Comparison of the Development of SARS-Coronavirus-2-Specific Cellular Immunity, and Central Memory CD4+ T-Cell Responses Following Infection versus Vaccination

Abstract

Memory T-cell responses following infection with coronaviruses are reportedly long-lived and provide long-term protection against severe disease. Whether vaccination induces similar long-lived responses is not yet clear since, to date, there are limited data comparing memory CD4+ T-cell responses induced after SARS-CoV-2 infection versus following vaccination with BioNTech/Pfizer BNT162b2. We compared T-cell immune responses over time after infection or vaccination using ELISpot, and memory CD4+ T-cell responses three months after infection/vaccination using activation-induced marker flow cytometric assays. Levels of cytokine-producing T-cells were remarkably stable between three and twelve months after infection, and were comparable to IFNγ+ and IFNγ+IL-2+ T-cell responses but lower than IL-2+ T-cell responses at three months after vaccination. Consistent with this finding, vaccination and infection elicited comparable levels of SARS-CoV-2 specific CD4+ T-cells after three months in addition to comparable proportions of specific central memory CD4+ T-cells. By contrast, the proportions of specific effector memory CD4+ T-cells were significantly lower, whereas specific effector CD4+ T-cells were higher after infection than after vaccination. Our results suggest that T-cell responses-as measured by cytokine expression-and the frequencies of SARS-CoV-2-specific central memory CD4+T-cells-indicative of the formation of the long-lived memory T-cell compartment-are comparably induced after infection and vaccination.

Keywords: COVID-19; SARS-CoV-2; cellular immunity; memory T-cells; vaccination.

Figure 1

Comparison of cells producing IFNγ (A), IL-2 (B), or both IFNγ and IL-2 (C) measured as stimulation index by ELISpot after stimulation with SARS-CoV-2 antigens at different time points following vaccination (cohort 1) or infection (cohorts 2 and 3). Horizontal bars indicate mean values. The dotted line shows the lower cut-off of SI = 3, below which all results are negative. * indicates significant differences with p < 0.05, n.s. indicates non-significant differences.

Figure 2

Comparison of SARS-CoV-2-specific memory CD4+ T-cell subsets following vaccination and infection. (A) Gating strategy: expression of CD25 and CD134 on CD4+ T-cells from stimulated (above left) or unstimulated (below left) PBMC from a representative vaccinated staff member. The percentages of SARS-CoV-2-specific CD4+ T-cells were calculated by subtracting the values of CD25^hiCD134^hi cells in gate P3 of stimulated PBMC from unstimulated PBMC. The whole CD4+ T-cell gate from stimulated PBMC was used set quadrants on T-cell subsets based on CCR7 and CD45RO expression (below right). Applying these quadrants to cells in the stimulated CD25^hi CD134^hi gate P3 allowed quantification of SARS-CoV-2-specific memory CD4+ T-cell subsets as percentage of specific CD4+ T-cells (above right). (B) Comparison of the frequencies of SARS-CoV-2-specific CD4+ T-cells, specific central memory CD4+ T-cells (Tcm), specific effector memory CD4+ T-cells (Tem), and specific effector T-cells (Teff) as a percentage of total CD4+ T-cells three months after vaccination and infection. Horizontal bars indicate mean values. (C) Comparison of the proportions of memory T-cell subsets as percentage of SARS-CoV-2-specific CD4+ T-cells at three months after vaccination and infection.