ADCC responses to H3N2 HA proteins in unvaccinated macaques compared to vaccinated H1N1-infected macaques and H1N1-infected macaques after H3N2 challenge. (A and B) Frequencies of NK cells expressing IFN-γ and CD107a in response to plasma from 6 unvaccinated/H1N1 challenge macaques (circles), 6 H1N1-infected macaques (squares), and 6 naive control macaques (triangles) prior to H3N2 challenge. (C and D) Frequencies of NK cells expressing IFN-γ and CD107a in response to plasma from 6 unvaccinated/H1N1 challenge macaques (circles), 6 H1N1-infected macaques (squares), and 6 naive control macaques (triangles) 4 weeks after H3N2 infection. (E and F) Frequencies of NK cells expressing IFN-γ and CD107a in the presence of plasma from 6 unvaccinated/H1N1 challenge macaques (circles), 6 H1N1-infected macaques (squares), and 6 naive control macaques (triangles) to recombinant HA proteins from seasonal H3N2 strains A/Wyoming/03/2003 (WY/06), A/Aichi/2/1968 (AI/68), and A/Brisbane/10/2007 (BR/07) 4 weeks after H3N2 infection. ADCC responses to the recombinant HA proteins from seasonal H3N2 and H1N1 influenza viruses A/Solomon Islands/03/2006 (SI/06) and A/Wyoming/03/2003 were also measured. The horizontal lines represent the median values of groups. Statistical analysis was performed by 2 separate Kruskal-Wallis tests, followed by separate Mann-Whitney U tests. All samples were corrected for background based on their responses to wells containing plasma but with no plate-bound antigen.