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.