SP3 and infection: To test whether CD4+ T cell help was required for lung MBC heterotypic recall responses to Sp3 challenge, we treated Sp-experienced B6 mice with the CD4+-cell depleting antibody GK1.5 or IgG2b isotype control before and during respiratory Sp3 infection to deplete circulating and parenchymal CD4+ cells (Figure 8A) (12).