Evaluation of the humoral immune response to a multicomponent recombinant vaccine against S. aureus in healthy pregnant heifers

After calving, heifers in the VacR group produced higher total IgG (IgGtotal) titers against each component, in both serum (rBt, 3.4 × 105; rClf, 3.1 × 105; rFnBP, 2.3 × 105) and milk (rBt, 2.6 × 104; rClf, 1.3 × 104; rFnBP, 1.1 × 104), than control heifers (P < 0.0001). There were increased concentrations of IgG1 and IgG2 in VacR group (P < 0.05), in both serum and milk. Humoral responses remained high throughout the period most susceptible to intramammary infections (P < 0.01). Antibodies produced against S. aureus rClf and rFnBP reduced bacterial adherence to fibronectin and fibrinogen by 73% and 67%, respectively (P < 0.001). Milk antibodies against these adhesins inhibited S. aureus invasion of a mammary epithelial cell line (MAC-T), resulting in 15.7% of bacteria internalized (P < 0.0001). There was an approximately 6-fold reduction in the hemolysis titer for the native hemolysin in the VacR group compared to the control group (P < 0.0001) and a significantly increase in the proportion of positive neutrophils (VacR, 29.7%; PBS, 13.1%) and the mean fluorescent index (VacR, 217.4; PBS, 152.6; P < 0.01) in the VacR group. The results suggest that VacR is a valuable vaccine candidate against S. aureus infections, and merits further field trials and experimental challenges.