Objectives
Recent outbreak of influenza A/H5N1 viral infections in birds and mammals worldwide prompted renewed concerns over an emerging H5N1-related influenza pandemic in the generally immunologically-naive global population. Cross-reactive CD4 T cells between A/H1N1 and A/H5N1 subtypes targeting internal and surface influenza proteins have been identified in humans, indicating some protection against severe and fatal A/H5N1 infection.+

Methods
To detect T-cell responses towards H1 and H5 viruses, we stimulated PBMCs from patients hospitalised with seasonal influenza A viruses, alongside healthy individuals, with overlapping peptide pools spanning hemagglutinin (HA) and neuraminidase (NA) of A/Vietnam/1203/2004 (H5N1) and A/New York/18/2009 (H1N1-pdm09-like) viruses.

Results
Majority of acutely infected and healthy participants (100% and 75%, respectively) had detectable CD4 and CD8 T-cell responses towards H1-HA and H5-HA, whereas 30–80% of individuals had detectable H5-NA responses. H1 and H5 HA-specific CD4 T-cell responses highly correlated, suggesting some level of cross-reactivity between T-cell responses directed against H1 and H5 subtypes. HA-specific CD4 T-cell responses were increased in influenza patients compared to healthy participants. We found no differences in H1- and H5-specific T cell responses between those born pre- and post-1968, irrespective of infection status. Sequence alignment of H1 and H5 viruses identified high conservation in the HA2 stalk subunit (81.53%). CD4 T-cell immunity towards immunodominant HA+++++306–318 epitope, restricted by several HLA-DRB1 molecules, revealed limited cross-reactivity between Group 1 and Group 2 HA-subtypes.

Conclusion
We define correlative H1- and H5-specific CD4 T-cell responses during seasonal influenza A virus infection, likely reflecting some level of H1/H5 T cell cross-reactivity.