Significance: The severity of the novel H7N9 influenza A virus (IAV) and the lack of neutralizing antibodies raise real pandemic concerns. In this scenario, CD8 ⁺ T lymphocytes (CTLs) may provide a layer of protection against the H7N9 virus. Our study dissects the extent of preexisting CTL immunity with the potential to respond to H7N9. We identified conserved immunogenic peptides with the capacity to elicit robust CTL responses against any human IAV, including the H7N9 virus, as well as the mutations that abolish CTL recognition. The human leukocyte antigen class I molecules that present these peptides vary in prevalence depending on the ethnicity. Such analyses found that the Alaskan and Australian Indigenous people may be particularly vulnerable to the H7N9 influenza disease.

Significance: Influenza is a rapidly spreading acute respiratory infection that causes profound morbidity and mortality. Established CD8 ⁺ T-lymphocyte (CTL) immunity directed at conserved viral regions provides protection against distinct influenza A viruses (IAVs). In this study, we show that public T-cell receptors (TCRs) specific for the most prominent human CTL epitope (M1 _58–66 restricted by HLA-A*0201) are capable of recognizing sporadically emerging variant IAVs. We also identify the structural mechanisms that enable promiscuous TCR recognition in this context. Our analysis suggests that preexisting cross-reactive TCRs may limit the spread of newly emerging pandemic IAVs.

As a major arm of the cellular immune response, CD4⁺ T cells are important in the control and clearance of infections. Primarily described as helpers, CD4⁺ T cells play an integral role in the development and activation of B cells and CD8⁺ T cells. CD4⁺ T cells are incredibly heterogeneous, and can be divided into six main lineages based on distinct profiles, namely T helper 1, 2, 17 and 22 (Th1, Th2, Th17, Th22), regulatory T cells (Treg) and T follicular helper cells (Tfh). Recent advances in structural biology have allowed for a detailed characterisation of the molecular mechanisms that drive CD4⁺ T cell recognition. In this review, we discuss the defining features of the main human CD4⁺ T cell lineages and their role in immunity, as well as their structural characteristics underlying their detection of pathogens.