Discussion
Assessing the composition, scope, and durability of protective immunity generated after SARS-CoV-2 infection or vaccination are critical for control of the pandemic and future vaccination strategies. It is likely that analyzing immune responses to SARS-CoV-2 has garnered more attention and information than any other human infection in history. Traditional assessments have included antibody responses which are often transient or rapidly declining in patients with moderate infections. However, we now know that T-cell immunity against SARS-CoV-2 is more diverse and cross-reactive with peptides expressed on other Coronaviruses9,11-12. These observations suggest that robust T-cell responses are an important and essential element of long-term immunity to SARS-CoV-2. However, assessments of T-cell immunity to SARS-CoV-2 are not readily available. In this regard, we present data from a flow cytometry-based assay detecting dual cytokine-producing, SARS-CoV-2-antigen-specific memory T-cells which demonstrates specificity and accuracy for detection of CD4+/CD8+ T-cell responses to SARS-CoV-2 peptides and differentiates infected and vaccinated individuals from those not exposed to SARS-CoV-2.
Finally, analysis of T-cell responses to an important VOC (B.1.1.7) showed that exposure to SARS-CoV-2 infection or BNT162b2 vaccine elicited nearly equivalent T-cell responses. Recent observations suggest that IgG responses to SARS-CoV-2 infection did not reduce viremia in patients infected with the B.1.1.7 variant8 however, SARS-CoV-2 T-cell responses were not explored in that study. Long term analysis of immune responses will be important since memory responses differ greatly from acute responses, especially at the antibody level7,14-15. Here, dormancy of memory T-cells, B-cells, and plasma cells that can rapidly be activated upon re-exposure to SARS-CoV-2 exposure are likely to have an important role in preventing SARS-CoV-2 infection and possibly infection from current and emerging VOC13.
Acknowledgements :
We wish to thank the members of the Transplant Immunology Laboratory and Division of Infectious Diseases at Cedars-Sinai Medical Center for their hard work and dedication to this project. We also want to express our gratitude to the patients with COVID-19 and vaccine participants for donating their time and blood for this study.
Figure Legends :
Figure 1. Detection of SARS-CoV-2-specific T-cells in whole blood. Fresh whole blood from participants was stimulated by SARS-CoV-2 Spike peptides. Activated CD4+ T-cells were identified as CD45+CD3+CD4+IL-2+/TNF-α+ cells while activated CD8+ T-cell were CD45+CD3+CD8+TNF-α+/IFN-γ+ cells. Also shown is Blood + PHA which is positive control and Blood only which is negative control.
Figure 2. T cell immune response in SARS-CoV-2 infected patients and vaccinated individuals. A.CD4+ and CD8+ T-cell immune responses to SARS-CoV-2 peptides from 134 patients with confirmed SARS-CoV-2 infection. T-cells were stimulated separately using 5 major CoV-2 peptides: Spike, VME, NCAP, AP3A, NS7A. Activated CD4+ and CD8+ T cells were enumerated in Figure 1. Each dot represents one individual reading. B. CD4+ and CD8+ T-cell immune responses to SARS-CoV-2 Spike peptides in healthy, infected and vaccinated individuals. C. The correlation of Nucleocapsid-specific IgG levels with CD4+ T-cell immune responses to one or more of 5 major SARS-CoV-2 peptides in 25 patients. D. The correlation between Spike-specific CD4+ T-cell immune responses and Spike-specific IgG levels in 13 patients with elevated CD4+ Spike-specific T-cell immune responses (IL-2+/TNF-α+ cell% in CD4+ > 0.3%).
Figure 3. Immunogenicity of variant B.1.1.7 spike peptides. A&B. CD4+ & CD8+ T-cell immune responses to Spike-specific peptides are shown in infected/recovered and vaccinated patients. T cells were stimulated by the original SARS-CoV-2 Spike (Wuhan) or variant B.1.1.7 Spike peptides. Activated CD4+ (A) and CD8+ T- cell (B) in 19 infected patients and 18 vaccinated individuals are shown. C&D.The paired data for immune responses to SARS-CoV-2 Spike peptides and UK (B.1.1.7)Spike peptides for each individual.
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