Control of CD8+ T cells through interference of Cytomegalovirus with the "peptide loading complex" (PLC)

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MHC I antigen presentation to CD8+ T-cells is a major mechanism of immune defense against virally infected cells. The endogenous antigens are loaded onto MHC I in the ER by the peptide loading complex (PLC), consisting of MHC I, the peptide transporter TAP, and chaperones including tapasin. Tapasin brings MHC I and TAP in close vicinity and together with the oxidoreductase ERp57 the peptide-receptive state of the MHC peptide-binding groove is regulated. HCMV can cause severe disease in immunocompromised persons, reflecting the delicate balance between the immune system reacting on the infection and the evasion of it by the virus itself. To cope with MHC I antigen presentation HCMV encodes for several post-translational strategies which have been extensively studied in transfected cells. We have analysed the PLC in naturally HCMV-infected cells and monitored the composition of the PLC throughout HCMV replication. Metabolic labeling experiments have revealed not only the absence of MHC I incorporation into the PLC during early time points of infection, but also lack of tapasin starting at early but being most pronounced at late times of infection. In contrast, Western blot analysis demonstrated only a slow decline of tapasin steady state levels in infected cells, suggesting a blocked synthesis rather than degradation. We found, after an initial induction at 8 hrs p.i., a strong inhibition of tapasin transcription at 24 hrs p.i. that persisted throughout the replication cycle. Furthermore, also reduction of TAP1 and TAP2 transcription was observed contrasting the elevated levels of MHC I transcripts. The project now follows two further objectives. First, the molecular basis of tapasin dissociation from MHC class I during HCMV infection will be analysed. This effect precedes the repression of tapasin transcription and occurs independently from the known HCMV inhibitors of the MHC class I pathway of antigen presentation, US2, US3 and US6. The goal is to identify the responsible HCMV gene that is supposed to exert a key function in the disruption of the PLC during HCMV infection. Identification of PLC regulating factors will allow for the second objective which is the analysis of MHC I presented antigens during HCMV infection after repairing PLC assembly and function. This will be informative for the evaluation of the actual escape of CD8+ T cell recognition of HCMV antigens and HCMV vaccine design.