High titers and low fucosylation of early human anti-SARS-CoV-2 IgG promote inflammation by alveolar macrophages.

Patients diagnosed with coronavirus disease 2019 (COVID-19) become critically ill primarily around the time of activation of the adaptive immune response. Here, we provide evidence that antibodies play a role in the worsening of disease at the time of seroconversion. We show that early-phase severe acute respiratory distress syndrome coronavirus 2 (SARS-CoV-2) spike protein-specific immunoglobulin G (IgG) in serum of critically ill COVID-19 patients induces excessive inflammatory responses by human alveolar macrophages. We identified that this excessive inflammatory response is dependent on two antibody features that are specific for patients with severe COVID-19. First, inflammation is driven by high titers of anti-spike IgG, a hallmark of severe disease. Second, we found that anti-spike IgG from patients with severe COVID-19 is intrinsically more proinflammatory because of different glycosylation, particularly low fucosylation, of the antibody Fc tail. Low fucosylation of anti-spike IgG was normalized in a few weeks after initial infection with SARS-CoV-2, indicating that the increased antibody-dependent inflammation mainly occurs at the time of seroconversion. We identified Fcγ receptor (FcγR) IIa and FcγRIII as the two primary IgG receptors that are responsible for the induction of key COVID-19-associated cytokines such as interleukin-6 and tumor necrosis factor. In addition, we show that anti-spike IgG-activated human macrophages can subsequently break pulmonary endothelial barrier integrity and induce microvascular thrombosis in vitro. Last, we demonstrate that the inflammatory response induced by anti-spike IgG can be specifically counteracted by fostamatinib, an FDA- and EMA-approved therapeutic small-molecule inhibitor of Syk kinase.

Biological and structural characterization of murine TRALI antibody reveals increased Fc-mediated complement activation.

Transfusion-related acute lung injury (TRALI) remains a leading cause of transfusion-related deaths. In most cases, anti-leukocyte antibodies in the transfusion product trigger TRALI, but not all anti-leukocyte antibodies cause TRALI. It has been shown that the anti-major histocompatibility complex (MHC) class I antibody 34-1-2S (anti-H-2Kd) causes TRALI in BALB/c mice (MHC class I haplotype H-2Kd), whereas SF1.1.10 (anti-H-2Kd) does not. In C57BL/6 mice (MHC class I haplotype H-2Kb), TRALI only occurs when anti-MHC class I antibody AF6-88.5.5.3 (anti-H-2Kb) is administered together with a high dose of 34-1-2S. It remains unknown which specific antibody characteristics are responsible for eliciting TRALI. We therefore investigated several biological and structural features of 34-1-2S compared with other anti-MHC class I antibodies, which on their own do not cause TRALI: SF1.1.10 and AF6-88.5.5.3. No substantial differences were observed between the TRALI-causing 34-1-2S and the TRALI-resistant SF1.1.10 regarding binding affinity to H-2Kd. Regarding binding affinity to H-2Kb, only AF6-88.5.5.3 potently bound to H-2Kb, whereas 34-1-2S exhibited weak but significant cross-reactivity. Furthermore, the binding affinity to FcγRs as well as the Fc glycan composition seemed to be similar for all antibodies. Similar Fc glycosylation profiles were also observed for human TRALI-causing donor anti-HLA antibodies compared with human anti-HLA antibodies from control donors. 34-1-2S, however, displayed superior complement activation capacity, which was fully Fc dependent and not significantly dependent on Fc glycosylation. We conclude that TRALI induction is not correlated with Fab- and Fc-binding affinities for antigen and FcγRs, respectively, nor with the composition of Fc glycans; but increased Fc-mediated complement activation is correlated with TRALI induction.

Natural killer cell activation by respiratory syncytial virus-specific antibodies is decreased in infants with severe respiratory infections and correlates with Fc-glycosylation.

OBJECTIVES: Respiratory syncytial virus (RSV) is a major cause of severe lower respiratory tract infections in infants, and there is no vaccine available. In early life, the most important contributors to protection against infectious diseases are the innate immune response and maternal antibodies. However, antibody-mediated protection against RSV disease is incompletely understood, as both antibody levels and neutralisation capacity correlate poorly with protection. Since antibodies also mediate natural killer (NK) cell activation, we investigated whether this functionality correlates with RSV disease. METHODS: We performed an observational case-control study including infants hospitalised for RSV infection, hernia surgery or RSV-negative respiratory viral infections. We determined RSV antigen-specific antibody levels in plasma using a multiplex immunoassay. Subsequently, we measured the capacity of these antibodies to activate NK cells. Finally, we assessed Fc-glycosylation of the RSV-specific antibodies by mass spectrometry. RESULTS: We found that RSV-specific maternal antibodies activate NK cells in vitro. While concentrations of RSV-specific antibodies did not differ between cases and controls, antibodies from infants hospitalised for severe respiratory infections (RSV and/or other) induced significantly less NK cell interferon-γ production than those from uninfected controls. Furthermore, NK cell activation correlated with Fc-fucosylation of RSV-specific antibodies, but their glycosylation status did not significantly differ between cases and controls. CONCLUSION: Our results suggest that Fc-dependent antibody function and quality, exemplified by NK cell activation and glycosylation, contribute to protection against severe RSV disease and warrant further studies to evaluate the potential of using these properties to evaluate and improve the efficacy of novel vaccines.