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

ObjectivesMonitoring of SARS-CoV-2 spread and vaccination strategies have relied on antibody (Ab) status as a correlate of protection. We used QuantiFERON (TM) (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). MethodsTwenty-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-gamma) 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. ResultsIn convalescents, substantial agreement was observed between QFN and AIM assays. IFN-gamma 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. ConclusionsAlbeit 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)).

Automated summary

This study used QuantiFERON (TM) (QFN) and Activation-Induced Marker (AIM) assays to measure memory T-cell reactivity in unvaccinated individuals with prior symptomatic infection and fully vaccinated asymptomatic donors. The results showed that there was substantial agreement between QFN and AIM assays in convalescents. The frequency of AIM(+) CD4(+) T-cells correlated with IFN-gamma concentrations, antibody levels, and AIM(+) CD8(+) T-cell frequencies.