GB virus-C infection in type 1 autoimmune hepatitis

Incorporating the Molecular Mimicry of Environmental Antigens into the Causality of Autoimmune Hepatitis

Molecular mimicry between foreign and self-antigens has been implicated as a cause of autoimmune hepatitis in experimental models and cross-reacting antibodies in patients. This review describes the experimental and clinical evidence for molecular mimicry as a cause of autoimmune hepatitis, indicates the limitations and uncertainties of this premise, and encourages investigations that assess diverse environmental antigens as sources of disease-relevant molecular mimics. Pertinent articles were identified in PubMed using multiple search phrases. Several pathogens have linear or conformational epitopes that mimic the self-antigens of autoimmune hepatitis. The occurrence of an acute immune-mediated hepatitis after vaccination for severe acute respiratory syndrome (SARS)-associated coronavirus 2 (SARS-CoV-2) has suggested that vaccine-induced peptides may mimic disease-relevant tissue antigens. The intestinal microbiome is an under-evaluated source of gut-derived antigens that could also engage in molecular mimicry. Chaperone molecules may enhance the pathogenicity of molecular mimics, and they warrant investigation. Molecular mimics of immune dominant epitopes within cytochrome P450 IID6, the autoantigen most closely associated with autoimmune hepatitis, should be sought in diverse environmental antigens and assessed for pathogenicity. Avoidance strategies, dietary adjustments, vaccine improvement, and targeted manipulation of the intestinal microbiota may emerge as therapeutic possibilities. In conclusion, molecular mimicry may be a missing causality of autoimmune hepatitis. Molecular mimics of key immune dominant epitopes of disease-specific antigens must be sought in diverse environmental antigens. The ubiquity of molecular mimicry compels rigorous assessments of peptide mimics for immunogenicity and pathogenicity in experimental models. Molecular mimicry may complement epigenetic modifications as causative mechanisms of autoimmune hepatitis.

Missing Causality and Heritability of Autoimmune Hepatitis

BACKGROUND

Autoimmune hepatitis has an unknown cause and genetic associations that are not disease-specific or always present. Clarification of its missing causality and heritability could improve prevention and management strategies.

AIMS

Describe the key epigenetic and genetic mechanisms that could account for missing causality and heritability in autoimmune hepatitis; indicate the prospects of these mechanisms as pivotal factors; and encourage investigations of their pathogenic role and therapeutic potential.

METHODS

English abstracts were identified in PubMed using multiple key search phases. Several hundred abstracts and 210 full-length articles were reviewed.

RESULTS

Environmental induction of epigenetic changes is the prime candidate for explaining the missing causality of autoimmune hepatitis. Environmental factors (diet, toxic exposures) can alter chromatin structure and the production of micro-ribonucleic acids that affect gene expression. Epistatic interaction between unsuspected genes is the prime candidate for explaining the missing heritability. The non-additive, interactive effects of multiple genes could enhance their impact on the propensity and phenotype of autoimmune hepatitis. Transgenerational inheritance of acquired epigenetic marks constitutes another mechanism of transmitting parental adaptations that could affect susceptibility. Management strategies could range from lifestyle adjustments and nutritional supplements to precision editing of the epigenetic landscape.

CONCLUSIONS

Autoimmune hepatitis has a missing causality that might be explained by epigenetic changes induced by environmental factors and a missing heritability that might reflect epistatic gene interactions or transgenerational transmission of acquired epigenetic marks. These unassessed or under-evaluated areas warrant investigation.

GB virus C and TT virus infections in Japanese patients with autoimmune hepatitis

The association of the newly identified viruses, GB virus C (GBV-C) and TT virus (TTV), with autoimmune hepatitis remains to be elucidated. Sera from 20 Japanese patients with autoimmune hepatitis and 50 volunteer blood donors were assayed for GBV-C RNA, antibodies to the GBV-C second envelope protein (E2), and TTV DNA. GBV-C RNA was examined by reverse-transcription polymerase chain reaction (PCR). Anti-GBV-C E2 (a marker of past infection) was tested by an enzyme-linked immunosorbent assay. TTV DNA was amplified by PCR using two different sets of primers: one derived from the original N22 sequence (Set A) and the other from the untranslated region (Set B). None of the patients or controls had GBV-C RNA. Anti-GBV-C E2 was found significantly more often in patients with autoimmune hepatitis (3/20) than in controls (1/50; P = 0.034). The prevalence of TTV DNA detected by primers Set A and that detected with either Set A or B were similar among patients with autoimmune hepatitis (4/20 and 16/20, respectively) and controls (9/50 and 40/50, respectively). Clinical characteristics did not differ in association with any of these viral markers. Of the 13 TTV isolates amplified with Set A, seven were classified as genotype 1a, four as genotype 1b, and 2 as genotype 3; no particular strain was associated with autoimmune hepatitis. These findings provide no compelling evidence that GBV-C or TTV has a pathogenic role in autoimmune hepatitis.

What do we need to know about non-A-to-E viral hepatitis?

The list of potential hepatotrophic viruses continues to grow, with the recent discovery of the GB virus-C, the TT virus, and the SEN virus. Prevalence rates of the GB virus-C have ranged from 1.2% to 13% among healthy blood donors from all over the world. Higher prevalence rates have been reported among intravenous drug users. Similarly, the TT virus has a global distribution. However, in spite of numerous reports of the presence of both of these viruses in various kinds of liver diseases, definite evidence linking it to a specific disease or illness is lacking. The SEN virus is thought to be a novel viral agent that may be linked to cryptogenic chronic hepatitis, but data are awaited.