CD4 and infection: We have previously demonstrated that following secondary infection of X-31 (H3N2) infected mice with x139, a recombinant virus containing the HA, NA, nucleoprotein, and polymerase basic 1 proteins of A/New Caledonia/20/99 (H1N1) with all other proteins derived from the X-31 viral strain, there was a selective loss in CD4 T cell responses directed against novel influenza peptide-epitopes contained predominately within HA protein [14].