T lymphocyte proliferative capacity and CD4+/CD8+ ratio in primiparous and pluriparous lactating cows

T cells play a central role in specific immunity; their populations and phenotypes could be affected by number of lactation in high-yielding dairy cows. To investigate the effects of parity on the dynamics of T lymphocytes, lymphoproliferative capacity, T lymphocyte subsets and CD4(+)/CD8(+) ratio were studied in peripheral blood of primiparous and pluriparous dairy cows during mid-late lactation. A non-radioactive technique was also adapted for a detailed lymphoproliferation assay. Compared with the primiparous cows, the pluriparous cows exhibited weaker lymphoproliferative activity, larger number of CD4(+) cells and substantially greater CD4(+)/CD8(+) ratio in their blood circulation. The increase of the CD4(+)/CD8(+) ratio in the blood of pluriparous dairy cows was mainly due to the rise in the proportion of CD4(+) cells and decline in the proportion of CD8(+) cells. This increase of the CD4(+)/CD8(+) ratio coincided with the decrease of mitogen-induced proliferation capacity of T lymphocytes. Of four lymphocyte divisions or generations during the lymphoproliferation assay, maximal lymphocyte proliferation capacity at generation 3 in primiparous cows was markedly greater than in pluriparous cows. With an alternatively safer, faster and more reproducible assay (compared with H-3-thymidine scintillation assay) we showed for the first time that aging in dairy cows leads to a decreased mitogen-induced lymphoproliferation and disturbed proportion between CD4(+) and CD8(+) T cells. This CD4(+)-CD8(+) imbalance together with diminished lymphoproliferative capacity may lead to a weaker T cytotoxic-mediated immunity and increased susceptibility to infectious diseases in pluriparous lactating cows. Our study also emphasizes further application of the methods in farm animals.