Down regulation of MHC class I cell surface expression in Marek's disease virus (MDV) infection and its relevance in vivo
Final Report Abstract
Currently, the viral factors responsible for downregulation of MHC class I on the surface of MDV- infected cells are not identified. However, the MDV homologue of the alphaherpesvirus pUL49.5, which is a potent inhibitor of TAP in other species, is a likely candidate. Earlier publications suggested the involvement of pUL49.5 in MHC downregulation with infection as well as transient transfection assays. However, if this downregulation was a consequence of TAP interference could not be answered in the past. Our investigations of MDV pUL49.5’s capacity to act on TAP in a similar fashion were prevented by an obvious infection- and/or cell-dependent detectability of the target protein in various assays (western blotting, immunofluorescence). However, cellular degradation processes as a cause for posttranslational instability of pUL49.5 could be excluded by treatment of transfected cells with different inhibitors. Whether the puzzling observations regarding pUL49.5 expression result from epitope-masking by potential interaction partners remains an open question. We reassessed some of the earlier results in slightly different experimental setups and were not able to fully reproduce described effects in our investigations. Nevertheless, a stably expressed Flag-tagged pUL49.5 construct led to a mild downregulation of MHC class I surface levels in preliminary flow cytometry-based analysis of transfected chicken cells. Since no significant differences in TAP expression levels could be observed in these cells, mechanisms other than degradation of TAP are likely responsible for the inhibition. Therefore, future investigations will focus on TAP peptide transport assays as well as co-immunoprecipitation studies to identify potential interaction partners of pUL49.5. However, contrary to earlier publications, the MDV pUL49.5 is not essential for viral replication and does not contribute to MHC class I downregulation during infection of chicken embryo cells in vitro. Future in vivo infection studies will make use of our MDV UL49.5 knockout mutant. We aim to elucidate potential synergistic effects with other viral immunoevasins and the relevance of their functions for MDV pathogenesis and tumour formation.
Publications
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Varicellovirus UL 49.5 proteins differentially affect the function of thetransporter associated with antigen processing, TAP. PLoS Pathog. 4, e1000080 (2008)
Koppers-Lalic, D. et al.
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Down-regulation of MHC class I by the Marek’s disease virus (MDV) UL49.5 gene product mildly affects virulence in a haplotype-specific fashion. Virology 405, 457–63 (2010)
Jarosinski, K. W., Hunt, H. D. & Osterrieder, N.
