic, maturative and reserve pools. Within the blood vasculature, neutrophils are either freely circulating or remain marginated along vessel walls in capillary beds, particularly in the lungs, liver and spleen. However, neutrophils so distributed are in a dynamic equilibrium, and after a brief sojourn in blood, they migrate into body cavities and tissues constituting a physiologic barrier to microbial invasion. Neutrophils from the marrow reserve and marginal pools rapidly and preferentially mobilize into areas of acute inflammation in response to chemotactic factors generated locally. There, they phagocytize, kill, and digest bacteria in a complex but sophisticated manner utilizing energy and granular constituents. During this process neutrophils also release certain agents which by themselves are inflammatory. Studies in developing calves have shown that (a), there is a sizable marrow reserve of neutrophils, (b) newly formed neutrophils appear in blood in about 7 days, (c) the circulating and marginal pools of neutrophils are about equal in size, and (d) neutrophils disappear from blood randomly with a half-time of about 9 hours. Such information on mature cows is not available. Experimental studies on mastitis in normal and neutropenic cows have demonstrated the dual role of neutrophils also in this species viz. , in protection against mammary infections and as incitors of the inflammatory reaction. Neutrophils rapidly mobilize into the mammary parenchyma and subsequently into the milk in glands experiencing acute inflammatory reaction and continue to emigrate for a variable period after disappearance of clinical signs of mastitis. While much has become known about neutrophil chemotaxis, phagocytosis, and bacterial killing in other species, little is known about bovine neutrophils in this regard and, particularly, in relation to mastitis. The bovine mammary gland is an excellent model to study inflammation and the cow has a remarkable ability to adapt to a heavy daily loss of neutrophils in milk.