Generation Of Major Histocompatibility Complex Class Research Articles

Quantitative measurement of the requirement of diverse protein degradation pathways in MHC class I peptide presentation.

Peptides from degradation of intracellular proteins are continuously displayed by major histocompatibility complex (MHC) class I.To better understand origins of these peptides, we performed a comprehensive census of the class I peptide repertoire in the presence and absence of ubiquitin-proteasome system (UPS) activity upon developing optimized methodology to enrich for and quantify these peptides. Whereas most class I peptides are dependent on the UPS for their generation, a surprising 30%, enriched in peptides of mitochondrial origin, appears independent of the UPS. A further ~10% of peptides were found to be dependent on the proteasome but independent of ubiquitination for their generation. Notably, clinically achievable partial inhibition of the proteasome resulted in display of atypical peptides. Our results suggest that generation of MHC class I•peptide complexes is more complex than previously recognized, with UPS-dependent and UPS-independent components; paradoxically, alternative protein degradation pathways also generate class I peptides when canonical pathways are impaired.

Tissue-specific proteasomes in generation of MHC class I peptides and CD8+ T cells.

Thymoproteasomes and immunoproteasomes are two types of tissue-specific proteasomes, which contribute to the production of major histocompatibility complex (MHC) class I (MHC-I)-associated peptides that are important for the development and function of CD8+ cytotoxic T cells. Thymoproteasomes are specifically expressed by cortical thymic epithelial cells and are important for MHC-I-dependent positive selection of developing thymocytes, whereas immunoproteasomes are abundant in many other cells, including hematopoietic cells and medullary thymic epithelial cells. Here we summarize the role of these two tissue-specific proteasomes, focusing on their functions in the development of CD8+ T cells in the thymus.

Generation of MHC class I ligands in the secretory and vesicular pathways.

CD8(+) T lymphocytes screen the surface of all cells in the body to detect pathogen infection or oncogenic transformation. They recognize peptides derived from cellular proteins displayed at the plasma membrane by major histocompatibility complex (MHC) class I molecules. Peptides are mostly by-products of cytosolic proteolytic enzymes. Peptidic ligands of MHC class I molecules are also generated in the secretory and vesicular pathways. Features of protein substrates, of proteases and of available MHC class I molecules for loading peptides in these compartments shape a singular collection of ligands that also contain different, longer, and lower affinity peptides than ligands produced in the cytosol. Especially in individuals who lack the transporters associated with antigen processing, TAP, and in infected and tumor cells where TAP is blocked, which thus have no supply of peptides derived from the cytosol, MHC class I ligands generated in the secretory and vesicular pathways contribute to shaping the CD8(+) T lymphocyte response.

The role of the proteasome in the generation of MHC class I ligands and immune responses

The ubiquitin–proteasome system (UPS) degrades intracellular proteins into peptide fragments that can be presented by major histocompatibility complex (MHC) class I molecules. While the UPS is functional in all mammalian cells, its subunit composition differs depending on cell type and stimuli received. Thus, cells of the hematopoietic lineage and cells exposed to (pro)inflammatory cytokines express three proteasome immunosubunits, which form the catalytic centers of immunoproteasomes, and the proteasome activator PA28. Cortical thymic epithelial cells express a thymus-specific proteasome subunit that induces the assembly of thymoproteasomes. We here review new developments regarding the role of these different proteasome components in MHC class I antigen processing, T cell repertoire selection and CD8 T cell responses. We further discuss recently discovered functions of proteasomes in peptide splicing, lymphocyte survival and the regulation of cytokine production and inflammatory responses.

Novel Strategies in the Treatment of Multiple Myeloma: From Proteasome Inhibitors to Immunotherapy

The proteasome serves as the catalytic core of the Ubiquitin (Ub) protein degradation pathway and has become an intriguing target in drug development and cancer therapy. Successful pharmacologic inhibition of the proteasome with the small molecule bortezomib led to US Food and Drug Administration (FDA) regulatory approval for the treatment of mantle cell lymphoma and multiple myeloma (MM) and has been extended to a steadily increasing number of clinical trials to assess effi cacy and safety in other hematologic malignances and solid tumors. Proteasome inhibition results in the accumulation of multi-ubiquitinated proteins, which are normally degraded through the tightly regulated Ub pathway. The Ub-Proteasome pathway is responsible for the selective degradation of many proteins that regulate the cell cycle and growth. Inhibition of the proteasome generates the accumulation of multi-ubiquitinated proteins that eventually leads to apoptosis although the exact mechanism of cell death is not completely understood. A specialized form of the proteasome, known to as the immunoproteasome, processes intracellular and viral proteins to generate peptides that are then presented at the cell surface bound as antigens (Ags) bound to the Major Histocompatibility Complex (MHC) class I molecule receptor. Importantly, inhibitors of the immunoproteasome decrease the processing and generation of MHC class I Ags and alter tumor cell recognition by the principal cellular effectors of the immune system. Hence, proteasome inhibitors may be employed as therapeutics to regulate the production of tumor specifi c Ags and for the selective removal of tumor cells through recognition by cytotoxic T lymphocytes (CTLs), natural killer (NK) cells and dendritic cells (DC). Proteasome inhibitors have been validated as effective cytotoxic agents and may have further potential as novel immunotherapeutic strategies.

Enhanced Expression of Interferon-γ-Induced Antigen-Processing Machinery Components in a Spontaneously Occurring Cancer

In human tumors, changes in the surface expression and/or function of major histocompatibility complex (MHC) class I antigens are frequently found and may provide malignant cells with a mechanism to escape control of the immune system. This altered human lymphocyte antigen (HLA) class I phenotype can be caused by either structural alterations or dysregulation of genes encoding subunits of HLA class I antigens and/or components of the MHC class I antigen-processing machinery (APM). Herein we analyze the expression of several proteins involved in the generation of MHC class I epitopes in feline injection site sarcoma, a spontaneously occurring tumor in cats that is an informative model for the study of tumor biology in other species, including humans. Eighteen surgically removed primary fibrosarcoma lesions were analyzed, and an enhanced expression of two catalytic subunits of immunoproteasomes, PA28 and leucine aminopeptidase, was found in tumors compared to matched normal tissues. As a functional counterpart of these changes in protein levels, proteasomal activities were increased in tissue extracts from fibrosarcomas. Taken together, these results suggest that alterations in the APM system may account for reduced processing of selected tumor antigens and may potentially provide neoplastic fibroblasts with a mechanism for escape from T-cell recognition and destruction.

Protein sorting within the MHC class II antigen-processing pathway.

Major histocompatibility complex (MHC) class II molecules are required for the presentation of antigenic peptides that are derived predominantly from internalized proteins. The assembly of MHC class II/peptide complexes occurs within endosomal compartments of antigen-presenting cells (APCs). Therefore, for assembly to occur, MHC class II molecules, foreign proteins, and accessory molecules must be sorted to appropriate intracellular sites. My laboratory is trying to understand how proteins are sorted to various antigen-processing compartments as well as to conventional endosomal organelles. Using chimeric marker proteins and a variety of biochemical and genetic approaches, we are addressing the specificity of protein sorting and the mechanisms by which sorting signals are deciphered. By using a similar chimeric protein approach to target endogenous proteins to distinct compartments, we hope to address the role of processing events in each compartment in the generation of MHC class II ligands.