# T cell immunity to seasonal Influenza A and H5N1 viruses in laboratory workers receiving annual seasonal Influenza vaccines

## TL;DR

This study shows that annual influenza vaccines can generate T cell responses that cross-react with dangerous H5N1 viruses, offering broader protection than previously thought.

## Contribution

The study demonstrates cross-reactive T cell immunity to H5N1 in vaccinated individuals, revealing insights into influenza vaccine-induced immune responses.

## Key findings

- T cell responses to influenza were lower than to SARS-CoV-2, but cross-reactive HA-specific memory cells were common.
- T cells expanded with H1 or H5 HA showed comparable cytokine profiles and cross-recognized homologous epitopes.
- The findings highlight differences in T cell immunity between influenza and SARS-CoV-2, with implications for vaccine design.

## Abstract

Emerging threats such as highly pathogenic influenza strains like H5N1 emphasize the need for vaccines that induce cross-reactive immunity against conserved epitopes. Existing influenza vaccines primarily elicit strain-specific responses, leaving gaps in protection against pandemic subtypes. This study aimed to evaluate T cell responses to seasonal influenza A and H5N1 and compare them to SARS-CoV-2 specific T cell responses to understand differences shaped by distinct exposure histories and vaccination strategies.

T cell responses were assessed in 41 laboratory workers who received annual seasonal influenza vaccines using ELISpot to quantify responses to peptide pools derived from influenza (H1N1 hemagglutinin [HA], H3N2 HA, H5N1 HA, matrix protein 1 [MP1], nucleoprotein [NP]) and SARS-CoV-2 (spike [S2S], nucleocapsid [S2N]). Ten-day expansion assays were used to evaluate functional cross-reactivity between H1, H3, and H5 HA. Intracellular cytokine staining was performed to assess antigen-specific T cell functionality. We used the IFN-γ ELISpot assay and intracellular cytokine staining to evaluate T cell responses to H5N1 HA peptides and assessed cross-reactivity and functional similarity in H1N1 HA-expanded cells.

The percentage of individuals with effector T cell responses to influenza peptide pools, was markedly lower than the percentage of individuals with S2S-specific T cells. However, HA-specific memory cells that cross-recognized H1, H3, and H5 HA were present in many individuals. T cells expanded with H1 or H5 HA proteins cross-recognized homologous epitopes in the 2 proteins and cytokine production profiles were comparable between H1- and H5-expanded T cells.

These results highlight the potential for influenza vaccines to elicit cross-reactive immunity against H5N1 viruses. These findings also demonstrate differences between T cell responses to influenza and SARS-CoV-2, highlighting distinct immune profiles that could inform future vaccine strategies.