Year: 2015
Type: article
Abstract: (See the major article by Bird et al on pages 703–11.) Work on Ebola virus has gained momentum over the past year, largely due to the ongoing, unprecedented, and devastating outbreak in West Africa, with >28 500 clinical Ebola cases and 11 300 deaths [1]. Scientists, infectious diseases professionals, and the media have repeatedly warned that the West African virus strain, designated Ebola virus Makona, may be mutating more rapidly, with the potential for increased virulence or transmissibility [2]. Mutations could, indeed, affect important countermeasures, such as diagnostics, therapeutics, and vaccines, although there is currently little evidence for increased virulence or transmissibility [3]. Thus, it is of paramount importance to further investigate the West African Ebola virus strain, as well as other ebolaviruses, with the goal to better define the biological virus phenotypes and to develop advanced countermeasures. The best way to study the complex interactions between viruses and the host is by using animal models of disease. Several animal models exist for the study of Ebola virus pathogenesis [4, 5]; Table 1 summarizes the critical features of these models. Nonhuman primates (NHPs), especially cynomolgus and rhesus macaques, are considered the gold standard animal models for Ebola [6]. NHPs have the advantage of being more closely related than rodents to humans, and, thus, data gleaned from experiments are likely highly applicable to humans. Disease in NHPs best recapitulates the features seen in humans among all established animal models. Obvious drawbacks to the use of NHPs include ethical considerations, cost, and size. Before NHPs are used for experimentation, analysis of rodent models of Ebola, such as the mouse [7], the guinea pig [8], and the hamster [9], is generally promoted. Characteristics of Animal Models for Ebola Virus Characteristics of Animal Models for Ebola Virus Immunocompetent rodents, in particular the mouse, are inexpensive small-animal models of Ebola; however, many aspects of these models make it difficult to draw conclusions about general pathogenesis and the efficacy of countermeasures in humans. The disease course is often different in rodents, with coagulopathies being a major hallmark in humans but almost nonexistent in mice or limited in guinea pigs. A few studies have used immunocompromised mouse models deficient in certain adaptive or innate immune responses (ie, interferon α/β receptor-knockout, STAT1-knockout, or severe combined immunodeficiency [SCID] mouse models) [10, 11]. These animals show lethal disease upon infection with adapted or wild-type Ebola virus, but disease progression is often slower with wild-type Ebola virus isolates. These models are not well established and are considered of limited use because of their immunocompromised status. However, until now, they represent the only rodent models that are susceptible to wild-type isolates. The more recently developed hamster model seems to best recapitulate human disease among rodent models, but the use of hamsters, as well as guinea pigs, is hampered by the lack of species-specific immunological tools [12, 13]. A major drawback with all immunocompetent rodent models is that ebolaviruses, as well as the closely related marburgviruses, must be passaged or reverse engineered to generate an adapted isolate that causes disease. Genetic changes in the rodent-adapted viruses have been mapped to genes with interferon antagonistic functions, and adapted viruses are able to efficiently antagonize the rodent's immune response [14, 15]. This brings into question whether the relationship between humans (and NHPs) and wild-type viruses is the same as between rodents and adapted viruses. The article by Bird et al in this issue of The Journal of Infectious Diseases describes the use of humanized mice as a model for Ebola virus infection and disease progression, using both a historic Central African isolate, as well as a contemporary Makona isolate. They show that mice devoid of their native immune response and reconstituted with a human innate and adaptive immune system are susceptible to infection and die of disease within approximately 2 weeks after inoculation with wild-type Ebola virus. They observed histological changes in the liver and upregulation of cytokines and chemokines, consistent with Ebola hemorrhagic fever. Unlike for native rodent models, wild-type Ebola virus was able to cause disease, indicating that interactions between the virus and the immune system models what is seen in humans. One of the biggest differences in the divergence between mice and humans is variation in the adaptive and innate immune systems [16, 17], bringing into question the applicability of data generated from rodents to the human condition. NHPs are more closely related to humans with respect to their immune responses, yet significant differences exist, and their usefulness as a model for examining how infectious agents interact with the human immune system is not ideal [18, 19]. This is highlighted by the observation that both mice and NHPs rarely faithfully replicate all, or even most, features of a human pathogen-specific clinical course of disease. To circumvent this, humanized mice have been used over the last several decades as a tool to assess interactions between pathogens and the human immune system. Recent advances in the generation of humanized mice come from both efficient ablation of the native immune system and reconstitution of a functional human immune system. The development of NOD.Cg-Prkdcscid Il2rgtm1Wjl/SzJ SCID mice (termed “NSG mice”) that lack the common γ chain of the interleukin 2 receptor, which is required for signaling by many cytokines, has allowed for a mouse that is highly susceptible to engraftment with human cells and tissues [20]. Following irradiation, these mice can be engrafted with hematopoietic CD34+ stem cells, which reconstitute the myeloid and lymphoid compartments. The addition of fetal liver and thymus tissue (bone marrow/liver/thymus [BLT]) allows for the development of a human hematopoietic and lymphoid immune system that can undergo HLA-restricted thymic education [21]. Although the myeloid and lymphoid systems of NSG-BLT mice are replaced by their respective human counterparts, several issues persist that hinder this model. Despite thymic education, the human lymphocytes are unable to interact with the mouse major histocompatibility complex (MHC) and, therefore, are unable to recognize antigen presented on MHC I molecules on infected mouse cells. Furthermore, these mice often eventually develop a wasting disease that resembles graft versus host disease [22]. Another limitation is that although they possess myeloid and lymphoid cells, these populations are highly plastic, and cell proportions fluctuate over time, providing a narrow window for studies that can be performed with physiologic relevance. Nevertheless, humanized mice, as described by Bird et al, as well as in a recent study of NSG mice engrafted with human hematopoietic stem cells but without human liver or thymic tissue [23], offer unique tools for the study of Ebola virus pathogenesis. Unlike many viruses, Ebola virus interacts with myeloid cells as initial targets of infection, and pathogenesis is thought to be dependent on the virus' ability to modulate the activation of these initial target cells, namely dendritic cells and macrophages [24–27]. Interactions with these cell types augment the development of adaptive immune responses, and there is much to be learned during the initial stages of infection that eventually dictates the overall quality and quantity of the immune response to, and ultimately, the outcome of infection. As infection progresses, cell types outside of the myeloid and lymphoid compartments, namely endothelial cells and hepatocytes, become important for the pathogenic process [28, 29]. The advantage of the humanized mouse as a model for Ebola virus infection is that now it is possible to examine the interaction of Ebola virus in the context of an intact human immune system. Regardless of whether this animal model faithfully recapitulates human disease, which the preliminary yet limited analysis by Bird et al suggests that it does in part, early events in infection and the complex responses to Ebola virus by human myeloid and lymphoid cells can now be assessed in vivo. As infection progresses, it is yet to be determined how useful this model will be as a pathogenesis model. Advances in humanized mouse technology, such as development of mice transgenic for human HLA genes and engrafted with human thymic tissue, will provide a more powerful tool for examining these questions. Another advantage of these models described by Bird et al and Lüdtke et al [23] is the ability to examine the activity of filovirus-specific therapeutics and vaccines that directly target cells derived from hematopoietic stem cells. For example, the effect of exposure of humanized mice to vaccines that modulate the activation and immune responses of myeloid and lymphoid cells can now be studied in this model. Future studies should be aimed to assess the efficacy of vaccines in this model to determine its predictive power. Recently, the use of fetal tissue for the purpose of research and therapy has been questioned, potentially affecting fundamental duties of medicine and public health [30–32]. The generation of humanized mice such as NSG-BLT mice is an example of technology that uses fetal tissue that otherwise would be disposed as medical waste. Obligatory compliance with all applicable laws and ethical guidelines regarding the use of human fetal tissue should provide the foundation for continuing use of these and other important technologies to serve the medical and public health sectors in developing treatments against important acute and chronic diseases, including those caused by highly virulent pathogens such as Ebola virus. Disclaimer. The opinions presented in this article are those of the authors and do not necessarily reflect those of the National Institute of Allergy and Infectious Diseases (NIAID), National Institutes of Health (NIH). Financial support. This work was supported by the Intramural Research Program, NIAID, NIH. Potential conflict of interest. Both authors: No reported conflicts. Both authors have submitted the ICMJE Form for Disclosure of Potential Conflicts of Interest. Conflicts that the editors consider relevant to the content of the manuscript have been disclosed.
Authors: Joseph Prescott, Heinz Feldmann
Language: English
Subfield: Infectious Diseases
Field: Medicine
Domain: Health Sciences
Open Access status: hybrid
Estimated APC paid: $4,763
OpenAlex ID: W2249015911