Retinoids and steroids in bovine mammary gland immunobiology

Abstract

Major variations in systemic retinoid and steroid levels during the periparturient period suggest that these potent signaling molecules play a role in the decreased innate immune response observed at early lactation. This period coincides with a marked increase in the incidence of coliform mastitis. Two critical parameters have been identified within the cow's first line of defense during acute coliform mastitis: efficient mammary epithelium remodeling and functional polymorphonuclear leukocytes. The overall aim of this review is to relate the actual knowledge on retinoids and steroids to bovine mammary gland immunobiology. Recent insights on the potential role of retinoic acid, estradiol and progesterone in the development and maintenance of a normal glandular epithelial architecture are provided. In parallel, their influence on myelopoiesis and PMN functionality is also discussed. Retinoic acid and steroid hormones predominantly serve as ligands that activate specific transcription factors belonging to a nuclear receptor superfamily and thereby control gene expression. Additionally, both retinoids and steroids may act via non-nuclear receptor mediated pathways. There are also indications on a subtle interplay between retinoic acid and estrogen pathways. These messengers probably affect cell proliferation and apoptosis through interaction with local growth factors e.g., the insulin-like growth factor /insulin-like growth factor binding protein-axis, and with the extracellular matrix e.g., by modulating matrix metalloproteinase activity. In conclusion, arguments are provided supporting the importance of retinoids and sex steroid hormones in the mammary gland innate immune defense.