Using Primary Human Cells to Analyze Human Cytomegalovirus Biology

The battle between host antiviral innate immunity and immune evasion by cytomegalovirus

Cytomegalovirus (CMV) has successfully established a long-lasting latent infection in humans due to its ability to counteract the host antiviral innate immune response. During coevolution with the host, the virus has evolved various evasion techniques to evade the host's innate immune surveillance. At present, there is still no vaccine available for the prevention and treatment of CMV infection, and the interaction between CMV infection and host antiviral innate immunity is still not well understood. However, ongoing studies will offer new insights into how to treat and prevent CMV infection and its related diseases. Here, we update recent studies on how CMV evades antiviral innate immunity, with a focus on how CMV proteins target and disrupt critical adaptors of antiviral innate immune signaling pathways. This review also discusses some classic intrinsic cellular defences that are crucial to the fight against viral invasion. A comprehensive review of the evasion mechanisms of antiviral innate immunity by CMV will help investigators identify new therapeutic targets and develop vaccines against CMV infection.

Repurposing an endogenous degradation domain for antibody-mediated disposal of cell-surface proteins

The exquisite specificity of antibodies can be harnessed to effect targeted degradation of membrane proteins. Here, we demonstrate targeted protein removal utilising a protein degradation domain derived from the endogenous human protein Proprotein Convertase Subtilisin/Kexin type 9 (PCSK9). Recombinant antibodies genetically fused to this domain drive the degradation of membrane proteins that undergo constitutive internalisation and recycling, including the transferrin receptor and the human cytomegalovirus latency-associated protein US28. We term this approach PACTAC (PCSK9-Antibody Clearance-Targeting Chimeras).

MORC3 represses the HCMV major immediate early promoter in myeloid cells in the absence of PML nuclear bodies

Human cytomegalovirus (HCMV) can undergo either a latent or a lytic infection in cells of the myeloid lineage. Whilst the molecular mechanisms which determine the outcome of infection are far from clear, it is well established that a key factor is the differential regulation of the major immediate early promoter (MIEP) responsible for driving lytic immediate early gene expression. Using a myelomonocytic cell line stably transduced with a GFP reporter under the control of the MIEP, which recapitulates MIEP regulation in the context of virus infection, we have used an unbiased CRISPR-Cas9 sub-genomic, epigenetic library screen to identify novel cellular factors involved in MIEP repression during establishment and maintenance of latency in myeloid cells. One such cellular factor identified was MORC3. Consistent with MORC3 being a robust repressor of the MIEP, we show that THP1 cells devoid of MORC3 fail to establish latency. We also show that MORC3 is induced during latent infection, recruited to the MIEP and forms MORC3 nuclear bodies (MORC3-NBs) which, interestingly, co-localize with viral genomes. Finally, we show that the latency-associated functions of MORC3 are regulated by the deSUMOylase activity of the viral latency-associated LUNA protein likely to prevent untimely HCMV reactivation.

The Human Cytomegalovirus Latency-Associated Gene Product Latency Unique Natural Antigen Regulates Latent Gene Expression

Human cytomegalovirus (HCMV) infection can lead to either lytic or latent infection, which is dependent on the regulation of the viral major immediate early promoter (MIEP). Suppression of the MIEP is a pre-requisite for latency and is driven by repressive epigenetic modifications at the MIEP during latent infection. However, other viral genes are expressed during latency and this is correlated with activatory epigenetic modifications at latent gene promoters. Yet the molecular basis of the differential regulation of latent and lytic gene expression by epigenetics is unclear. LUNA, a latent viral transcript, has been suggested to be important for HCMV latency and has also been shown to be important for efficient reactivation likely through its known deSUMOylase activity. Intriguingly, we and others have also observed that LUNA enhances latency-associated expression of the viral UL138 gene. Here, we show that in the absence of LUNA, the expression of multiple latency-associated transcripts is reduced during latent infection, which is correlated with a lack of activatory marks at their promoters. Interestingly, we also show that LUNA interacts with the hematopoietic transcription factor GATA-2, which has previously been shown to bind to a number of latency-associated gene promoters, and that this interaction is dependent on the deSUMOylase domain of LUNA. Finally, we show that the deSUMOylase activity of LUNA is required for the establishment and/or maintenance of an open chromatin configuration around latency-associated gene promoters. As such, LUNA plays a key role in efficient latency-associated viral gene expression and carriage of viral genome during latent carriage.

HCMV carriage in the elderly diminishes anti-viral functionality of the adaptive immune response resulting in virus replication at peripheral sites

Human cytomegalovirus (HCMV) infection and periodic reactivation is, generally, well controlled by adaptative immune responses in the healthy. In older people, overt HCMV disease is rarely seen despite the association of HCMV with increased risk of mortality; evidence from studies of unwell aged populations suggest that HCMV seropositivity is an important co-morbidity factor. HCMV genomes have been detected in urine from older donors, suggesting that the immune response prevents systemic disease but possibly immunomodulation due to lifelong viral carriage may alter its efficacy at peripheral tissue sites. Previously we have demonstrated that there were no age-related expansions of T cell responses to HCMV or increase in latent viral carriage with age and these T cells produced anti-viral cytokines and viremia was very rarely detected. To investigate the efficacy of anti-HCMV responses with increasing age, we used an in vitro Viral Dissemination Assay (VDA) using autologous dermal fibroblasts to determine the anti-viral effector capacity of total PBMC, as well as important subsets (T cells, NK cells). In parallel we assessed components of the humoral response (antibody neutralization) and combined this with qPCR detection of HCMV in blood, saliva and urine in a cohort of young and old donors. Consistent with previous studies, we again show HCMV specific cIL-10, IFNγ and TNFα T cell responses to peptides did not show an age-related defect. However, assessment of direct anti-viral cellular and antibody-mediated adaptive immune responses using the VDA shows that older donors are significantly less able to control viral dissemination in an in vitro assay compared to young donors. Corroborating this observation, we detected viral genomes in saliva samples only from older donors, these donors had a defect in cellular control of viral spread in our in vitro assay. Phenotyping of fibroblasts used in this study shows expression of a number of checkpoint inhibitor ligands which may contribute to the defects observed. The potential to therapeutically intervene in checkpoint inhibitor pathways to prevent HCMV reactivation in the unwell aged is an exciting avenue to explore.

Latency-associated upregulation of SERBP1 is important for the recruitment of transcriptional repressors to the viral major immediate early promoter of human cytomegalovirus during latent carriage

Suppression of human cytomegalovirus (HCMV) major immediate early gene (IE) expression from the viral major immediate early promoter (MIEP) is known to be crucial for the establishment and maintenance of HCMV latency in myeloid progenitor cells and their undifferentiated derivatives. This suppression of the MIEP during latent infection is known to result from epigenetic histone modification imparting a repressive chromatin structure around the MIEP in undifferentiated myeloid cells. In contrast, reactivation, resulting from, e.g., myeloid cell differentiation, is associated with activatory chromatin marks around the MIEP. Recently, recruitment of the transcriptional repressor SETDB1, via KAP1, to latent HCMV genomes was shown to be involved in latency-associated MIEP suppression in CD34+ progenitor cells. KAP1 is also known to associate with Chromodomain-helicase-DNA-binding protein 3 (CHD3) as part of the NuRD complex which can aid transcriptional silencing. We now show that the cellular protein Plasminogen activator inhibitor 1 RNA-binding protein (SERBP1), a known interactor of CHD3, is significantly upregulated during HCMV latency and that this protein is required for MIEP suppression during latent infection of myeloid cells. We further show that SERBP1 mediates CHD3 association with the MIEP as well as KAP1 association with viral genomic DNA. We suggest that SERBP1 functions as a scaffold protein to recruit transcriptional repressors to the latent viral genome and to mediate transcriptional silencing of the MIEP during latent carriage.

The complex biology of human cytomegalovirus latency

While many viral infections are limited and eventually resolved by the host immune response or by death of the host, other viruses establish long-term relationships with the host by way of a persistent infection, that range from chronic viruses that may be eventually cleared to those that establish life-long persistent or latent infection. Viruses infecting hosts from bacteria to humans establish quiescent infections that must be reactivated to produce progeny. For mammalian viruses, most notably herpesviruses, this quiescent maintenance of viral genomes in the absence of virus replication is referred to as latency. The latent strategy allows the virus to persist quiescently within a single host until conditions indicate a need to reactivate to reach a new host or, to re-seed a reservoir within the host. Here, I review common themes in viral strategies to regulate the latent cycle and reactivate from it ranging from bacteriophage to herpesviruses with a focus on human cytomegalovirus (HCMV). Themes central to herpesvirus latency include, epigenetic repression of viral gene expression and mechanisms to regulate host signaling and survival. Critical to the success of a latent program are mechanisms by which the virus can "sense" fluctuations in host biology (within the host) or environment (outside the host) and make appropriate "decisions" to maintain latency or re-initiate the replicative program. The signals or environments that indicate the establishment of a latent state, the very nature of the latent state, as well as the signals driving reactivation have been topics of intense study from bacteriophage to human viruses, as these questions encompass the height of complexity in virus-host interactions-where the host and the virus coexist.