A neoantigenic determinant in the D-dimer fragment of fibrin

Fibrin Strands Will Grow from Soluble Fibrin and Hang Up in an In Vitro Microcirculatory Viscoelastic Model: Is This a Major Cause of COVID-19 Associated Coagulopathy?

Viscoelastic testing (VET) by both TEG and ROTEM has demonstrated hypercoagulability early in corona virus disease 2019 (COVID-19) associated coagulopathy (CAC). Additional VET studies demonstrated fibrinolytic shutdown late in a majority of severely ill COVID-19 patients with an associated elevation of d-dimer. Elevated d-dimer confirms that coagulation, followed by fibrinolysis, has occurred. These findings imply that, during CAC, three enzymes—thrombin, Factor XIIIa and plasmin—must have acted in sequence. However, limitations in standard VET analyses preclude exploration of the earliest phases of clot induction, as well as clot formation and clot dissolution in flowing blood. Herein, we describe a novel method illuminating aspects of this unexplored area. In addition, we created an in vitro blood flow model in which the interactions of thrombin, Factor XIII and plasmin with fibrinogen can be studied, allowing the determination of soluble fibrin (SF), the highly unstable form of fibrin that precedes the appearance of a visible clot. This model allows the determination of the SF level at which fibrin microclots begin to form.

[Inventive activity of the Departments of Protein Structure and Function, and Molecular Immunology of the Palladin Institute of Biochemistry of NAS of Ukraine. Part II. National breakthrough in the study and diagnostics of human hemostasis system]

The scientists of Protein Structure and Function, and Molecular Immunology Departments of the Palladin Institute of Biochemistry (NAS of Ukraine) under the supervision of member of NASU and NAMSU, prof. S. V. Komisarenko and corresponding member of NASU prof. E. V. Lugovskoy have made the real breakthrough in the field of research of the mechanisms of fibrin polymerization and formation of fibrin framework of thrombi. The immunodiagnostic test-systems for the evaluation of the risk of thrombus formation were developed for the first time. Researches have obtained the monoclonal antibodies to fibrinogen, fibrin, D-dimer and their fragments. These monoclonal antibodies were used as molecular probes for the localization of newly detected fibrin polymerization sites. Obtained antibodies with high affinity interact with fibrinogen, D-dimer and soluble fibrin – main markers of the risk of thrombus formation. They were used for the development of the immunodiagnostic test-systems to quantify these markers in human blood plasma for the evaluation of the state of haemostasis system, detection of prethrombotic states, disseminated intravascular coagulation, detection of thrombosis and monitoring of antithrombotic and fibrinolytic therapy. The successful trial of developed test-systems was carried out in clinics of Ukraine, and the State registration was obtained for the implementation of them into the clinical practice. Presented works were awarded State prize of Ukraine in Science and technology.

A neoantigenic determinant in coiled coil region of human fibrin beta-chain

INTRODUCTION

The transformation of fibrinogen into fibrin by thrombin exposes neoantigenic determinants that are buried in fibrinogen. Fibrin-specific monoclonal antibodies (mAbs) to these neoantigenic determinants, which don't react with fibrinogen, can be obtained. The aim of our investigation was to obtain fibrin-specific mAbs, to study their influence on fibrin polymerization and to use them for quantification of soluble fibrin in human blood plasma.

MATERIALS AND METHODS

Human fibrin desAABB in 2 M urea was used as an antigen. Standard hybridoma technique was used for production of mAbs. Turbidity analysis and transmission electron microscopy were used to study the effect of mAbs and their Fab-fragment on fibrin polymerization. The localization of epitope for mAb in fibrin molecule was determined using ELISA and immunoblot analysis with fibrinogen, fibrin desAA, fibrin desAABB and various fibrin(ogen) fragments.

RESULTS

A mAb FnI-3C has been obtained that doesn't bind to fibrinogen and reacts with fibrin desAA and fibrin desAABB with K(D) value of 9.7610(-10) M. The epitope for this mAb proved to be localized in the fibrin fragment Bbeta118-134. MAb FnI-3C and its Fab retarded specifically the stage of fibrin protofibril lateral association.

CONCLUSIONS

A fibrin-specific mAb FnI-3C has been obtained to fibrin fragment Bbeta118-134 which may be a contact site taking a part in protofibril lateral association. MAb FnI-3C was used as a "catch"-one in double-sandwich ELISA for soluble fibrin quantification in human blood plasma.

Novel monoclonal antibody that recognizes new neoantigenic determinant of D-dimer

Our novel monoclonal antibody (mAb) B4 reacted with only D-dimer but not intact fibrinogen, or fibrinogen degradation products (FgDP) such as D-monomer, E fragment on ELISA. B4 didn't react with denatured D-dimer, while it reacted well with denatured D-monomer rather than the native form, indicating that B4 recognizes some neoconformational epitope in D-dimer. In our epitope study, B4 recognized the N-terminal (Bbeta134-142) of D-dimer, which corresponds to the most flexible segment of coiled coil backbone. It was confirmed by inhibition assay of B4 binding to D-dimer using the synthesized peptides with this sequence. As the other evidence, B4 didn't bind to some D-dimer species produced from a particular fibrinogen variant. This fibrinogen variant is mutated BbetaLys133 residue to Gln133 thus it doesn't produce the particular N-terminal epitope of D134 approximately by plasmin. Finally, our mAb was useful for clinical application. ELISA using our mAbs was well correlated with other commercial D-dimer ELISAs and in some clinical samples it was preferable to them. These results suggest that the epitope for B4 is another neoantigenic determinant in native D-dimer as distinct from native D-monomer.

d-Dimer as marker for microcirculatory failure: correlation with LOD and APACHE II scores

The relevance of plasma d-dimer levels as marker for morbidity and organ dysfunction in severely ill patients is largely unknown. In a prospective study we determined d-dimer plasma levels of 800 unselected patients at admission to our intensive care unit. In 91% of the patients' samples d-dimer levels were elevated, in some patients up to several hundredfold as compared to normal values. The highest mean d-dimer values were present in the patient group with thromboembolic diseases, and particularly in non-survivors of pulmonary embolism. In patients with circulatory impairment (r=0.794) and in patients with infections (r=0.487) a statistically significant correlation was present between d-dimer levels and the APACHE II score (P<0.001). The logistic organ dysfunction score (LOD, P<0.001) correlated with d-dimer levels only in patients with circulatory impairment (r=0.474). On the contrary, patients without circulatory impairment demonstrated no correlation of d-dimer levels to the APACHE II or LOD score. Taking all patients together, no correlations of d-dimer levels with single organ failure or with indicators of infection could be detected. In conclusion, d-dimer plasma levels strongly correlated with the severity of the disease and organ dysfunction in patients with circulatory impairment or infections suggesting that elevated d-dimer levels may reflect the extent of microcirculatory failure. Thus, a therapeutic strategy to improve the microcirculation in such patients may be monitored using d-dimer plasma levels.

Two monoclonal antibodies to D-dimer-specific inhibitors of fibrin polymerization

D-dimer of human fibrin was used as antigen to obtain monoclonal antibodies (mAbs). We have obtained 16 hybridomas producing mAbs of different specificity. Only two of these mAbs inhibited fibrin polymerization. They are of the IgG-class. One mAb (II-4d) inhibited fibrin polymerization to 100% and another (II-3b) to 60% at a molar ratio mAb/fibrin=1.0. Fab-fragments of these mAbs inhibited fibrin polymerization completely at the same molar ratio. The epitopes for the mAbs studied are situated in the NH2-terminal part of the gamma-chain in fibrin D-domain. Electron microscopy showed that fibrin was in monomeric form in the presence of these mAbs or their Fab-fragments. Thus, these mAbs stop the initial step of fibrin polymerization, i.e. protofibril formation. Only one site of protofibril formation is known now in COOH-terminal half of the D-domain gamma chain named "a" site, which is complementary to the "A" site in the central E-domain of fibrin molecule. Our experiment with immobilized GPRP showed that the "a" site in fibrin D-fragment preserved its binding activity to GPRP when the D-fragment was complexed with mAbs-inhibitors of fibrin polymerization. Thus, these two mAbs inhibit fibrin polymerization not by blocking the sites "a" but either by blocking another (not "a") specific site in D-domain or by steric hindrance of highly organized fibrin polymerization process.

Generation and characterization of a novel single-chain antibody fragment specific against human fibrin clots from phage display antibody library

A novel single-chain fragment variable (scFv) antibody was developed directed against human fibrin clots by using the human single fold scFv libraries I+J (Tomlinson I+J). Three positively binding scFvs were evaluated by scFv-phage enzyme-linked immunosorbent assay (ELISA) and DNA sequencing. Then the positive scFv was expressed in soluble form in Escherichia coli HB2151 and purified by immobilized metal affinity chromatography (IMAC) with a yield of about 1 mg/l, the expression of soluble scFv was verified by Western blot analysis. The purified scFv could specifically recognize human fibrin clots and indicate no binding ability with human fibrinogen shown by ELISA. Furthermore, we will amplify the gene of the positive scFv by polymerase chain reaction (PCR) for future study of its role in diagnosis and therapy of thrombus-correlated diseases.