Hepatic microcirculation – A critical but neglected factor for the outcome of viral hepatitis☆
Article Outline
Aggravation of viral hepatitis by platelet-derived serotonin. Lang PA, Contaldo C, Georgiev P, El-Badry AM, Recher M, Kurrer M, Cervantes-Barragan L, Ludewig B, Calzascia T, Bolinger B, Merkler D, Odermatt B, Bader M, Graf R, Clavien PA, Hegazy AN, Löhning M, Harris NL, Ohashi PS, Hengartner H, Zinkernagel RM, Lang KS.
More than 500 million people worldwide are persistently infected with hepatitis B virus or hepatitis C virus. Although both viruses are poorly cytopathic, persistence of either virus carries a risk of chronic liver inflammation, potentially resulting in liver steatosis, liver cirrhosis, end-stage liver failure or hepatocellular carcinoma. Virus-specific T cells are a major determinant of the outcome of hepatitis, as they contribute to the early control of chronic hepatitis viruses, but they also mediate immunopathology during persistent virus infection. We have analyzed the role of platelet-derived vasoactive serotonin during virus-induced CD8(+) T cell-dependent immunopathological hepatitis in mice infected with the noncytopathic lymphocytic choriomeningitis virus. After virus infection, platelets were recruited to the liver, and their activation correlated with severely reduced sinusoidal microcirculation, delayed virus elimination and increased immunopathological liver cell damage. Lack of platelet-derived serotonin in serotonin-deficient mice normalized hepatic microcirculatory dysfunction, accelerated virus clearance in the liver and reduced CD8(+) T cell-dependent liver cell damage. In keeping with these observations, serotonin treatment of infected mice delayed entry of activated CD8(+) T cells into the liver, delayed virus control and aggravated immunopathological hepatitis. Thus, vasoactive serotonin supports virus persistence in the liver and aggravates virus-induced immunopathology.
[Abstract reproduced by permission of Nat Med 2008;14:756–761].
Serotonin is a biogenic amine which binds to many different receptors, thus exerting multiple actions as a neurotransmitter in the central and autonomous nervous system. Beyond its role for the regulation of enteric motility and neurotransmission, serotonin has been recognized increasingly as an important modulator of hepatic microcirculation affecting both liver damage and repair [1], [2]. Enterochromaffin cells in the gut constitute the major source of serotonin, which is then sequestered into platelets. The article by Lang et al. [3] describes a pivotal role for serotonin release from platelets affecting the outcome of viral hepatitis due to altered hepatic microcirculation.
The authors studied experimental infection with the noncytopathic lymphocytic choriomeningitis virus (LCMV), which provides a well-established animal model to study mechanisms of cellular immunity closely resembling those in hepatitis B and C [4], [5]: LCMV-infection leads to viral hepatitis, which is either cleared rapidly or can take a more chronic course depending on the exact experimental conditions. Virus-specific T lymphocytes are a key determinant in this model, because they contribute to control over hepatitis viruses but also mediate immunopathology during persistent infection.
In an elegant series of experiments Lang et al. now have identified impaired hepatic microcirculation as a further key factor determining the outcome of hepatic LCMV infection. They demonstrated platelets in the periportal infiltrates which had been recruited and activated by CD8+ T cells and revealed intensive platelet/ endothelial cell interactions associated with reduced perfusion of hepatic sinusoids. The close correlation between CD8+ T cell-dependent platelet recruitment and impaired hepatic microcirculation was associated with extensive tissue damage, e.g. reduced tissue oxygen tension as well as increased serum lactate and bilirubin levels.
To demonstrate that intrahepatic platelets in LCMV infection exerted their effects via serotonin release, Lang et al. studied mice lacking tryptophan hydroxylase 1 (Tph1−/− mice), which are unable to synthesize serotonin in peripheral tissues. Despite similar platelet activation after LCMV infection Tph1−/− mice showed normal red blood cell velocity and sinusoidal perfusion supporting the concept that serotonin from activated platelets had been the mediator of impaired microcirculation in wild-type animals. These findings were strengthened by experiments with the serotonin re-uptake inhibitor fluoxetine which prevents sequestering of serotonin in platelet storage pools. Similar to Tph1−/− mice pharmacological serotonin depletion in platelets resulted in preserved hepatic microcirculation and reduced hepatic damage after LCMV infection. Of note, although initial viral loads had been identical, Tph1−/− mice consistently controlled LCMV infection after 6 days, whereas the virus was still present in the wild-type control animals. The authors ruled out differences in innate immunity as a potential cause of viral persistence and in depletion experiments confirmed that differential viral clearance was dependent on functional CD8+ T lymphocytes. A detailed analysis failed to reveal differences in phenotype, interferon-γ production, cytotoxic activity and numbers of antigen-specific T cells but in Tph1−/− mice disclosed greater fractions CD8+ T lymphocytes in the hepatic lobules versus portal areas consistent with the concept of impaired hepatic microcirculation in serotonin releasing wild-type animals. This finding resembled human hepatitis B where a rather similar distribution of T lymphocytes had been associated with enhanced control over viral replication and reduced immunopathology [6]. Finally, the authors also studied serotonin-treated animals which exhibited delayed intrahepatic entry of LCMV-specific T lymphocytes, enhanced LCMV replication and ultimately increased liver damage resulting from prolonged local CD8+ T cell cytotoxicity following persistent antigenic stimulation.
Taken together, Lang et al. clearly clarified that serotonin release from platelets during hepatitis can be a cause of sinusoidal microcirculatory failure, which delays recruitment of antiviral T lymphocytes thereby protracting viral clearance and ultimately enhancing tissue damage from continued antigenic activation of T cells. In analogy to this animal model serotonin released during acute viral hepatitis might favor virus replication to a level that enables chronic persistence in man. Thus, the paper directs attention to the potentially dominant role of hepatic microcirculation not only for the onset of chronic viral hepatitis but also other liver pathology. The manuscript of Lang et al. already provides some hint at this possibility, because consistent with reduced liver damage, Tph1−/− mice exhibited less fibrosis after LCMV infection than wild-type mice. Moreover, LCMV infection experiments in newborn mice further confirmed that in a dose-dependent fashion, serotonin promoted several aspects of chronic viral hepatitis such as fibrosis and steatosis. In its extremes, failure of the hepatic microvasculature might result in fulminant hepatic failure. Indeed several observers have linked altered sinusoidal cell functions in Fas ligand- and T cell-induced liver damage to severe organ failure [7], [8]. However, the exact role of platelets and serotonin release in this process has not yet been clarified.
The paper by Lang et al. leaves open several important issues. First, the authors did not identify the exact sinusoidal cells (hepatic stellate cells, sinusoidal endothelial cells?) and their particular type of serotonin receptors responding to serotonin from intrahepatic platelets. Although their observations are well in keeping with similar data from other groups [9], impaired hepatic microcirculation has mainly been characterized in descriptive terms without clarifying the changes in cell biology and ultrastructure that alter hepatic perfusion at the cellular level. Although T lymphocytes are major players in the pathophysiology of human viral hepatitis B and C, other factors probably reflected less well in LCMV hepatitis must be also taken into account. For instance, Lang et al. claimed that altered microcirculation did not affect natural immunity, because interferon responses were identical in serotonin-depleted and wild-type animals. However, in men epidemiological evidence exists that activation of natural killers also contributes to viral clearance in hepatitis C [10]. Unlike murine LCMV infection altered hepatic microcirculation in man quite likely also affects recruitment of natural killer cells. Thus, it must still be confirmed in how far reduced microcirculation in LCMV infected mice actually corresponds to human viral hepatitis.
Should the LCMV model prove valid, it might become an intriguing issue to see whether pharmacological serotonin antagonists could counteract the inflammatory changes in hepatic microcirculation to support antiviral immune responses. Currently, this strategy is severely hampered by our incomplete understanding of hepatic serotonin receptor expression in viral hepatitis and the functional changes transmitted by them to sinusoidal cells. Beyond that, many serotonin antagonists exhibit only insufficient receptor specificity and also bind to dopaminergic, adrenergic or cholinergic receptors. Owing to unacceptable side effects from such “dirty compounds”, serotonin receptor blockade has meanwhile been abandoned in the field of portal hypertension [1] and will not become available as supportive treatment in viral hepatitis before more specific compounds have been identified.
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☆ The author declares that he does not have anything to disclose regarding funding from industries or conflict of interest with respect to this paper.
PII: S0168-8278(08)00789-7
doi:10.1016/j.jhep.2008.12.004
© 2009 European Association for the Study of the Liver. Published by Elsevier Inc. All rights reserved.
