Effect of parity on lymphocytes in peripheral blood and colostrum of healthy Holstein dairy cows.

Investigation of the bovine systemic and mammary gland immune cells at calving might provide crucial information about the susceptibility of the mammary gland to infection. This study investigated the leukocyte population and cytokine mRNA levels in peripheral blood mononuclear cells (PBMCs) and colostrum mononuclear cells (CCs) obtained from healthy cows soon after calving. Fifty dairy cows that did not show clinical diseases were divided into 4 groups on the basis of parity: heifer (group 1, n = 10), 2nd calving (group 2, n = 11), 3rd calving (group 3, n = 14), and more than 3rd calving (group 4, n = 15). In the peripheral blood the numbers of CD3(+)TcR1-N12(+), CD3(+), CD4(+), and major histocompatibility complex class II(+)CD14(-) lymphocytes were significantly higher in group 1 than in group 4, whereas in the colostrum the percentages of CD4(+) and CD4(+)CD26(+) lymphocytes and the CD4(+)/CD8(+) ratio were significantly lower in group 1 than in group 4. There were no significant differences in the cytokine mRNA levels of PBMCs among the 4 groups; however, in the CCs the ratio of interferon gamma to interleukin 4 was significantly lower in group 1 than in group 3. These results suggest that the cellular immune function of PBMCs is lower, whereas mammary gland immune cells are more active, in cows with higher parity compared with heifers at calving.