Metabolic regulation of hepatic immunopathology by myeloid-derived suppressor cells

Around one third of the world's population are estimated to have been exposed to hepatitis B virus (HBV) and at least 350 million people remain chronically infected. The resulting liver diseases include cirrhosis and liver cancer and are responsible for more than a million deaths every year. Existing therapies rarely achieve a cure. Patients are therefore consigned to long-term treatment, which remains unaffordable for many of the countries where the infection is most prevalent and carries risks of viral resistance and toxicity. Liver cancer is the third commonest cause of cancer-related deaths and treatments for it remain very limited.

Our work is focused on understanding how to boost the body's natural immunity in order to achieve control of HBV and complications like liver cancer. We think that this should be achievable since many adults who get infected with HBV, naturally develop an immune response that controls it successfully for the rest of their lives. It is well known that T cells are critical for controlling viruses like HBV and that their numbers and function are drastically impaired in patients with chronic infection. We have found that one reason their T cells don't expand and work properly is that they are being starved of key nutrients like arginine. Our new findings reveal that another specialised immune cell type called "myeloid-derived suppressor cells" deprives T cells of nutrients and thereby stunts their responsiveness. These suppressor cells help to damp-down immune responses in the liver to stop them damaging this vital organ but in doing so, impair the control of pathogens like HBV.

We now propose to define this process more comprehensively and investigate whether it can be manipulated to treat liver infections and other conditions such as tumours. A thorough understanding of the pathways and mechanisms involved will allow us to tailor therapies that can specifically target myeloid-derived suppressor cells or reverse the defects in "starved" T cells. This approach will be tested in the lab using cells extracted from the blood and liver of patients, in special 3D liver culture systems and in a new model of HBV infection. All samples are taken with fully informed consent and ethical board approval. Liver samples are only taken when there is surplus tissue left from procedures required for diagnosis or treatment.

Our results will not only help with the development of better treatments for HBV and liver cancer, they will also enhance our understanding of how the liver suppresses immune responses. Our findings could therefore be applied to the treatment of other infections that target the liver like malaria and could conversely be harnessed to prevent rejection of liver transplants or to treat autoimmune liver disease.

The liver has evolved a highly tolerising environment which preserves the functional integrity of this vital organ in the face of the high antigenic load it receives through the portal venous system. Hepatotropic infections and tumours take advantage of this niche to subvert immune responses. In this proposal we explore the metabolic regulation of hepatic T cell immunity by granulocytic myeloid-derived suppressor cells (gMDSC), based on our previous work implicating amino acid starvation in chronic hepatitis B pathogenesis (CHB) (Das, JEM 2008). Our unpublished data suggest that the disparate degrees of immune-mediated tissue injury manifested in this highly prevalent infection may be determined by the differential expansion of gMDSC. Arginase-expressing gMDSC expand transiently in acute resolving HBV, their decrease coinciding with the peak of hepatic injury. In CHB, gMDSC are likewise most increased in those patients replicating HBV without evidence of immunopathology, their expansion correlating with increases in circulating arginase and a depletion of L-arginine.
gMDSC accumulate in the liver and our preliminary data indicate a role for activated hepatic stellate cells in sustaining them. We now propose to characterise which pathological insults drive their expansion in the liver and test the hypothesis that stellate cells, HBV-infected hepatocytes and hypoxia make mechanistic contributions. We find that gMDSC can potently inhibit T cell responses by depriving them of L-arginine; we will further probe the impact of this starvation on T cell signaling, function and amino acid transporters. We will test the postulate that T cells can undergo compensatory changes in amino acid transporters promoting metabolic reprogramming. To investigate the therapeutic potential of gMDSC, we will deplete them (and their metabolic mediators) in mouse models of HBV infection. Our findings will have broad relevance for the understanding and manipulation of hepatic tolerance.

Increasing the responsiveness of patients with immunotolerant HBV infection to therapy
This proposal aims to address the major health and wealth implications resulting from the burden of chronic infection with hepatitis B virus (HBV). Of the estimated 350 million people persistently infected with HBV, a large proportion live in countries that will not be able to bear the cost of long-term suppressive antiviral therapy. Maintenance treatment with costly new drugs for this infection also has major resource implications for the UK, where the prevalence of HBV infection has increased dramatically as a result of immigration patterns. Existing antivirals are potentially toxic in the longterm, particularly since many patients are relatively young and of child-bearing age. This work could allow development of an immunotherapeutic boost to the natural antiviral T cell response that will mean only a finite course of treatment is required. Importantly it raises the possibility of making the large reservoir of highly infectious patients with "immunotolerant phase" HBV infection amenable to treatment.

Improving therapy for HCC
Liver cancer is the third commonest cause of cancer-related deaths worldwide and treatment remains very limited. Myeloid-derived suppressor cells, the subject of this proposal, also play an important role in subverting the immune control of liver cancer and liver metastases. A better understanding of the factors drivign their accumulation and their capacity to regulate T cell metabolims and function could therefore help inform new therapeutic approaches for liver cancers. For example, as part of a multinational collaboration funded by the MRC and A/Star, we recently pioneered the first-in-man use of genetically engineered T cells directed against an HBV antigen expressed by the liver cancer cells. The work in this proposal could allow genetic optimisation of the survival and function of such TCR re-directed T cells by altering their response to the metabolic challenges of the liver environment.

Industrial links
My close links with industry will facilitate commercialisation of any findings generated by this work; a number of leading pharmaceutical companies as well as small biotechs are now investing in the development of new immunotherapeutic approaches as a major growth area in viral hepatitis.

Promoting women in science
As chair of our Athena Swan Self Assessment Team I led a successful application for a Divisional Silver Award (2014) and have become very involved in the promotion of women in science. I plan to extend my mentoring role to help more junior female scientists to pursue their careers and reduce the current gender imbalance in senior positions.

Public engagement
UCL is one of only 6 centres in the UK to receive funding from the Beacons of Public Engagement programme set up by HEFCE, Research Councils UK and the Wellcome Trust. We have a Public Engagement Unit that gives support in our interactions with those outside the university.
The MRC Centre for Medical Molecular Virology at UCL is actively committed to public engagement in order to develop wider public understanding of the work we do and to encourage community participation. As part of this centre, my team and I participate in public engagement activities such as school visits, student placements and public talks. In particular we have helped with visits of classes from several primary schools, introducing the children to concepts about viruses and liver disease and showing them the flow cytometer in action.