Integrated effect of seasons and lactation stages on the plasma inflammatory cytokines, function and receptor expression of milk neutrophils in Sahiwal (Bos indicus) cows

Mastitis is a highly prevalent and one of the costliest diseases of dairy cows affecting the mammary gland. Milk neutrophils present in the mammary gland serve as an integral part of the mammary immunity, and their performance is influenced by different environmental conditions and lactation stages. To investigate the combined effects of seasons and lactation stages on the mammary immunity, milk and blood samples were collected from three groups of high producing indigenous Sahiwal cows. Function and receptor expression of milk neutrophils together with cortisol and inflammatory interleukins concentration in blood were studied. The first group of cows started their lactation in winter and completed their lactation in hot-humid season; the second group started their lactation in hot-dry season and completed it in winter. The third group started their lactation in hot-humid and completed by the hot-dry season. Plasma cortisol levels were very high during early lactation in all seasons. An inverse relationship was observed between cortisol levels and glucocorticoid receptor. Elevated phagocytic activity and plasma interleukin-2 levels were seen in winter and during mid lactation of all seasons. A positive correlation was noticed between plasma IL-8, the percentage of milk neutrophils and expression of chemokine receptors (CXCR1 and CXCR2). The highest expression of toll-like receptors (TLR2 and TLR4) and chemokine receptors was in hot-humid season. Reduction in the phagocytic activity of neutrophils, pro-inflammatory cytokines and elevated levels of cortisol in cows which started their lactation and attained peak lactation during hot-humid season indicated more stress in them. Integrated influence of both seasons and lactation stages on the activity of milk neutrophils along with plasma interleukins and cortisol levels may be used to develop suitable managemental strategies to improve mammary health and increase milk production in indigenous dairy breeds experiencing harsh environmental conditions.