Viral proteins counteracting host defenses

Since the first observations of viral interference with antigen presentation in the MHC class-I pathway, the field has advanced to a detailed analysis. We know numerous genes and for some of them we have profound information on their mechanistic function. The antigen presentation pathway is affected at all stages starting from proteasomal degradation of an antigenic viral protein, as shown for EBV, transfer of the proteasomal cleavage products as peptides in the ER by specific transporters, the loading of the nascent MHC class-I molecule, and finally the transport of the complex to the surface and presentation in a normal or deranged form. All these different steps of the MHC class-I antigen presentation pathway are targets for viral proteins. Not only MHC class-I but also MHC class-II proteins are a target of viral influence either by direct downregulation and degradation of proteins or by interference of signal transduction pathways.