Summary Hepatitis A virus (HAV) infection is the major cause of acute liver failure in paediatric patients. The clinical spectrum of infection is variable, and liver injury is determined by altered hepatic enzyme function and bilirubin concentration. We recently reported differences in cytokine profiles between distinct HAV‐induced clinical courses, and bilirubin has been recognized as a potential immune‐modulator. However, how bilirubin may affect cytokine profiles underlying the variability in the course of infection has not been determined. Herein, we used a transcription factor (TF) binding site identification approach to retrospectively analyse cytokine expression in HAV‐infected children and to predict the entire set of TFs associated with the expression of specific cytokine profiles. The results suggested that modulation of the activity of signal transducers and activators of transcription proteins (STATs) may play a central role during HAV infection. This led us to compare the degree of STAT phosphorylation in peripheral blood lymphoid cells (PBLCs) from paediatric patients with distinct levels of conjugated bilirubin (CB). Low CB levels in sera were associated with increased STAT‐1 and STAT‐5 phosphorylation. A positive correlation was observed between the serum interleukin‐6 (IL‐6) content and CB values, whereas higher levels of CB correlated with reduced serum IL‐8 values and with a reduction in the proportion of PBLCs positive for STAT‐5 phosphorylation. When CB was used to stimulate patients' PBLCs in vitro, the levels of IL‐6 and tumour necrosis factor‐α were increased. The data showed that bilirubin plays a role in STAT function and affects cytokine profile expression during HAV infection.

The immune response is known as a physiological mechanism to protect the body, providing defense to different systems that compose it and allowing its proper functioning. The ability to keep the organism free from foreign agents depends on the mechanisms of natural resistance or innate immunity, as well as the resistance that can develop over time through adaptive immunity. However, when these defense mechanisms fail, it can trigger injuries and diseases in the tissues, such as hypersensitivity, which is characterized as an excessive and undesirable reaction, produced by the immune system; as well as autoimmunity, which refers to the failure of the mechanisms of immunological tolerance, causing the reaction of the immune system against the body itself.

Hepatitis A virus (HAV) and hepatitis C virus (HCV) are both viruses with hepatotropic lifestyles. HAV induces an acute infection that results in the elimination of the virus by the host whereas HCV is typically able to establish a persistent infection that may result in cirrhosis and hepatocellular carcinoma. The mechanisms responsible for this difference are unknown. However, given HAV and HCV are both non-cytophatic viruses, the observed symptoms and liver injury during the infections are the result of specific immune responses under the control of cytokines. Thus, the production of cytokines during hepatotropic viral infections may constitute a mechanism leading to different outcomes. Therefore, understanding the differences in the cytokine patterns induced in response to HAV and HCV is likely to provide important insights into the cytokine-mediated mechanisms underlying the long-term persistence of HCV, the broad spectrum of clinical manifestations induced by HAV and the resolution of HAV infection during the acute phase. Herein, we focus on discoveries that hold promise in identifying cytokines as therapeutic targets for the treatment of viral hepatitis.