In this issue of Transplantation, Seale et al1 describe the use of bioengineered devices that facilitate vascularization for the transplantation of primary human hepatocytes. This has the potential to significantly improve the outcome in scenarios where cell transplant may be an approach to supporting and improving liver function.
The liver is the largest internal organ which performs a complex array of metabolic, synthetic, immunologic, and detoxification functions.2 Consequently, loss of liver function is a very serious clinical challenge, and it is not rare. Liver failure is the fourth most common cause of death for middle-aged adults in the United States.2 Interestingly, the liver has a unique capacity for regeneration, even after the majority of the cells are damaged or destroyed.2 However, identification of patients that may resolve liver failure through a regenerative response remains difficult.2 Liver transplantation is the only current treatment that can alter mortality from end-stage liver failure. However, despite surgical advances, such as split liver transplants, the discrepancy between the need for transplantation and the availability of organs continues to grow.3 Therefore, a number of approaches are being developed, ranging from direct hepatocyte injection, development of engineered implantable liver tissue constructs to development of whole organ engineering, to try and improve outcomes for patients with liver failure (Figure 1).
Direct hepatocyte transplantation has failed to show long-term clinical benefits due to poor cell engraftment and the limited survival time of the transplanted hepatocytes. Therefore, functional substrates and scaffolds capable of providing a more appropriate microenvironment, such as described by Seale et al,1 need to be developed for the use of hepatocytes in liver tissue engineering, cell therapy and transplantation.2-5 Overall, attempts to address obstacles, including testing different sources of cells to obtain the necessary cell numbers, continue.5
Other major obstacles remain in the development of cell-based approaches for organ-specific regeneration of liver and other organs,6 even with engineered microenvironments. One of these is the fact that transplanting large numbers of cells in biomaterials is limited by the nutrient supply. Hepatocytes are metabolically very active and are normally in close contact with an extensive sinusoidal vascular network. Vascularization is one of the critically recognized but unmet needs of engineered microtissues.7,8 Improved vascularization allows the engineering of larger tissues with better viability and would alleviate the functional limits caused by nutrient insufficiency.2,9 Implantation in highly vascularized sites, such as peritoneum or under the kidney capsule, has been noted to improve engraftment and function.2 However, these sites are not readily accessed and require invasive surgery.
The article by Seale et al provides some intriguing observations that may indicate a way to start addressing this issue. They microencapsulate primary human hepatocytes in hydrogels, but use a dual-compartment hydrogel with a macroporous outer compartment. Primary human hepatocytes are embedded in a fibrin gel and placed in the hollowed inner compartment, whereas human umbilical vein cells (HUVECs), alone or combined with mouse embryonic fibroblasts (MEFs) or human mesenchymal stem cells (hMSCs) that support hepatocytes and facilitate host cell infiltration and vascularization, are seeded in the polyethylene glycol diacrylate/hyaluronic acid methacrylate hydrogel outer compartment, which has been engineered as a porous scaffold. The experimental results presented in their article shows the appearance of blood vessels (visible as blood-filled vessels macroscopically and confirmed by immunohistochemistry) in the inserts that were simply placed subcutaneously in mice. Blood vessel numbers continue to increase throughout the time the inserts are implanted in mice. In these initial assays, hepatocyte function was tested by assaying human albumin in blood of the host mice, which allowed for repeated sampling. Human albumin levels rose over the first week and showed stable levels during the remainder the 1-month observation period. Although not rising to statistical significance, HUVECs supplemented with MEF or hMSC resulted in higher albumin levels than HUVECs alone.
Obvious questions are raised by the results presented in this report. About stability: How long will the cells in these inserts be viable and functional? In the presented experiments, cells in inserts were viable and functional for the full 1 month observation time. About scalability: How many cells can a single insert contain and support? It is still a long way from the current 20 million to the 1 to 10 billion cells needed for functional rescue, even when keeping in mind that these can be introduced on multiple inserts. About function: How many of the essential liver functions can be augmented by this approach? Eventually, about use in patients: Optimized implantation routes (the subject of active investigations, eg10) and efficacy in restoring liver functions. The device developed by Seale et al provides a promising starting point to begin answering these questions.
1. Seale N, Ramaswamy S, Shih Y-R, et al. Macroporous dual-compartment hydrogels for minimally invasive transplantation of primary human hepatocytes. Transplantation
2. Bhatia SN, Underhill GH, Zaret KS, et al. Cell and tissue engineering for liver disease. Sci Transl Med
3. Mazza G, Al-Akkad W, Rombouts K, et al. Liver tissue engineering: from implantable tissue to whole organ engineering. Hepatol Commun
4. Hughes RD, Mitry RR, Dhawan A. Current status of hepatocyte transplantation. Transplantation
5. Boylan JM, Francois-Vaughan H, Gruppuso PA, et al. Engraftment and repopulation potential of late gestation fetal rat hepatocytes. Transplantation
6. Mandrycky C, Phong K, Zheng Y. Tissue engineering toward organ-specific regeneration and disease modeling. MRS Commun
7. Fu J, Wang DA. In Situ Organ-Specific Vascularization in Tissue Engineering. Trends Biotechnol
. 2018; Published online March 16, 2018. DOI:10.1016/j.tibtech.2018.02.012.
8. Sekiya S, Shimizu T. Introduction of vasculature in engineered three-dimensional tissue. Inflamm Regen
9. Lee GH, Lee JS, Wang X, et al. Bottom-up engineering of well-defined 3D microtissues using microplatforms and biomedical applications. Adv Healthc Mater
10. Pourcher G, El-Kehdy H, Kanso F, et al. Volumetric portal embolization: a new concept to improve liver regeneration and hepatocyte engraftment. Transplantation