Cross-Reactivity of Antiphospholipid Antibodies with Gut Commensal Proteins in Antiphospholipid Syndrome

Background: Antiphospholipid syndrome (APS) is an autoimmune disease characterized by the persistent presence of antiphospholipid antibodies (aPL), mainly targeted against β2 glycoprotein 1 (β2GP1). The autoimmune response to β2GP1 is aimed at several B-cell and T-cell epitopes. Molecular mimicry of these epitopes by gut commensal proteins, so-called mimotopes, causing cross-immunization, might contribute to the formation of aPL. Objective: To study the potential role of gut microbiome cross-immunization in APS by examining cross-reactivity of aPL with gut commensal mimotope-containing proteins. Methods: Fecal microbial metagenome of APS patients was determined using shotgun sequencing. An in-house developed in silico pipeline was used to identify gut commensal proteins that show sequence homology with known β2GP1 B and T cell epitopes in the metagenomic data. An enzyme-linked immunosorbent assay was used to test the identified microbial proteins for IgG cross-reactivity, with plasma of 21 APS patients and 17 control participants. Results: The in silico pipeline resulted in the identification of six gut commensals with a B cell and T cell β2GP1 epitope homologue. Of these, YjjG family noncanonical pyrimidine nucleotidase, one of the candidate-β2GP1 B cell mimicking proteins, showed significantly increased IgG reactivity in APS patients compared to control participants, as well as higher binding of a specific anti-β2GP1 monoclonal antibody than a negative control. Conclusion: in APS patients. Insights into the origins of antibody formation may yield new therapeutic targets for improvement of APS treatment.