Viral hepatitis, secondary to infection with hepatitis A, B, C, D, and E viruses, are a major public health problem and an important cause of morbidity and mortality. Despite the huge medical advances achieved in recent years, there are still points of conflict concerning the pathogenesis, immune response, development of new and more effective vaccines, therapies, and treatment. This review focuses on the most important research topics that deal with issues that are currently being solved, those that remain to be solved, and future research directions. For hepatitis A virus we will address epidemiology, molecular surveillance, new susceptible populations as well as environmental and food detections. In the case of hepatitis B virus, we will discuss host factors related to disease, diagnosis, therapy, and vaccine improvement. On hepatitis C virus, we will focus on pathogenesis, immune response, direct action antivirals treatment in the context of solid organ transplantation, issues related to hepatocellular carcinoma development, direct action antivirals resistance due to selection of resistance-associated variants, and vaccination. Regarding hepatitis D virus, we describe diagnostic methodology, pathogenesis, and therapy. Finally, for hepatitis E virus, we will address epidemiology (including new emerging species), diagnosis, clinical aspects, treatment, the development of a vaccine, and environmental surveillance.

The immune response is critical in NAFLD pathogenesis, but the liver infiltrate’s composition and the role of each T cell population is still up for debate. To characterize liver pathogenesis in pediatric and adult cases, frequency and localization of immune cell populations [Cytotoxic T Lymphocytes (CD8+), T helper Lymphocytes (CD4+), Regulatory T lymphocytes (Foxp3+) and Th17 (IL-17A+)] were evaluated. In portal/periportal (P/P) tracts, both age groups displayed a similar proportion of CD8+ and CD4+ lymphocytes. However, comparable Foxp3+ and IL-17A+ cell frequencies were observed in pediatric cases, meanwhile, in adults Foxp3+ was higher than IL-17A+ cells. Interestingly, IL-17A+ lymphocytes seemed to be nearly exclusive of P/P area in both age groups. In intralobular areas, both pediatric and adult cases showed CD8+ lymphocytes predominance with lower frequencies of CD4+ lymphocytes followed by Foxp3+ . Severe inflammation was associated with higher intralobular Foxp3+ lymphocytes (p = 0.026) in children, and lower P/P Foxp3+ and higher IL-17A+ lymphocytes in adults. All cases with fibrosis ≥ 2 displayed P/P low Foxp3+ and high IL-17A+ lymphocyte counts. Pediatric cases with worse steatosis showed high P/P CD4+ (p = 0.023) and intralobular CD8+ (p = 0.027) and CD4+ cells (p = 0.012). In NAFLD cases, the lymphocyte liver infiltrate composition differs between histological areas. Treg and Th17 balance seems to condition damage progression, denoting their important role in pathogenesis.

In Chronic hepatitis B (CHB) infection, virus and immune response interplay is thoughtto be responsible for pathogenesis. Yet, the impact of each immune cell population and viral protein expression in liver damage is still unknown. Our aim was to study the interplay between intrahepatic immune response and viral activity in relation to CHB liver damage. Immunostaining was performed in 29 liver biopsies from untreated CHB patients to characterize liver infiltrate [Th (CD4+), CTL (CD8+), Treg (FoxP3+), Th17 (IL-17A+) and Th1 (T-bet+)] and viral antigen expression (HBsAg and HBcAg).Inflammatory activity and fibrosis were assessed using the HAI and METAVIR scoring system. All studied populations were identified in the portal-periportal (P-P) areas with a CD4+ lymphocyte predominance, while only CD8+ and FoxP3+ cells were observed in the intralobular area. Both P-P CD4+ and intralobular CD8+ cell frequencies were increased among severe hepatitis cases. Concerning HBsAg and HBcAg expression, a mutually exclusive pattern was observed. HBcAg was mainly detected among HBeAg-positive patients and was associated with hepatitis severity and higher frequency of P-P FoxP3+, intralobular CD8+ and FoxP3+ cells. HBsAg was identified among HBeAg-negative cases with less severe hepatitis grade and lower frequency of P-P CD4+ and intralobular FoxP3+ lymphocytes. In conclusion, the HBV antigen profile expression seen during CHB infection may be reflecting different stages of viral replication which impacts the host immune response and liver damage process. While HBcAg might be an inducer of a regulatory microenvironment, the intralobular CTL population seemed to have a key role in hepatitis severity. K E Y W O R D Schronic hepatitis B, inflammatory infiltrate, liver damage, viral antigen