CD28 and influenza: Thus, we next examined AT2 in vivo flu peptide/MHCII complex formation, by co-culturing in vivo-infected AT2s from wild-type or MHCII−/− B6 mice taken 4 days post influenza infection with polyclonal splenic CD4+ or CD8+ T cells taken 9 days post-infection, in the presence of soluble anti-CD28 (Fig. 5d).