Evidence that a Single Peptide–MHC Complex on a Target Cell Can Elicit a Cytolytic T Cell Response

Recognition of tumor-associated Ags (TAAs) on tumor cells by CTLs and the subsequent tumor cell death are assumed to be dependent on TAA protein expression and to correlate directly with the level of peptide displayed in the binding site of the HLA class I molecule. In this study we evaluated whether the levels of Her-2/neu protein expression on human tumor cell lines directly correlate with HLA-A*0201/Her2/neu peptide presentation and CTL recognition. We developed a TCR mimic (TCRm) mAb designated 1B8 that specifically recognizes the HLA-A2.1/Her2/neu peptide (369–377) (Her2(369)-A2) complex. TCRm mAb staining intensity varied for the five human tumor cell lines analyzed, suggesting quantitative differences in levels of the Her2(369)-A2 complex on these cells. Analysis of tumor cell lines pretreated with IFN-γ and TNF-α for Her2/neu protein and HLA-A2 molecule expression did not reveal a direct correlation between the levels of Her2/neu Ag, HLA-A2 molecule, and Her2(369)-A2 complex expression. However, compared with untreated cells, cytokine-treated cell lines showed an increase in Her2(369)-A2 epitope density that directly correlated with enhanced tumor cell death (p = 0.05). Although a trend was observed between tumor cell lysis and the level of the Her2(369)-A2 complex for untreated cells, the association was not significant. These findings suggest that tumor cell susceptibility to CTL-mediated lysis may be predicted based on the level of specific peptide-MHC class I expression rather than on the total level of TAA expression. Further, these studies demonstrate the potential of the TCRm mAb for validation of endogenous HLA-peptide epitopes on tumor cells.

Section: Discussionmentioning

confidence: 99%

Levels of Specific Peptide-HLA Class I Complex Predicts Tumor Cell Susceptibility to CTL Killing

Weidanz

Nguyen²,

Woodburn³

et al. 2006

“…Previous studies have generally shown that the number of peptide-MHC complexes per target cell required for recognition and cytolysis by differentiated CTL varies from several thousand to Ͻ10 (26 -30). Sykulev et al previously provided more indirect evidence that in target cells pulsed with an average of three peptide epitope molecules per cell, cells expressing a single peptide-MHC complex could be recognized by CTL (31). Consistent with our quantitation is the observation that unmanipulated HLA-B27 ϩ B-LCL are poorly recognized by RRIY-DLIEL-specific CTL, even though these effectors are of extremely high avidity (SD 50 ϭ 1-5 pM) based on peptide dose-response curves (data not shown).…”

Section: Discussionmentioning

confidence: 99%

Immunodominance Among EBV-Derived Epitopes Restricted by HLA-B27 Does Not Correlate with Epitope Abundance in EBV-Transformed B-Lymphoblastoid Cell Lines

Crotzer

Christian

Brooks

et al. 2000

Using synthetic peptides, the HLA-B27-restricted CTL response to EBV in asymptomatic virus carriers has been mapped to four epitope regions in EBV latent cycle Ags. One of these peptide-defined epitopes (RRIYDLIEL) tends to be immunodominant and is recognized in the context of all three B27 subtypes studied, B*2702, B*2704, and B*2705. The other peptide-defined epitopes induce responses only in the context of one subtype, the immunogenic combinations being RRARSLSAERY/B*2702, RRRWRRLTV/B*2704, and FRKAQIQGL/B*2705. We used immunoaffinity chromatography to isolate the naturally presented viral peptides associated with these MHC class I molecules on the surface of EBV-transformed B-LCL. Using CTL reconstitution assays in conjunction with mass spectrometry, we established that the naturally processed and presented peptides are identical with the previously identified synthetic sequences. Despite the subtype-specific immunogenicity of three of the four epitopes, all four epitope peptides were found in association with each of the three different HLA-B27 subtypes. Indeed, those peptides that failed to induce a response in the context of a particular HLA-B27 subtype were frequently presented at greater abundance by that subtype than were the immunogenic peptides. Furthermore, among the peptides that did induce a response, immunodominance did not correlate with epitope abundance; in fact the immunodominant RRIYDLIEL epitope was least abundant, being present at less than one copy per cell. The relationship of this unexpected finding to the persistence of EBV is discussed.

“…Their existence suggests a broader functional TCR repertoire via an expanded pool of potential antigenic epitopes. Although cryptic epitopes may be present in small abundance relative to traditional epitopes, the extreme sensitivity of CTL may compensate to allow efficient TCR detection and triggering of effector function (6).…”

mentioning

confidence: 99%

Cytolytic CD8+ T Cells Directed against a Cryptic Epitope Derived from a Retroviral Alternative Reading Frame Confer Disease Protection

Green

2006

Cytolytic CD8+ T cells (CTL) are key to the immune response that controls virus infections and mediates disease protection. The ability of CTL to induce apoptosis of infected cells and/or limit viral replication is determined by recognition of processed viral peptide epitopes on the surface of the target cell. An understudied source of MHC class I-presented peptides is the aptly named “cryptic epitopes,” defined by their nontraditional methods of generation, including derivation from alternative reading frames (ARFs). Although ARF-encoded epitopes have now been documented in a few systems, their potential functional relevance in vivo has been debated. In this study, we demonstrate the physiological significance of an ARF-derived CTL epitope in a retrovirus-induced disease model. We show that disease-susceptible CD8-deficient mice reconstituted with CTL specific for the retroviral ARF-derived SYNTGRFPPL epitope controlled an infection by the LP-BM5 retrovirus isolate, evidently at the level of viral clearance, resulting in protection of these mice from disease. These data indicate that ARF-derived epitopes are indeed relevant inducers of the immune system and demonstrate the importance of atypically generated peptides as functional Ag with a physiologic role in disease protection.