Low avidity T cell responses to SARS-CoV-2 in unexposed individuals and severe COVID-19

Pre-existing memory T-cells against SARS-CoV-2 are present in a fraction of unexposed individuals and their induction by common cold corona viruses (CCCoVs) infection is suggested. Here we demonstrate that SARS-CoV-2-reactive T-cells were present in the memory compartment of virtually all unexposed individuals but possessed only low functional avidity. They harbored multiple, highly variable cross-reactivities that were not restricted to CCCoVs. Cross-reactivity to CCCoV was almost absent in COVID-19 patients. This was irrespective of strong T-cell memory against CCCoV in all donors. In severe but not mild COVID-19, SARS-CoV-2-specific T-cells also displayed low functional avidity and reduced clonal expansion, despite strongly increased frequencies. Our data question a major protective role of CCCoV for COVID-19. Instead, we suggest that a low avidity pre-existing T-cell memory may contribute to the excessive but low avidity T-cell responses, which we identified here as a hallmark of severe COVID-19. Overall design: Single-cell sequencing of 6 unexposed and 14 COVID-19 patients