Human cytomegalovirus (HCMV) is a betaherpesvirus which establishes a lifelong persistent infection, underpinned by its ability to establish latent infection in early myeloid lineage cells, in the infected host. Although well controlled by a healthy immune system, HCMV causes pathological and life threatening disease in individuals with a compromised or immature immune response, which can come from primary HCMV infection or reactivation of latent infection. Although progress is being made in understanding the mechanisms by which HCMV maintains latency and reactivates, a better understanding is essential towards the aim of targeting and killing latently infected cells.

In this thesis, I will present evidence that the HCMV-encoded chemokine receptor homologue US28, which is expressed during latent infection of CD14+ monocytes, is necessary for maintaining HCMV latency in these monocytes and, in the absence of US28 protein expression, HCMV undergoes lytic infection. US28 expression was found to attenuate cellular signalling pathways in latently infected cells; in particular, MAP kinase and NFκB. Interestingly, deletion of the US28 gene or inhibition of the US28 protein resulted in the expression of lytic antigens which allowed detection of infected monocytes by the immune system. This observation may lead to a potential new immunotherapeutic strategy against latent HCMV.

Having demonstrated that US28 protein is expressed on the surface of latently infected monocytes, I tested whether a new fusion-toxin protein, called F49A-FTP, which binds US28 protein, could be used to target and kill latently infected cells. I developed a protocol for treating latently infected monocytes with F49A-FTP which resulted in a significant reduction in virus reactivation after monocyte differentiation to dendritic cells. I was also able to show that this treatment kills CD34+ progenitor cells, which were experimentally latently infected with HCMV, as well as latently infected monocytes from a healthy, seropositive blood donor.

Finally, during my investigations into the role of US28 during HCMV latency, a mass spectrometry screen was performed to measure changes in cellular protein expression when US28 protein is expressed in isolation, in THP-1 monocyte-like cell line. This identified CTCF, a transcription factor which appears to be modified by US28 in THP-1 cells. I showed that CTCF has a repressive effect on the HCMV MIEP, and that CTCF likely plays a role in HCMV latency.

In summary, this work provides insights into the role of US28 during HCMV latency, and proposes potential novel therapeutic strategies to kill latently infected cells.