Background Antiphospholipid syndrome (APS) is an autoimmune thrombophilia characterised by recurrent thromboembolism and or pregnancy morbidity in the presence of antiphospholid antibodies (aPL). Major auto antigen is B2 Glycoprotein I (Β2GPI). APL form immunocomplexes with B2GPI and activate platelets leading to thrombus formation. Previous studies have shown that APS patients have higher circulating levels of platelet derived chemokines. Among them CXCL4, a proinflammatory chemokine with antiangiogenic properties, has already been proven to bind and dimerize B2GPI leading to the formation of a triple complex, aPl-B2GPI-CXCL4, which strongly activates platelets. Therefore, we aimed to investigate the pharmaceutical disruption of the B2GPI-CXCL4 interaction.
Methods An in vitro binding assay was developed for the study of B2GPI-CXCL4 interaction. Briefly, 1 µg/ml B2GPI was coated overnight on high binding polystyrene plates and CXCL4 was incubated in various concentrations for 2 hours. Detection of CXCL4 was performed using a biotinylated polyclonal goat anti-human antibody against CXCL4 followed by Streptavidin-HRP incubation with TMB as substrate.
Next in silico molecular docking experiments using the MAESTRO and GLIDE (Schrodinger Inc.) software determined the exact interaction residues of CXCL4 and B2GPI. A number of oligopeptide CXCL4 fragments was screened according to their capacity to bind on B2GPI on the interaction interface with CXCL4. The amino acid sequence of the most promising was further modified in order to achieve high binding strength and optimal solubility properties. Finally, four oligopeptides were designed and synthesised. These oligopeptides were tested using the aforementioned assay for their ability to bind on B2GPI and consequently to inhibit B2GPI-CXCL4 interaction. B2GPI was incubated with the oligopeptides prior to the addition of 10 ng/ml of CXCL4.
Results Preincubation of B2GPI with the peptides resulted in partial inhibition of B2GPI-CXCL4 interaction in all cases. Among them one presented the highest inhibiting properties which resulted in 40% inhibition of CXCL4 binding on B2GPI.
Conclusions Preliminary results show that the designed peptides exhibit inhibiting properties. Further experiments and possibly amino acid alterations are required to determine their exact efficiency. Once desirable inhibition and solubility properties are achieved these oligopeptides could be tested as lead compounds for potential therapeutic agents.