Several evidences to support this hypothesis were presented in this study: 1) Levels of IFN-γ and IL-17A/F were significantly increased in the early stage of bloodstream infection (Fig 4); 2) Both IFN-γ+CD3+CD4+ Th1 cells and IL-17+CD3+CD4+ Th17 cells significantly increased in MAP27-immunized mice after infection (Fig 5); 3) Our in vitro assay showed that splenocytes from MAP27-immunized mice specifically recognized S. aureus and produced IFN-γ and IL-17 (Fig 7). Here, CD4 is linked to infection.