Towards a small animal model for hepatitis C

Alexander Ploss & Charles M. RiceEMBO reports (2009) 10, 1220 - 1227

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Hepatitis C virus

•Hepatitis C virus (HCV) causes chronic liver diseases.•Can lead to cirrhosis, liver failure and liver cancer.•130 million people infected worldwide•Current Interferon treatment is often ineffective and poorly tolerated•No vaccine available till date.

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HCV life cycle

Drawbacks to HCV researchChimpanzee are the only available immunocompetent experimental system.

Use limited by ethical constraint, restricted availability and high costs

Preferred animal model due to easy availability and handling, economical and complete genome information available

Approaches towards animal model

Viral adaptation to infect non human cells

Genetic host humanization to adapt the rodent tissues

Xenotransplation of human tissues

Viral adaptation Sera inoculated from HCV infected individual resisted due to block in HCV entry

CD81 and OCLN must be of human origin to permit HCV infection

Expression of mouse CD81 in human hepatoma cells causes very low infection probably due to reduced affinity

Mouse CD81 is 90% homologous to human; 4 unconserved key residues

HCV can be adapted in vitro to use mouse CD81;

Selection of three mutations in E1 and E2 enhanced mouse CD81 dependent uptake to levels comparable to human orthologue

Viral adaptationStudies show HCV entry and replication in murine environment; assembly and release of infectious virions are unclear

Increasing viral replication can allow detection of infectious virus by expression of cDNA transgene encoding entire HCV genome

Generation of proper 5’ and 3’ ends of HCV genome crucial, production of infectious HCV from stably HCV cDNA-transfected HepG2 has been reported

Additional adaptation steps necessary to select for genomes with assembly competence in a mouse cell environment

Viral adaptation

The efficiency of murine tropic HCV infection to that of human cells expressing the mouse entry factors is uncertain

Post entry additional restriction might exist, thus virally encoded replication machinery might not adapt to murine counterpart

Modifications affecting species tropism might have significant efforts on immune responses and pathogenesis

Xenotranplantation modelChimeric mice harboring HCV permissive tissue can be obtained, susceptible to human hepatotropic pathogens

Alb-uPa transgene overexpression results in severe liver damage rescued by transplanting non transgenic (human) hepatocytes

Xenotransplanted humanized mice are potential for studying drug and vaccine efficacy as well as pharmacokinetics and toxicity

Application limited by less production, substantial variability and logistical constraints.

Pathogenesis and immunity studies impeded by absence of functional immune system-required to avoid xenograft rejection

Xenotransplantation model

Combination of humanized liver model with mice harbouring a human haematolymphoid system

Generation by engrafting suspensions of human haematopoetic progenitor stem cells into immunodeficient animals, virus-specific immune response can be elicited on successful human immune reconstitution

Merging hepatic and haematolymphoid reconstitution in a single recipient will allow studies of pathogenesis, immune correlates and mechanisms of persistence of HCV and other human hepatotropic pathogens

The generation of these types of humanized mice requires substantial infrastructure and advanced technical skills

Host adaptation

An inbred mouse model with inheritable susceptibility to HCV would overcome the technical difficulties of the xenotransplantation model

Efforts for genetic adaptation of human CD81-transgenic mice were resistant to HCV infection

Functional cDNA complementation screen approach could identify critical human factors required for efficient HCV RNA replication in mouse cells.

Liver specific miRNAs- miR122 and miR199a have shown to affect HCV RNA translation and replication

Dominant-negative restriction factors causing altered innate antiviral responses or inability of HCV proteins to overcome murine defences could impair HCV replication in mouse.

Host adaptationSeveral genetic adaptations to humanize the murine host are required to create an inbred mouse model for HCV infection which could be a platform for further improvements designed to mimic more closely the clinical features of human hepatitis.

Studies to identify crucial human factors is dubious due to minimal overlap and relevance of HCV interactions yet to be demonstrated

Sequence divergence of identified factors might reduce the infectious capacity and affect tropism.

ConclusionHCV infection aggravated by insufficient therapy and lack of proper vaccine

A robust animal model easily propagated and reproducing the viral life cycle would be instrumental in understanding HCV pathogenesis and to guide drug development

Deciphering the barriers to HCV replication in mouse cells will provide rich insights into virus host interactions directing towards a sustainable mouse model of HCV infection.