Wen, Renren

Abstract Thromboembolic complication is common in severe coronavirus disease 2019 (COVID-19), leading to an investigation into the presence of prothrombotic antibodies akin to those found in heparin-induced thrombocytopenia (HIT). In a study of samples from 130 hospitalized patients, collected 3.6 days after COVID-19 diagnosis, 80% had immunoglobulin G (IgG) antibodies recognizing complexes of heparin and platelet factor 4 (PF4; PF4/H), and 41% had antibodies inducing PF4-dependent P-selectin expression in CpG oligodeoxynucleotide–treated normal platelets. Unlike HIT, both PF4/H-reactive and platelet-activating antibodies were found in patients with COVID-19 regardless of recent heparin exposure. Notably, PF4/H-reactive IgG antibodies correlated with those targeting the receptor-binding domain (RBD) of the severe acute respiratory syndrome coronavirus 2 spike protein. Moreover, introducing exogenous RBD to or removing RBD-reactive IgG from COVID-19 plasma or IgG purified from COVID-19 plasma significantly reduced their ability to activate platelets. RBD-specific antibodies capable of platelet activation were cloned from peripheral blood B cells of patients with COVID-19. These antibodies possessed sequence motifs in the heavy-chain complementarity-determining region 3 (HCDR3), resembling those identified in pathogenic HIT antibodies. Furthermore, IgG+ B cells having these HCDR3 signatures were markedly expanded in patients with severe COVID-19. Importantly, platelet-activating antibodies present in patients with COVID-19 were associated with a specific elevation of platelet α-granule proteins in the plasma and showed a positive correlation with markers for inflammation and tissue damage, suggesting a functionality of these antibodies in patients. The demonstration of functional and structural similarities between certain RBD-specific antibodies in patients with COVID-19 and pathogenic antibodies typical of HIT suggests a novel mechanism by which RBD-specific antibodies might contribute to thrombosis in COVID-19.

AbstractHeparin-induced thrombocytopenia (HIT) is a serious adverse drug reaction characterized by antibodies that recognize platelet factor 4/heparin complexes (PF4/H) and activate platelets to create a prothrombotic state. Although a high percentage of heparin-treated patients produce antibodies to PF4/H, only a subset also makes antibodies that are platelet activating (PA). A close correlation between PA antibodies and the likelihood of experiencing HIT has been demonstrated in clinical studies, but how PA (presumptively pathogenic) and nonactivating (NA) (presumptively benign) antibodies differ from each other at the molecular level is unknown. To address this issue, we cloned 7 PA and 47 NA PF4/H-binding antibodies from 6 patients with HIT and characterized their structural and functional properties. Findings showed that PA clones differed significantly from NA clones in possessing 1 of 2 heavy chain complementarity-determining region 3 (HCDR3) motifs, RX1-2R/KX1-2R/H (RKH) and YYYYY (Y5), in an unusually long complementarity-determining region 3 (≥20 residues). Mutagenic studies showed that modification of either motif in PA clones reduced or abolished their PA activity and that appropriate amino acid substitutions in HCDR3 of NA clones can cause them to become PA. Repertoire sequencing showed that the frequency of peripheral blood IgG+ B cells possessing RKH or Y5 was significantly higher in patients with HIT than in patients without HIT given heparin, indicating expansion of B cells possessing RKH or Y5 in HIT. These findings imply that antibodies possessing RKH or Y5 are relevant to HIT pathogenesis and suggest new approaches to diagnosis and treatment of this condition.

Kanack and colleagues analyze anti-platelet factor 4 antibodies from 5 patients with vaccine-induced thrombotic thrombocytopenia (VITT) secondary to COVID-19 adenoviral vaccination and antibodies from patients with spontaneous heparin-induced thrombocytopenia (HIT) and classical HIT. VITT antibodies are monoclonal or oligoclonal, similar to spontaneous HIT, whereas classical HIT antibodies are polyclonal. Heparin inhibits antibody-induced platelet activation in VITT, suggesting that heparin should be considered for the treatment of VITT.