The cytotoxic T-lymphocyte-mediated killing of virus-infected cells requires previous recognition of short viral antigenic peptides bound to human leukocyte antigen class I molecules that are exposed on the surface of infected cells. The cytotoxic T-lymphocyte response is critical for the clearance of human respiratory syncytial virus infection. In this study, naturally processed viral human leukocyte antigen class I ligands were identified with mass spectrometry analysis of complex human leukocyte antigen-bound peptide pools isolated from large amounts of human respiratory syncytial virus-infected cells. Acute antiviral T-cell response characterization showed that viral transcription determines both the immunoprevalence and immunodominance of the human leukocyte antigen class I response to human respiratory syncytial virus.
Newly synthesized HLA class II molecules from antigenpresenting cells associate with the class II invariant chain (Ii). These complexes are eventually transported to specialized endosomal compartments where the Ii is progressively proteolyzed until only a fragment known as the class-II-associated invariant chain peptide (CLIP) 1 remains bound in the HLA class II peptide-binding groove to prevent it from binding to cellular peptides or pathogen peptides from the endogenous pathways. Interaction of HLA class II/CLIP complexes with the accessory molecule HLA-DM induces conformational changes in HLA class II molecules. Additionally, the release of CLIP results in peptide-receptive HLA class II molecules. This compartment fuses with a late endosome that contains exogenous proteins and/or viral particles that were previously endocytosed. Thus, the binding of antigen-processed peptides of different lengths, but with specific major anchor residues that can be deeply accommodated into specific pockets of the antigen recognition site of the HLA class II molecule, produces the stabilization of the nascent HLA class II/peptide complexes and allows for their subsequent transport to the cell membrane where they are exposed for T cell recognition (1).Human respiratory syncytial virus (HRSV) (2), which is included in the Paramyxoviridae family of the Mononegavirales order, presents a single-stranded, negative-sense RNA genome that codes for 11 proteins. This enveloped pneumovirus causes repeat infections throughout life, and although in healthy adults mild infections are generally reported, the health risk in infected pediatric, immunocompromised, and elderly populations is much more serious. HRSV is the main cause of hospitalization for bronchiolitis and pneumonia in infants and young children, with infection rates approaching 70% in the first year of life (3). Worldwide, at least 3.4 million hospital admissions each year are associated with severe HRSV disease, and the global mortality rate was estimated at more than a quarter of a million deaths in 2010, mainly in developing countries (4).The immune mechanisms involved in HRSV disease and protection are not completely understood; however, it is known that infection induces mucosal and systemic humoral and cellular responses. Studies evaluating CD4 ϩ and CD8 ϩ T-lymphocyte subsets individually or together showed that both effector MHC class I-and helper MHC class II-restricted cellular responses are particularly important in clearing infections (5). Previously, some HRSV epitopes that are restricted by different HLA class II molecules were identified using T cells from seropositive individuals (6 -9). However, these experiments were performed with overlapping synthetic pepFrom the ‡Centro Nacional
The transporter associated with antigen processing (TAP) translocates the cytosol-derived proteolytic peptides to the endoplasmic reticulum lumen where they complex with nascent human leukocyte antigen (HLA) class I molecules. Non-functional TAP complexes and viral or tumoral blocking of these transporters leads to reduced HLA class I surface expression and a drastic change in the available peptide repertoire. Using mass spectrometry to analyze complex human leukocyte antigen HLA-bound peptide pools isolated from large numbers of TAP-deficient cells, we identified 334 TAP-independent ligands naturally presented by four different HLA-A, -B, and -C class I molecules with very different TAP dependency from the same cell line. The repertoire of TAP-independent peptides examined favored increased peptide lengths and a lack of strict binding motifs for all four HLA class I molecules studied. The TAP-independent peptidome arose from 182 parental proteins, the majority of which yielded one HLA ligand. In contrast, TAP-independent antigen processing of very few cellular proteins generated multiple HLA ligands. Comparison between TAP-independent peptidome and proteome of several subcellular locations suggests that the secretory vesicle-like organelles could be a relevant source of parental proteins for TAP-independent HLA ligands. Finally, a predominant endoproteolytic peptidase specificity for Arg/Lys or Leu/Phe residues in the P1 position of the scissile bond was found for the TAP-independent ligands. These data draw a new and intricate picture of TAP-independent pathways.
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