The impact of a human cytomegalovirus subviral particle vaccine on the host cell proteome and on virus replication
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Abstract
The development of a vaccine against the human cytomegalovirus (HCMV), suitable to prevent congenital disease in neonates and to reduce morbidity and mortality in immunocompromised patients has been ranked as a top-priority medical goal. Non-infectious HCMV particles, termed Dense Bodies (DBs) are a promising platform to develop such a vaccine. The particles are a unique product of cytomegalovirus infected cells and are concomitantly released with infectious virions. Purified DBs are highly immunogenic in laboratory animals, as they elicit both neutralizing antibodies as well as antiviral T lymphocytes. A DB-based vaccine has, however, not yet been tested in humans. In preparation of clinical studies, this project was designed to investigate the molecular interactions of DBs with different target cells and to study the impact of DBs on HCMV replication.
Proteomic analyses were performed on different culture cells, like fibroblasts or endothelial cells that were exposed to DBs. A striking result of this was that exposure of cells to DBs resulted in the upregulation of an array of interferon (IFN) regulated genes (IRGs), likely leading to the establishment of an antiviral state in these cells. The inhibition of the IFN JAK/STAT signalling cascade during DB incubation demonstrated that upregulation of IRGs like MX1, IFIT3 and ISG15 was dependent on type I IFN signalling. Another remarkable finding was that DBs also promoted the upregulation of components of the major histocompatibility complex (MHC) antigen presentation pathways and connected cellular processes, namely ubiquitination and autophagy. This adds significantly to our understanding about the particular feature of DBs to efficiently induce antiviral CD4- and CD8- T lymphocytes in the absence of viral gene expression. Besides this, proteins involved in other biological processes like cell-cycle regulation and chromosome organisation were upregulated in fibroblasts and endothelial cells, but also in haematopoietic cells like monocytes and dendritic cells (DCs), showing the complex pattern of cellular protein regulation following DB-exposure.
As a DB-vaccine would be applied to millions of individuals, part of whom would be latently infected, a second prompting question was, if these particles would support or restrict HCMV replication. Enhancement of viral replication would be an adverse function of these particles that likely would prevent vaccine licensing. Exposure of fibroblasts to DBs however indeed reduced HCMV progeny production. The effect was most prominent when the cells were “primed” by DBs prior to infection. This fosters the hypothesis that DBs are produced by cells as an antiviral mechanism, similar to IFNs that induce an antiviral state in neighbouring cells and thus protects them against subsequent HCMV infection. This work provides the basis to further test this hypothesis.