Relative deficiency in interferon-γ-secreting CD4+ T cells is strongly associated with poorer COVID-19 vaccination responses in older adults

Abstract

Although the two-dose mRNA vaccination regime provides protection against SARS-CoV-2, older adults have been shown to exhibit poorer vaccination responses. In addition, the role of vaccine-induced T-cell responses is not well characterised. We aim to assess the impact of age on immune responses after two doses of the BNT162b2 mRNA vaccine, focussing on antigen-specific T-cells. A prospective 3-month study was conducted on 15 young (median age 31 years, interquartile range (IQR) 25-35 years) and 14 older adults (median age 72 years, IQR 70-73 years). We assessed functional, neutralising antibody responses against SARS-CoV-2 variants using ACE-2 inhibition assays, and changes in B and T-cell subsets by high-dimensional flow cytometry. Antigen-specific T-cell responses were also quantified by intracellular cytokine staining and flow cytometry. Older adults had attenuated T-helper (Th) response to vaccination, which was associated with weaker antibody responses and decreased SARS-CoV-2 neutralisation. Antigen-specific interferon-γ (IFNγ)-secreting CD4+ T-cells to wild-type and Omicron antigens increased in young adults, which was strongly positively correlated with their neutralising antibody responses. Conversely, this relationship was negative in older adults. Hence, older adults' relative IFNγ-secreting CD4+ T cell deficiency might explain their poorer COVID-19 vaccination responses. Further exploration into the aetiology is needed and would be integral in developing novel vaccination strategies and improving infection outcomes in older adults.

Keywords: COVID‐19 mRNA vaccine; IFN‐γ; antigen‐specific T‐cell response; immunosenescence; older adults.

FIGURE 1

Immune responses to SARS‐CoV2 mRNA vaccination. For humoral responses: (a) Anti‐Spike IgG titres and nucleocapsid (NC) test of infection (b) Virus neutralisation assays against SARS‐CoV2 wild‐type, Omicron B.1.1.529 and Omicron BA.2 variants for young (n = 15) and old (n = 12) pre/post vaccinees. (c) Frequency of total B‐cells (CD19 + CD3‐CD14‐), (d) IgG+ (CD19+ CD27+ IgG+ IgM‐) and (e) IgA+ (CD19+ CD27+ IgA+ IgG‐ IgM‐) memory B‐cells in pre/post vaccinees. For cellular responses: (f) Frequency of ICOS+ PD‐1+ Tfh cells (ICOS+PD‐1 + CXCR5 + CD4+) (g) Th17 (CCR6 + CXCR3‐CCR4‐CD4+), (h) TEMRA (CD45RA + CD27‐CD8+) and (i) naive cells (CD45RA + CD27 + CD8+) in young (n = 15) and old (n = 7) pre/post vaccinees. Sidak's multiple comparison correction was used for comparing young versus old, Tukey's multiple comparison correction was used for comparing time‐points within the age groups. p values were determined by two‐way ANOVA with Sidak's and Tukey's multiple comparison corrections; *p < 0.05; **p < 0.01; ***p < 0.001. Bars represent mean ± standard deviation.

FIGURE 2

Antigen‐specific T‐cell responses. (a) CD4+ T cell cytokine expression after stimulation with WT, Omicron BA.2 and nucleocapsid antigens. Proportion of CD4+ T cells expressing 1, 2, 3 or 4 cytokines after stimulation with WT and Omicron BA.2 spike proteins, and nucleocapsid. p‐values were determined by Boolean analysis; *p < 0.05. Spike‐specific CD4+ T‐cell effector responses to SARS‐COV2 mRNA vaccination: frequencies of CD4+ T‐cells expressing (b) all 4 cytokines (GM‐CSF + IFNγ+IL‐2 + TNFα+ among CD4+), (c) IFNγ and TNFα (GM‐CSF‐IFNγ+IL‐2‐TNFα+ among CD4+) and (d) IFN‐γ only (GM‐CSF‐IFNγ+IL‐2‐TNFα‐ among CD4+) after stimulation with WT and Omicron BA.2 whole spike proteins, and whole nucleocapsid. Sidak's multiple comparison correction was used for comparing young versus old, and Tukey's multiple comparison correction was used for comparing time‐points within the age groups. p values were determined by two‐way ANOVA with Sidak's and Tukey's multiple comparison corrections; *p < 0.05; **p < 0.01; ***p < 0.001; ****p < 0.0001. Bars represent mean ± standard deviation. (e) Correlation between percentage of spike‐specific CD4+ cells expressing IFNγ and neutralisation against WT and Omicron BA.2 antigens at day 100 in young (n = 15) and old (n = 14) vaccinees. Pearson's normally distributed correlation coefficient for linear data was used.