ABSTRACT
A dysregulated immune response with high levels of SARS-CoV-2 specific IgG antibodies is a common and distinctive feature of severe or critical COVID-19. Although a robust IgG response is typically considered to be beneficial, an overshooting activation mediated by immune receptors recognizing the Fc part of IgG (Fc{gamma}Rs) is thought to be detrimental and associated with immunopathology. However, direct evidence of Fc{gamma}R driven immunopathology in COVID-19 is still sparse. Here, we used a cell-based Fc{gamma}R activation reporter system to systematically analyze SARS-CoV-2 specific IgG responses and IgG-mediated Fc{gamma}RIII (CD16) activation profiles in COVID-19 patient cohorts categorized by severity of disease. We found that increased CD16 activation by SARS-CoV-2 specific IgG is associated with a known pro-inflammatory IgG modification, namely afucosylation. Further, we identified CD16-reactive soluble IgG immune complexes (sICs) to be present in the serum of COVID-19 patients and show that the resulting CD16 activation by sICs is strongly related to disease severity. Our results provide evidence that CD16 activation by pro-inflammatory SARS-CoV-2 specific IgG together with circulating sICs is a major contributor to COVID-19 immunopathology. These findings highlight the importance of Fc{gamma}R driven immunopathology in COVID-19. Further, our data highly warrant the development of targeted intervention strategies against IgG driven immunopathology following SARS-CoV-2 infection.
Subject(s)
COVID-19ABSTRACT
As the SARS-CoV-2 pandemic is still ongoing and dramatically influences our life, the need for recombinant proteins for diagnostics, vaccine development, and research is very high. The spike (S) protein, and particularly its receptor binding domain (RBD), mediates the interaction with the ACE2 receptor on host cells and may be modulated by its structural features. Therefore, well characterized recombinant RBDs are essential. We have performed an in-depth structural and functional characterization of RBDs expressed in Chinese hamster ovary (CHO) and human embryonic kidney (HEK293) cells. To structurally characterize the native RBDs (comprising N- and O-glycans and additional posttranslational modifications) a multilevel mass spectrometric approach was employed. Released glycan and glycopeptide analysis were integrated with intact mass analysis, glycan-enzymatic dissection and top-down sequencing for comprehensive annotation of RBD proteoforms. The data showed distinct glycosylation for CHO- and HEK293-RBD with the latter exhibiting antenna fucosylation, higher level of sialylation and a combination of core 1 and core 2 type O-glycans. Additionally, from both putative O-glycosylation sites, we could confirm that O-glycosylation was exclusively present at T323, which was previously unknown. For both RBDs, the binding to SARS-CoV-2 antibodies of positive patients and affinity to ACE2 receptor was addressed showing comparable results. This work not only offers insights into RBD structural and functional features but also provides a workflow for characterization of new RBDs and batch-to-batch comparison.