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Experimental Intramammary Challenge with Staphylococcus chromogenes in Dairy Heifers with Specific CXCR1 Genotypes
The CXCR1 gene encodes one of the two receptors for interleukin 8 (IL-8), named chemokine (C-X-C motif) receptor 1 (CXCR1). Foregoing experiments suggested that single nucleotide polymorphism (SNP) CXCR1c.980A>G (rs43323012) influences mammary gland immunity and mastitis resistance. To further investigate these findings, 8 mid-lactating Holstein heifers were challenged intramammarily with coagulase-negative staphylococci (CNS) species. Four heifers had genotype c.980AG and 4 heifers had genotype c.980GG. Each heifer was inoculated with 1.0 x 106 colony-forming units (CFU) of Staphylococcus chromogenes, originating from a chronically infected quarter. The quarter bacterial count (qBC) and quarter milk somatic cell count (qSCC) were measured at 4, 6, 9, 12, 18, 24, 28, 32, 36, 48, 54, 60, 72 and 78h post inoculation. Additionally, quarter milk production (qMP) was recorded at 12, 24, 36, 48, 60 and 72h post inoculation. Differences in the three outcome variables between c.980AG and c.980GG heifers were analyzed using linear mixed regression models with heifer as random effect and genotype, sampling time and their interaction as fixed effects. None of the quarters/heifers showed symptoms of clinical mastitis. All heifers cleared the inoculated S. chromogenes before the end of the trial. No significant differences in qBC and qMP were observed between c.980AG and c.980GG heifers. However, the change of qSCC over time tended to be associated with the heifer’s genotype (interaction sampling time x genotype: P = 0.06). The increase in qSCC was more pronounced and persistent in c.980AG heifers compared to c.980GG heifers. In conclusion, increase in qSCC following an intramammary challenge with S. chromogenes tended to be associated with SNP CXCR1c.980A>G. Heifers expressing genotype c.980AG showed a higher immune response than heifers expressing genotype c.980GG.
Keywords: CXCR1 genotype, Staphylococcus chromogenes, intramammary challenge