Could the activation of FcRIII/CD16 by anti-spike SARS-CoV-2 antibodies independently determine COVID-19 severity?
A team of scientists from Germany recently investigated whether hyperactivation of immune receptors that recognize the Fc part of an IgG molecule (FcR) can lead to immunopathologic changes in coronavirus disease 2019 (COVID-19) patients. They have observed that hyperactivation of FcRIII (CD16) by proinflammatory IgG antibodies against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) can significantly contribute to COVID-19 related immunopathology. The study is currently available on the bioRxiv* preprint server. Background  Hyperinflammation is a hallmark of severe COVID-19. In critically ill COVID-19 patients, aggravation of symptoms often correlates with high levels of anti-SARS-CoV-2 IgG antibodies and proinflammatory cytokines, including IL-6 and TNF alpha. During viral infection, activation of FcRs by circulating soluble antigen-antibody complexes and subsequent release of cytokines is considered as a potent defense mechanism against invading viruses. However, hyperactivation of FcRs can lead to heightened inflammatory response, which in turn can contribute to immunopathology, as observed in critically ill COVID-19 patients. In this context, studies have shown that N297-dependent modification of glycans such as afucosylation within the constant region of anti-SARS-CoV-2 IgG antibodies can increase their binding to FcRs, which in turn can trigger hyperinflammatory responses. In the current study, the scientists have analyzed the correlation between antigen-specific IgG responses and the activation profile of immune receptor FcRIII (CD16) in hospitalized COVID-19 patients. They have investigated whether hyperactivation of CD16 is associated with COVID-19 severity. The Fc receptor CD16, which is expressed by natural killer cells, monocyte-derived macrophages, or neutrophils, recognizes circulatory soluble antigen-antibody complexes to mediate antibody-dependent cellular cytotoxicity and antibody-dependent cytokine secretion, which are vital cellular processes to combat invading pathogens. Study design  To determine the kinetics of anti-SARS-CoV-2 IgG antibodies, serum samples were collected from 41 hospitalized COVID-19 patients. Of all patients, 27 were critically ill with severe pneumonia requiring invasive mechanical ventilation, and 14 were severely ill with pneumonia. The most common comorbidity was hypertension, which was present in 85% of patients. The serum samples were collected from the patients at different time points within 6  25 days after symptom onset and analyzed for anti-spike and anti-nucleocapsid IgG antibodies. Anti-SARS-CoV-2 antibody kinetics  In most of the patients, anti-SARS-CoV-2 IgG antibodies were detected within 9  14 days after symptom onset, which gradually increased over time, reaching a constant level at 18  20 days. Specifically, seroconversion for anti-nucleocapsid antibodies occurred earlier than that for anti-spike antibodies. Regarding neutralizing antibodies, peak titers were observed in all patients at 18  20 days post symptom onset. Taken together, these observations indicate that there is no significant difference in antibody kinetics between severely ill and critically ill COVID-19 patients. Activation of CD16  Cell-based reporter system was utilized to examine at which extent anti-SARS-CoV-2 IgG antibodies impact CD16 activation. The findings revealed that both anti-spike and anti-receptor binding domain (RBD) antibodies caused a significantly increased activation of CD16 in critically ill patients compared to severely ill patients. However, no such difference was observed for anti-nucleocapsid antibody-mediated activation of CD16. Overall, in all critically ill patients, a sustained activation of CD16 by spike and RBD-specific antibodies was observed, indicating the involvement of CD16 in determining COVID-19 severity. For further evaluation, IgG core fucosylation (the process of adding fucose sugar to the antibody Fc region) of anti-spike and anti-nucleocapsid antibodies were examined in serum samples obtained from critically and severely ill patients. The findings reveal that compared to patients with a low CD16 activation profile, patients with high CD16 activity had significantly lower levels of core fucosylation. However, no difference in fucosylation level was observed between critically and severely ill patients. Overall, these observations indicate that lower fucosylation of anti-spike and anti-nucleocapsid antibodies is associated with enhanced CD16 activation in patients with critical or severe COVID-19. Soluble immune complexes and COVID-19 severity  The quantification of CD16 activation by IgG-specific soluble immune complexes was carried out by a cell-based reporter assay. The findings revealed significantly higher levels of CD16-reactive soluble immune complexes in critically ill patients compared to that in severely ill patients. To further validate whether soluble immune complexes represent the CD16-reactive component in the serum, a series of experiments were conducted, which collectively revealed that the serum levels of CD-16-reactive immune complexes independently determine COVID-19 severity. Study significance  The study reveals that activation of CD16 by soluble immune complexes strongly correlates with COVID-19 severity. Based on the study findings, the scientists suggest that saturation of Fc receptors, including CD16 by intravenous administration of immunoglobulins, can be effective in mitigating COVID-19 related immunopathology. *Important notice  bioRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.