T cells play a fundamental role in the early control and clearance of many
viral infections of the respiratory system. In SARS-CoV-2-infected individuals,
lymphopenia with drastically reduced CD4+ and CD8+ T cells correlates with
Coronavirus disease 2019 (COVID-19)-associated disease severity and mortality. In this study, we characterized cellular and humoral immune responses induced in patients with mild, severe and critical
COVID-19. Peripheral blood mononuclear cells of 37 patients with mild, severe and critical
COVID-19 and 10 healthy individuals were analyzed by IFNγ ELISpot and multi-color flow cytometry upon stimulation with
peptide pools covering complete immunodominant SARS-CoV-2 matrix, nucleocapsid and spike
proteins. In addition SARS-CoV-2 antibody levels, neutralization abilities and
anaphylatoxin levels were evaluated by various commercially available ELISA platforms. Our data clearly demonstrates a significantly stronger induction of SARS-CoV-2 specific CD8+ T lymphocytes and higher IFNγ production in patients with mild compared to patients with severe or critical
COVID-19. In all patients SARS-CoV-2-specific
antibodies with similar neutralizing activity were detected, but highest titers of total IgGs were observed in critical patients. Finally, elevated
anaphylatoxin C3a and C5a levels were identified in severe and critical
COVID-19 patients probably caused by aberrant
immune complex formation due to elevated antibody titers in these patients. Crucially, we provide a full picture of cellular and humoral immune responses of
COVID-19 patients and prove that robust polyfunctional CD8+ T cell responses concomitant with low
anaphylatoxin levels correlate with mild
infections. In addition, our data indicates that high SARS-CoV-2 antibody titers are associated with severe
disease progression.