Role of neutrophil polymorphonuclear leukocytes during bovine coliform mastitis: physiology or pathology?

The review compiles some major findings concerning the inflammatory reaction in the mammary gland of dairy cows within the physiological context of the lactation cycle. The dual role of the PMN leukocyte in defense and tissue damage during experimentally induced coliform mastitis, especially around parturition and during early lactation, is highlighted. This disease affects many high producing cows in dairy herds and may cause several cases of death per year in the most severe cases. Most researchers now accept that the PMN is a key factor in the cows' defense against intramammary infection with E coli. During diapedesis of PMN into the mammary gland, several functionally important receptors are up-regulated, allowing for a more efficient phagocytosis and killing of invading pathogens. While PMN are phagocytosing and destroying the invading pathogens, they inadvertently release chemical mediators which induces swelling of secretory epithelium cytoplasm, sloughing of secretory cells, and decreased secretory activity. Permanent scarring will result in a loss of milk production. PMN's act as friends and as foes and are important components in the balance between mammary defense and damage. The mammary gland is a complex open self-regulatory system with a continuous flow of matter, energy and information. Metabolically, it has absolute priority over many other tissues except the brain. Self-regulation with change over time is characterized by a dynamic equilibrium between two mechanisms: homeostatic and homeorhetic. The defense against invaders by innate immunity and auto-repair of the damaged tissues are covered by homeostatic mechanisms while colostrogenesis and maintenance of milk secretion are controlled by mainly homeorhetic mechanisms. However, also innate immunity has to function and develop in time, depending on the lactation cycle, and its behavior and evolution in time in such a dynamical system is a challenge and a problem at the same time. In such a complex dynamic situation it is not surprising that physiology is not far away from pathology. E. coli mastitis can be a severe problem during the beginning of lactation whereas it is completely self-curing after peak lactation (8 weeks). The approach to focus on the PMN doesn't mean that the defense of the mammary gland is more simple than in other tissues. The defense of mammary gland is characterized by its complexity and over the last years many data show that there are tight connections with the mononuclear cells in mammary gland tissue. Today it is known that T cells play a central role in orchestrating the immune response. However, because of the peculiar interest in the PMN of the authors during the last 10 years, the immunobiology of the mononuclear cells in the mammary gland is not covered.