Cells and cytokines in inflammatory secretions of bovine mammary gland.

In response to invading bacteria, the mammary gland is protected by a variety of defence mechanisms, which can be separated into two distinct categories: innate immunity and specific immunity. Milk somatic cells consist of several cell types, including neutrophils, macrophages, lymphocytes and a smaller percentage of epithelial cells. In the healthy lactating mammary gland, macrophages are the predominant cell type whereas neutrophils are the major cell population during early inflammation. Following a bacteria invasion, neutrophil recruitment is elicited by inflammatory mediators that are produced in the infected gland by cells, possibly macrophages, activated by bacteria phagocytosis or responding to bacterial toxins or metabolites. Several cytokines, including interleukin- (IL-) 1 beta, IL-6, IL-8, tumour necrosis factor- (TNF-) alpha and interferon- (IFN-) gamma are known to be important to elicit the acute phase response and allow the accumulation of leukocytes at the site of infection. In addition to their role in early non-specific defences, macrophages also play a key role in the specific immune system, as antigen processing and presenting cells for the T cells. Few lymphocytes are found in milk of healthy glands where the predominant phenotype is CD8+ T cells. During the inflammatory reaction, T cells are recruited in milk and CD4+ cells become the predominant phenotype. The understanding of the specific and nonspecific immune mechanisms involved in the mammary gland defence against invading bacteria may lead to the development of new vaccines and to the use of cytokines to design immunomodulatory strategies for the control of bovine mastitis.