Bovine colostral cells - the often forgotten component of colostrum

Abstract

At birth, neonatal calves have all the essential immune components they need as adult cattle; however, it takes at least 2 to 4 weeks for those components to become fully functional.1,2 The immune system does not become completely mature until the animal is 5 to 8 months old; for example, it takes 8 months for T cells (CD4+, CD8+, and T-cell receptor γδ+ cells) to reach peak numbers.3 Thus, the immature immune system of young calves likely moderates rather than prevents disease. This does not imply that young (< 8 months old) calves cannot respond to antigens; it simply means that the response to those antigens is weaker, slower, and easier to overcome than it will be when the immune system is mature.4 Cattle and other ruminants have a syndesmochorial placenta, which forms a syncytium between the maternal endometrium and the fetal trophectoderm, separating the maternal and fetal blood supplies and preventing the transmission of immunologic components from the dam to the fetus in utero. During the immediate preparturient period, pregnant cows initiate colostrogenesis, and blood cells and other colostral components migrate to the mammary gland5-7 where their phenotype and function are altered.8-11 Because bovine maternal and fetal blood supplies are kept separate in utero, calves are immunologically naïve immediately after birth, which puts them at high risk for disease. The acquisition of passive immunity, generally through the ingestion of colostrum, helps protect calves against disease during this critical period. Cattle producers routinely feed newborn calves fresh (ie, never frozen) or frozen colostrum, which contains antibodies against pathogens within the local environment; however, antibodies are not the sole constituents of colostrum and passively acquired immunity.12 In addition to maternally derived antibodies, colostrum contains immunologically important cytokines and a large number of maternally derived (maternal) leukocytes, which collectively contribute to the immunoprotection of the neonate.13-16 Although the role of colostral antibodies in the protection of neonates against disease has been well documented,17-20 the role of colostral cellular components in passive immunity is less well understood.21-28 In the early 1970s, researchers began to suggest that maternal lymphocytes in colostrum were primed antigen-responsive cells that, when ingested soon after birth, penetrate the permeable intestinal wall and provide transient local or systemic cell-mediated immunity, the breadth of which reflects the antigenic exposure of the cow from which the colostrum was acquired.28,29.