Local Th17 immunity upon mammary immunization is protective against <em>E. coli</em> mastitis

International audienceCurrent vaccines to Escherichia coli mastitis have shown some albeit limited efficacy. Their mode of action is poorly documented and the immune response protecting the mammary gland against E. coli is not completely understood. To improve our knowledge of mammary gland immune defenses, we compared the response of three groups of six cows that received either an intramuscular or an intramammary protocol of immunizationagainst E. coli P4 before a homologous challenge. The control group received adjuvant only. Local immunization modified favorably the course of infection, by improving bacterial clearance while limiting inflammation. Systemic clinical score was also reduced and mammary secretion was preserved (Herry, 2017). High-throughput profiling using a newly developed cytokine 15-plex assay indicated a diminished TNFα production while increased IFNγ was detected in the immunized groups. Antibody response did not correlate with protection, but cellular immunity better related to protection of the mammary gland. Indeed, a transcriptome analysis performed using RNA sequencing on blood samples collected during immunization and infection phases shows that the lymphocyte response was activated in all groups 12 hours after inoculation and correlated with lower clinical scores as shown by weighted correlation network analysis. At the same time, neutrophils were produced and recruited with a moderate neutropenia in immunized groups. Type I interferon response correlated with intensity and persistence of inflammation during the late phase of mastitis. Furthermore, to assess the local T cell response in mammary tissues of locally immunized and control cows, CD4pos T cells were isolated by fluorescence cell sorting upon mammary tissue digestion of challenged and control glands, and their transcriptome determined by RNA sequencing. Results show that IL-17 expression was increased during E. coli infection and Th17 response was significantly enriched in immunized glands. These findings indicate that protective mechanisms linked to local immunization rely on IL-17-mediated immunity