Immunosuppression in Cows following Intramammary Infusion of Mycoplasma bovis

Mycoplasma bovis is a destructive pathogen that causes large economic losses in rearing cattle for beef and dairy worldwide. M. bovis causes suppression of and evades the host immune response; however, the mechanisms of host immune function involved in M. bovis mastitis have not been elucidated. The purpose of this study was to elucidate the characteristics of the bovine immune response to mycoplasmal mastitis. We evaluated the responsiveness of the bovine mammary gland following infusion of M. bovis Somatic cell counts and bacterial counts in milk from the infected quarter were increased. However, the proliferation of peripheral blood mononuclear cells (blood MNCs) and mononuclear cells isolated from M. bovis-stimulated mammary lymph nodes (lymph node MNCs) did not differ from that in the unstimulated cells. Transcriptome analysis revealed that the mRNA levels of innate immune system-related genes in blood MNCs, complement factor D (CFD), ficolin 1 (FCN1), and tumor necrosis factor superfamily member 13 (TNFSF13) decreased following intramammary infusion of M. bovis The mRNA levels of immune exhaustion-related genes, programmed cell death 1 (PD-1), programmed cell death-ligand 1 (PD-L1), lymphocyte activation gene 3 (LAG3), and cytotoxic T-lymphocyte-associated protein 4 (CTLA4) of milk mononuclear cells (milk MNCs) in the infected quarter were increased compared with those before infusion. Increase in immune exhaustion-related gene expression and decrease in innate immune response-related genes of MNCs in quarters from cows were newly characterized by M. bovis-induced mastitis. These results suggested that M. bovis-induced mastitis affected the immune function of bovine MNCs, which is associated with prolonged duration of infection with M. bovis.