P6022 Transcriptomic study of bovine macrophages infected in vitro with Streptococcus agalactiae

Abstract

Mastitis is a costly disease in dairy cattle, impairing animal welfare and increasing the use of antibiotics. It is a multi-factorial disease, affected by the load and virulence of the infecting pathogen, the milking hygiene, and the immune status and genetics of the host. Streptococcus agalactiae has re-emerged as a potential threat to udder health in Norway, especially in large herds and herds using automatic milking systems. Some cows develop chronic subclinical mastitis with high somatic cell count (SCC). Such animals shed the bacteria and contribute to spreading the infection to other cows and herds. Hence, investigation of why these become chronically infected is important. Macrophages are critical effectors and regulators of inflammation serving as the first line of defense against invading pathogens. Intramammary infections will activate macrophages to produce proinflammatory cytokines required to kill intracellular pathogens, however the balance between pro- and antiinflammatory signals is crucial for immune regulation of inflammation and preventing chronic conditions. The objective of this study is to increase the understanding of the host genetics and immune response to Streptococcus agalactiae causing chronic subclinical mastitis in Norwegian Red (NRF) dairy herds. In genotyped cows we will examine the transcriptomic responses of macrophages infected with high and low virulent strains of Streptococcus agalactiae. Seven animals were selected for a pilot study of in vitro challenge of blood monocyte-derived macrophages with two strains of Streptococcus agalactiae differing in virulence. Transcript analyses of several selected cytokines and chemokines were performed by qPCR. Four thousand NRF cows is currently genotyped using a customized 55K bovine SNP-chip (a typing tool for 55 000 Single Nucleotide Polymorphism markers) and genomic breeding values (GEBVs) for SCC will be estimated to evaluate the contrasts between the groups. Based on this genotyping cows with long duration of high SCC (= chronically infected animals) and unfavorable GEBV for SCC, and cows with very low SCC (= control) and favorable GEBV for SCC will be selected for in vitro challenge of blood monocyte-derived macrophages with the two bacterial strains. For each selected individual transcriptome analyses, i.e., mRNA sequencing, will be performed to identify the macrophage response to the infections in challenged and control sets of cells. This will generate information on genes and splice variants involved in immune responses during the infection with high and low virulent strains of Streptococcus agalactiae.