Pre-existing T-cell immunity to SARS-CoV-2 in unexposed healthy controls in Ecuador, as detected with a COVID-19 Interferon-Gamma Release Assay

• Published in: International journal of infectious diseases Vol. 105; pp. 21 - 25
• Main Authors: Echeverría, Gustavo, Guevara, Ángel, Coloma, Josefina, Ruiz, Alison Mera, Vasquez, María Mercedes, Tejera, Eduardo, de Waard, Jacobus H.
• Format: Journal Article
• Language: English
• Published: Canada Elsevier Ltd 01.04.2021; The Authors. Published by Elsevier Ltd on behalf of International Society for Infectious Diseases; Elsevier
• Subjects: Adult; COVID-19; COVID-19 - immunology; ELISPOT; Female; Humans; Interferon-Gamma (IFN-γ) release assay (IGRA); Interferon-gamma Release Tests; Male; Middle Aged; Nucleocapsid protein; Pre-existing immunity; SARS-CoV-2; SARS-CoV-2 - immunology; Spike protein; T cells; T-Lymphocytes - immunology; COVID-19; SARS-CoV-2; Spike protein; Pre-existing immunity; Interferon-Gamma (IFN-γ) release assay (IGRA); T cells; Nucleocapsid protein; ELISPOT
• ISSN: 1201-9712; 1878-3511; 1878-3511
• DOI: 10.1016/j.ijid.2021.02.034

•We used an IGRA to analyze a T-cell response in COVID-19 patients and healthy unexposed controls.•We show that 80% of the convalescent COVID-19 patients had reactive T cells to SARS-CoV-2 antigens.•Surprisingly, 44% of the unexposed controls also had a strong T-cell response to these antigens.•We suggest a pre-existing immunity because of T-cells, primed against SARS-CoV-2-like antigens before the start of the pandemic. Studies of T-cell immune responses against SARS-CoV-2 are important in understanding the immune status of individuals or populations. Here, we use a simple, cheap, and rapid whole blood stimulation assay - an Interferon-Gamma Release Assay (IGRA) - to study T-cell immunity to SARS-CoV-2 in convalescent COVID-19 patients and in unexposed healthy contacts from Quito, Ecuador. Interferon-gamma (INF-γ) production was measured in the heparinized blood of convalescent and unexposed subjects after stimulation for 24 h with the SARS-CoV-2 Spike S1 protein, the Receptor Binding Domain (RBD) protein or the Nucleocapsid (NP) protein, respectively. The presence of IgG-RBD protein antibodies in both study groups was determined with an “in-house” ELISA. As measured with INF-γ production, 80% of the convalescent COVID-19 patients, all IgG-RBD seropositive, had a strong T-cell response. However, unexpectedly, 44% of unexposed healthy controls, all IgG-RBD seronegative, had a strong virus-specific T-cell response with the COVID-19 IGRA, probably because of prior exposure to common cold-causing coronaviruses or other viral or microbial antigens. The high percentage of unexposed healthy subjects with a pre-existing immunity suggests that a part of the Ecuadorian population is likely to have SARS-CoV-2 reactive T-cells. Given that the IGRA technique is simple and can be easily scaled up for investigations where high numbers of patients are needed, this COVID-19 IGRA may serve to determine if the T-cell only response represents protective immunity to SARS-CoV-2 infection in a population-based study.