Antiviral T-cell frequencies in a healthy population: Reference values for evaluating antiviral immune cell profiles in immunocompromised patients and their application during the SARS-CoV-2 pandemic

ABSTRACT

Background: Viral infections and reactivations (e.g. cytomegalovirus (CMV)) are a major cause of morbidity and mortality after hematopoietic stem cell transplantation (HSCT) and solid organ transplantation (SOT) and in patients with immunodeficiencies. Here, antiviral drugs are the mainstay of treatment, but they have side effects and cannot achieve complete viral clearance without prior reconstitution of functional virus-specific T cells (VSTs). Method(s) We performed serological testing and measured VST frequencies against 23 viral protein-derived peptide pools of 11 clinically relevant human viruses by Interferon-gamma (IFN-gamma) ELISpot assay in a cohort of healthy donors (n=151). Moreover, we performed in-depth immune profiling in patients actively infected with SARS-CoV-2 (n=92) and in unvaccinated, recovered COVID-19 patients (n=204) by ELISA, ELISpot, multicolor flow cytometry and multiplex analysis. Result(s) Based on serological testing, IFN-gamma ELISpot results, age and sex, we established normal ranges for VST frequencies in healthy donors for better interpretation of VST frequencies observed in immunocompromised patients. While in SARS-CoV-2 recovered patients, the antiviral immune response was characterized by a broad specificity, significantly lower T-cell responses were observed during active infection. Comparison with the previously established reference values for VST frequencies revealed an overall reduced T-cell functionality based on the lack of CMV-pp65-reactive T cells in CMV-seropositive COVID-19 patients, which was associated with an inflammatory milieu, expression of inhibitory molecules, and effector caspase activity in T cells. Conclusion(s) The established reference values are an invaluable tool for immune response assessment, therapeutic agent intensity and decision making in immunocompromised patients. Further, we provide evidence that the low T-cell response observed during SARS-CoV-2 infection is not exclusively due to lymphopenia, but rather due to checkpoint- and cell death-associated mechanisms, suggesting that these patients may benefit from SARS-CoV-2-specific T-cell therapy.