Disrupting HIT immune complexes as a new treatment for HIT Free

In this issue of Blood,Rauova et al¹ show that universal heparin reversal agents (UHRAs)² inhibit ultralarge immune complex (ULIC) formation, dissociate preformed complexes, and suppress thrombosis in heparin-induced thrombocytopenia (HIT).

HIT is a serious immune reaction to heparins.^3,4 It is mediated by an autoantibody that reacts with its antigen, platelet factor 4 (PF4)/heparin,^3,4 or PF4/glycosaminoglycan (GAG)⁵ complex, to form ULICs.⁶ ULICs cause thrombocytopenia and initiate a chain of multicellular prothrombotic processes, including platelet activation and aggregation,^4-7 neutrophil activation and NETosis,⁷ and blood coagulation activation and thrombin generation,⁴ and these result in severe thrombosis (see figure). If ULICs are eliminated, thrombosis and thrombocytopenia will not occur.

Although heparin use has declined, it is still needed, particularly for cardiovascular surgery. Patients with HIT still experience recurrent thrombosis and disastrous consequences, such as limb gangrene (3%-11%) and deaths (10%-18%)^4,8 (see figure). Anticoagulant treatment for HIT additionally causes or exacerbates bleeding,⁸ even resulting in fatal hemorrhage. Therefore, current treatments are only partially effective and may cause lethal complications. There is an unmet clinical need for more effective and safer treatments.

Anticoagulant treatments are inadequate because they target only blood coagulation and not the overall prothrombotic processes (ie, platelet, neutrophil, monocyte, blood coagulation activations, and others)^5,7 or their trigger (ie, ULICs). In this study, Rauova et al posed the following key questions: (1) Will UHRAs inhibit formation of ULICs, disintegrate these immune complexes, suppress the HIT prothrombotic processes, and, as a consequence, block thrombus formation and progression in HIT? (2) If the answers are yes, what are the mechanisms? (3) Will UHRAs prevent thrombocytopenia in HIT? The authors performed a series of cleverly designed, elegant experiments to address these questions.

Their experiments showed UHRAs inhibited formation of PF4/heparin and PF4/GAG complexes, and ULICs, and, if already formed, induced their dissociation.¹ Using enzyme-linked immunosorbent assay, the authors demonstrated that UHRAs prevented binding of HIT patient antibodies to heparin/PF4 or PF4/GAG complexes, including complexes on platelets. UHRAs also blocked platelet activation and aggregation. The authors further showed that UHRAs effectively suppressed HIT antibody-induced platelet adhesion to the endothelium ex vivo using Bioflux microfluidic (blood flow) devices.^1,9 

Importantly, the investigators found that UHRAs not only inhibited HIT antibody-induced prothrombotic processes in vitro and ex vivo, but also in vivo using a murine HIT model.¹⁰ In this model, HIT antibody injected into the mice interacted with antigenic complexes of PF4 and endothelium-bound GAG, to form ULICs at a laser-induced vascular injury site. This led to thrombus formation. When UHRA was injected into the mice, it dissociated the HIT antibody and PF4 from the thrombus.¹ Subsequent additional HIT antibody injection led to secondary thrombus growth. This was prevented by coadministration of UHRA and HIT antibody. Altogether, the results indicate that UHRA can prevent thrombus formation and its propagation.¹ 

HIT antibody injection into the mice also resulted in thrombocytopenia. Coadministration of UHRA and the antibody showed a trend toward reduced severity of thrombocytopenia with UHRA-7 and complete prevention of thrombocytopenia development with UHRA-10.

The findings of this research have clinical significance and therapeutic implication. The research showed that UHRAs not only suppressed thrombus formation but also prevented progression of preformed thrombus.¹ The latter is clinically relevant in treatment of HIT as thrombi are usually present at diagnosis. Importantly, the study also provided functional mechanisms whereby the cationic molecules inhibited thrombosis. In addition, the research showed that UHRAs were able to prevent thrombocytopenia development.⁴ 

It is known that PF4/heparin or PF4/GAG antigenic complex is formed by ionic interaction between cationic PF4 and polyanionic heparin^1,5 or GAG.⁶ It seems obvious that polycationic or polyanionic molecules can break up the antigenic complex. Yet, no published studies have shown this. The authors’ findings are clearly novel and have significantly advanced the field in identifying a robust therapeutic target for treatment of HIT.

UHRA is attractive as a drug because it lacks anticoagulant activity and, therefore, is unlikely to cause or enhance existing bleeding. Current nonheparin anticoagulant treatments are insufficiently efficacious. Furthermore, they cause or exacerbate bleeding,^3,8 particularly in patients with HIT with severe thrombocytopenia or significant renal or liver impairment. Thus, UHRA has the potential to be developed as a new, more efficacious, and safer drug for the treatment of HIT. If so, it will improve the clinical outcomes of patients with HIT, will prevent recurrent thrombosis, and, importantly, will prevent disastrous consequences, such as limb gangrene, serious bleeding, and death.⁸ It may fulfill the unmet clinical need in HIT treatment.

However, this study has some limitations. It has not investigated other important prothrombotic mechanisms and roles of other cells other than platelets, whereby UHRA may inhibit thrombosis. These include cells such as neutrophils, monocytes, and endothelial cells,^4,5,7 and prothrombotic processes, including NETosis^4,7 and monocyte etosis. Neutrophil activation and NETosis have been identified as the major drivers of thrombosis in HIT.⁷ Certainly, it is understandable that all aspects of the field were not fully investigated for this report.

Regarding future directions, publication of this study will prompt other investigators to study the effects of UHRA on thrombosis mediated by neutrophils and other cells as well as NETosis and other prothrombotic processes. Additionally, we may expect the publication of similar studies using additional polycations beyond UHRA, and polyanions. The latter are as effective as polycations in disrupting ULIC. Polyanions have the additional advantage of having the ability of neutralizing prothrombotic/tissue toxic effects of positively charged histones, released during NETosis. They may possibly be more efficacious in the treatment of HIT.

In summary, UHRAs dissociate PF4/heparin or PF4/GAG antigenic complexes, thus blocking HIT antibody-antigen interaction, inhibiting formation of HIT immune complexes, and consequently reducing thrombosis. In a murine model of HIT, UHRAs suppress thrombosis and prevent the development of thrombocytopenia. UHRA is a good potential candidate for the development of a new efficacious treatment for HIT.

Conflict-of-interest disclosure: The author declares no competing financial interests.