Comparison of T-cell immune responses to SARS-CoV-2 spike (S) and nucleocapsid (N) protein using an in-house flow-cytometric assay in laboratory employees with and without previously confirmed COVID-19 in South Africa: nationwide cross-sectional study

Abstract

Aims: To compare specific T-cell responses between laboratory employees in South Africa with and without previously diagnosed SARS-CoV-2 infection.

Methods: Employees at a private pathology laboratory in South Africa were invited to participate in a nationwide cross-sectional study. T-cell proliferation to SARS-CoV-2 nucleocapsid (N)-proteins and spike (S)-proteins was measured by flow cytometry and compared between participants.

Results: Based on classification according to SARS-CoV-2 reverse transcription (RT)-PCR results, a total of 81% (42/52) of positive participants demonstrated T-cell proliferation to SARS-CoV-2 N-proteins or S-proteins (95% CI 67.5% to 90.4%), while 62% (68/110) of negative participants also had detectable T-cell responses to SARS-CoV-2 proteins (95% CI 52.1% to 70.9%). When classified according to SARS-CoV-2 serology results, 92.6% (50/54) of positive participants demonstrated T-cell proliferation to SARS-CoV-2 proteins (95% CI 82.1 to 97,9 %), while 56% (60/108) of negative participants demonstrated T-cell proliferation (95% CI 45.7% to 65.1%). The magnitude of the T-cell responses as determined by a stimulation index, was significantly higher in the group previously infected by SARS-CoV-2 than in the negative group. A statistically significant difference in T-cell proliferation was noted between high risk and low risk groups for exposure to SARS-CoV-2 within the negative group, but no significant difference in magnitude of the response.

Conclusions: A significant proportion of South African laboratory employees who were not previously diagnosed with COVID-19 demonstrated T-cell reactivity to SARS-CoV-2 N-proteins and S-proteins. The pre-existing T-cell proliferation responses may be attributable to cross-reactive immune responses to other human coronaviruses, or possibly asymptomatic infection.

Keywords: COVID-19; diagnostic techniques and procedures; immunology; virology.