From the AGA Journals

Engineered liver models to study human hepatotropic pathogens

Publish date: March 1, 2018

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Engineered liver models to study human hepatotropic pathogens

Gural et al. present a timely and outstanding review of the advances made in the engineering of human-relevant liver culture platforms for investigating the molecular mechanisms of infectious diseases (e.g., hepatitis B/C viruses and Plasmodium parasites that cause malaria) and developing better drugs or vaccines against such diseases. The authors cover a continuum of platforms with increasing physiological complexity, such as 2-D hepatocyte monocultures on collagen-coated plastic, 2-D cocultures of hepatocytes and nonparenchymal cells, (both randomly distributed and patterned into microdomains to optimize cell-cell contact), 3-D cultures/cocultures housed in biomaterial-based scaffolds, perfusion-based bioreactors to induce cell growth and phenotypic stability, and finally rodents with humanized livers. Cell sourcing considerations for building human-relevant platforms are discussed, including cancerous cell lines, primary human hepatocytes, and stem cell–derived hepatocytes (e.g., induced pluripotent stem cells).

Dr. Salman Khetani

From the discussions of various studies, it is clear that this field has benefitted tremendously from advances in tissue engineering, including microfabrication tools adapted from the semiconductor industry, to construct human liver platforms that last for several weeks in vitro, can be infected with hepatitis B/C virus and Plasmodium parasites with high efficiencies, and are very useful for high-throughput and high-content drug screening applications. The latest protocols in isolating and cryopreserving primary human hepatocytes and differentiating stem cells into hepatocyte-like cells with adult functions help reduce the reliance on abnormal or cancerous cell lines for building platforms with higher relevance to the clinic. Ultimately, continued advances in microfabricated human liver platforms can aid our understanding of liver infections and spur further drug/vaccine development.

Salman R. Khetani, PhD, is associate professor, department of bioengineering, University of Illinois at Chicago. He has no conflicts of interest.


 

FROM CELLULAR AND MOLECULAR GASTROENTEROLOGY AND HEPATOLOGY

Although 2D cultures offer ease of use and monitoring of infection, they often lack the complexity of the liver microenvironment and impact of different cell types on liver infections. A 3D radial-flow bioreactor (cylindrical matrix) was able to maintain and amplify human hepatoma cells (for example, Huh7 cells), by providing sufficient oxygen and nutrient supply, supporting productive HCV infection for months. Other 3D cultures of hepatoma cells using polyethylene glycol–based hydrogels, thermoreversible gelatin polymers, alginate, galactosylated cellulosic sponges, matrigel, and collagen have been developed and shown to be permissive to HCV or HBV infections. Although 3D coculture systems exhibit better hepatic function and differential gene expression profiles in comparison to 2D counterparts, they require a large quantity of cells and are a challenge to scale up. Recently, several liver-on-a-chip models have been created that mimic shear stress, blood flow, and the extracellular environment within a tissue, holding great potential for modeling liver-specific pathogens.

Humanized mouse models with ectopic human liver structures have been developed in which primary HHs are transplanted following liver injury. Chimeric mouse models including Alb-uPA/SCID (HHs transplanted into urokinase-type plasminogen activator-transgenic severe combined immunodeficient mice), FNRG/FRG (HHs transplanted into Fah[-/-], Rag2[-/-], and Il2rg[-/-] mice with or without a nonobese diabetic background), and TK-NOG (HHs transplanted into herpes simplex virus type-1 thymidine kinase mice) were validated for HCV, HBV, P. falciparum, and P. vivax infections. It is, however, laborious to create and maintain chimeric mouse models and monitor infection processes in them.

It is important to note that the selection of model system and the readout modality to monitor infection will vary based on the experimental question at hand. Tissue engineering has thus far made significant contributions to the knowledge of hepatotropic pathogens; a continued effort to develop better liver models is envisioned.

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