Short communication: Th1 immunity is associated with Staphylococcus chromogenes intramammary infection

Staphylococcus chromogenes is one of the most prevalent non-aureus staphylococci associated with persistent intramammary infections (IMIs) in dairy cattle. However, the local cellular immune response to naturally occurring S. chromogenes IMI remains poorly characterized. Thus, this study aimed to assess milk T cell subsets, macrophages, neutrophils, and key intracellular cytokine production by milk T cells in mammary gland quarters naturally infected with S. chromogenes. Milk samples were used from non-infected and S. chromogenes-infected quarters from clinically healthy Holstein-Friesian dairy cows. The intensity of intracellular IFN-γ production by CD4⁺ (P = 0.048) and CD8⁺ (P = 0.035) T lymphocytes was higher in S. chromogenes-infected quarters than in non-infected controls, with a similar trend observed for γδ T cells (P = 0.06). Furthermore, the proportion of milk γδ T cells was higher in S. chromogenes-infected quarters (P = 0.05). Differences were observed neither in intracellular IL-17A production by T cell subsets nor in the proportions of milk CD4⁺ and CD8⁺ T lymphocytes, macrophages and neutrophils. Our findings identify T cell responses as a key component of the mammary immune response to S. chromogenes IMI and open new avenues for future studies investigating its potential in vivo protective response against major mastitis pathogens.