Nonrheumatoid IgM in human hepatitis C virus-associated type II cryoglobulinemia recognize mimotopes of the CD4-like LAG-3 protein.

Type II mixed cryoglobulinemia (CryoII) is an autoimmune disorder frequently associated to human hepatitis C virus (HCV) infection and characterized by the presence of cold-insoluble immunocomplexes containing IgM with rheumatoid activity. To identify disease-related epitopes, we screened a phage-displayed random peptide library using purified IgM from patients with HCV-associated CryoII(CryoII/HCV). A dominant population of phage isolates bearing the HPLAP pentapeptide consensus motif was identified and shown to be recognized by a nonrheumatoid IgM species strongly associated to CryoII/HCV. The phage-borne mimotopes (phagotopes) displayed a strong homology with an exposed extra-loop region of human lymphocyte activation 3 gene (LAG-3) product. Consistently, rabbit sera raised against a synthetic LAG-3 peptide efficiently recognized the selected phagotopes. Furthermore, one such phagotope was revealed to be a good immunogenic mimic of LAG-3 when injected into rabbits. IgM purified from CryoII/HCV patients' sera specifically reacted with the LAG-3 peptide in ELISA, and this binding was inhibited by the selected phagotopes. These results provide experimental support for a general strategy to identify novel autoantigens.