CD4 and infection: To determine if heterologous infection or immunization priming of CD4+ T cells markedly impacted the B cell clonal repertoire selection compared to mice infected with only influenza, we used the priming and influenza challenge experimental setups as previously described in Fig 2A, then 100 days after influenza challenge, mice were immunized i.p. with 10 μg recombinant HA (rHA) from PR8 influenza without adjuvant (Fig 7A) to preferentially engage HA-specific memory B cells to analyze secondary plasmablasts derived from the recalled HA-specific B cells.