ABSTRACT Staphylococcus aureus is a leading cause of infection worldwide and a major driver of antibiotic resistance. Although many staphylococcal vaccines have been developed, all vaccines tested to date in human trials have failed for unclear reasons. Unlike humans who are infected or colonized with S. aureus at an early age, laboratory animals are rarely exposed to the human pathogen. Therefore, we queried if prior S. aureus exposure, in the form of infection, modifies protective immunity conferred by IsdB vaccination. Strikingly, prior staphylococcal infection in mice interferes with the induction of anti-staphylococcal immunity by IsdB vaccination. The mechanism appears to be driven by IL-10 and S. aureus-experienced B cells. These findings led us to hypothesize that S. aureus infection induces an ineffective B cell response associated with IL-10 that is preferentially recalled when subsequent IsdB vaccine is administered (original antigenic sin). To address our hypothesis, in Aim 1, we will determine how S. aureus- activated B cells and IL-10 modulate the host immune response to IsdB vaccine and nullify anti-staphylococcal protection. In Aim 2, we will determine what modifications to the IsdB-specific antibodies, induced by prior S. aureus exposure, make the antibodies non- protective. In Aim 3, we will determine the broader clinical relevance of our findings by testing if vaccine interference occurs in humanized BLT mice and in various clinically relevant settings.