Short-interval longitudinal analysis of immune cell dynamics in heavy draft foals after colostrum intake

Maternal cells and antibodies are present in equine colostrum, but their relationship with immune cells in foals after colostrum ingestion remains unclear. This study longitudinally analyzed immune cells in the peripheral blood of neonatal foals at short intervals and investigated their relationship with colostral immune cells. Eleven clinically healthy draft mares and their foals were examined. Foal blood was collected before colostrum intake (0 hr) and at 2-3, 6, 12, 36-48 hr, and 3 and 7 days after ingestion. Mare blood and milk (0, 36-48 hr; 0 hr milk defined as colostrum) were also collected. Mononuclear cells were analyzed by flow cytometry using cluster of differentiation (CD) 5, CD4, CD8, CD21, CD172a, and CD45RO, with double-staining for major histocompatibility complex class II (MHC II). Total leukocytes and CD4⁺, CD5⁺, CD8⁺, and CD45RO⁺ cells significantly increased during the first week of life, whereas MHC II⁺ CD5⁺, MHC II⁺ CD8⁺, and MHC II⁺ CD45RO⁺ cells increased earlier at 12 hr. CD5⁺, CD4⁺, and CD8⁺ cells were more abundant in colostrum than in milk. CD8⁺ cells were more abundant in colostrum than in blood, whereas CD21⁺ and CD45RO⁺ cells were more abundant in blood than in colostrum, with no correlation. An early increase in CD8⁺ MHC II⁺ T cells before gut closure is a novel finding and suggests possible migration of colostrum-derived CD8⁺ T cells into neonatal circulation.