Differentiation-stage specific self-peptides bound by major histocompatibility complex class I molecules

Peptides bound to major histocompatibility complex molecules

Peptides are the means by which immune effector T cells recognize and defend against the foreign proteins of pathogens. T cell recognition of these molecules, however, is strictly dependent on peptide binding to the receptor-like molecules of the major histocompatibility complex (MHC) locus. The basic unit of recognition is a trimolecular complex consisting of the T cell antigen receptor, the MHC molecule, and the MHC-bound peptide ligand. The multistep process that culminates in MHC presentation of peptides to T cells begins in the last phases of protein catabolism. While the individual roles of many key molecules involved in peptide presentation have recently been defined, there still remain many questions regarding processing of proteins into MHC-bound peptides. This review summarizes the recent developments in peptide antigen processing for MHC molecules, with focus on how proteins are believed to be sampled and selected for degradation into peptides.

Naturally Processed Tissue- and Differentiation Stage-Specific Autologous Peptides Bound by HLA Class I and II Molecules of Chronic Myeloid Leukemia Blasts

Structural analysis of naturally processed peptides bound to the HLA class I and class II molecules of chronic myeloid leukemia (CML) blast cells was performed to characterize the antigen processing and autoantigen repertoire in this hematopoietic malignancy. Self-peptides derived from the carboxy-terminal end of the breakpoint cluster region (bcr) protein, as well as several differentiation stage- and tissue-specific self-antigens characteristic of early stages of myeloid differentiation, such as c-fes, c-pim, granulocyte-macrophage colony-stimulating factor receptor α chain, proteinase 3, and cathepsin G, were identified. A common characteristic of several of the high copy-number self-peptides identified in this study is the participation of their parent proteins in signal transduction or myeloid effector function. Because bcr-abl junctional peptides bind to a limited number of major histocompatibility complex (MHC) class I alleles, an effective peptide-based immunotherapy strategy for CML requires identification of further tumor-associated or tissue-specific peptide antigens binding to common MHC alleles such as HLA-A2. The differentiation stage- and tissue-specific MHC-bound peptides found in this study, as well as the naturally processed proteins from which they are derived, may represent autoantigens towards which T-cell responses may potentially be developed for immunotherapy of hematopoietic malignancies such as CML.

Naturally Processed Tissue- and Differentiation Stage-Specific Autologous Peptides Bound by HLA Class I and II Molecules of Chronic Myeloid Leukemia Blasts

Structural analysis of naturally processed peptides bound to the HLA class I and class II molecules of chronic myeloid leukemia (CML) blast cells was performed to characterize the antigen processing and autoantigen repertoire in this hematopoietic malignancy. Self-peptides derived from the carboxy-terminal end of the breakpoint cluster region (bcr) protein, as well as several differentiation stage- and tissue-specific self-antigens characteristic of early stages of myeloid differentiation, such as c-fes, c-pim, granulocyte-macrophage colony-stimulating factor receptor α chain, proteinase 3, and cathepsin G, were identified. A common characteristic of several of the high copy-number self-peptides identified in this study is the participation of their parent proteins in signal transduction or myeloid effector function. Because bcr-abl junctional peptides bind to a limited number of major histocompatibility complex (MHC) class I alleles, an effective peptide-based immunotherapy strategy for CML requires identification of further tumor-associated or tissue-specific peptide antigens binding to common MHC alleles such as HLA-A2. The differentiation stage- and tissue-specific MHC-bound peptides found in this study, as well as the naturally processed proteins from which they are derived, may represent autoantigens towards which T-cell responses may potentially be developed for immunotherapy of hematopoietic malignancies such as CML.

MHC class I antigen processing pathways

The multistep process that culminates in major histocompatibility complex (MHC) class I presentation of foreign of self-peptides begins in the last phases of protein catabolism. Although the individual roles of many key molecules-such as proteasomes, the transporter associated with antigen processing, and various endoplasmic reticulum chaperones-have recently been elucidated, there still remain many questions regarding processing of proteins into MHC class I bound peptides. This review summarizes the recent developments in antigen processing for MHC class I molecules, with a focus on how proteins are believed to be sampled and selected for degradation.

Major histocompatibility complex class I presentation of exogenous and endogenous protein-derived peptides by a transfected human monocyte cell line

Monocyte/macrophages are professional antigen-presenting cells of the cellular immune system, serving to generate peptides for major histocompatibility complex (MHC) class II-restricted recognition by CD4+ T-lymphocyte effector cells. Antigen presentation by these cells involves the internalization of extracellular proteins and their fragmentation within vacuolar compartments. The resulting peptides become associated with MHC class II molecules. The final destination of exogenous peptide antigens, however, is not absolute in monocytes. Processed peptides, derived from exogenous proteins, can also associate with MHC class I molecules. To study simultaneous presentation of peptides derived from exogenous and endogenous proteins by human leucocyte antigen (HLA) class I molecules, we isolated the peptides from a human immunodeficiency virus nef transfected U937 monocytic cell line. The HLA class I-bound peptides were separated by reverse phase-high performance liquid chromatography. Comparison of the peptide sequence data with protein databases revealed that the peptides derived from extracellular, as well as intracellular, proteins, suggesting that monocytes have a more generalized MHC class I antigen-processing pathway than previously documented.

Analysis of naturally processed human histocompatibility leukocyte antigen class I-bound peptides from hepatocellular carcinoma tissues in vivo

Naturally processed self-peptides bound to human histocompatibility leukocyte antigens (HLA) class I molecules of human hepatocellular carcinoma tissues (HLA-A2.1, -B44, -B13) in vivo were isolated for sequence analysis. Acid-eluted peptides were subjected to reversed-phase high-performance liquid chromatographic separation and single-fraction sequencing was performed by Edman degradation. The peptides were found to be octamers or nonamers and they were derived from the processing of intracellular proteins. Three independent sequences were obtained from HLA-A2.1 molecules. One of the peptides showed sequence homology to the hepatitis B virus (HBV) pre-S protein, one to aldehyde dehydrogenase, and the other to no known protein. Two independent sequences were obtained from HLA-B44, B13 molecules: one showed sequence homology to the human c-abl protein, the other showed no homology to any known protein. A synthetic biotinylated peptide based on the HBV pre-S peptide sequence was confirmed to bind to HLA-A2.1 gene-transfected L cells. These data suggested that peptides potentially recognized by cytotoxic T cells can bind to HLA class I molecules on tumor cells in vivo.

Cell surface expression of major histocompatibility class I antigens is modulated by P-glycoprotein transporter

P-glycoprotein (Mdr1), a member of the ABC superfamily, is a pump able to transport several compounds across plasma membranes. It displays a high level of similarity with the MHC-linked transporters TAP1 and TAP2 which are involved in the delivery of immunogenic peptides across the endoplasmic reticulum. In the present study we analyze the P-glycoprotein's ability to interfere with the biosynthetic pathway of the MHC class I molecules. Our results show that P-glycoprotein is involved in the modulation of the MHC class I expression in multidrug-resistant tumor cell lines, COS1 cells transfected with mdr1 gene, and human T lymphocytes. Epitope screening evokes the possibility that P-glycoprotein induces a modulation of the different MHC class I forms expressed on the cell surface. We propose that P-glycoprotein is involved in the transport of antigenic protein fragments from the cytosol into the endoplasmic reticulum. The suggested mechanism could be physiologically relevant in tissues displaying a high Mdr1 activity, where this transporter could contribute to the regulation of locoregional immune responses.

Identification of a naturally processed HLA A0201-restricted viral peptide from cells expressing human papillomavirus type 16 E6 oncoprotein

Human papillomavirus (HPV) DNA encoding the oncogenic proteins E6 and E7 is usually retained in cervical carcinomas, implicating these proteins as potential target antigens for immune recognition in this virally associated tumor. We have characterized endogenously processed peptides eluted from major histocompatibility complex class I molecules in cells infected with a recombinant vaccinia expressing the HPV-16 E6 oncoprotein. The reverse-phase chromatography profile of peptides eluted from isolated HLA-A0201 molecules in cells expressing the E6 oncoprotein differs from that of cells not expressing E6. Sequential Edman degradation of novel peaks found in the peptide profiles from cells expressing HPV-16 E6 led to the identification of a naturally processed HLA-A0201-restricted E6 peptide of sequence KLPQLCTEL. This approach has allowed the identification of a viral peptide which is processed and presented by cells expressing the E6 oncoprotein and is a likely target for cytotoxic T lymphocyte recognition in HLA-A0201-positive patients.

HLA class I self peptides isolated from a T-cell leukemia reveal the allele-specific motif of HLA-B38

Naturally-processed self peptides bound to HLA class I molecules of a T-cell leukemia (HLA-A1, A31, B38, B58) were isolated for sequence analysis. Acid-eluted peptides were subjected to reversed-phase HPLC separation and single-fraction sequencing was performed by Edman degradation. The peptides were found to be mostly nonamers and could be grouped into three distinct structural motifs. One of the peptide groups consistently displayed histidine at position 2 and a bulky hydrophobic residue at the C-terminus (XHXPXXXXY/F). The only HLA class I structure expressed by this T-cell leukemia which is consistent with the binding of peptides carrying this sequence motif is HLA-B38. A peptide binding assay confirmed this assignment. HLA-B38 is present in 10-12% of the Jewish population and is associated with several autoimmune disorders. The HLA-B38 motif may be an important tool for identifying potential T-cell epitopes involved in these diseases and for designing peptide vaccines.

Major histocompatibility complex-restricted recognition of autologous chronic lymphocytic leukemia by tumor-specific T cells

From the peripheral blood of a patient with chronic lymphocytic leukemia (CLL) we generated a T-cell line and clones which recognized autologous CLL. The line comprised T-cell clones which responded to the CLL as well as to autologous Epstein-Barr virus (EBV)-transformed B cells in an HLA-DR-restricted fashion. In addition, the line comprised clones which were CLL-specific and showed no reactivity against EBV-transformed B cells and against autologous peripheral blood mononuclear cells obtained during remission. The proliferative response of the CLL-specific T-cell clone was inhibited by monoclonal antibodies to HLA-DR11, the major histocompatibility complex (MHC)-restrictive element. These results indicate that the MHC class-II molecule of CLL binds a tumor-specific peptide which is recognized by autologous T cells in an MHC class-II-restricted fashion. Such a peptide may serve as a target for immunotherapy.