CD4 and cystic fibrosis: Intriguingly, while no difference in the ability of CD4 and CD8 T cells to produce IFN-γ and TNF following Cas9 or positive control CEF (cytomegalovirus [CMV], Epstein-Barr virus [EBV], influenza virus) peptides was observed, there was a significant reduction in the frequency of IFN-γ+/TNF+ CD4 and CD8 T cells in CF compared to non-CF controls when stimulated with phorbol 12-myristate 13-acetate and ionomycin (PMA/I).