Memory CD4⁺ and CD8⁺ T lymphocyte proliferation in vaccinated dairy cows with different histories of Staphylococcus...

Staphylococcus aureus mastitis constitutes a serious threat to dairy cows. The reasons why available vaccines are not fully effective remain poorly understood; thus, in the present study, we investigated CD4⁺ and CD8⁺ T lymphocyte proliferation in dairy cows vaccinated with a polyvalent mastitis vaccine that had distinct precedent Staphylococcus aureus mastitis. We studied 17 S. aureus-infected dairy cows (11 vaccinated and six unvaccinated) and eight vaccinated healthy dairy cows with no previous S. aureus mastitis infections. Flow cytometry was used to assess lymphocyte proliferation using an anti-Ki67 antibody, and monoclonal antibodies were used to identify T cell subsets. S. aureus-infected cows exhibited reduced overall lymphocyte proliferation, including CD4⁺ T lymphocyte proliferation, and memory lymphocyte proliferation in response to S. aureus isolate stimulus. Immunization did not influence the expansion of blood lymphocyte populations. Furthermore, CD8⁺ T cells, memory CD8⁺ T lymphocytes, and effector memory CD8⁺ T lymphocytes displayed reduced proliferation 21 days after the third vaccine dose compared with before vaccination at time zero. The present data demonstrates an overall negative regulation of the T-cell response suggesting its detrimental impact leading to the persistence of S. aureus intramammary infections. Furthermore, the lack of vaccination effect on T-cell mediated immunity (e.g., proliferation) may be related to poor vaccine efficacy.