Identifying Mechanisms Associated with Innate Immunity in Cows Genetically Susceptible to Mastitis

Mastitis, or mammary gland inflammation, causes the greatest loss in profit for dairy producers. Mastitis susceptibility differs among cows due to environmental, physiological, and genetic factors. Prior research identified a genetic marker in a chemokine receptor, CXCR1, associated with mastitis susceptibility and decreased neutrophil migration. Current research seeks to identify reasons behind mastitis susceptibility by validating this model through in vivo challenge with Streptococcus uberis and studying specific mechanisms causing impaired neutrophil migration. Holstein cows with GG (n=19), GC (n=28), and CC (n=20) genotypes at CXCR1+777 were challenged intramammarily with S. uberis strain UT888. After challenge 68% of quarters from GG genotype, 74% from CC genotype and only 47% from GC genotype cows had ≥10 colony forming units/ml S. uberis for at least two sampling time points (P<0.05). However, among infected cows, number of S. uberis, somatic cell count, rectal temperature, milk scores and mammary scores were comparable among genotypes throughout infection. These findings suggest that cows with GC genotypes may be more resistant to S. uberis mastitis, but have similar responses if infected. To better understand the mechanisms associated with disease resistance, migration patterns in neutrophils from cows with different CXCR1+777 genotypes were evaluated. Neutrophils from cows with GG (n=11) and CC (n=11) genotypes were isolated and stimulated with zymosan activated sera (ZAS). Cells were fixed and stained for F-actin and evaluated for F-actin content, distribution, and cell morphology. Neutrophils from CC cows had significantly lower average F-actin polymerization than GG cows v (P=0.05). Directed migration of neutrophils from GG (n=10) and CC (n=10) genotypes was imaged and tracking data was analyzed for individual cells. Cells from GG genotype traveled further on an X axis and had higher X/Y movement towards IL8 compared to CC genotype, meaning they moved more directly towards IL8. Our findings suggest lower F-actin polymerization in combination with lower ability to directly move towards IL8 could impair neutrophil response to infection in cows with a CC genotype and may contribute to increased mastitis susceptibility. Finding what makes certain cows more susceptible to mastitis could lead to strategies aimed at improved prevention and treatment of mastitis.