Discriminating promiscuous from target-specific autoantibodies in COVID-19.

Diverse autoantibodies were suggested to contribute to severe outcomes of COVID-19, but their functional implications are largely unclear. ACE2, the SARS-CoV-2 receptor and a key regulator of blood pressure, was described to be one of many targets of autoantibodies in COVID-19. ACE2 in its soluble form (sACE2) is highly elevated in the blood of critically ill patients, raising the question of whether sACE2:spike complexes induce ACE2 reactivity. Screening 247 COVID-19 patients, we observed elevated sACE2 and anti-ACE2 IgG that were poorly correlated. Interestingly, levels of IgGs recognizing ACE2, IFNα2, and CD26 strongly correlated in severe COVID-19, with 15% of sera showing polyreactivity versus 4.1% exhibiting target-directed autoimmunity. Promiscuous autoantibodies failed to impair the activity of ACE2 and IFNα2, while only specific anti-IFNα2 IgG compromised cytokine function. Our study suggests that the detection of autoantibodies in COVID-19 is often attributed to a promiscuous reactivity, potentially misinterpreted as target-specific autoimmunity with functional impact.

Soluble ACE2 correlates with severe COVID-19 and can impair antibody responses.

Identifying immune modulators that impact neutralizing antibody responses against severe acute respiratory syndrome coronavirus type 2 (SARS-CoV-2) is of great relevance. We postulated that high serum concentrations of soluble angiotensin-converting enzyme 2 (sACE2) might mask the spike and interfere with antibody maturation toward the SARS-CoV-2-receptor-binding motif (RBM). We tested 717 longitudinal samples from 295 COVID-19 patients and showed a 2- to 10-fold increase of enzymatically active sACE2 (a-sACE2), with up to 1 µg/mL total sACE2 in moderate and severe patients. Fifty percent of COVID-19 sera inhibited ACE2 activity, in contrast to 1.3% of healthy donors and 4% of non-COVID-19 pneumonia patients. A mild inverse correlation of a-sACE2 with RBM-directed serum antibodies was observed. In silico, we show that sACE2 concentrations measured in COVID-19 sera can disrupt germinal center formation and inhibit timely production of high-affinity antibodies. We suggest that sACE2 is a biomarker for COVID-19 and that soluble receptors may contribute to immune suppression informing vaccine design.