Whole blood-based cytokine release assay identifies IL-2 as a biomarker for rapid determination of vaccine-induced SARS-CoV-2-specific T cell immune responses

ABSTRACT

Background: A simple, accurate and rapid whole blood-based T cell test was previously developed to determine SARS-CoV-2-specific T cell immunity. The test was established by comparing cytokine production from naturally infected convalescent donors with covid-19 negative donors. The data revealed IL-2 production to be the most indicative of prior SARS-CoV-2 infection. However, accurately identifying vaccine-induced SARS-CoV-2-specific T cell immunity via this method was still to be confirmed. Herein, we sort to address if this was possible. Method: A cohort of unvaccinated healthy individuals was recruited to donate a single blood sample for an overnight in vitro stimulation with peptides spanning immunodominant regions specific for SARS-CoV-2. Blood plasma samples were harvested and analysed for a broad panel of cytokines using ELISA for IFN-g and Luminex xMAP cytokine arrays for IL-2 and other TH1/TH2 cytokines. The same cohort were then asked to donate a second blood sample following SARS-CoV-2 vaccinations, and the same stimulations and analyses were performed. In addition, plasma anti-SARS-CoV-2 IgG levels were assessed in both pre-and post-vaccination samples by direct ELISA against the whole spike protein. Results: A multiplex cytokine array revealed IL-2 to be the most reliable biomarker in indicating a vaccine-induced SARS-CoV-2-specific T cell response, with 100% of post-vaccinated donors mounting a significant IL-2 response above a pre-determined cut off level for positivity of 19.91pg/ml. All donors demonstrated a considerable increase in magnitude of IL-2 responses from pre-vaccination to post-vaccination, with results ranging from ∼125% change to >36,000% change. In addition, IFN-g and plasma IgG ELISAs revealed both to be reliable biomarkers, with post-vaccination levels of each being significantly raised above pre-vaccination levels. However, the magnitude of these responses was not as greatly increased as those observed with IL-2, nor did they achieve an increase in 100% of donors assessed. Conclusion: This standardisable, rapid, and accurate T cell test approach can be utilised to make accurate and comparable assessments of vaccine-induced T cell immunity across multiple population cohorts. This could provide valuable insight into the extremely important question of how long vaccine-induced immunity may last, and aid decision making around if and when vaccine boosters should be administered.