HCV Research Discussion: Dr. Alexander Ploss

Posted on April 27, 2015

What are your interests in the field of HCV Research?

My lab is interested in understanding what makes hepatitis C virus (HCV) a virus that infects basically only humans or chimpanzees. There are fairly basic questions that surround the host tropism of the virus, and we want to get a better understanding of the biology, how the virus interacts with its host cell, and also to use this knowledge to build animal models that you could employ to test, for example, preclinical drug and vaccine candidates to improve treatment in the future.

Could you please elaborate further?

We take different approaches to better understand the host range of the hepatitis C virus. HCV has a narrow host range limited to robust infection in humans and chimpanzees. The basis for this limited host range is incompletely understood.  We try to decipher why HCV does not infect species that are more commonly used in biomedical research such as mice or rodents in general. We initially performed genetic screens to identify host factors that are similar or have common features, but are somehow different, between mice and men.

For example, we discovered that certain molecules that are required for hepatitis C virus entry, specifically tetraspanin CD81 and a tight junction molecule called occludin, slightly differ in their sequences between humans and mice. When you express the human versions of these molecules in mouse cells, you can actually facilitate hepatitis C entry. We then showed in our subsequent studies that expression of these molecules in a mouse liver enables HCV uptake into murine hepatocytes in vivo. When CD81 and occludin are transgenically expressed in mice with a genetic disruption of parts of their innate immune system, you can even get the virus to replicate at low levels and produce new virions in vivo. We call this a “genetically humanized mouse”, which can be used to study the interaction of the hepatitis C virus with the mammalian host. It’s one of the approaches we used to develop a better animal for the study of hepatitis C/host biology, other than chimpanzees.

Another approach to this issue also focuses on host adaptation. In this case, we transplant the relevant tissues, specifically human liver and the human immune system, into rodent recipients. We basically transplant the human liver to provide a reservoir where the hepatitis C virus can replicate, and then at the same time provide an immune system that is able to respond to the infection. Specifically, we start with mice that are lacking endogenous B-cells, T-cells, and NK cells, which makes them unable to destroy transplanted human liver cells. The genetic background of the xenorecipient strain allows us to inflict injury to the mouse liver. Thereby, when you transplant human hepatocytes into these animals, you can selectively expand the human cells and end up with a mouse that basically “grows a human liver”. Since these mice are highly immunocompromised, they don’t have an immune system to respond to and clear the HCV infection. This model is frequently used for challenge studies but is adequate for studying HCV pathogenesis. During chronic HCV infection patients develop progressive liver disease meaning fibrosis, cirrhosis, and eventually hepatocellular carcinoma. Liver pathogenesis is thought to be largely immune mediated. So we try to improve our hepatitis C mouse model by transplanting an optimally donor-matched human immune system into the mice to mimic disease features observed in humans. You can do this by simply injecting human hematopoietic stem cells into the immunocompromised animals that have also received the liver transplant. The co-injected blood-forming stem cells give rise to many of the cell lineages that you can find in the human immune system. This platform is still under development but the long term goal is use this model to study immunopathogenesis mediated by hepatitis C virus.

A third complimentary approach to studying the HCV-host interaction aims at adapting HCV to replicate in a usually non-permissive species. So instead of modifying the host, you’re basically selecting for (a) viral variants that has acquired the ability to, for example, engage potential host factors from the hepatocytes of another species, such as mice or monkeys, and infect that species more efficiently.

What do you think are the greatest unmet needs in hepatitis C research that could one day affect treatment?

The treatment for hepatitis C has improved substantially over the last few years. Treatment used to be a combination of the long-lasting form of interferon, pegylated interferon, which was combined with a nucleoside analog called ribavirin. This regimen was effective in about half of the patients who received it, which is not bad but still, a lot of patients failed treatment. Unfortunately, this kind of therapy was associated with considerable side effects and many patients wouldn’t tolerate it. Then over the last few years, substantial improvements were made with the introduction of molecules, so called directly acting antivirals, that specifically interfere with enzymes and proteins that the virus codes. And now the best therapies that are out there can effectively cure hepatitis C virus in the majority of patients. They seem to work in most patient populations, even in those who have progressed pretty far with the disease, meaning they have advanced fibrosis or even cirrhosis. That’s great, but currently these treatments are very expensive, and there are a few issues that still need to be addressed.

If you think about it, about 170 million people are infected with hepatitis C worldwide, and many of those live in countries that simply can’t afford these very expensive therapies, or live in areas where you can’t definitely ensure that they will have access to these effective therapies. So having a vaccine available that effectively prevents hepatitis C infection would certainly be of great benefit. I think that is an important area for continuing research.

An additional area that we know very little about is hepatitis C virus pathogenesis. How does hepatitis C virus exactly cause the disease? Clearly, the immune system plays an important role in this process. New directly acting antiviral hold promise to cure even those patients who have developed severe liver disease. However, in many of those liver function may not revert to normal baseline levels. So gaining a better understanding of HCV pathogenesis could not only be stopped but also how the damage could be turned back so the liver basically recovers after successful treatment, is probably another important area of research.

Alexander Ploss, Ph.D. completed his Bachelor’s and Master’s degree in biochemistry at the University of Tübingen, Germany including additional training the Howard Hughes Medical Institute at the University of Washington, Seattle, and at the German Cancer Research Center in Heidelberg, Germany. Dr. Ploss completed his Ph.D. in Immunology at Memorial Sloan-Kettering Cancer Center/Cornell University and postdoctoral training at the Rockefeller University. Prior to joining the Department of Molecular Biology at Princeton University in 2013 he was a research associate professor at the Center for the Study of Hepatitis C at the Rockefeller University. His research focuses on immune responses and pathogenesis to human infectious diseases, including hepatitis viruses, related flaviviruses, and malaria. His group combines tissue engineering, molecular virology/pathogenesis, and animal construction, to create and apply innovative technologies including humanized mouse models for the study and intervention of human hepatotropic infections. In recognition of his work he received a Kimberly Lawrence Cancer Research Discovery Fund Award, the Astellas Young Investigator Award of the Infectious Diseases Society of America and the Liver Scholar Award from the American Liver Foundation. Professor Ploss is a member of the Genomic Instability and Tumor Progression Program at the Cancer Institute of NJ.

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