Presentation of viral peptide antigens by Human Leukocyte Antigen (HLA) class I. Human immunodeficiency virus type 1 (HIV-1) proviral gene expression, including RNA transcription (a) and protein translation (b); generates functional viral proteins (c) as well as truncated or mis-folded proteins that are degraded by the cellular proteasome complex to form short antigenic peptides (d); These peptides are transported from the cytoplasm into the endoplasmic reticulum (ER) (e) where they can be loaded onto HLA-I molecules.Peptide/HLA complexes traffic from the ER through the Golgi and secretory vesicle (SV) network to the plasma cell membrane, where the peptide antigens are presented to circulating cytotoxic T lymphocytes (CTL) (f); The viral Nef protein shuttles HLA molecules located at the cell surface or within the trans-Golgi network into lysosomal compartments (g); where they are degraded.In the absence of Nef-mediated HLA down-regulation, antigen-specific CTL respond to stimulation by releasing cytotoxic molecules, including perforin and granzymes, resulting in elimination of the virus-infected cell (h).