Identification and characterization of factor XI autoantibodies in 2 patients with systemic lupus erythematosus: insights into mechanisms of acquired factor XI deficiency.

Background: Factor (F)XI is a zymogen that contributes to thrombin generation through activation of FIX. Patients with a complete absence of FXI are prone to developing alloantibody inhibitors after replacement therapy. Acquired FXI autoantibodies are less common, and data regarding their mechanisms of action are lacking. Objectives: We describe 2 patients with severe acquired FXI deficiency and identify the FXI domains to which the autoantibodies bind. Methods: FXI and prekallikrein (PK) are homologs with similar structures. We prepared recombinant human FXI and PK, as well as chimeric molecules in which individual domains within FXI or PK are replaced with the corresponding domain from the other protein. Patient plasma and normal plasma were used as antibody sources, and their capacities to recognize recombinant proteins on Western blots were compared. Results: Patients 1 and 2 were females with systemic lupus erythematous and no bleeding history. FXI activity in both cases was undetectable by one-stage clotting assay, with autoantibody titers of 64 Bethesda Units and 11.4 Bethesda Units, respectively. In both cases, the autoantibody appeared to clear FXI protein from plasma. Immunoglobulin G in patient 1 targeted the FXI catalytic domain, while the autoantibody in patient 2 was likely oligoclonal with components that recognized the FXI apple 2 and apple 3 domains. Conclusion: These autoantibodies inhibited FXI function and promoted its clearance. The inhibitors targeted the 2 most important FXIa domains for FIX activation and demonstrated properties similar to those described in patients with FXI alloantibody inhibitors.