Staphylococcus aureus (S. aureus) is a gram-positive bacterium that causes mastitis in bovines and leads to financial losses to the dairy industry. Although antibody response plays a role in immune defense against S. aureus, cellular responses are of interest for vaccine development. A vaccine that stimulates both antibody and cellular responses could promote memory cell formation and provide effective protection against S. aureus. The superantigens and virulence factors secreted by live S. aureus (LSA) can interfere with immune responses and memory cell formation. Because irradiation reduces the metabolic activity and secretion of proteins, including S. aureus superantigens and hemolysins, we hypothesized the irradiated S. aureus (ISA) could drive immune cell responses.

Dendritic cells (DC) were co-cultured with lymphocytes to study the cellular responses to ISA and LSA. Dendritic cells present antigens and polarize lymphocytes into different helper T (Th) cell types that drive cellular immune responses. The DC loaded with either ISA or LSA induced increased mRNA transcription of Th17-related cytokines and cytotoxic effector memory cell formation during antigen recall experiments. Lymphocytes co-cultured with LSA-loaded DC exhibited a higher fold-change in interferon (IFN) γ mRNA compared to ISA-loaded DC, suggesting the secreted antigens and the metabolic activity of S. aureus play a role in Th1 polarization.

Th1 polarization can drive excessive inflammation and suppress beneficial Th17 responses. Bovine DC were stimulated with a mutant α-toxin deletion S. aureus strain to evaluate if α-toxin-mediated NOD2 receptor signaling activates Th1 polarization in response to S. aureus, which revealed that NOD2 mRNA transcription in DC was independent of α-toxin and that the deletion of α-toxin had no effect on the transcription of the cytokine IL-12 or the production of IFNγ by lymphocytes, events that drive Th1 polarization, in co-cultures. The deletion of accessory gene regulator (agr), which controls α-toxin production, reduced IFNγ production in lymphocytes co-cultured with the S. aureus-loaded DC, indicating that agr controlled the ability of S. aureus antigens to drive the Th1 polarization of lymphocytes.

Overall, this thesis demonstrates that ISA is a promising source of antigens that stimulate memory cells formation and Th17 polarization in bovine immune cells. The reduced Th1 cytokine response to S. aureus was not dependent on α-toxin, but other virulence factors controlled by agr should be screened to determine the source of Th1 stimulation.