Staphylococcus aureus is a leading cause of bovine intramammary infections (IMI). Standard antibiotic treatments are not very effective and currently available vaccines lack tangible efficacy. Developing a vaccine formulation for S. aureus mastitis is challenging and selection of target antigens is critical. The gene products of six S. aureus genes that are highly expressed during IMI were selected as antigens for this study. The vaccine contained six recombinant proteins formulated with Emulsigen®-D, a CpG oligodeoxynucleotide and indolicidin. Nine cows in mid-lactation received the vaccine while ten received saline (placebo). Two immunizations were performed 10 weeks apart. All the antigens induced an immune response. A balanced immune response (IgG2/IgG1 ratio of 1) was observed for antigen SACOL0442 while a predominant Th2 response was observed for the other antigens (IgG2/IgG1 ratio <1). Immunizations induced CD4+ cell proliferation in response to SACOL0442, SACOL0029, SACOL0720 and SACOL1912 while a CD8+ cell proliferation was induced by SACOL0720. Four weeks after the second immunization, three quarters per animal were experimentally infused with ∼60 CFU of S. aureus. Although no difference in S. aureus counts was observed between the two groups after this robust infectious challenge, a sustained reduction in milk somatic cells counts (SCC) was observed in vaccinated cows. A correlation between SCC and S. aureus counts in milk was also observed. Altogether, this indicates that the collective immune responses induced by the antigens certainly contribute to the observed benefits of the whole vaccine. More work is needed to understand how different antigens stimulate a different response using the same adjuvant.
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