Interaction between stress hormones and phagocytic cells and its effect on the health status of dairy cows: A review

Abstract

Dairy cows are exposed to various stressors during their production cycle that makes them more susceptible to various diseases. Phagocytes (neutrophils and macrophages) are important soldiers of the innate immune system. Neutrophils are the first responders to an inflammatory response and stress and kill pathogens by generating reactive oxygen species and by the release of various antimicrobial peptides, enzymes, neutrophil extracellular trap formation, etc. Macrophages, the other phagocytes, are also the cleanup crew for the innate immune system that removes debris, pathogens, and dead neutrophils later on after an inflammatory response. The neuroendocrine system along with phagocytes exhibits an immunomodulatory potential during stressful conditions. Neuroendocrine system directly affects the activity of phagocytes by communicating bidirectionally through shared receptors and messenger molecules such as hormones, neurotransmitters, or cytokines. Different immune cells may show variable responses to each hormone. Short time exposure to stress can be beneficial, but repeated or extended exposure to stress may be detrimental to the overall health and well-being of an animal. Although some stresses associated with farming practices in dairy cows are unavoidable, better understanding of the interactions occurring between various stress hormones and phagocytic cells can help to reduce stress, improve productivity and animal welfare. This review highlights the role played by various stress hormones in modulating phagocytic cell performance of dairy cattle under inflammatory conditions.

Keywords: dairy cattle; inflammatory diseases; leukocyte trafficking; macrophages; neutrophils.

Figure-1

Diagrammatic representation of phagocytic cells activation and subsequent migration at the site of infection. Initially, macrophages in the tissue detect the presence of pathogens through CD-14 and toll-like receptors and secrete inflammatory cytokines such as IL-1 and tumor necrosis factor which further stimulate the release of interleukin (IL-8) from Weibel-Palade bodies as well as expression of both P-selectin and E-selectin on the surface of endothelial cells (ECs). IL-8 binds to the chemokine receptors (CXCR1 and CXCR2) on the surface of polymorphonuclear leukocytes (PMNs) which causes PMNs activation. Activated neutrophils use L-selectin and integrin for slowing down and tethering to the ECs, respectively. Matrix metalloproteinases help in transmigration and chemokine receptors help PMNs to move toward the chemotactic gradients and perform their functions by various mechanisms. (Source: Mohanned Naif Alhussien and Ajay Kumar Dang).

Figure-2

Stress modulation of the hormonal profile by the central nervous system. On experiencing an adverse stimulus (stressor), the hypothalamic–pituitary–adrenal axis and the sympathetic nervous system are activated resulting in the release of glucocorticoids and catecholamines, which can modulate and suppress various receptors and adhesion molecules involved in the activity of phagocytic cells. Furthermore, the pituitary hormones (prolactin and growth hormone) are also released which, however, enhance phagocytic cells activity. GHRH: Growth hormone-releasing hormone; PRH: Prolactin-releasing hormone; CRH: Corticotropin-releasing hormone; ACTH: Adrenocorticotropic hormone. Negative (−) sign indicates an inhibitory effect, while positive (+) sign indicates a stimulatory effect. (Source: Mohanned Naif Alhussien and Ajay Kumar Dang).

Figure-3

Diagrammatic representation of activation of the hypothalamic–pituitary–adrenal axis during stress/inflammation and its subsequent effects on the activity of phagocytes. (Source: Mohanned Naif Alhussien and Ajay Kumar Dang).

Figure-4

Diagrammatic representation of neuroendocrine activation of the growth hormone axis and its subsequent effects on the activity of phagocytes. IGF-1: Insulin-like growth factor 1. Negative (−) sign indicates an inhibitory effect while positive (+) sign indicates a stimulatory effect. (Source: Mohanned Naif Alhussien and Ajay Kumar Dang).