Mechanistic insight into complement C3 regulation during chronic HBV infection: effect on viral persistence and host...

The complement system (CS) is central to antiviral defence, yet many viruses subvert it to escape clearance. Complement-component C3 is the key effector molecule of CS that plays diverse roles in pathogen elimination, inflammation and immune responses, but its regulation during chronic HBV infection (CHI) remains poorly understood. This study examined the mechanisms governing C3 expression in hepatocytes and immune cells during CHI and evaluated the impact of altered C3 levels on viral persistence and host immunity.

C3 expression was evaluated in blood and liver-tissue samples from patients with CHI and healthy controls using ELISA and RT-PCR. HBV-transfected hepatoma cells were used to examine epigenetic regulation of C3, including promoter methylation, histone deacetylation, and transcription factor phosphorylation. Autophagy-related functions of C3 were assessed by immunofluorescence. Flow cytometry was used to determine the intracellular C3 levels in immune cells of HBV-infected patients and the influence of viral and host factors on C3 expression. Additionally, cytokine production by immune cells and the modulating effect of recombinant C3a on these cytokines was studied. The effect of Tenofovir therapy on C3 concentration was also evaluated.

C3 level was found to be significantly diminished in sera and liver-tissues of HBV-infected patients and HBV-expressing hepatoma cells relative to controls. HBX suppressed C3 transcription by restricting the availability of the crucial transcription factor, C/EBPβ through its promoter hypermethylation and by enhancing histone deacetylation at C3-promoter including C/EBPβ binding-sites whereas, HBV surface proteins (HBS) further blocked C/EBPβ phosphorylation essential for transcription. Functionally, C3 deficiency disrupted autophagosome-lysosome fusion and prevented autophagic degradation of viral proteins, leading to enhanced HBV release. Reduced intracellular C3 was noted in monocytes and virus-specific T cells of HBV-infected patients that could be attributed to exposure to HBS and cytokines (IL-4/IL-1β). The decline in C3 skewed the immune responses toward an anti-inflammatory phenotype with diminished pro-inflammatory activities, which were reversed by recombinant C3a. One-year of Tenofovir therapy partially restored serum but not immune-cell C3 levels.

HBV downregulates C3 through epigenetic and signalling mechanisms, facilitating viral persistence and dampening antiviral immunity. Targeting complement regulation may represent a novel therapeutic strategy in CHI.