Cell subpopulations and cytokine expression in cow milk in response to chronic Staphylococcus aureus infection

Staphylococcus aureus is a major pathogen in bovine intramammary infections of subclinical and chronic nature. Persistent infection with S. aureus has been postulated to be associated with an impaired immune response. This st udy was designed to define changes in peripheral blood and milk cell subpop ulations during chronic S. aureus infection. The expression of specific ant igens on the surface of lymphocytes and neutrophils was studied by flow cyt ometry. Cytokines and cytokine transcripts elaborated by the milk-derived c ells were also investigated, using ELISA and reverse transcription polymera se chain reaction, respectively. The results indicated that cell subpopulat ions in blood from infected cows were not modified. In contrast, changes oc curred in infected milk: neutrophils were the main cell population, but the y were not in a highly activated state; the CD8+ T-lymphocytes were mainly recruited compared with the CD4+ T-lymphocytes, suggesting that CD8+ T-lymp hocytes play an important role in chronic S. aureus infection. Also, the pr oportion of the B-lymphocytes among the total lymphocyte population was inc reased, suggesting that a humoral response developed, and no change was obs erved in the gamma delta subset. No cytokine mRNA was found in milk cells f rom uninfected mammary glands. In contrast, interleukin (IL)-1 alpha, IL-1 beta, IL-6 and tumor necrosis factor alpha pro-inflammatory cytokine and IL -10 and IL-12 regulatory cytokine mRNA were synthesized in cells derived fr om infected mammary glands, whereas no IL-2 nor IL-4 mRNA were found. There fore, cells present in milk during chronic S. aureus infection were activat ed, but did not reveal any polarization of the immune response.