Assessing SARS-CoV-2-specific T-cell reactivity in late convalescents and vaccinees: Comparison and combination of QuantiFERON and activation-induced marker assays, and relation with antibody status

OBJECTIVES: Monitoring of SARS-CoV-2 spread and vaccination strategies have relied on antibody (Ab) status as a correlate of protection. We used QuantiFERON™ (QFN) and Activation-Induced Marker (AIM) assays to measure memory T-cell reactivity in unvaccinated individuals with prior documented symptomatic infection (late convalescents) and fully vaccinated asymptomatic donors (vaccinees). METHODS: Twenty-two convalescents and 13 vaccinees were enrolled. Serum anti-SARS-CoV-2 S1 and N Abs were measured using chemiluminescent immunoassays. QFN was performed following instructions and interferon-gamma (IFN-γ) measured by ELISA. AIM was performed on aliquots of antigen-stimulated samples from QFN tubes. SARS-CoV-2-specific memory CD4(+)CD25(+)CD134(+), CD4(+)CD69(+)CD137(+) and CD8(+)CD69(+)CD137(+) T-cell frequencies were measured by flow cytometry. RESULTS: In convalescents, substantial agreement was observed between QFN and AIM assays. IFN-γ concentrations and AIM(+) (CD69(+)CD137(+)) CD4(+) T-cell frequencies correlated with each other, with Ab levels and AIM(+) CD8(+) T-cell frequencies, whereas AIM(+) (CD25(+)CD134(+)) CD4(+) T-cell frequencies correlated with age. AIM(+) CD4(+) T-cell frequencies increased with time since infection, whereas AIM(+) CD8(+) T-cell expansion was greater after recent reinfection. QFN-reactivity and anti-S1 titers were lower, whereas anti-N titers were higher, and no statistical difference in AIM-reactivity and Ab positivity emerged compared to vaccinees. CONCLUSIONS: Albeit on a limited sample size, we confirm that coordinated, cellular and humoral responses are detectable in convalescents up to 2 years after prior infection. Combining QFN with AIM may enhance detection of naturally acquired memory responses and help stratify virus-exposed individuals in T helper 1-type (T(H)1)-reactive (QFN(pos) AIM(pos) Abs(high)), non-T(H)1-reactive (QFN(neg) AIM(pos) Abs(high/low)), and pauci-reactive (QFN(neg) AIM(neg) Abs(low)).