Unlike cytosolic processing and presentation of viral Ags by virus-infected cells， Ags first expressed in infected nonprofessional APCs， such as CD4 T cells in the case of HIV， are taken up by dendritic cells and cross-presented. This generally requires entry through the endocytic pathway， where endosomal proteases have first access for processing. Thus， understanding virus escape during cross-presentation requires an understanding of resistance to endosomal proteases， such as cathepsin S (CatS). We have modified HIV-1 gp120 by mutating a key CatS cleavage site (ThrThr) in the V3 loop of the immunodominant epitope IGPGRAFY to IGPGRAFY to prevent digestion. We found this mutation to facilitate cross-... More
Unlike cytosolic processing and presentation of viral Ags by virus-infected cells， Ags first expressed in infected nonprofessional APCs， such as CD4 T cells in the case of HIV， are taken up by dendritic cells and cross-presented. This generally requires entry through the endocytic pathway， where endosomal proteases have first access for processing. Thus， understanding virus escape during cross-presentation requires an understanding of resistance to endosomal proteases， such as cathepsin S (CatS). We have modified HIV-1 gp120 by mutating a key CatS cleavage site (ThrThr) in the V3 loop of the immunodominant epitope IGPGRAFY to IGPGRAFY to prevent digestion. We found this mutation to facilitate cross-presentation and provide evidence from MHC binding and X-ray crystallographic structural studies that this results from preservation of the epitope rather than an increased epitope affinity for the MHC class I molecule. In contrast， when the protein is expressed by a vaccinia virus in the cytosol， the wild-type protein is immunogenic without this mutation. These proof-of-concept results show that a virus like HIV， infecting predominantly nonprofessional presenting cells， can escape T cell recognition by incorporating a CatS cleavage site that leads to destruction of an immunodominant epitope when the Ag undergoes endosomal cross-presentation.
