Understanding human cytomegalovirus latency as a means to target the latent reservoir

Human cytomegalovirus (HCMV) is a type of herpesvirus which is carried without symptoms by the majority of the population. However, it can cause serious disease in infants born to mothers who acquire the infection in pregnancy. Similarly, in people whose immune systems are suppressed, for instance when they undergo organ transplantation or stem cell grafts, HCMV can be life threatening. There has been a continual call for increases in the number of transplants in the UK (see http://www.bbc.co.uk/news/health-40564329 and http://www.bbc.co.uk/news/health-11688102); this means that HCMV disease is likely to become more and more of a problem, particularly, in transplant patients.
Unlike many other viruses, HCMV is never cleared after its initial infection but persists for the life-time of the individual. At least in part, this is due to the ability of HCMV to avoid the immune system by expression of virus genes which help it avoid immune responses. However, the life-long persistence of this virus is also a biological property of all herpesviruses because the virus is able to undergo a so-called latent infection. During latency, the virus hides in the cell without making new virus particles but is still able to reawaken, generating new infectious virus which can cause disease and can also be passed on to other uninfected people.
Understanding how the virus succeeds in maintaining itself as a latent infection in people without causing disease and how this relationship breaks down to cause disease should help develop better methods of treating the virus and for designing a vaccine (there is currently no vaccine available).
Our research is aimed at determining (i) the mechanisms by which the virus maintains a silent infection (latency) in specialised cells of the immune system (ii) how the virus re-awakens (reactivation) and (ii) how the immune system is prevented from eliminating the virus from the body. This will help us understand how the virus is able to hijack the cell in order for the virus to be carried latently and will inform us of novel therapies to target latently infected cells to prevent disease in transplant patients. In addition to advancing our understanding of HCMV, in particular, this research may also lead to a better understanding of other virus infections which persist in the body, how the human host controls them as well as help identify common strategies for their elimination.

HCMV primary infection of the immunocompetent is asymptomatic. However, infection of individuals with compromised or immature immune systems can be life threatening. Furthermore, HCMV persists for the lifetime of the host which, due to its ability to avoid immunesurveillance and, ultimately, establish a latent infection. Cellular latency is defined maintenance of viral genomes in the absence of infectious virus production. Consequently, current therapies targeting HCMV replication will not target the latent virus; yet reactivation from latency is a major cause of disease in immune suppressed transplant patients. Work from our laboratory has been instrumental in identifying some of the viral genes expressed during latency, the host immune responses to these antigens and how these genes modify the latently infected cell. However, the viral transcriptome is likely far more complex than initially thought and how latent virus modifies the latently infected cell will be a result of a complex array of interactions between multiple latency-associated viral gene products and a multitude of host cell functions. On this basis, we have now made considerable efforts to try to analyse changes in the total cell proteome resulting from latent infection by unbiased approaches which has helped identify important phenotypic changes resulting from latent infection. We also have developed unbiased screening technologies to identify viral and cellular factors necessary for the establishment and maintenance of latency, as well as reactivation, in the myeloid lineage. Such analyses will allow a more comprehensive understanding of how this human pathogen persists for the lifetime of the host and will also help us to exploit the phenotypic changes in the latently infected cell for chemotherapeutic and immunological targets to lead to credible strategies aimed at eliminating latent virus in vivo and in solid organ donor tissue.

HCMV is universally distributed in human populations and, like all herpesviruses, persists throughout life after primary infection. HCMV remains an important human pathogen, particularly in the settings of congenital infection; it is the commonest infective cause of congenital central nervous system damage and is also a major cause of disease in immunocompromised subjects, especially in the context of organ and stem cell allografting. Whilst antiviral therapy is available, side effects and drug resistance occur. Additionally, current antivirals do not target the latent reservoir and so do not eliminate virus which can reactivate and cause morbidity, particularly in transplant patients receiving tissue from HCMV seropositive donors or in recipients who are already seropositive for the virus. Consequently, HCMV headed the list of needed vaccines in the US Institute of Medicine on Priorities for Vaccines report (Arvin Clin Infect Dis 2004), in part reflecting the weighting given to quality life years saved.
Understanding virus pathogenesis at a molecular level during lytic infection and latency, and how viral gene products interact with host immune responses, is crucial for design of novel antiviral strategies and approaches to vaccines. For HCMV, this is increasingly important not least because of the increasing calls for transplant organs by the BMA and Government but also because there is opinion that long-term persistence of HCMV, underpinned by virus reactivation, is associated with long-term diseases such as atherosclerosis and neoplasias.
This research will directly benefit UK and international research academics in the herpesvirus community as well as those interested in immunity and immune evasion. Similarly, the National Institute of Health Research (NIHR) community clearly benefits from any successful outcomes of the proposed research programme through decreased transplantation-mediated HCMV disease. Cambridge University and Cambridge University Hospitals NHS Foundation Trust, as one of the 20 NIHR Comprehensive Biomedical Research Centres (BRC) in the UK, are ideally placed to adopt new technologies, techniques and treatments for improving health and to translate fundamental biomedical research into clinical research that benefits patients: Antimicrobial resistance and Transplantation and regenerative medicine are two of the themes in our BRC, which will help facilitate the translational research associated with this application. Our research should also inform industrial partners interested in vaccine/drug development as it will provide key information both for vaccine design and drug targeting. For instance, a successful vaccine program will have clear health impacts through the prevention of congenital infections and subsequent long-term associated health care costs and the ability to target the latent viral would have far-reaching clinical benefits in transplant recipients.
Our proposed programme will support 2 technicians and 2 postdoctoral scientists, all are supported by our present programme of work. Long-term funding has allowed, and will continue to allow, our research technicians to become highly skilled laboratory workers. Their in-house training will also enable them to move into higher roles vital for the effective running of research groups and University Departments. The training that our post-doctoral researchers receive, not only in laboratory based research but also in student teaching will enable them to move on to academic posts to develop their own research programs as well as to become well trained scientists who would also be highly valued by industry.