Although migration of leukocytes into the mammary gland is pivotal for a cow’s response against intramammary invading pathogens, the contribution of lymphocyte subsets to this response remains unclear. To investigate the dynamics of lymphocyte populations during Escherichia coli mastitis, T-lymphocyte subsets, CD4+/ CD8+ ratio, CD21+ cells, and lymphoproliferation were studied in blood and milk of primiparous cows exposed to different quantities of bacteria. The cows were intramammarily inoculated with 104 cfu of E. coli (group A) and 106 cfu (group B). Compared with group A, a much greater number of lymphocytes migrated into the infected quarters at postinfection hour (PIH) 6 to 24 in group B, and the CD8+ cells were the first-recruited T cells in the milk. There was a significant decline in the CD4+/CD8+ ratios at PIH 6 to 24 in group B. The decrease of CD4+/CD8+ ratios at PIH 6 to 24 resulted mainly from greater CD8+ cell concentrations in milk. In contrast, at PIH 72, CD4+/CD8+ ratios increased about 2-fold in both groups. This increase was mainly due to the increase in CD4+ cell concentration. The increased concentration of CD4+ cells coincided with an increase in the CD21+ cell population in the milk. In blood, the increase of CD8+ cells appeared much faster in group B (PIH 6 and 12) than in group A. The results from lymphoproliferation also indicated a greater increase in the proliferative response in both blood and milk lymphocytes of group B. Our study demonstrates for the first time that an increase of E. coli inoculum dose accelerates the trafficking of CD8+ cells during initiation of E. coli mastitis, and these cells are the predominant T cells in milk during the early hours of bovine E. coli mastitis.

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