IgG antibodies are crucial for protection against invading pathogens. A highly conserved N-linked glycan within the IgG-Fc tail, essential for IgG function, shows variable composition in humans. Afucosylated IgG variants are already used in anti-cancer therapeutic antibodies for their elevated activity through Fc receptors (FcγRIIIa). Here, we report that afucosylated IgG (~6% of total IgG in humans) are specifically formed against enveloped viruses but generally not against other antigens. This mediates stronger FcγRIIIa responses, but also amplifies brewing cytokine storms and immune-mediated pathologies. Critically ill COVID-19 patients, but not those with mild symptoms, had high levels of afucosylated IgG antibodies against SARS-CoV-2, amplifying pro-inflammatory cytokine release and acute phase responses. Thus, antibody glycosylation plays a critical role in immune responses to enveloped viruses, including COVID-19.

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 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 pro-inflammatory because of different glycosylation, particularly low fucosylation, of the antibody Fc tail. Notably, 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. Finally, 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.

Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP1) family members mediate receptor- and tissue-specific sequestration of infected erythrocytes (IEs) in malaria. Antibody responses are a central component of naturally acquired malaria immunity. PfEMP1-specific IgG likely protects by inhibiting IE sequestration and through IgG-Fc Receptor (FcγR) mediated phagocytosis and killing of antibody-opsonized IEs. The affinity of afucosylated IgG to FcγRIIIa is up to 40-fold higher than fucosylated IgG, resulting in enhanced antibody-dependent cellular cytotoxicity. Most IgG in plasma is fully fucosylated, but afucosylated IgG is elicited in response to enveloped viruses and to paternal alloantigens during pregnancy. Here we show that naturally acquired PfEMP1-specific IgG is strongly afucosylated in a stable and exposure-dependent manner, and efficiently induces FcγRIIIa-dependent natural killer (NK) cell degranulation. In contrast, immunization with a subunit PfEMP1 (VAR2CSA) vaccine results in fully fucosylated specific IgG. These results have implications for understanding protective natural- and vaccine-induced immunity to malaria.

For yet unknown reasons, severely ill COVID-19 patients often become critically ill around the time of activation of adaptive immunity. Here, we show that anti-Spike IgG from serum of severely ill COVID-19 patients induces a hyper-inflammatory response by human macrophages, which subsequently breaks pulmonary endothelial barrier integrity and induces microvascular thrombosis. The excessive inflammatory capacity of this anti-Spike IgG is related to glycosylation changes in the IgG Fc tail. Moreover, the hyper-inflammatory response induced by anti-Spike IgG can be specifically counteracted in vitro by use of the active component of fostamatinib, an FDA- and EMA-approved therapeutic small molecule inhibitor of Syk.One sentence summaryAnti-Spike IgG promotes hyper-inflammation.

27IgG antibodies are crucial for protection against invading pathogens. A highly conserved N-28 linked glycan within the IgG-Fc-tail, essential for IgG function, shows variable composition in 29 humans. Afucosylated IgG variants are already used in anti-cancer therapeutic antibodies for 30 their elevated binding and killing activity through Fc receptors (FcγRIIIa). Here, we report that 31 afucosylated IgG which are of minor abundance in humans (~6% of total IgG) are specifically 32 formed against surface epitopes of enveloped viruses after natural infections or 33 immunization with attenuated viruses, while protein subunit immunization does not elicit 34 this low fucose response. This can give beneficial strong responses, but can also go awry, 35 resulting in a cytokine-storm and immune-mediated pathologies. In the case of COVID-19, 36 the critically ill show aggravated afucosylated-IgG responses against the viral spike protein. In 37 contrast, those clearing the infection unaided show higher fucosylation levels of the anti-38 spike protein IgG. Our findings indicate antibody glycosylation as a potential factor in 39 inflammation and protection in enveloped virus infections including COVID-19.40Main Text: 41 Antibodies have long been considered functionally static, mostly determined by their isotype 42 and subclass. The presence of a conserved N-linked glycan at position 297, in the so called 43 constant Fc-domain of IgG, is essential for effector functions (1-3). Moreover, it is now 44 generally accepted that the composition of this glycan is highly variable and has functional 45 consequences (2-4). This is especially true for the core fucose attached to the Fc glycan. The 46 discovery that IgG variants without core fucosylation cause elevated antibody dependent 47 cellular cytotoxicity (ADCC), via increased IgG-Fc-receptor IIIa (FcγRIIIa) affinity (5, 6), 48 resulted in next-generation glyco-engineered monoclonal antibodies (mAb) without core 49 fucosylation for targeting tumors (7). 50Generally, changes in the Fc glycans are associated with age, sex and autoimmune diseases 51 (8). Serum IgG are highly fucosylated at birth and slightly decrease to ~94% fucosylation at 52 adulthood (9). Until now, no strong clues on how IgG core fucosylation is controlled have 53 come forward. 54 We have previously observed that alloantibodies against red blood cells (RBC) and platelets 55 show remarkably low IgG-Fc-fucosylation in most patients, even down to 10% in several 56 cases (10-12), whereas the overall serum IgG Fc-fucosylation show consistently normal high 57 levels. Moreover, we have reported the lowered IgG-Fc fucosylation to be one of the factors 58 determining disease severity in pregnancy associated alloimmunizations, resulting in 59 excessive thrombocytopenia's and blood cell destruction when targeted by afucosylated 60 antibodies (11)(12)(13). In addition to the specific afucosylated-IgG response against platelets 61 and RBC antigens, this response has also been identified against HIV and Dengue virus (...

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Clinical immunity to malaria is associated with the acquisition of IgG specific for members of the Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP1) family of clonally variant antigens on the surface of infected erythrocytes (IEs). The VAR2CSA subtype of PfEMP1 mediates IE binding in the placenta.

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