Human Influenza A Virus-Specific CD8+ T-Cell Response Is Long-lived

Abstract

Animal and human studies have demonstrated the importance of influenza A virus (IAV)-specific CD8(+) cytotoxic T lymphocytes (CTLs) in heterosubtypic cross-protective immunity. Using peripheral blood mononuclear cells obtained intermittently from healthy HLA-typed blood donors between 1999 and 2012, we were able to demonstrate that IAV-specific CTLs are long-lived. Intercurrent IAV infections transiently increase the frequency of functionally distinct subsets of IAV-specific CTLs, in particular effector and effector memory T cells.

Keywords: CD8+ T cells; human; immunity; influenza A virus; longevity.

Figure 1.

Defining influenza A virus (IAV)-specific CD8⁺ T cells, using flow cytometry. A, The frequency of IAV-specific CD8⁺ T cells was determined after stimulating peripheral blood mononuclear cells with IAV H3N2. Fluorochrome-labeled antibodies were used to identify the CD8⁺ T-cell population expressing CD3, interferon γ, and CD69. Background values of nonstimulated control cells were subtracted. B, In addition, the frequency of IAV-specific CD8⁺ T cells was determined using Dextramer (Dm) staining (red gate). Various T-cell subsets were further assessed on the basis of expression of CD45RA, CD28, CCR7, and CD27. Owing to the low frequency of CD8⁺Dm⁺ T cells, the gating strategy was based on the whole CD3⁺CD8⁺ T-cell population (blue gate).

Figure 2.

Phenotyping influenza A virus (IAV)-specific CD8⁺ T cells. The frequency of IAV-specific CD8⁺ T cells was determined in peripheral blood mononuclear cells (PBMCs) obtained from healthy blood donors in the indicated years. Bars indicate the frequency of Dextramer (Dm)–expressing CD8⁺ T cells (left y-axis). Based on the expression of CD45RA, CD28, CCR7, and CD27, the IAV-specific CD8⁺Dm⁺ T cells were further subdivided into various T-cell subsets to determine the proportion of naive T cells, effector T cells, effector memory T cells (T_EM), effector memory RA T cells (T_EMRA), and central memory T cells (T_CM). Cells not belonging to any of these subsets were defined as “other”. The red lines indicate the frequency of IAV-specific CD8⁺ T-cell population expressing interferon γ (IFN-γ) and CD69, as determined by intracellular IFN-γ staining, after stimulation with infectious IAV (right y-axis). Above each graph, the year of isolation is indicated for the IAV that circulated during the influenza season preceding the time point of PBMC collection and against which antibody responses were detected by a virus neutralization assay. NA, plasma sample was not available for that time point; −, no seroconversion against an IAV strain circulating in the preceding influenza season was detected.