Alexander Ploss

Infectious diseases account for at least 15 million deaths each year - almost a quarter of all human deaths worldwide. Against a constant background of established infections, epidemics of new and old infectious diseases periodically emerge, greatly magnifying the global burden of infections. Many pathogens causing disease in humans exhibit nearly unique human tropism, posing additional challenges for studying host-pathogen interaction and for efficiently testing anti-microbial intervention strategies. Humanized mice, i.e. mice expressing human genes or containing human tissues, have emerged as powerful systems to model human infections in vivo. We have and continue to construct humanized hosts genetically or via xenotransplantation to analyze previously intractable human (hepatotropic) pathogens. These systems offer unprecedented opportunities to effectively study host-pathogen interactions in vivo and to preclinically evaluate drug and vaccine candidates.

The research of my lab focuses on immune responses and pathogenesis to human pathogens infecting the liver, including hepatitis B (HBV) and C viruses (HCV), yellow fever and Dengue viruses and plasmodial parasites. My group combines tissue engineering, molecular virology/pathogenesis, and animal construction, to create and apply innovative technologies for the study and intervention of human hepatotropic infections.

Determinants of interspecies tropism of human hepatotropic pathogens

Many of these pathogens display unique human tropism, and the development of novel intervention strategies has been hampered by the lack of robust, cost effective and predictive models that accurately reproduce the hallmarks of human infections. While rodents and non-human primates have been employed in biomedical research and drug/vaccine development, they often do not yield reliable pre-clinical results that translate into effective human treatments. Two important factors contribute to this failure: on the microbial side, surrogate pathogens often differ significantly from highly restricted human counterparts; on the host side, the immune correlates of protection in non-human mammalian species often diverge from human responses.

My group uses several independent but possibly complementary approaches to overcome current species barriers and generate a small animal model for microbial pathogenesis: 1. Adaptation of microbial genomes to infect hepatocytes of non-human origin (mice and/or smaller non-human primates). 2. Humanization of the mouse liver and immune system by transplanting human hematopoietic stem cells and hepatocytes into a single murine recipient, thus allowing studies of pathology, immune correlates, and mechanisms of pathogen persistence. 3. Genetic host adaptation to create inbred murine models for viral pathogens. The latter approach encompasses systematic screens to identify and overcome additional species restrictions. We apply genome-engineering techniques to render the murine host environment more conducive to infections with the respective human pathogens.

Pathogenesis of viral hepatitis

Five distinct viruses, hepatitis A, B, C, delta and E viruses, are known to cause hepatitis in humans. HAV and HEV infections cause by-and-large acute, spontaneously resolving infections. In contrast, in excess of 500 million people are persistently infected with HBV/HDV and HCV. Chronic carriers are at risk of developing severe liver disease, including fibrosis, cirrhosis and hepatocellular carcinoma. An effective vaccine has been developed to prevent HBV infection, however, established infections are currently incurable and require life-long suppression with antiviral drugs. HBV pathogenesis is frequently exacerbated by co-infection with HDV. HDV is generally considered to be a subviral satellite because it can propagate only in the presence of the HBV. HBV persist in host cells as covalently closed circular DNA (cccDNA). Mechanisms of HBV persistence are incompletely understood.

Likewise, HCV has a high propensity of establishing chronic infections in human. A protective vaccine for HCV has not yet been developed. Current treatment - although projected to be improving considerably in the future - is only partially effective and plagued with side-effects but can result in complete elimination of the virus. We aim to shed light on the mechanisms governing HCV persistence.

Molecular characterization of plasmodial dormancy

Malaria accounts for more than 2% of all human deaths world-wide. A call was made for the global elimination of malaria, involving the eradication of all human malaria parasite species. Malaria is an arthropod borne infection caused in humans by five different species of Plasmodium, of which primarily P. vivax (Pv) – responsible for 70-80 million infections annually - has a high propensity for developing dormant stages - hypnozoites. In any attempt to eradicate malaria it will be necessary to know how, when and where to attack hypnozoites.

Due to the lack of experimental systems hypnozoites are almost completely undefined and mechanisms of their formation and/or reactivation are unknown. The primate parasite P. cynomolgi (Pcy) offers a model for dormancy and relapse but large-scale experiments in non-human primates, e.g. to identify new drugs, are restricted for ethical and financial reasons. To bridge this gap we have generated human and simian liver chimeric mice, which are powerful tools to Pv and Pcy in their native environment. We will profile Pv and Pcy liver stages and their host cells using transcriptomic and/or proteomic approaches. Molecular signatures of malarial dormancy will be critical for identifying putative therapeutic intervention points.

Generation of humanized for the study of human infectious disease

"Humanized" mice are versatile tools in the investigation of human disease. These are amenable small animal models transplanted with human cells or tissues (and/or equipped with human transgenes) that may be ideally suited for direct investigation of human infectious agents. Successful engraftment depends on avoiding rejection and maximizing tissue function, ensured by correct localization and appropriate tissue support by host factors. Despite the challenges, humanized mouse technology has made rapid progress over the last few years and it is now possible to achieve high levels of human chimerism in various host organs/tissues, particularly the immune system and liver. Such humanized mice provide a new opportunity to perform pre-clinical studies of intractable human pathogens. Despite their promise as challenge models, immune responses to infection remain suboptimal in humanized mice. In order to improve immune function, we employ a combination strategies including enhancing the ablation of endogenous mouse subsets to create "space" for human cells, providing exogenous cytokines to overcome impaired biological cross-reactivity between mouse and human, counteracting active graft destruction, and expressing human MHC molecules to ensure proper T cell education and homeostasis.