MHC-II expressing neutrophils circulate in blood and milk during mastitis and show high microbicidal activity

Bovine mastitis are mainly caused by bacterial infection. They are responsible for economic losses and have an impact on the health and welfare of animals. The increase in the somatic cell count in milk during mastitis is mainly due to the influx of neutrophils which have a crucial role in the elimination of pathogens. For a long time, these first line defenders has been view as microbes’ killers with limited role in the orchestration of the immune response. However, their role is more complex and we recently characterized a MHC-II expressing neutrophil subset with regulatory capacities in cattle. In this study, we questioned the implication of different neutrophils subsets in the mammary gland immunity during clinical and subclinical mastitis. Here, we described for the first time that, in blood as in milk, neutrophils are a heterogeneous population and encompass at least two subsets distinguishable with their expression of MHC-II. We observed higher bactericidal capacities of milk MHC-IIpos neutrophils as compared to their classical counterparts, due to a higher production of ROS and phagocytosis ability. MHC-IIpos neutrophils are enriched in milk during a subclinical mastitis as compared to blood. Moreover, we observed a positive and highly significant correlation between MHC-IIpos neutrophils and T lymphocytes present in milk during subclinical mastitis. To conclude, our study could open the way to the discovery of new biomarkers of mastitis inflammation. ### Competing Interest Statement The authors have declared no competing interest.