Differential macrophage immune checkpoint expression triggered by bovine-associated staphylococci

This study investigated whether distinct bovine-associated staphylococci can trigger expression of PD-1 and its ligand PD-L1 in RAW 264.7 murine macrophage cell line. Four well-studied strains, Staphylococcus aureus [n = 2; one from a bovine nose (SN) and one udder-adapted strain isolated from a persistent intramammary infection (IMI; S. aureus IM)] and S. chromogenes (n = 2; one udder-adapted strain isolated from a persistent IMI (S. chromogenes IM) and one originating from a teat apex; S. chromogenes TA), were used. The expression of PD-1 and PD-L1, was measured through flow cytometry. The macrophage infection was performed over time using a multiplicity of infection (MOI) of 1, 5, and 10, respectively, up to 6 h. All staphylococcal strains induced higher macrophage expression of PD-1 compared to an unstimulated control. The PD-1 expression was MOI-dependent. Most notably, the S. aureus SN triggered a drastic increase in the macrophage PD-1 expression over time. In contrast, PD-1 expression induced by both staphylococci IM strains dropped significantly over time, whereas the S. chromogenes TA exhibited no significant variation over time. A significant increase, though subtle, in PD-L1 expression was triggered by the S. aureus IM and S. chromogenes IM. A higher intensity of PD-L1 expression, significant yet relatively subtle, was observed during the earlier stages of infection. Thus, our findings underscore strain- and niche-specific patterns of macrophage modulation by staphylococci, encouraging further studies to investigate the potential of PD-1 and/or PD-L1 blockade as a non-antibiotic strategy to tackle bovine IMIs. However, as this study used a murine macrophage cell line, further validation in primary bovine macrophages is required.