Human hepatocytes and hematolymphoid dual reconstitution in treosulfan-conditioned uPA-NOG mice

Engineering Mice to Study Human Immunity

Humanized mice, which carry a human hematopoietic and immune system, have greatly advanced our understanding of human immune responses and immunological diseases. These mice are created via the transplantation of human hematopoietic stem and progenitor cells into immunocompromised murine hosts further engineered to support human hematopoiesis and immune cell growth. This article explores genetic modifications in mice that enhance xeno-tolerance, promote human hematopoiesis and immunity, and enable xenotransplantation of human tissues with resident immune cells. We also discuss genetic editing of the human immune system, provide examples of how humanized mice with humanized organs model diseases for mechanistic studies, and highlight the roles of these models in advancing knowledge of organ biology, immune responses to pathogens, and preclinical drugs tested for cancer treatment. The integration of multi-omics and state-of-the art approaches with humanized mouse models is crucial for bridging existing human data with causality and promises to significantly advance mechanistic studies.

Humanized Mouse Systems to Study Viral Infection: A New Era in Immunology Research

For decades, scientists have relied on traditional animal models to study viral infection and the immune response. However, these models have limitations, and the search for more accurate and reliable ways to study the human-pathogen interphase has led to the development of humanized mouse systems. These revolutionary models have transformed how we understand viral infection and the human immune system's interactions with viruses to control or exacerbate disease. They are also paving the way for new treatments and therapies. In this article, we explore the history and development of humanized mouse systems and their advantages, limitations, and applications in viral immunology research. We describe the different types of humanized mouse models, including their generation and utility for studying human pathogens, with an emphasis on human-specific viruses. In addition, we discuss areas for further refinement and future applications.

Current Treatment Regimens and Promising Molecular Therapies for Chronic Hepatobiliary Diseases

Chronic hepatobiliary damage progressively leads to fibrosis, which may evolve into cirrhosis and/or hepatocellular carcinoma. The fight against the increasing incidence of liver-related morbidity and mortality is challenged by a lack of clinically validated early-stage biomarkers and the limited availability of effective anti-fibrotic therapies. Current research is focused on uncovering the pathogenetic mechanisms that drive liver fibrosis. Drugs targeting molecular pathways involved in chronic hepatobiliary diseases, such as inflammation, hepatic stellate cell activation and proliferation, and extracellular matrix production, are being developed. Etiology-specific treatments, such as those for hepatitis B and C viruses, are already in clinical use, and efforts to develop new, targeted therapies for other chronic hepatobiliary diseases are ongoing. In this review, we highlight the major molecular changes occurring in patients affected by metabolic dysfunction-associated steatotic liver disease, viral hepatitis (Delta virus), and autoimmune chronic liver diseases (autoimmune hepatitis, primary biliary cholangitis, and primary sclerosing cholangitis). Further, we describe how this knowledge is linked to current molecular therapies as well as ongoing preclinical and clinical research on novel targeting strategies, including nucleic acid-, mesenchymal stromal/stem cell-, and extracellular vesicle-based options. Much clinical development is obviously still missing, but the plethora of promising potential treatment strategies in chronic hepatobiliary diseases holds promise for a future reversal of the current increase in morbidity and mortality in this group of patients.

Mice Engrafted with Human Liver Cells

Rodents are commonly employed to model human liver conditions, although species differences can restrict their translational relevance. To overcome some of these limitations, researchers have long pursued human hepatocyte transplantation into rodents. More than 20 years ago, the first primary human hepatocyte transplantations into immunodeficient mice with liver injury were able to support hepatitis B and C virus infections, as these viruses cannot replicate in murine hepatocytes. Since then, hepatocyte chimeric mouse models have transitioned into mainstream preclinical research and are now employed in a diverse array of liver conditions beyond viral hepatitis, including malaria, drug metabolism, liver-targeting gene therapy, metabolic dysfunction-associated steatotic liver disease, lipoprotein and bile acid biology, and others. Concurrently, endeavors to cotransplant other cell types and humanize immune and other nonparenchymal compartments have seen growing success. Looking ahead, several challenges remain. These include enhancing immune functionality in mice doubly humanized with hepatocytes and immune systems, efficiently creating mice with genetically altered grafts and reliably humanizing chimeric mice with renewable cell sources such as patient-specific induced pluripotent stem cells. In conclusion, hepatocyte chimeric mice have evolved into vital preclinical models that address many limitations of traditional rodent models. Continued improvements may further expand their applications.

Mathematical Models of Early Hepatitis B Virus Dynamics in Humanized Mice

Analyzing the impact of the adaptive immune response during acute hepatitis B virus (HBV) infection is essential for understanding disease progression and control. Here we developed mathematical models of HBV infection which either lack terms for adaptive immune responses, or assume adaptive immune responses in the form of cytolytic immune killing, non-cytolytic immune cure, or non-cytolytic-mediated block of viral production. We validated the model that does not include immune responses against temporal serum hepatitis B DNA (sHBV) and temporal serum hepatitis B surface-antigen (HBsAg) experimental data from mice engrafted with human hepatocytes (HEP). Moreover, we validated the immune models against sHBV and HBsAg experimental data from mice engrafted with HEP and human immune system (HEP/HIS). As expected, the model that does not include adaptive immune responses matches the observed high sHBV and HBsAg concentrations in all HEP mice. By contrast, while all immune response models predict reduction in sHBV and HBsAg concentrations in HEP/HIS mice, the Akaike Information Criterion cannot discriminate between non-cytolytic cure (resulting in a class of cells refractory to reinfection) and antiviral block functions (of up to 99 % viral production 1-3 weeks following peak viral load). We can, however, reject cytolytic killing, as it can only match the sHBV and HBsAg data when we predict unrealistic levels of hepatocyte loss.

A Monocytic Barrier to the Humanization of Immunodeficient Mice

Mice with severe immunodeficiencies have become very important tools for studying foreign cells in an in vivo environment. Xenotransplants can be used to model cells from many species, although most often, mice are humanized through the transplantation of human cells or tissues to meet the needs of medical research. The development of immunodeficient mice is reviewed leading up to the current state-of-the-art strains, such as the NOD-scid-gamma (NSG) mouse. NSG mice are excellent hosts for human hematopoietic stem cell transplants or immune reconstitution through transfusion of human peripheral blood mononuclear cells. However, barriers to full hematopoietic engraftment still remain; notably, the survival of human cells in the circulation is brief, which limits overall hematological and immune reconstitution. Reports have indicated a critical role for monocytic cells - monocytes, macrophages, and dendritic cells - in the clearance of xenogeneic cells from circulation. Various aspects of the NOD genetic background that affect monocytic cell growth, maturation, and function that are favorable to human cell transplantation are discussed. Important receptors, such as SIRPα, that form a part of the innate immune system and enable the recognition and phagocytosis of foreign cells by monocytic cells are reviewed. The development of humanized mouse models has taken decades of work in creating more immunodeficient mice, genetic modification of these mice to express human genes, and refinement of transplant techniques to optimize engraftment. Future advances may focus on the monocytic cells of the host to find ways for further engraftment and survival of xenogeneic cells.

Persistent hepatitis B virus and HIV coinfections in dually humanized mice engrafted with human liver and immune system

Chronic hepatitis B (CHB), caused by hepatitis B virus (HBV), remains a major medical problem. HBV has a high propensity for progressing to chronicity and can result in severe liver disease, including fibrosis, cirrhosis, and hepatocellular carcinoma. CHB patients frequently present with viral coinfection, including human immunodeficiency virus type (HIV) and hepatitis delta virus. About 10% of chronic HIV carriers are also persistently infected with HBV, which can result in more exacerbated liver disease. Mechanistic studies of HBV-induced immune responses and pathogenesis, which could be significantly influenced by HIV infection, have been hampered by the scarcity of immunocompetent animal models. Here, we demonstrate that humanized mice dually engrafted with components of a human immune system and a human liver supported HBV infection, which was partially controlled by human immune cells, as evidenced by lower levels of serum viremia and HBV replication intermediates in the liver. HBV infection resulted in priming and expansion of human HLA-restricted CD8+ T cells, which acquired an activated phenotype. Notably, our dually humanized mice support persistent coinfections with HBV and HIV, which opens opportunities for analyzing immune dysregulation during HBV and HIV coinfection, and preclinical testing of novel immunotherapeutics.

Targeted viral adaptation generates a simian-tropic hepatitis B virus that infects marmoset cells

Hepatitis B virus (HBV) only infects humans and chimpanzees, posing major challenges for modeling HBV infection and chronic viral hepatitis. The major barrier in establishing HBV infection in non-human primates lies at incompatibilities between HBV and simian orthologues of the HBV receptor, sodium taurocholate co-transporting polypeptide (NTCP). Through mutagenesis analysis and screening among NTCP orthologues from Old World monkeys, New World monkeys and prosimians, we determined key residues responsible for viral binding and internalization, respectively and identified marmosets as a suitable candidate for HBV infection. Primary marmoset hepatocytes and induced pluripotent stem cell-derived hepatocyte-like cells support HBV and more efficient woolly monkey HBV (WMHBV) infection. Adapted chimeric HBV genome harboring residues 1-48 of WMHBV preS1 generated here led to a more efficient infection than wild-type HBV in primary and stem cell derived marmoset hepatocytes. Collectively, our data demonstrate that minimal targeted simianization of HBV can break the species barrier in small NHPs, paving the path for an HBV primate model.

A spatiotemporally controlled recombinant cccDNA mouse model for studying HBV and developing drugs against the virus

Covalently closed circular (ccc) DNA is the template for hepatitis B virus (HBV) replication. The lack of small animal models for characterizing chronic HBV infection has hampered research progress in HBV pathogenesis and drug development. Here, we generated a spatiotemporally controlled recombinant cccDNA (rcccDNA) mouse model by combining Cre/loxP-mediated DNA recombination with the liver-specific "Tet-on/Cre" system. The mouse model harbors three transgenes: a single copy of the HBV genome (integrated at the Rosa26 locus, RHBV), H11-albumin-rtTA (spatiotemporal conditional module), and (tetO)₇-Cre (tetracycline response element), and is named as RHTC mouse. By supplying the RHTC mice with doxycycline (DOX)-containing drinking water for two days, the animals generate rcccDNA in hepatocytes, and the rcccDNA supports active HBV gene expression and can maintain HBV viremia persistence for over 60 weeks. Persistent HBV gene expression induces intrahepatic inflammation, fibrosis, and dysplastic pathology, which closely mirrors the disease progression in clinical patients. Bepirovirsen, an antisense oligonucleotide (ASO) targeting all HBV RNA species, showed dose-dependent antiviral effects in the RHTC mouse model. The spatiotemporally controlled rcccDNA mouse is convenient and reliable, providing versatile small animal model for studying cccDNA-centric HBV biology as well as evaluating antiviral therapeutics.

Emerging Preclinical Applications of Humanized Mouse Models in the Discovery and Validation of Novel Immunotherapeutics and Their Mechanisms of Action for Improved Cancer Treatment

Cancer therapeutics have undergone immense research over the past decade. While chemotherapies remain the mainstay treatments for many cancers, the advent of new molecular techniques has opened doors for more targeted modalities towards cancer cells. Although immune checkpoint inhibitors (ICIs) have demonstrated therapeutic efficacy in treating cancer, adverse side effects related to excessive inflammation are often reported. There is a lack of clinically relevant animal models to probe the human immune response towards ICI-based interventions. Humanized mouse models have emerged as valuable tools for pre-clinical research to evaluate the efficacy and safety of immunotherapy. This review focuses on the establishment of humanized mouse models, highlighting the challenges and recent advances in these models for targeted drug discovery and the validation of therapeutic strategies in cancer treatment. Furthermore, the potential of these models in the process of uncovering novel disease mechanisms is discussed.

A novel cDNA-uPA/SCID/Rag2-/- /Jak3-/- mouse model for hepatitis virus infection and reconstruction of human immune system

Although human hepatocyte-transplanted immunodeficient mice support infection with hepatitis viruses, these mice fail to develop viral hepatitis due to the lack of an adaptive immune system. In this study, we generated new immunodeficiency cDNA-urokinase-type plasminogen activator (uPA)/SCID/Rag2-/- /Jak3-/- mice and established a mouse model with both a humanized liver and immune system. Transplantation of human hepatocytes with human leukocyte antigen (HLA)-A24 resulted in establishment of a highly replaced liver in cDNA-uPA/SCID/Rag2-/- /Jak3-/- mice. These mice were successfully infected with hepatitis B virus (HBV) and hepatitis C virus (HCV) for a prolonged period and facilitate analysis of the effect of anti-HCV drugs. Administration of peripheral blood mononuclear cells (PBMCs) obtained from an HLA-A24 donor resulted in establishment of 22.6%-81.3% human CD45-positive mononuclear cell chimerism in liver-infiltrating cells without causing graft-versus-host disease in cDNA-uPA/SCID/Rag2-/- /Jak3-/- mice without human hepatocyte transplantation. When mice were transplanted with human hepatocytes and then administered HLA-A24-positive human PBMCs, an alloimmune response between transplanted human hepatocytes and PBMCs occurred, with production of transplanted hepatocyte-specific anti-HLA antibody. In conclusion, we succeeded in establishing a humanized liver/immune system characterized by an allo-reaction between transplanted human immune cells and human liver using a novel cDNA-uPA/SCID/Rag2-/- /Jak3-/- mouse. This mouse model can be used to generate a chronic hepatitis mouse model with a human immune system with application not only to hepatitis virus virology but also to investigation of the pathology of post-transplantation liver rejection.

Can next-generation humanized mice that reconstituted with both functional human immune system and hepatocytes model the progression of viral hepatitis to hepatocarcinogenesis?

Hepatitis B virus (HBV) and Hepatitis C virus (HCV) chronic infections cause liver immunopathological diseases such as hepatitis, fibrosis, cirrhosis, and hepatocellular carcinomas, which are difficult to treat and continue to be major health problems globally. Due to the species-specific hepato-tropism of HBV and HCV, conventional rodent models are limited in their utility for studying the infection and associated liver immunopathogenesis. Humanized mice reconstituted with both functional human immune system and hepatocytes (HIS-HuHEP mice) have been extremely instrumental for in vivo studies of HBV or HCV infection and human-specific aspects of the progression of liver immunopathogenesis. However, none of the current HIS-HuHEP mice can model the progression of viral hepatitis to hepatocarcinogenesis which may be a notorious result of HBV or HCV chronic infection in patients, suggesting that they were functionally compromised and that there is still significant space to improve and establish next-generation of HIS-HuHEP mice with more sophisticated functions. In this review, we first summarize the principal requirements to establish HIS-HuHEP mice. We then discuss the respective protocols for current HIS-HuHEP mice and their applications, as well as their advantages and disadvantages. We also raise perspectives for further improving and establishing next-generation HIS-HuHEP mice.

Preclinical models of idiosyncratic drug-induced liver injury (iDILI): Moving towards prediction

Idiosyncratic drug-induced liver injury (iDILI) encompasses the unexpected harms that prescription and non-prescription drugs, herbal and dietary supplements can cause to the liver. iDILI remains a major public health problem and a major cause of drug attrition. Given the lack of biomarkers for iDILI prediction, diagnosis and prognosis, searching new models to predict and study mechanisms of iDILI is necessary. One of the major limitations of iDILI preclinical assessment has been the lack of correlation between the markers of hepatotoxicity in animal toxicological studies and clinically significant iDILI. Thus, major advances in the understanding of iDILI susceptibility and pathogenesis have come from the study of well-phenotyped iDILI patients. However, there are many gaps for explaining all the complexity of iDILI susceptibility and mechanisms. Therefore, there is a need to optimize preclinical human in vitro models to reduce the risk of iDILI during drug development. Here, the current experimental models and the future directions in iDILI modelling are thoroughly discussed, focusing on the human cellular models available to study the pathophysiological mechanisms of the disease and the most used in vivo animal iDILI models. We also comment about in silico approaches and the increasing relevance of patient-derived cellular models.

Animal Models of Hepatitis B Virus Infection-Success, Challenges, and Future Directions

Chronic hepatitis B virus (HBV) infection affects more than 250 million people worldwide, which greatly increases the risk for terminal liver diseases, such as liver cirrhosis and hepatocellular carcinoma (HCC). Even though current approved antiviral therapies, including pegylated type I interferon (IFN) and nucleos(t)ide analogs, can effectively suppress viremia, HBV infection is rarely cured. Since HBV exhibits a narrow species tropism and robustly infects only humans and higher primates, progress in HBV research and preclinical testing of antiviral drugs has been hampered by the scarcity of suitable animal models. Fortunately, a series of surrogate animal models have been developed for the study of HBV. An increased understanding of the barriers towards interspecies transmission has aided in the development of human chimeric mice and has greatly paved the way for HBV research in vivo, and for evaluating potential therapies of chronic hepatitis B. In this review, we summarize the currently available animal models for research of HBV and HBV-related hepadnaviruses, and we discuss challenges and future directions for improvement.

Establishment of Humanized Mice for the Study of HBV

Viral hepatitis particularly Hepatitis B Virus (HBV) is still an ongoing health issue worldwide. Despite the vast technological advancements in research and development, only HBV vaccines, typically given during early years, are currently available as a preventive measure against acquiring the disease from a secondary source. In general, HBV can be cleared naturally by the human immune system if detected at low levels early. However, long term circulation of HBV in the peripheral blood may be detrimental to the human liver, specifically targeting human hepatocytes for cccDNA integration which inevitably supports HBV life cycle for the purpose of reinfection in healthy cells. Although there is some success in using nucleoside analogs or polyclonal antibodies targeting HBV surface antigens (HBsAg) in patients with acute or chronic HBV+ (CHB), majority of them would either respond only partially or succumb to the disease entirely unless they undergo liver transplants from a fully matched healthy donor and even so may not necessarily guarantee a 100% chance of survival. Indeed, in vitro/ex vivo cultures and various transgenic animal models have already provided us with a good understanding of HBV but they primarily lack human specificity or virus-host interactions in the presence of human immune surveillance. Therefore, the demand of utilizing humanized mice has increased over the last decade as a pre-clinical platform for investigating human-specific immune responses against HBV as well as identifying potential immunotherapeutic strategies in eradicating the virus. Basically, this review covers some of the recent developments and key advantages of humanized mouse models over other conventional transgenic mice platforms.

Innovations, challenges, and minimal information for standardization of humanized mice

Mice xenotransplanted with human cells and/or expressing human gene products (also known as "humanized mice") recapitulate the human evolutionary specialization and diversity of genotypic and phenotypic traits. These models can provide a relevant in vivo context for understanding of human-specific physiology and pathologies. Humanized mice have advanced toward mainstream preclinical models and are now at the forefront of biomedical research. Here, we considered innovations and challenges regarding the reconstitution of human immunity and human tissues, modeling of human infections and cancer, and the use of humanized mice for testing drugs or regenerative therapy products. As the number of publications exploring different facets of humanized mouse models has steadily increased in past years, it is becoming evident that standardized reporting is needed in the field. Therefore, an international community-driven resource called "Minimal Information for Standardization of Humanized Mice" (MISHUM) has been created for the purpose of enhancing rigor and reproducibility of studies in the field. Within MISHUM, we propose comprehensive guidelines for reporting critical information generated using humanized mice.

Animal Models Used in Hepatitis C Virus Research

The narrow range of species permissive to infection by hepatitis C virus (HCV) presents a unique challenge to the development of useful animal models for studying HCV, as well as host immune responses and development of chronic infection and disease. Following earlier studies in chimpanzees, several unique approaches have been pursued to develop useful animal models for research while avoiding the important ethical concerns and costs inherent in research with chimpanzees. Genetically related hepatotropic viruses that infect animals are being used as surrogates for HCV in research studies; chimeras of these surrogate viruses harboring specific regions of the HCV genome are being developed to improve their utility for vaccine testing. Concurrently, genetically humanized mice are being developed and continually advanced using human factors known to be involved in virus entry and replication. Further, xenotransplantation of human hepatocytes into mice allows for the direct study of HCV infection in human liver tissue in a small animal model. The current advances in each of these approaches are discussed in the present review.

Animal Models of Hepatitis C Virus Infection

Hepatitis C virus (HCV) is an important and underreported infectious disease, causing chronic infection in ∼71 million people worldwide. The limited host range of HCV, which robustly infects only humans and chimpanzees, has made studying this virus in vivo challenging and hampered the development of a desperately needed vaccine. The restrictions and ethical concerns surrounding biomedical research in chimpanzees has made the search for an animal model all the more important. In this review, we discuss different approaches that are being pursued toward creating small animal models for HCV infection. Although efforts to use a nonhuman primate species besides chimpanzees have proven challenging, important advances have been achieved in a variety of humanized mouse models. However, such models still fall short of the overarching goal to have an immunocompetent, inheritably susceptible in vivo platform in which the immunopathology of HCV could be studied and putative vaccines development. Alternatives to overcome this include virus adaptation, such as murine-tropic HCV strains, or the use of related hepaciviruses, of which many have been recently identified. Of the latter, the rodent/rat hepacivirus from Rattus norvegicus species-1 (RHV-rn1) holds promise as a surrogate virus in fully immunocompetent rats that can inform our understanding of the interaction between the immune response and viral outcomes (i.e., clearance vs. persistence). However, further characterization of these animal models is necessary before their use for gaining new insights into the immunopathogenesis of HCV and for conceptualizing HCV vaccines.

Human macrophages and innate lymphoid cells: Tissue-resident innate immunity in humanized mice

Macrophages and innate lymphoid cells (ILCs) are tissue-resident cells that play important roles in organ homeostasis and tissue immunity. Their intricate relationship with the organs they reside in allows them to quickly respond to perturbations of organ homeostasis and environmental challenges, such as infection and tissue injury. Macrophages and ILCs have been extensively studied in mice, yet important species-specific differences exist regarding innate immunity between humans and mice. Complementary to ex-vivo studies with human cells, humanized mice (i.e. mice with a human immune system) offer the opportunity to study human macrophages and ILCs in vivo within their surrounding tissue microenvironments. In this review, we will discuss how humanized mice have helped gain new knowledge about the basic biology of these cells, as well as their function in infectious and malignant conditions. Furthermore, we will highlight active areas of investigation related to human macrophages and ILCs, such as their cellular heterogeneity, ontogeny, tissue residency, and plasticity. In the near future, we expect more fundamental discoveries in these areas through the combined use of improved humanized mouse models together with state-of-the-art technologies, such as single-cell RNA-sequencing and CRISPR/Cas9 genome editing.

Establishment of the Dual Humanized TK-NOG Mouse Model for HIV-associated Liver Pathogenesis

Despite the increased life expectancy of patients infected with human immunodeficiency virus-1 (HIV-1), liver disease has emerged as a common cause of their morbidity. The liver immunopathology caused by HIV-1 remains elusive. Small xenograft animal models with human hepatocytes and human immune system can recapitulate the human biology of the disease's pathogenesis. Herein, a protocol is described to establish a dual humanized mouse model through human hepatocytes and CD34+ hematopoietic stem/progenitor cells (HSPCs) transplantation, to study liver immunopathology as observed in HIV-infected patients. To achieve dual reconstitution, male TK-NOG (NOD.Cg-Prkdcscid Il2rgtm1Sug Tg(Alb-TK)7-2/ShiJic) mice are intraperitoneally injected with ganciclovir (GCV) doses to eliminate mouse transgenic liver cells, and with treosulfan for nonmyeloablative conditioning, both of which facilitate human hepatocyte (HEP) engraftment and human immune system (HIS) development. Human albumin (ALB) levels are evaluated for liver engraftment, and the presence of human immune cells in blood detected by flow cytometry confirms the establishment of human immune system. The model developed using the protocol described here resembles multiple components of liver damage from HIV-1 infection. Its establishment could prove to be essential for studies of hepatitis virus co-infection and for the evaluation of antiviral and antiretroviral drugs.

Mice with Chimeric Human Livers and Their Applications

The complete life cycle of the hepatitis C virus (HCV) can be recapitulated in vivo using immunodeficient mice that have had their livers extensively repopulated with human hepatocytes. These human liver chimeric mouse models have enabled the study of many aspects of the HCV life cycle, including antiviral interventions that have helped to shape the curative landscape that is available today. The first human liver chimeric mouse model capable of supporting the HCV life cycle was generated in SCID-uPA mice. Although other human liver chimeric mouse models have since been developed, the SCID-uPA mouse model remains one of the most robust in vivo systems available for HCV studies. This chapter reviews development, validation and application of the SCID-uPA mouse model, and discusses their potential application for studying other liver-centric diseases and pathogens and for the design and testing of vaccine candidates for the eradication of HCV.

Mouse Models for Studying HCV Vaccines and Therapeutic Antibodies

In spite of the immense progress in hepatitis C virus (HCV) research, efforts to prevent infection, such as generating a vaccine, have not yet been successful. The high price tag associated with current treatment options for chronic infection and the spike in new infections concurrent with growing opioid abuse are strong motivators for developing effective immunization and understanding neutralizing antibodies' role in preventing infection. Humanized mice-both human liver chimeras as well as genetically humanized models-are important platforms for testing both possible vaccine candidates as well as antibody-based therapies. This chapter details the variety of ways humanized mouse technology can be employed in pursuit of learning how HCV infection can be prevented.

Humanized Mouse Models for the Study of Infection and Pathogenesis of Human Viruses

The evolution of infectious pathogens in humans proved to be a global health problem. Technological advancements over the last 50 years have allowed better means of identifying novel therapeutics to either prevent or combat these infectious diseases. The development of humanized mouse models offers a preclinical in vivo platform for further characterization of human viral infections and human immune responses triggered by these virus particles. Multiple strains of immunocompromised mice reconstituted with a human immune system and/or human hepatocytes are susceptible to infectious pathogens as evidenced by establishment of full viral life cycles in hope of investigating viral–host interactions observed in patients and discovering potential immunotherapies. This review highlights recent progress in utilizing humanized mice to decipher human specific immune responses against viral tropism.

Human fetal liver cultures support multiple cell lineages that can engraft immunodeficient mice

During prenatal development the liver is composed of multiple cell types with unique properties compared to their adult counterparts. We aimed to establish multilineage cultures of human fetal liver cells that could maintain stem cell and progenitor populations found in the developing liver. An aim of this study was to test if maturation of fetal hepatocytes in short-term cultures supported by epidermal growth factor and oncostatin M can improve their ability to engraft immunodeficient mice. Fetal liver cultures supported a mixture of albumin⁺ cytokertin-19⁺ hepatoblasts, hepatocytes, cholangiocytes, CD14⁺⁺CD32⁺ liver sinusoidal endothelial cells (LSECs) and CD34⁺CD133⁺ haematopoietic stem cells. Transplantation of cultured cells into uPA-NOG or TK-NOG mice yielded long-term engraftment of hepatocytes, abundant LSEC engraftment and multilineage haematopoiesis. Haematopoietic engraftment included reconstitution of B-, T- and NK-lymphocytes. Colonies of polarized human hepatocytes were observed surrounded by human LSECs in contact with human CD45⁺ blood cells in the liver sinusoids. Thus, fetal liver cultures support multiple cell lineages including LSECs and haematopoietic stem cells while also promoting the ability of fetal hepatocytes to engraft adult mouse livers. Fetal liver cultures and liver-humanized mice created from these cultures can provide useful model systems to study liver development, function and disease.

Yellow Fever Virus: Knowledge Gaps Impeding the Fight Against an Old Foe

Yellow fever (YF) was one of the most dangerous infectious diseases of the 18th and 19th centuries, resulting in mass casualties in Africa and the Americas. The etiologic agent is yellow fever virus (YFV), and its live-attenuated form, YFV-17D, remains one of the most potent vaccines ever developed. During the first half of the 20th century, vaccination combined with mosquito control eradicated YFV transmission in urban areas. However, the recent 2016-2018 outbreaks in areas with historically low or no YFV activity have raised serious concerns for an estimated 400-500 million unvaccinated people who now live in at-risk areas. Once a forgotten disease, we highlight here that YF still represents a very real threat to human health and economies. As many gaps remain in our understanding of how YFV interacts with the human host and causes disease, there is an urgent need to address these knowledge gaps and propel YFV research forward.

Animal Models to Study Hepatitis C Virus Infection

With more than 71 million chronically infected people, the hepatitis C virus (HCV) is a major global health concern. Although new direct acting antivirals have significantly improved the rate of HCV cure, high therapy cost, potential emergence of drug-resistant viral variants, and unavailability of a protective vaccine represent challenges for complete HCV eradication. Relevant animal models are required, and additional development remains necessary, to effectively study HCV biology, virus–host interactions and for the evaluation of new antiviral approaches and prophylactic vaccines. The chimpanzee, the only non-human primate susceptible to experimental HCV infection, has been used extensively to study HCV infection, particularly to analyze the innate and adaptive immune response upon infection. However, financial, practical, and especially ethical constraints have urged the exploration of alternative small animal models. These include different types of transgenic mice, immunodeficient mice of which the liver is engrafted with human hepatocytes (humanized mice) and, more recently, immunocompetent rodents that are susceptible to infection with viruses that are closely related to HCV. In this review, we provide an overview of the currently available animal models that have proven valuable for the study of HCV, and discuss their main benefits and weaknesses.

The use of humanized mice for studies of viral pathogenesis and immunity

Humanized mice, that is, animals engrafted with human tissues and/or expressing human genes, have been instrumental in improving our understanding of the pathogenesis and immunological processes that define some of the most challenging human-tropic viruses. In particular, mice engrafted with components of a human immune system (HIS) offer unprecedented opportunities for mechanistic studies of human immune responses to infection. Here, we provide a brief overview of the current panel of HIS mouse models available and cite recent examples of how such humanized animals have been used to study immune responses and pathogenesis elicited by human-tropic viruses. Finally, we will outline some of the challenges that lay ahead and strategies to improve and refine humanized mice with the goal of more accurately recapitulating human immune responses to viral infection.

Human hepatocyte depletion in the presence of HIV-1 infection in dual reconstituted humanized mice

Human immunodeficiency virus type 1 (HIV-1) infection impairs liver function, and liver diseases have become a leading cause of morbidity in infected patients. The immunopathology of liver damage caused by HIV-1 remains unclear. We used chimeric mice dually reconstituted with a human immune system and hepatocytes to address the relevance of the model to pathobiology questions related to human hepatocyte survival in the presence of systemic infection. TK-NOG males were transplanted with mismatched human hematopoietic stem/progenitor cells and hepatocytes, human albumin concentration and the presence of human immune cells in blood were monitored for hepatocytes and immune reconstitution, and mice were infected with HIV-1. HIV-1-infected animals showed a decline in human albumin concentration with a significant reduction in percentage of human hepatocytes compared to uninfected mice. The decrease in human albumin levels correlated with a decline in CD4+ cells in the liver and with an increase in HIV-1 viral load. HIV-1 infection elicited proinflammatory response in the immunological milieu of the liver in HIV-infected mice compared to uninfected animals, as determined by upregulation of IL23, CXCL10 and multiple toll-like receptor expression. The inflammatory reaction associated with HIV-1 infection in vivo could contribute to the depletion and dysfunction of hepatocytes. The dual reconstituted TK-NOG mouse model is a feasible platform to investigate hepatocyte-related HIV-1 immunopathogenesis.This article has an associated First Person interview with the first author of the paper.

Viral Load Affects the Immune Response to HBV in Mice With Humanized Immune System and Liver

BACKGROUND & AIMS

Hepatitis B virus (HBV) infects hepatocytes, but the mechanisms of the immune response against the virus and how it affects disease progression are unclear.

METHODS

We performed studies with BALB/c Rag2^-/-Il2rg^-/-Sirpa^NODAlb-uPA^tg/tg mice, stably engrafted with human hepatocytes (HUHEP) with or without a human immune system (HIS). HUHEP and HIS-HUHEP mice were given an intraperitoneal injection of HBV. Mononuclear cells were isolated from spleen and liver for analysis by flow cytometry. Liver was analyzed by immunohistochemistry and mRNA levels were measured by quantitative reverse transcription polymerase chain reaction (PCR). Plasma levels of HBV DNA were quantified by PCR reaction, and antigen-specific antibodies were detected by immunocytochemistry of HBV-transfected BHK-21 cells.

RESULTS

Following HBV infection, a complete viral life cycle, with production of HBV DNA, hepatitis B e (HBe), core (HBc) and surface (HBs) antigens, and covalently closed circular DNA, was observed in HUHEP and HIS-HUHEP mice. HBV replicated unrestricted in HUHEP mice resulting in high viral titers without pathologic effects. In contrast, HBV-infected HIS-HUHEP mice developed chronic hepatitis with 10-fold lower titers and antigen-specific IgGs, (anti-HBs, anti-HBc), consistent with partial immune control. HBV-infected HIS-HUHEP livers contained infiltrating Kupffer cells, mature activated natural killer cells (CD69+), and PD-1+ effector memory T cells (CD45RO+). Reducing the viral inoculum resulted in more efficient immune control. Plasma from HBV-infected HIS-HUHEP mice had increased levels of inflammatory and immune-suppressive cytokines (C-X-C motif chemokine ligand 10 and interleukin 10), which correlated with populations of intrahepatic CD4+ T cells (CD45RO+PD-1+). Mice with high levels of viremia had HBV-infected liver progenitor cells. Giving the mice the nucleoside analogue entecavir reduced viral loads and decreased liver inflammation.

CONCLUSION

In HIS-HUHEP mice, HBV infection completes a full life cycle and recapitulates some of the immunopathology observed in patients with chronic infection. Inoculation with different viral loads led to different immune responses and levels of virus control. We found HBV to infect liver progenitor cells, which could be involved in hepatocellular carcinogenesis. This is an important new system to study anti-HBV immune responses and screen for combination therapies against hepatotropic viruses.

Modeling hepatitis virus infections and treatment strategies in humanized mice

Hepatitis viruses cause chronic liver diseases such as fibrosis, cirrhosis and hepatocellular carcinomas that are difficult to treat and constitute a global health problem. Species-specific viral tropism has limited the usefulness of small animal models to study the impact of viral hepatitis. Immunodeficient mice grafted with human hepatocytes are susceptible to hepatitis viruses B, C, D and E (HBV, HCV, HDV and HEV), developing full viral life cycles, and delivering a means to investigate virus-host interactions and antiviral treatments. These chimeric humanized mouse models have been further grafted with humanized immune systems to decipher immune responses following hepatotropic viral infections, the ensuing pathophysiology, and to test novel therapeutic strategies.

Animal models of HBV infection

The mechanisms determining hepatitis B virus (HBV) persistence and pathogenesis are not fully elucidated, but appear to be multi-factorial. Current medication to repress viral replication is available; however, the unique replication strategies employed by HBV enable the virus to persist within the infected hepatocytes. Consequently, cure is rarely achieved. Progresses in HBV research and preclinical testing of antiviral agents have been limited by the narrow species- and tissue-tropism of the virus, the paucity of infection models available and the restrictions imposed by the use of chimpanzees, the only animals fully susceptible to HBV infection. Mice are not HBV permissive but major efforts have focused on the development of mouse models of HBV replication and infection, such as the generation of humanized mice. By presenting the different animal models available, this review will highlight the most important and clinically relevant findings that have been retrieved from the respective systems.

Development of mouse models for analysis of human virus infections

Viruses usually exhibit strict species-specificity as a result of co-evolution with the host. Thus, in mouse models, a great barrier exists for analysis of infections with human-tropic viruses. Mouse models are unlikely to faithfully reproduce the human immune response to viruses or viral compounds and it is difficult to evaluate human therapeutic efficacy with antiviral reagents in mouse models. Humans and mice essentially have different immune systems, which makes it difficult to extrapolate mouse results to humans. In addition, apart from immunological reasons, viruses causing human diseases do not always infect mice because of species tropism. One way to determine tropism would be a virus receptor that is expressed on affected cells. The development of gene-disrupted mice and Tg mice, which express human receptor genes, enables us to analyze several viral infections in mice. Mice are, indeed, susceptible to human viruses when artificially infected in receptor-supplemented mice. Although the mouse cells less efficiently permit viral replication than do human cells, the models for analysis of human viruses have been established in vivo as well as in vitro, and explain viral pathogenesis in the mouse systems. In most systems, however, nucleic acid sensors and type I interferon suppress viral propagation to block the appearance of infectious manifestation. We herein review recent insight into in vivo antiviral responses induced in mouse infection models for typical human viruses.

Modeling Natural Killer Cell Targeted Immunotherapies

Animal models have extensively contributed to our understanding of human immunobiology and to uncover the underlying pathological mechanisms occurring in the development of diseases. However, mouse models do not reproduce the genetic and molecular complexity inherent in human disease conditions. Human immune system (HIS) mouse models that are susceptible to human pathogens and can recapitulate human hematopoiesis and tumor immunobiology provide one means to bridge the interspecies gap. Natural killer cells are the founding member of the innate lymphoid cell family. They exert a rapid and strong immune response against tumor and pathogen-infected cells. Their antitumor features have long been exploited for therapeutic purposes in the context of cancer. In this review, we detail the development of highly immunodeficient mouse strains and the models currently used in cancer research. We summarize the latest improvements in adoptive natural killer (NK) cell therapies and the development of novel NK cell sources. Finally, we discuss the advantages of HIS mice to study the interactions between human NK cells and human cancers and to develop new therapeutic strategies.

The Application of Humanized Mouse Models for the Study of Human Exclusive Viruses

The symbiosis between humans and viruses has allowed human tropic pathogens to evolve intricate means of modulating the human immune response to ensure its survival among the human population. In doing so, these viruses have developed profound mechanisms that mesh closely with our human biology. The establishment of this intimate relationship has created a species-specific barrier to infection, restricting the virus-associated pathologies to humans. This specificity diminishes the utility of traditional animal models. Humanized mice offer a model unique to all other means of study, providing an in vivo platform for the careful examination of human tropic viruses and their interaction with human cells and tissues. These types of animal models have provided a reliable medium for the study of human-virus interactions, a relationship that could otherwise not be investigated without questionable relevance to humans.

Higher Serum Alanine Transaminase Levels in Male Urokinase-Type Plasminogen Activator-Transgenic Mice Are Associated With Improved Engraftment of Hepatocytes but not Liver Sinusoidal Endothelial Cells

The effects of sex on the degree of liver damage and human cell engraftment were investigated in immunodeficient urokinase-type plasminogen activator-transgenic (uPA-NOG) mice. Liver damage, measured by serum alanine transaminase (ALT) levels, was compared in male and female uPA-NOG mice of different ages. Male mice had significantly higher ALT levels than females with a median of 334 versus 158 U/L in transgenic homozygous mice, respectively. Mice were transplanted with human adult hepatocytes or fetal liver cells and analyzed for any correlation of engraftment of hepatocytes, liver sinusoidal endothelial cells (LSECs), and hematopoietic cells with the degree of liver damage. Hepatocyte engraftment was measured by human albumin levels in the mouse serum. Higher ALT levels correlated with higher hepatocyte engraftment, resulting in albumin levels in male mice that were 9.6 times higher than in females. LSEC and hematopoietic cell engraftment were measured by flow cytometric analysis of the mouse liver and bone marrow. LSEC and hematopoietic engraftment did not differ between male and female transplant recipients. Thus, the sex of uPA-NOG mice affects the degree of liver damage, which is reflected in the levels of human hepatocyte engraftment. However, the high levels of LSEC engraftment observed in uPA-NOG mice are not further improved among male mice, suggesting that a lower threshold of liver damage is sufficient to enhance endothelial cell engraftment. Previously described sex differences in human hematopoietic stem cell engraftment in immunodeficient mice were not observed in this model.

Hepatocarcinogenesis associated with hepatitis B, delta and C viruses

Globally, over half a billion people are persistently infected with hepatitis B (HBV) and/or hepatitis C viruses. Chronic HBV and HCV infection frequently lead to fibrosis, cirrhosis and hepatocellular carcinoma (HCC). Co-infections with hepatitis delta virus (HDV), a subviral satellite requiring HBV for its propagation, accelerates the progression of liver disease toward HCC. The mechanisms by which these viruses cause malignant transformation, culminating in HCC, remain incompletely understood, partially due to the lack of adequate experimental models for dissecting these complex disease processes in vivo.

Humanized mice efficiently engrafted with fetal hepatoblasts and syngeneic immune cells develop human monocytes and NK cells

BACKGROUND & AIMS

Human liver chimeric mice are useful models of human hepatitis virus infection, including hepatitis B and C virus infections. Independently, immunodeficient mice reconstituted with CD34(+) hematopoietic stem cells (HSC) derived from fetal liver reliably develop human T and B lymphocytes. Combining these systems has long been hampered by inefficient liver reconstitution of human fetal hepatoblasts. Our study aimed to enhance hepatoblast engraftment in order to create a mouse model with syngeneic human liver and immune cells.

METHODS

The effects of human oncostatin-M administration on fetal hepatoblast engraftment into immunodeficient fah(-/-) mice was tested. Mice were then transplanted with syngeneic human hepatoblasts and HSC after which human leukocyte chimerism and functionality were analyzed by flow cytometry, and mice were challenged with HBV.

RESULTS

Addition of human oncostatin-M enhanced human hepatoblast engraftment in immunodeficient fah(-/-) mice by 5-100 fold. In contrast to mice singly engrafted with HSC, which predominantly developed human T and B lymphocytes, mice co-transplanted with syngeneic hepatoblasts also contained physiological levels of human monocytes and natural killer cells. Upon infection with HBV, these mice displayed rapid and sustained viremia.

CONCLUSIONS

Our study provides a new mouse model with improved human fetal hepatoblast engraftment and an expanded human immune cell repertoire. With further improvements, this model may become useful for studying human immunity against viral hepatitis.

LAY SUMMARY

Important human pathogens such as hepatitis B virus, hepatitis C virus and human immunodeficiency virus only infect human cells which complicates the development of mouse models for the study of these pathogens. One way to make mice permissive for human pathogens is the transplantation of human cells into immune-compromised mice. For instance, the transplantation of human liver cells will allow the infection of these so-called "liver chimeric mice" with hepatitis B virus and hepatitis C virus. The co-transplantation of human immune cells into liver chimeric mice will further allow the study of human immune responses to hepatitis B virus or hepatitis C virus. However, for immunological studies it will be crucial that the transplanted human liver and immune cells are derived from the same human donor. In our study we describe the efficient engraftment of human fetal liver cells and immune cells derived from the same donor into mice. We show that liver co-engraftment resulted in an expanded human immune cell repertoire, including monocytes and natural killer cells in the liver. We further demonstrate that these mice could be infected with hepatitis B virus, which lead to an expansion of natural killer cells. In conclusion we have developed a new mouse model that could be useful to study human immune responses to human liver pathogens.

Mouse Systems to Model Hepatitis C Virus Treatment and Associated Resistance

While addition of the first-approved protease inhibitors (PIs), telaprevir and boceprevir, to pegylated interferon (PEG-IFN) and ribavirin (RBV) combination therapy significantly increased sustained virologic response (SVR) rates, PI-based triple therapy for the treatment of chronic hepatitis C virus (HCV) infection was prone to the emergence of resistant viral variants. Meanwhile, multiple direct acting antiviral agents (DAAs) targeting either the HCV NS3/4A protease, NS5A or NS5B polymerase have been approved and these have varying potencies and distinct propensities to provoke resistance. The pre-clinical in vivo assessment of drug efficacy and resistant variant emergence underwent a great evolution over the last decade. This field had long been hampered by the lack of suitable small animal models that robustly support the entire HCV life cycle. In particular, chimeric mice with humanized livers (humanized mice) and chimpanzees have been instrumental for studying HCV inhibitors and the evolution of drug resistance. In this review, we present the different in vivo HCV infection models and discuss their applicability to assess HCV therapy response and emergence of resistant variants.

In vivo models of hepatitis B and C virus infection

Globally, more than 500 million individuals are chronically infected with hepatitis B (HBV), delta (HDV), and/or C (HCV) viruses, which can result in severe liver disease. Mechanistic studies of viral persistence and pathogenesis have been hampered by the scarcity of animal models. The limited species and cellular host range of HBV, HDV, and HCV, which robustly infect only humans and chimpanzees, have posed challenges for creating such animal models. In this review, we will discuss the barriers to interspecies transmission and the progress that has been made in our understanding of the HBV, HDV, and HCV life cycles. Additionally, we will highlight a variety of approaches that overcome these barriers and thus facilitate in vivo studies of these hepatotropic viruses.

Experimental in vitro and in vivo models for the study of human hepatitis B virus infection

Chronic infection with the hepatitis B virus (HBV) affects an estimate of 240 million people worldwide despite the availability of a preventive vaccine. Medication to repress viral replication is available but a cure is rarely achieved. The narrow species and tissue tropism of the virus and the lack of reliable in vitro models and laboratory animals susceptible to HBV infection, have limited research progress in the past. As a result, several aspects of the HBV life cycle as well as the network of virus host interactions occurring during the infection are not yet understood. Only recently, the identification of the functional cellular receptor enabling HBV entry has opened new possibilities to establish innovative infection systems. Regarding the in vivo models of HBV infection, the classical reference was the chimpanzee. However, because of the strongly restricted use of great apes for HBV research, major efforts have focused on the development of mouse models of HBV replication and infection such as the generation of humanized mice. This review summarizes the animal and cell culture based models currently available for the study of HBV biology. We will discuss the benefits and caveats of each model and present a selection of the most important findings that have been retrieved from the respective systems.

Chimeric antigen receptor (CAR)-engineered T cells redirected against hepatitis C virus (HCV) E2 glycoprotein

OBJECTIVE

The recent availability of novel antiviral drugs has raised new hope for a more effective treatment of hepatitis C virus (HCV) infection and its severe sequelae. However, in the case of non-responding or relapsing patients, alternative strategies are needed. To this end we have used chimeric antigen receptors (CARs), a very promising approach recently used in several clinical trials to redirect primary human T cells against different tumours. In particular, we designed the first CARs against HCV targeting the HCV/E2 glycoprotein (HCV/E2).

DESIGN

Anti-HCV/E2 CARs were composed of single-chain variable fragments (scFvs) obtained from a broadly cross-reactive and cross-neutralising human monoclonal antibody (mAb), e137, fused to the intracellular signalling motif of the costimulatory CD28 molecule and the CD3ζ domain. Activity of CAR-grafted T cells was evaluated in vitro against HCV/E2-transfected cells as well as hepatocytes infected with cell culture-derived HCV (HCVcc).

RESULTS

In this proof-of-concept study, retrovirus-transduced human T cells expressing anti-HCV/E2 CARs were endowed with specific antigen recognition accompanied by degranulation and secretion of proinflammatory and antiviral cytokines, such as interferon γ, interleukin 2 and tumour necrosis factor α. Moreover, CAR-grafted T cells were capable of lysing target cells of both hepatic and non-hepatic origin expressing on their surface the HCV/E2 glycoproteins of the most clinically relevant genotypes, including 1a, 1b, 2a, 3a, 4 and 5. Finally, and more importantly, they were capable of lysing HCVcc-infected hepatocytes.

CONCLUSIONS

Clearance of HCV-infected cells is a major therapeutic goal in chronic HCV infection, and adoptive transfer of anti-HCV/E2 CARs-grafted T cells represents a promising new therapeutic tool.

Generation of Human Liver Chimeric Mice for the Study of Human Hepatotropic Pathogens

Human liver chimeric mice have become valuable tools for the study of human hepatotropic pathogens and for the investigation of metabolism and pharmacokinetics of novel drugs. The evolution of the underlying mouse models has been rapid in the past years. The diverse fields of applications of those model systems and their technical challenges will be discussed in this chapter.

Liver-targeted antiviral peptide nanocomplexes as potential anti-HCV therapeutics

Great success in HCV therapy was achieved by the development of direct-acting antivirals (DAA). However, the unsolved issues such as high cost and genotype dependency drive us to pursue additional therapeutic agents to be used instead or in combination with DAA. The cationic peptide p41 is one of such candidates displaying submicromolar anti-HCV potency. By electrostatic coupling of p41 with anionic poly(amino acid)-based block copolymers, antiviral peptide nanocomplexes (APN) platform was developed to improve peptide stability and to reduce cytotoxicity associated with positive charge. Herein, we developed a facile method to prepare galactosylated Gal-APN and tested their feasibility as liver-specific delivery system. In vitro, Gal-APN displayed specific internalization in hepatoma cell lines. Even though liver-targeted and non-targeted APN displayed comparable antiviral activity, Gal-APN offered prominent advantages to prevent HCV association with lipid droplets and suppress intracellular expression of HCV proteins. Moreover, in vivo preferential liver accumulation of Gal-APN was revealed in the biodistribution study. Altogether, this work illustrates the potential of Gal-APN as a novel liver-targeted therapy against HCV.

Humanized Mouse Models to Study Cell-Mediated Immune Responses to Liver-Stage Malaria Vaccines

Malaria vaccine development is hampered by the lack of small animal models that recapitulate human immune responses to Plasmodium falciparum. We review the burgeoning literature on humanized mice for P. falciparum infection, including challenges in engraftment of human immune cells, hepatocytes, and erythrocytes. Recent advances in immune-compromised mouse models and stem cell technology have already enabled proof of concept that the entire parasite life cycle can be sustained in a murine model and that adaptive human immune responses to several parasite stages can be measured. Nonetheless, optimization is needed to achieve a reproducible and relevant murine model for malaria vaccine development. This review is focused on the complexities of T cell development in a mouse humanized with both a lymphoid system and hepatocytes. An understanding of this will facilitate the use of humanized mice in the development of liver-stage vaccines.

Genetic Dissection of the Host Tropism of Human-Tropic Pathogens

Infectious diseases are the second leading cause of death worldwide. Although the host multitropism of some pathogens has rendered their manipulation possible in animal models, the human-restricted tropism of numerous viruses, bacteria, fungi, and parasites has seriously hampered our understanding of these pathogens. Hence, uncovering the genetic basis underlying the narrow tropism of such pathogens is critical for understanding their mechanisms of infection and pathogenesis. Moreover, such genetic dissection is essential for the generation of permissive animal models that can serve as critical tools for the development of therapeutics or vaccines against challenging human pathogens. In this review, we describe different experimental approaches utilized to uncover the genetic foundation regulating pathogen host tropism as well as their relevance for studying the tropism of several important human pathogens. Finally, we discuss the current and future uses of this knowledge for generating genetically modified animal models permissive for these pathogens.

Animal models for the study of HCV

The development and evaluation of effective therapies and vaccines for the hepatitis C virus (HCV) and the study of its interactions with the mammalian host have been hindered for a long time by the absence of suitable small animal models. Immune compromised mouse models that recapitulate the complete HCV life cycle have been useful to investigate many aspects of the HCV life cycle including antiviral interventions. However, HCV has a high propensity to establish persistence and associated histopathological manifestations such as steatosis, fibrosis, cirrhosis and hepatocellular carcinoma (HCC). Better understanding of these processes requires the development of a permissive and fully immunocompetent small animal model. In this review we summarize the in vivo models that are available for the study of HCV.

Determinants of hepatitis B and delta virus host tropism

Hepatitis B virus (HBV) infections are a global health problem afflicting approximately 360 million patients. Of these individuals, 15-20 million are co-infected with hepatitis delta virus (HDV). Progress toward curative therapies has been impeded by the highly restricted host tropism of HBV, which is limited to productive infections in humans and chimpanzees. Here, we will discuss different approaches that have been taken to study HBV and HDV infections in vivo. The development of transgenic and humanized mice has lead to deeper insights into HBV pathogenesis. An improved understanding of the determinants governing HBV and HDV species tropism will aid in the construction of a small animal model with inheritable susceptible to HBV/HDV.

Experimental hepatocyte xenotransplantation--a comprehensive review of the literature

Hepatocyte transplantation (Tx) is a potential therapy for certain diseases of the liver, including hepatic failure. However, there is a limited supply of human livers as a source of cells and, after isolation, human hepatocytes can be difficult to expand in culture, limiting the number available for Tx. Hepatocytes from other species, for example, the pig, have therefore emerged as a potential alternative source. We searched the literature through the end of 2014 to assess the current status of experimental research into hepatocyte xenoTx. The literature search identified 51 reports of in vivo cross-species Tx of hepatocytes in a variety of experimental models. Most studies investigated the Tx of human (n = 23) or pig (n = 19) hepatocytes. No studies explored hepatocytes from genetically engineered pigs. The spleen was the most common site of Tx (n = 23), followed by the liver (through the portal vein [n = 6]) and peritoneal cavity (n = 19). In 47 studies (92%), there was evidence of hepatocyte engraftment and function across a species barrier. The data provided by this literature search strengthen the hypothesis that xenoTx of hepatocytes is feasible and potentially successful as a clinical therapy for certain liver diseases, including hepatic failure. By excluding vascular structures, hepatocytes isolated from genetically engineered pig livers may address some of the immunological problems of xenoTx.