Characterization of the WC1+ γδ T lymphocyte population after experimental intramammary infection of dairy cows with two Staphylococcus aureus strains with distinct adaptation genotypes

This study aimed to characterize the WC1⁺ γδ T lymphocyte population in an experimental intramammary infection (IMI) model using two Staphylococcus aureus strains differing in their ability to adapt to the mammary gland (MG). Two isolates (806 and 5011) from bovine IMIs with different genotypic and phenotypic profiles were selected. Strain 806 was associated with low adaptation to the MG (non-persistent-NP), whereas strain 5011 exhibited high adaptation (persistent-P). Three groups of clinically healthy cows (n = 4 per group) were inoculated intramammarily with strain 806 (NP), strain 5011 (P) and pyrogen-free saline solution. Strain 806 (NP) induced an early and robust inflammatory response, evidenced by alterations in circulating γδ T lymphocytes populations expressing activation (CD25⁺) and antigen-presenting cell markers (CD80⁺ and MHCII⁺), an increase in γδ T lymphocytes in milk, and elevated local IL-17 and IFN-γ levels. In contrast, strain 5011 (P) displayed an enhanced ability to evade these immune mechanisms, favoring the establishment of chronic infection. In this latter case, only changes in circulating CD86⁺ γδ T lymphocytes were detected, together with an increase in γδ T lymphocytes in milk and comparatively lower levels of IL-17 and IFN-γ. These findings demonstrate that γδ T lymphocytes contribute to S. aureus IMI by migrating to the MG, acting as antigen-presenting cells, and promoting the production of IL-17 and IFN-γ, cytokines that play a pivotal role in the immune response against this pathogen. The genotypic and phenotypic characteristics of S. aureus strains influenced the behavior of the γδ T lymphocyte population.