Autoantibody IgG subclasses to recombinant antigens and the role of bacterial stimuli in primary biliary cirrhosis

Aim: Serum antimitochondrial antibody (AMA) of the IgG2 and IgG3 subclasses has been reported to be predominant in patients with primary biliary cirrhosis from developed countries. No data are available as to the significance of AMA subtypes in Japanese primary biliary cirrhosis (PBC) patients who have previously manifested unique serological features, nor it is known whether AMA subclasses are influenced by bacterial stimuli, as suggested by the molecular theory of PBC. We undertook a three-step study to address these questions. Methods: First, Japanese PBC sera were tested using the established triple recombinant antigen (pML-MIT3) to find AMA subclass distribution. Second, we used the three recombinant mitochondrial antigens in PBC sera of Japanese and USA patients to explore the ethnic difference. Third, we used CpG oligodeoxynucleotides and a B cell mitogen to challenge ex vivo peripheral leukocytes from indirect immunofluorescence (IIF)-AMA-positive patients with Japanese PBC. Results: We detected most frequently IgG2-AMA followed by IgG3-AMA, with the latter being more common in IIF-AMA-positive cases, and demonstrated that the IgG3 reactivity against the dominant antigen was significantly higher in PBC sera from the USA. We determined that the bacterial stimulus was superior to the mitogen at inducing a predominant production of IgG2-AMA and CD20+ B cell activation. Conclusion: Our data cumulatively supported the hypothesis that IgG2 AMA subtypes are predominant in PBC and suggest that this might be favored by an innate immune reaction against bacterial particles, such as CpG DNA.