An implantable vascularized protein gel construct that supports human fetal hepatoblast survival and infection by hepatitis C virus in mice

Liver cell therapy: is this the end of the beginning?

The prevalence of liver diseases is increasing globally. Orthotopic liver transplantation is widely used to treat liver disease upon organ failure. The complexity of this procedure and finite numbers of healthy organ donors have prompted research into alternative therapeutic options to treat liver disease. This includes the transplantation of liver cells to promote regeneration. While successful, the routine supply of good quality human liver cells is limited. Therefore, renewable and scalable sources of these cells are sought. Liver progenitor and pluripotent stem cells offer potential cell sources that could be used clinically. This review discusses recent approaches in liver cell transplantation and requirements to improve the process, with the ultimate goal being efficient organ regeneration. We also discuss the potential off-target effects of cell-based therapies, and the advantages and drawbacks of current pre-clinical animal models used to study organ senescence, repopulation and regeneration.

Mice engrafted with human hematopoietic stem cells support a human myeloid cell inflammatory response in vivo

Mice engrafted with human CD34⁺ hematopoietic stem and progenitor cells (CD34⁺ -HSPCs) have been used to study human infection, diabetes, sepsis, and burn, suggesting that they could be highly amenable to characterizing the human inflammatory response to injury. To this end, human leukocytes infiltrating subcutaneous implants of polyvinyl alcohol (PVA) sponges were analyzed in immunodeficient NSG mice reconstituted with CD34⁺ -HSPCs. It was reported that human CD45⁺ (hCD45⁺ ) leukocytes were present in PVA sponges 3 and 7 days postimplantation and could be localized within the sponges by immunohistochemistry. The different CD45⁺ subtypes were characterized by flow cytometry and the profile of human cytokines they secreted into PVA wound fluid was assessed using a human-specific multiplex bead analyses of human IL-12p70, TNFα, IL-10, IL-6, IL1β, and IL-8. This enabled tracking the functional contributions of HLA-DR⁺ , CD33⁺ , CD19⁺ , CD62L⁺ , CD11b⁺ , or CX3CR1⁺ hCD45⁺ infiltrating inflammatory leukocytes. PCR of cDNA prepared from these cells enabled the assessment and differentiation of human, mouse, and uniquely human genes. These findings support the hypothesis that mice engrafted with CD34⁺ -HSPCs can be deployed as precision avatars to study the human inflammatory response to injury.

Micropatterned coculture of primary human hepatocytes and supportive cells for the study of hepatotropic pathogens

The development of therapies and vaccines for human hepatropic pathogens requires robust model systems that enable the study of host-pathogen interactions. However, in vitro liver models of infection typically use either hepatoma cell lines that exhibit aberrant physiology or primary human hepatocytes in culture conditions in which they rapidly lose their hepatic phenotype. To achieve stable and robust in vitro primary human hepatocyte models, we developed micropatterned cocultures (MPCCs), which consist of primary human hepatocytes organized into 2D islands that are surrounded by supportive fibroblast cells. By using this system, which can be established over a period of days, and maintained over multiple weeks, we demonstrate how to recapitulate in vitro hepatic life cycles for the hepatitis B and C viruses and the Plasmodium pathogens P. falciparum and P. vivax. The MPCC platform can be used to uncover aspects of host-pathogen interactions, and it has the potential to be used for drug and vaccine development.

Tissue-Engineered Microvasculature to Reperfuse Isolated Renal Glomeruli

Kidney transplantation is often the most effective therapy for end-stage renal disease, but there are not enough donor organs to meet the rising demand. Tissue engineering of kidneys is a potential solution to this organ shortage. Achieving microvascular perfusion has been a major barrier to engineering tissues beyond thin muscularized sheets such as the bladder wall. Our laboratory has previously reported that human umbilical vein endothelial cells (ECs) transduced with the antiapoptotic protein Bcl-2 will spontaneously organize into perfused microvessels within type I collagen gels when implanted in immunodeficient mice. To test if this system can be used to perfuse more complex structures, we combined Bcl-2-transduced ECs (Bcl-2-ECs) with renal glomeruli, the specialized vascular filtration units of the kidney. Microdissected green fluorescent protein-expressing rat glomeruli suspended in type I collagen gels were implanted within immunodeficient mice with or without the inclusion of Bcl-2-ECs. Survival of rat glomeruli was enhanced by coimplantation with Bcl-2-ECs. Intravital rhodamine dextran injections demonstrated that surviving glomeruli were perfused through Bcl-2-EC-derived microvessels. Perfused glomeruli maintained podocin staining, but transmission electron microscopy revealed endothelial swelling and podocyte foot process effacement. Anastomosis of microvessels derived from Bcl-2-ECs with glomerular capillaries provides proof of concept that self-assembled microvessels can perfuse specialized organ structures such as glomeruli, but that perfusion alone may be insufficient to maintain normal structure.

A hydrogel-endothelial cell implant mimics infantile hemangioma: modulation by survivin and the Hippo pathway

Microvascular endothelial cells cultured in three-dimensional hydrogel scaffolds form a network of microvessel structures when implanted subcutaneously in mice, inosculate with host vessels, and over time remodel into large ectatic vascular structures resembling hemangiomas. When compared with infantile hemangiomas, similarities were noted, including a temporal progression from a morphological appearance of a proliferative phase to the appearance of an involuted phase, mimicking the proliferative and involutional phases of infantile hemangioma. Consistent with the progression of a proliferative phase to an involuted phase, both the murine implants and human biopsy tissue exhibit reduced expression of Ajuba, YAP, and Survivin labeling as they progressed over time. Significant numbers of CD45+, CD11b+, Mac3+ mononuclear cells were found at the 2-week time point in our implant model that correlated with the presence of CD45+, CD68+ mononuclear cells observed in biopsies of human proliferative-phase hemangiomas. At the 4-week time point in our implant model, only small numbers of CD45+ cells were detected, which again correlated with our findings of significantly diminished CD45+, CD68+ mononuclear cells in human involutional-phase hemangiomas. The demonstration of mononuclear cell infiltration transiently in the proliferative phase of these lesions suggests that the vascular proliferation and/or regression may be driven in part by an immune response. Gross and microscopic morphological appearances of human proliferative and involutional hemangiomas and our implant model correlate well with each other as do the expression levels of Hippo pathway components (Ajuba and YAP) and Survivin and correlate with proliferation in these entities. Inhibitors of Survivin and Ajuba (which we have demonstrated to inhibit proliferation and increase apoptosis in murine hemangioendothelioma cell tissue culture) may have potential as other beneficial treatments for proliferating infantile hemangiomas. This implant model may have potential as a modest through-put screen for testing and development of therapeutics targeted at the proliferative phase of infantile hemangiomas, reducing the subsequent postinvolutional scarring or deformities sometimes associated with these lesions.

New Methods in Tissue Engineering: Improved Models for Viral Infection

New insights in the study of virus and host biology in the context of viral infection are made possible by the development of model systems that faithfully recapitulate the in vivo viral life cycle. Standard tissue culture models lack critical emergent properties driven by cellular organization and in vivo-like function, whereas animal models suffer from limited susceptibility to relevant human viruses and make it difficult to perform detailed molecular manipulation and analysis. Tissue engineering techniques may enable virologists to create infection models that combine the facile manipulation and readouts of tissue culture with the virus-relevant complexity of animal models. Here, we review the state of the art in tissue engineering and describe how tissue engineering techniques may alleviate some common shortcomings of existing models of viral infection, with a particular emphasis on hepatotropic viruses. We then discuss possible future applications of tissue engineering to virology, including current challenges and potential solutions.

Paracrine exchanges of molecular signals between alginate-encapsulated pericytes and freely suspended endothelial cells within a 3D protein gel

Paracrine signals, essential for the proper survival and functioning of tissues, may be mimicked by delivery of therapeutic proteins within engineered tissue constructs. Conventional delivery methods are of limited duration and are unresponsive to the local environment. We developed a system for sustained and regulated delivery of paracrine signals by encapsulating living cells of one type in alginate beads and co-suspending these cell-loaded particles along with unencapsulated cells of a second type within a 3D protein gel. This system was applied to vascular tissue engineering by placing human placental microvascular pericytes (PCs) in the particulate alginate phase and human umbilical vein endothelial cells (HUVECs) in the protein gel phase. Particle characteristics were optimized to keep the encapsulated PCs viable for at least two weeks. Encapsulated PCs were bioactive in vitro, secreting hepatocyte growth factor, an angiogenic protein, and responding to externally applied HUVEC-derived signals. Encapsulated PCs influenced HUVEC behavior in the surrounding gel by enhancing the formation of vessel-like structures when compared to empty alginate bead controls. In vivo, encapsulated PCs modulated the process of vascular self-assembly by HUVECs in 3D gels following implantation into immunodeficient mice. We conclude that alginate encapsulated cells can provide functional paracrine signals within engineered tissues.

Advances and challenges in studying hepatitis C virus in its native environment

Approximately 2% of the worldwide population is infected with hepatitis C virus (HCV), the major causative agent of non-A, non-B hepatitis. Although substantial progress has been made in developing tools to dissect the viral life cycle, most in vitro studies rely on hepatoma cell lines, which are functionally disparate from the natural in vivo target of the virus – hepatocytes. To gain insights into virus–host interactions, there is a need for HCV-model systems that more closely mimic the physiological environment of the liver. Here, we discuss recent advances in culture and detection systems that facilitate the study of HCV in primary cells. Use of these new models may help bridge the gap between in vitro studies and clinical research.