IFNG and infection: Almost no CD4T cells produced IFNγ spontaneously in unvaccinated mice prior to PbA challenge (naive mice: 0.42%, 51 × 103 cells, mean value), but a substantial portion of CD4T cells produced IFNγ in response to PbA infection (control PbA: 1.9%, 162 103 cells, mean value) (Figure 6C,D).