The neutrophil function and lymphocyte profile of milk from bovine mammary glands infected with Streptococcus dysgalactiae

Abstract

Streptococcus dysgalactiaeis a bacterium that accounts for a notable proportion of both clinical and subclinical intramammary infections (IMIs). Thus, the present study explores the function of milk neutrophils and the lymphocyte profile in mammary glands naturally infected withStreptococcus dysgalactiae. Here, we used 32 culture-negative control quarters from eight clinically healthy dairy cows with low somatic cell counts and 13S. dysgalactiae-infected quarters from six dairy cows. Using flow cytometry, we evaluated the percentage of milk monocytes/macrophages and neutrophils, expression of CD62L, CD11b and CD44 by milk neutrophils, the levels of intracellular reactive oxygen species (ROS) production and phagocytosis ofStaphylococcus aureusby milk neutrophils, and neutrophil viability. Furthermore, the percentages of B cell (CD21+) and T lymphocyte subsets (CD3+/CD4+/CD8−; CD3+/CD8+/CD4−; and CD3+/CD8−/CD4−), and the expression of CD25 by T milk lymphocytes (CD3+) and T CD4+milk cells were also assessed by flow cytometry using monoclonal antibodies. The present study showed a higher SCC and percentage of milk neutrophils, and a decrease in the percentage of milk monocytes/macrophages fromS. dysgalactiae-infected quarters when compared to uninfected ones. We also observed a higher expression of CD11b by milk neutrophils and a tendency toward a decrease in neutrophil apoptosis rate inS. dysgalactiae-infected quarters. In addition, theS. dysgalactiae-infected quarters had higher percentages of milk T cells (CD3+) and their subset CD3+CD8+CD4−cells. Overall, the present study provided new insights intoS. dysgalactiaeIMIs, including distinct lymphocyte profiles, and a tendency toward an inhibition of apoptosis in milk neutrophils.