Heterogeneous magnitude of immunological memory to SARS-CoV-2 in recovered individuals

Abstract

Objective: Although the adaptive immune response to SARS-CoV-2 has been characterised in the acute and early convalescent phase of the disease, few studies explore whether natural infection elicits long-lasting immunological memory in recovered individuals. In this work, we aimed to assess the maintenance of immunological memory to SARS-CoV-2.

Methods: We evaluated the long-term virus-specific cellular and humoral immune response in the members of an Italian Serie A football team, who experienced a cluster of COVID-19 in March 2020, which was strictly evaluated in the following months.

Results: Our results highlight a heterogeneous magnitude of immunological memory at 5 months after infection. Indeed, 20% of the subjects displayed a weak cellular and humoral memory to SARS-CoV-2, suggesting that they may be at higher risk of reinfection. In addition, a history of symptomatic COVID-19 was associated with higher levels of SARS-CoV-2-reactive CD4⁺ T cells and specific antibody levels than in asymptomatic individuals.

Conclusion: Collectively, these data demonstrate that immunity to SARS-CoV-2 is maintained five months postinfection even if the magnitude of response is heterogeneous among individuals. This finding suggests that some COVID-19-recovered subjects may benefit from vaccination.

Keywords: SARS‐CoV‐2; T cells; immunoglobulins; immunological memory.

Figure 1

Characterisation of the study cohort.

Figure 2

Characterisation of the immunological memory to SARS‐CoV‐2 in exposed individuals. Evaluation of IgG specific for a total virion extract (a) or spike protein (b) in uninfected individuals (n = 14), exposed group 1 (n = 15) and exposed group 2 (n = 15). (c) Evaluation of nucleoprotein‐specific total Ig in uninfected individuals (n = 14), exposed group 1 (n = 15) and exposed group 2 (n = 15). (d) Frequency of CD4⁺ T cells reactive to SARS‐CoV‐2, defined by the expression of CD154 and at least one cytokine among IFN‐γ, IL‐2 and TNF‐α (CD4⁺CD154⁺cytokine⁺) in uninfected individuals (n = 14), exposed group 1 (n = 15) and exposed group 2 (n = 15). Frequency of SARS‐CoV‐2‐specific CD4⁺ T cells expressing CD154 and IFN‐γ (e), IL‐2 (f) or TNF‐α (g) in uninfected individuals (n = 14), exposed group 1 (n = 15) and exposed group 2 (n = 15). Data in d–g are presented as percentages of CD4⁺ T cells, subtracted of background unstimulated negative control. Horizontal lines in a–g represent mean values. Grey area represents the cut‐off value. Correlation between total virion‐specific IgG (h) or spike‐specific IgG levels (i) with the frequency of SARS‐CoV‐2‐reactive CD4⁺CD154⁺cytokine⁺ T cells. (l) Uniform manifold approximation projection (UMAP) visualisation of uninfected (orange) and exposed (blue) individuals. Arrows indicate the three subjects hospitalised for COVID‐19. **P < 0.01; ***P < 0.001.

Figure 3

Different strength of immunological memory to SARS‐CoV‐2 in exposed individuals. Evaluation of IgG specific for a total virion extract (a) or spike protein (b) in individuals with a strong (n = 24) or weak (n = 6) signature of immunological memory to SARS‐CoV‐2. (c) Evaluation of nucleoprotein‐specific total Ig in individuals with a strong (n = 24) or weak (n = 6) signature of immunological memory to SARS‐CoV‐2. (d) Frequency of CD4⁺ T cells reactive to SARS‐CoV‐2, defined by the expression of CD154 and at least one cytokine among IFN‐γ, IL‐2 and TNF‐α (CD4⁺CD154⁺cytokine⁺) in individuals with a strong (n = 24) or weak (n = 6) signature of immunological memory to SARS‐CoV‐2. Frequency of SARS‐CoV‐2‐specific CD4⁺ T cells expressing CD154 and IFN‐γ (e), IL‐2 (f) or TNF‐α (g) in individuals with a strong (n = 24) or weak (n = 6) signature of immunological memory to SARS‐CoV‐2. Data in d–g are presented as percentages of CD4⁺ T cells, subtracted of background unstimulated negative control. Horizontal lines in a–g represent mean values. *P < 0.05; ***P < 0.001.

Figure 4

Immunological memory to SARS‐CoV‐2 in subjects with a history of symptomatic or asymptomatic COVID‐19. Evaluation of IgG specific for a total virion extract (a) or spike protein (b) in exposed individuals with a history of symptomatic (n = 20) or asymptomatic (n = 10) COVID‐19. (c) Evaluation of nucleoprotein‐specific total Ig in exposed individuals with a history of symptomatic (n = 20) or asymptomatic (n = 10) COVID‐19. (d) Frequency of CD4⁺ T cells reactive to SARS‐CoV‐2, defined by the expression of CD154 and at least one cytokine among IFN‐γ, IL‐2 and TNF‐α (CD4⁺CD154⁺cytokine⁺) in individuals with a history of symptomatic (n = 20) or asymptomatic (n = 10) COVID‐19. Frequency of SARS‐CoV‐2‐specific CD4⁺ T cells expressing CD154 and IFN‐γ (e), IL‐2 (f) or TNF‐α (g) in individuals with a history of symptomatic (n = 20) or asymptomatic (n = 10) COVID‐19. Data in d–g are presented as percentages of CD4⁺ T cells, subtracted of background unstimulated negative control. Horizontal lines in a–g represent mean values. (h) Uniform manifold approximation projection (UMAP) visualisation of uninfected (orange), symptomatic (blue) and asymptomatic (green) individuals. (i) Characterisation of SARS‐CoV‐2‐specific CD4⁺ T‐cell polyfunctionality in individuals with a history of symptomatic (n = 20) or asymptomatic (n = 10) COVID‐19. **P < 0.01.