Abstract
Major histocompatibility complex class I (MHC I) molecules present antigenic peptides for CD8(+) T-cell recognition. Prior to cell surface expression, proper MHC I loading is conducted by the peptide-loading complex (PLC), composed of the MHC I heavy chain (HC) and β(2)-microglobulin (β(2)m), the peptide transporter TAP, and several chaperones, including tapasin. Tapasin connects peptide-receptive MHC I molecules to the PLC, thereby facilitating loading of high-affinity peptides onto MHC I. To cope with CD8(+) T-cell responses, human cytomegalovirus (HCMV) encodes several posttranslational strategies inhibiting peptide transport and MHC I biogenesis which have been studied extensively in transfected cells. Here we analyzed assembly of the PLC in naturally HCMV-infected fibroblasts throughout the protracted replication cycle. MHC I incorporation into the PLC was absent early in HCMV infection. Subsequently, tapasin neosynthesis became strongly reduced, while tapasin steady-state levels diminished only slowly in infected cells, revealing a blocked synthesis rather than degradation. Tapasin mRNA levels were continuously downregulated during infection, while tapasin transcripts remained stable and long-lived. Taking advantage of a novel method by which de novo transcribed RNA is selectively labeled and analyzed, an immediate decline of tapasin transcription was seen, followed by downregulation of TAP2 and TAP1 gene expression. However, upon forced expression of tapasin in HCMV-infected cells, repair of MHC I incorporation into the PLC was relatively inefficient, suggesting an additional level of HCMV interference. The data presented here document a two-pronged coordinated attack on tapasin function by HCMV.