The mechanisms of immune protection to SARS-CoV-2 are still unclear and researchers around the world are attempting to address this critical question. In a preprint, not yet peer-reviewed, Dr. Tania Watts from the University of Toronto, unravels the characteristics and extent of B and T cell responses of our immune system after recovery from COVID-19. She also compares the profile of these COVID-19 immune responses in the same subjects to long-term immunity from a common respiratory viral infection: influenza A.

 

The current study examined the persistence and characteristics of T cell responses in 24 individuals who had recovered from COVID-19 by taking blood samples at two time points over a 9-month period.

Key points:

  • SARS-CoV-2 induces robust T cell and antibody responses in most recovered individuals.
  • SARS-CoV-2 induces anti-viral responses with some features that are distinct from those seen in long-term immunity to influenza A.

 

The researchers used peripheral blood mononuclear cells (PBMCs) from 24 individuals to study T cell responses against SARS-CoV-2 and compared them with the influenza A virus (IAV). While all subjects responded to at least one SARS-CoV-2 antigen, there were distinct differences in T cell responses between SARS-CoV-2 and influenza. For instance, SARS-CoV-2 induced a lower CD8:CD4 ratio and a lower ratio of IFN-γ: IL-2 CD4+ T cells compared to IAV. IAV also induced more multi-functional T cells, which are generally associated with a more effective antiviral response. Likewise, when looking at the cytokine response, they detected a lower IFN-γ:IL-6 ratio and an altered profile of cytotoxic molecules such as granzyme or perforin when PBMCs were stimulated with SARS-COV-2 peptides, that might indicate an altered anti-viral mechanism.

Finally, when analyzing antibody responses, they found that nearly 95% of subjects mounted an IgG response against the SARS-CoV-2 Spike (S) protein, with only 79% and 53% mounting responses against the virus’ RBD and N, respectively. In general, antibody responses to S lasted longer than those against RBD and N.

This study compared differences in immune responses between a new response to the SARS-CoV-2 virus and long-term immunity to IAV. Findings demonstrated that there are discernable differences in the elicited immune response against each virus. However, despite these differences, researchers observed the continued presence of immune activity against SARS-CoV-2 that persists 9-months after symptom onset among individuals who had COVID-19. Hence, continuous monitoring will be necessary to better understand the long-term immune response against SARS-COV-2 and how it might change with vaccination.

 

Law JC, Girard M, Chao GYC, Ward LA, Isho B, Rathod B, Colwill K, Li Z, Rini JM, Yue FY, Mubareka S, McGeer AJ, Ostrowski MA, Gommerman JL, Gingras A-C, Watts TH. Immunity to SARS-CoV-2 persists 9 months post-symptoms with an altered T cell phenotype compared to influenza A virus-specific memory. medRxiv. 2021 Jun 10. doi: 10.1101/2021.06.08.21258518.