Transport and quality control of MHC class I molecules in the early secretory pathway

The temporal expression pattern of classical MHC class I in sleep-restricted mice: Generalizations and broader implications

The intricate relationship between sleep and leukocyte trafficking has garnered intense attention, particularly their homing dynamics to secondary lymphoid organs under normal and restricted sleep (SR). Considering the scarcity of information regarding circadian rhythms in major histocompatibility class I (MHC-I) expression in SR, we designed a study that assessed the temporal expression of MHC-I in murine lymph nodes and spleen and the subsequent effects of sleep recovery. Male C57BL/6, housed in 12:12 light/dark cycle, were grouped into control (C) and SR. SR was carried for one week before lymphoid tissues were sampled at selected time points and assessed for leukocyte number and MHC-I expression. SR resulted in 21% decrease in granulocyte and 24% increase in agranulocyte numbers. In C, MHC-I expression pattern in lymph nodes was bimodal and relatively higher than splenocytes during the animal's active phase (110.2 ± 1.8 vs 81.9 ± 3.8, respectively; p = 0.002). Splenocytes; however, showed a bimodal pattern upon SR, with higher protein levels during the rest than the activity period (154.6 + 36.2 vs 99.5 + 15.9, respectively; p = 0.002), suggesting preparedness for a potential infection. Furthermore, SR caused a significant drop in MHC-I expression at the onset of rest with 57% and 30% reduction in lymph nodes and splenocytes, respectively. However, the overall protein expression collectively taken from both lymphoid tissues remained stable, emphasizing its indispensable role in immunological homeostasis. This stability coincided with the restoration of protein levels to baseline after a short sleep recovery period, resembling a reset for MHC-I antigen presentation following a week of SR. Understanding the interplay between MHC-I expression and contextual factors could enhance treatment protocols, refining the efficacy and time precision of glucocorticoid-based therapies in immune modulation.

HLA-I levels correlate with survival outcomes in response to immune checkpoint inhibitors in non-small cell lung cancer

OBJECTIVES

Immune checkpoint inhibitors (ICIs) have provided a breakthrough in the treatment of non-small cell lung cancer (NSCLC) patients, but only some patients benefit substantively. Identifying definitive predictive biomarkers could overcome this limitation.

MATERIALS AND METHODS

We selected 146 metastatic NSCLC patients treated with anti-PD-(L)1. Immunohistochemistry of HLA-I, PD-L1 and CD73 was performed in 122 tumor biopsies at diagnosis. The association with patients, tumor parameters, and the predictive value to ICI treatment were determined.

RESULTS

In our cohort, 42 %, 25 %, and 21 % of the tumors exhibited high levels of HLA-I, PD-L1, and CD73, respectively. Lung adenocarcinomas displayed elevated CD73 levels, compared with lung squamous cell carcinomas (P = 0.026). High PD-L1 was significantly correlated with high levels of HLA-I (P = 0.005) and of CD73 (P = 0.025). Patients with high-level HLA-I tumors exhibited more favorable clinical outcomes following ICI, with a median overall survival of 30.7 months (95 % confidence interval [CI]: 18.3 months-not reached), compared with 18.2 months (95 % CI: 12.4-25.2 months) in patients with low-level HLA-I tumors (P = 0.016). The median progression-free survival (PFS) for patients with high-level HLA-I tumors was 18.5 months (95 % CI: 11.1-57.1 months), longer than patients with low-level HLA-I tumors, whose median PFS was 9.2 months (95 % CI: 7.2-11.9 months) (P = 0.006). In a multivariable analysis, high-level HLA-I was independently associated with lower risk of progression to ICI (HR = 0.46, 95 % CI 0.24-0.87; P = 0.018).

CONCLUSIONS

High-level HLA-I were associated with better clinical outcomes to ICI in our cohort of NSCLC patients. Therefore, further investigations are warranted to refine this biomarker and validate its efficacy in prospective and larger set of patients.

Med23 deficiency reprograms the tumor microenvironment to promote lung tumorigenesis

BACKGROUND

Lung cancer is the leading cause of cancer-related death worldwide. We previously found that Mediator complex subunit 23 (MED23) is important for the tumourigenicity of lung cancer cells with hyperactive Ras activity in vitro, although the in vivo function of MED23 in lung tumourigenesis remains to be explored.

METHODS

In this study, we utilized well-characterized KrasG12D-driven non-small cell lung cancer mouse model to investigate the role of MED23 in lung cancer. The lung tumour progression was evaluated by H&E and IHC analysis. Western blotting and qRT-PCR assays were performed to detect changes in gene expression. Immune cells were analyzed by FACS technology. RNA-seq and reporter assays were conducted to explore the mechanism.

RESULTS

We observed that lung epithelial Med23 deletion by adeno-Cre resulted in a significant increase in KrasG12D tumour number and size, which was further verified with another mouse model with Med23 specifically deleted in alveolar type II cells. Mice with lung-specific Med23 deficiency also exhibited accelerated tumourigenesis, and a higher proliferation rate for tumour cells, along with increased ERK phosphorylation. Notably, the numbers of infiltrating CD4+ T cells and CD8+ T cells were significantly reduced in the lungs of Med23-deficient mice, while the numbers of myeloid-derived suppressor cells (MDSCs) and Treg cells were significantly increased, suggesting the enhanced immune escape capability of the Med23-deficient lung tumours. Transcriptomic analysis revealed that the downregulated genes in Med23-deficient lung tumour tissues were associated with the immune response. Specifically, Med23 deficiency may compromise the MHC-I complex formation, partially through down-regulating B2m expression.

CONCLUSIONS

Collectively, these findings revealed that MED23 may negatively regulate Kras-induced lung tumourigenesis in vivo, which would improve the precise classification of KRAS-mutant lung cancer patients and provide new insights for clinical interventions.

Overcoming Resistance Mechanisms to Immune Checkpoint Inhibitors: Leveraging the Anti-Tumor Immune Response

As far back as 3000 years ago, the immune system was observed to play a role in mediating tumor regression. Since then, many strategies have been developed to leverage the anti-tumor immune response. However, while many patients respond to ICIs up front some do not, and many of those that do eventually experience tumor progression. Currently, there are several predictive biomarkers of the immune checkpoint inhibitor response; however, no one test appears to be universally predictive and their application varies by disease site. There are many ways in which cancer cells develop primary or acquired resistance to immune checkpoint inhibitors. Efforts to reverse resistance include ways to combat T cell exhaustion, reprogram the tumor microenvironment, increase the availability of tumor neo-antigens, target alternative immune checkpoints, restore a normal/healthy patient gut microbiome, oncolytic viruses and tumor vaccines. The most studied and most promising methods include combining ICIs with therapies targeting alternative immune checkpoints and restoring a normal/healthy patient gut microbiome. This review will discuss T cell-mediated immunity, how this is leveraged by modern immunotherapy to treat cancer and mechanisms of immune checkpoint inhibitor resistance, while highlighting strategies to overcome primary and secondary resistance mechanisms.

Autophagy-dependent regulation of MHC-I molecule presentation

The major histocompatibility complex (MHC) class I molecules present peptide antigens to MHC class I-restricted CD8+ T lymphocytes to elicit an effective immune response. The conventional antigen-processing pathway for MHC-I presentation depends on proteasome-mediated peptide generation and peptide loading in the endoplasmic reticulum by members of the peptide loading complex. Recent discoveries in this field highlight the role of alternative MHC-I peptide loading and presentation pathways, one of them being autophagy. Autophagy is a cell-intrinsic degradative pathway that ensures cellular homoeostasis and plays critical roles in cellular immunity. In this review article, we discuss the role of autophagy in MHC class I-restricted antigen presentation, elucidating new findings on the crosstalk of autophagy and ER-mediated MHC-I peptide presentation, dendritic cell-mediated cross-presentation and also mechanisms governing immune evasion. A detailed molecular understanding of the key drivers of autophagy-mediated MHC-I modulation holds promising targets to devise effective measures to improve T cell immunotherapies.

Immunotherapy resistance in non-small-cell lung cancer: From mechanism to clinical strategies

The high primary resistance incidence and unavoidable secondary resistance are the major clinical obstacle to lasting long-term benefits in Non-small-cell lung cancer (NSCLC) patients treated with immunotherapy. The mechanisms of immunotherapy resistance in NSCLC are complex, mainly involving tumor cells and tumor microenvironment (TME) infiltrating immune cells, including TAMs, B cells, NK cells, and T cells. The selection of clinical strategies for NSCLC progression after immunotherapy resistance should depend on the progressive mode. The progression pattern of NSCLC patients after immunotherapy resistance can be divided into oligo-progression and systemic/multiple progression, which should be considered for further treatment selection. In the future, it needs to explore how to optimize the combined therapy and explore strategies to reprogram infiltrating immune cells under various genetic backgrounds of tumor cells and timely reshape TME during antitumor treatments.

Spotlight on TAP and its vital role in antigen presentation and cross-presentation

In the late 1980s and early 1990s, the hunt for a transporter molecule ostensibly responsible for the translocation of peptides across the endoplasmic reticulum (ER) membrane yielded the successful discovery of transporter associated with antigen processing (TAP) protein. TAP is a heterodimer complex comprised of TAP1 and TAP2, which utilizes ATP to transport cytosolic peptides into the ER across its membrane. In the ER, together with other components it forms the peptide loading complex (PLC), which directs loading of high affinity peptides onto nascent major histocompatibility complex class I (MHC-I) molecules that are then transported to the cell surface for presentation to CD8+ T cells. TAP also plays a crucial role in transporting peptides into phagosomes and endosomes during cross-presentation in dendritic cells (DCs). Because of the critical role that TAP plays in both classical MHC-I presentation and cross-presentation, its expression and function are often compromised by numerous types of cancers and viruses to evade recognition by cytotoxic CD8 T cells. Here we review the discovery and function of TAP with a major focus on its role in cross-presentation in DCs. We discuss a recently described emergency route of noncanonical cross-presentation that is mobilized in DCs upon TAP blockade to restore CD8 T cell cross-priming. We also discuss the various strategies employed by cancer cells and viruses to target TAP expression or function to evade immunosurveillance - along with some strategies by which the repertoire of peptides presented by cells which downregulate TAP can be targeted as a therapeutic strategy to mobilize a TAP-independent CD8 T cell response. Lastly, we discuss TAP polymorphisms and the role of TAP in inherited disorders.

Homotypic and heterotypic in cis associations of MHC class I molecules at the cell surface

Through the presentation of peptide antigens to cytotoxic T lymphocytes, major histocompatibility complex (MHC) class I molecules mediate the adaptive immune response against tumors and viruses. Additional non-immunological functions include the heterotypic association of class I molecules with cell surface receptors, regulating their activities by unknown mechanisms. Also, homotypic associations resulting in class I dimers and oligomers - of unknown function - have been related to pathological outcomes. In this review, we provide an overview of the current knowledge about the occurrence, biochemical nature, and dynamics of homotypic and heterotypic associations of class I molecules at the cell surface with special focus on the molecular species that take part in the complexes and on the evidence that supports novel biological roles for class I molecules. We show that both heterotypic and homotypic class I associations reported in the literature describe not one but several kinds of oligomers with distinctive stoichiometry and biochemical properties.

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Major histocompatibility complex class I molecules form homotypic and heterotypic associations at the cell surface.

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Associations show distinctive stoichiometry and biochemical properties.

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Associations might regulate immunological and non-immunological processes.

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Heterotypic association with cell surface receptors might regulate receptor's activity.

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Homotypic associations have been related to pathological outcomes.

Allotype-Specific Glycosylation and Cellular Localization of Human Leukocyte Antigen Class I Proteins

Presentation of antigens by human leukocyte antigen (HLA) complexes at the cell surface is a key process in the immune response. The α-chain, containing the peptide-binding groove, is one of the most polymorphic proteins in the proteome. All HLA class I α-chains carry a conserved N-glycosylation site, but little is known about its nature and function. Here, we report an in-depth characterization of N-glycosylation features of HLA class I molecules. We observe that different HLA-A α-chains carry similar glycosylation, distinctly different from the HLA-B, HLA-C, and HLA-F α-chains. Although HLA-A displays the broadest variety of glycan characteristics, HLA-B α-chains carry mostly mature glycans, and HLA-C and HLA-F α-chains carry predominantly high-mannose glycans. We expected these glycosylation features to be directly linked to cellular localization of the HLA complexes. Indeed, analyzing HLA class I complexes from crude plasma and inner membrane-enriched fractions confirmed that most HLA-B complexes can be found at the plasma membrane, while most HLA-C and HLA-F molecules reside in the endoplasmic reticulum and Golgi membrane, and HLA-A molecules are more equally distributed over these cellular compartments. This allotype-specific cellular distribution of HLA molecules should be taken into account when analyzing peptide antigen presentation by immunopeptidomics.

Acquired Resistance to Immune Checkpoint Blockades: The Underlying Mechanisms and Potential Strategies

The immune checkpoint blockade therapy has completely transformed cancer treatment modalities because of its unprecedented and durable clinical responses in various cancers. With the increasing use of immune checkpoint blockades in clinical practice, a large number of patients develop acquired resistance. However, the knowledge about acquired resistance to immune checkpoint blockades is limited and poorly summarized. In this review, we clarify the principal elements of acquired resistance to immune checkpoint blockades. The definition of acquired resistance is heterogeneous among groups or societies, but the expert consensus of The Society for Immunotherapy of Cancer can be referred. Oligo-progression is the main pattern of acquired resistance. Acquired resistance can be derived from the selection of resistant cancer cell clones that exist in the tumor mass before therapeutic intervention or gradual acquisition in the sensitive cancer cells. Specifically, tumor intrinsic mechanisms include neoantigen depletion, defects in antigen presentation machinery, aberrations of interferon signaling, tumor-induced exclusion/immunosuppression, and tumor cell plasticity. Tumor extrinsic mechanisms include upregulation of other immune checkpoints. Presently, a set of treatment modalities is applied to patients with similar clinical characteristics or resistance mechanisms for overcoming acquired resistance, and hence, further research is required.

Venus flytrap or pas de trois? The dynamics of MHC class I molecules

The peptide binding site of major histocompatibility complex (MHC) class I molecules is natively unfolded when devoid of peptides. Peptide binding stabilizes the structure and slows the dynamics, but peptide-specific and subtype-specific motions influence, and are influenced by, interaction with assembly chaperones, the T cell receptor, and other class I-binding proteins. The molecular mechanisms of cooperation between peptide, class I heavy chain, and beta-2 microglobulin are insufficiently known but are being elucidated by nuclear magnetic resonance and other modern methods. It appears that micropolymorphic clusters of charged amino acids, often hidden in the molecule interior, determine the dynamics and thus chaperone dependence, cellular fate, and disease association of class I.

Suppression of Non-Random Fertilization by MHC Class I Antigens

Hermaphroditic invertebrates and plants have a self-recognition system on the cell surface of sperm and eggs, which prevents their self-fusion and enhances non-self-fusion, thereby contributing to genetic variation. However, the system of sperm-egg recognition in mammals is under debate. To address this issue, we explored the role of major histocompatibility complex class I (MHC class I, also known as histocompatibility 2-Kb or H2-Kb and H2-Db in mice) antigens by analyzing H2-Kb-/-H2-Db-/-β2-microglobulin (β2M)-/- triple-knockout (T-KO) male mice with full fertility. T-KO sperm exhibited an increased sperm number in the perivitelline space of wild-type (WT) eggs in vitro. Moreover, T-KO sperm showed multiple fusion with zona pellucida (ZP)-free WT eggs, implying that the ability of polyspermy block for sperm from T-KO males was weakened in WT eggs. When T-KO male mice were intercrossed with WT female mice, the percentage of females in progeny increased. We speculate that WT eggs prefer fusion with T-KO sperm, more specifically X-chromosome-bearing sperm (X sperm), suggesting the presence of preferential (non-random) fertilization in mammals, including humans.

Covering All the Bases: Complementary MR1 Antigen Presentation Pathways Sample Diverse Antigens and Intracellular Compartments

The ubiquitously expressed, monomorphic MHC class Ib molecule MHC class I-related protein 1 (MR1) presents microbial metabolites to mucosal-associated invariant T (MAIT) cells. However, recent work demonstrates that both the ligands bound by MR1 and the T cells restricted by it are more diverse than originally thought. It is becoming increasingly clear that MR1 is capable of presenting a remarkable variety of both microbial and non-microbial small molecule antigens to a diverse group of MR1-restricted T cells (MR1Ts) and that the antigen presentation pathway differs between exogenously delivered antigen and intracellular microbial infection. These distinct antigen presentation pathways suggest that MR1 shares features of both MHC class I and MHC class II antigen presentation, enabling it to sample diverse intracellular compartments and capture antigen of both intracellular and extracellular origin. Here, we review recent developments and new insights into the cellular mechanisms of MR1-dependent antigen presentation with a focus on microbial MR1T cell antigens.

Multifunctional Chaperone and Quality Control Complexes in Adaptive Immunity

The fundamental process of adaptive immunity relies on the differentiation of self from nonself. Nucleated cells are continuously monitored by effector cells of the immune system, which police the peptide status presented via cell surface molecules. Recent integrative structural approaches have provided insights toward our understanding of how sophisticated cellular machineries shape such hierarchical immune surveillance. Biophysical and structural achievements were invaluable for defining the interconnection of many key factors during antigen processing and presentation, and helped to solve several conundrums that persisted for many years. In this review, we illuminate the numerous quality control machineries involved in different steps during the maturation of major histocompatibility complex class I (MHC I) proteins, from their synthesis in the endoplasmic reticulum to folding and trafficking via the secretory pathway, optimization of antigenic cargo, final release to the cell surface, and engagement with their cognate receptors on cytotoxic T lymphocytes.

Antigen processing and presentation

Dendritic cells are at the center of immune responses. They are defined by their ability to sense the environment, take up and process antigen, migrate to secondary lymphoid organs, where they present antigens to the adaptive immune system. In particular, they present lipids and proteins from pathogens, which they encountered in peripheral tissues, to T cells in order to induce a specific effector immune response. These complex antigens need to be broken down into peptides of a certain length in association with Major Histocompatibility Complex (MHC) molecules. Presentation of MHC/antigen complexes alongside costimulatory molecules and secretion of proinflammatory cytokines will induce an appropriate immune response. This interaction between dendritic cells and T cells takes place at defined locations within secondary lymphoid organs. In this review, we discuss the current knowledge and recent advances on the cellular and molecular mechanisms that underlie antigen processing and the subsequent presentation to T lymphocytes.

Determinants of immunological evasion and immunocheckpoint inhibition response in non-small cell lung cancer: the genetic front

The incorporation into clinical practice of immune-checkpoint inhibitors (ICIs), such as those targeting the cytotoxic T lymphocyte-associated antigen 4 (CTLA-4) and the programmed cell death 1 (PD-1) and its ligand (PD-L1), has represented a major breakthrough in non-small cell lung cancer (NSCLC) treatment, especially in cases where the cancer has no druggable genetic alterations. Despite becoming the standard of care in certain clinical settings, either alone or in combination with chemotherapy, a proportion of patients do not respond while others actually progress during treatment. Therefore, there is a clinical need to identify accurate predictive biomarkers and to develop novel therapeutic strategies based on ICIs. Although they have limitations, the current markers evaluated to select which patients will undergo ICI treatment are the levels of PD-L1 and the tumor mutational burden. In this paper we describe what is currently known about the dynamic interaction between the cancer cell and the immune system during carcinogenesis, with a particular focus on the description of the functions and gene alterations that preclude the host immunoresponse in NSCLC. We emphasize the deleterious gene alterations in components of the major histocompatibility complex (HLA-I or B2M) and of the response to IFNγ (such as JAK2) which are mutually exclusive and can affect up to one fifth of the NSCLCs. The participation of other gene alterations, such as those of common oncogenes and tumor suppressors, and of the epigenetic alterations will also be discussed, in detail. Finally, we discuss the potential use of the tumor's genetic profile to predict sensitivity to ICIs.

Endocytic Recycling of MHC Class I Molecules in Non-professional Antigen Presenting and Dendritic Cells

Major histocompatibility complex class I (MHC I) molecules are glycoproteins that display peptide epitopes at the cell surface of nucleated cells for recognition by CD8⁺ T cells. Like other cell surface receptors, MHC class I molecules are continuously removed from the surface followed by intracellular degradation or recycling to the cell surface, in a process likely involving active quality control the mechanism of which remains unknown. The molecular players and pathways involved in internalization and recycling have previously been studied in model cell lines such as HeLa. However, dendritic cells (DCs), which rely on a specialized endocytic machinery that confers them the unique ability to “cross”-present antigens acquired by internalization, may use distinct MHC I recycling pathways and quality control mechanisms. By providing MHC I molecules cross-presenting antigens, these pathways may play an important role in one of the key functions of DCs, priming of T cell responses against pathogens and tumors. In this review, we will focus on endocytic recycling of MHC I molecules in various experimental conditions and cell types. We discuss the organization of the recycling pathway in model cell lines compared to DCs, highlighting the differences in the recycling rates and pathways of MHC I molecules between various cell types, and their putative functional consequences. Reviewing the literature, we find that conclusive evidence for significant recycling of MHC I molecules in primary DCs has yet to be demonstrated. We conclude that endocytic trafficking of MHC class I in DCs remains poorly understood and should be further studied because of its likely role in antigen cross-presentation.

Regulation of the Cell Biology of Antigen Cross-Presentation

Antigen cross-presentation is an adaptation of the cellular process of loading MHC-I molecules with endogenous peptides during their biosynthesis within the endoplasmic reticulum. Cross-presented peptides derive from internalized proteins, microbial pathogens, and transformed or dying cells. The physical separation of internalized cargo from the endoplasmic reticulum, where the machinery for assembling peptide-MHC-I complexes resides, poses a challenge. To solve this problem, deliberate rewiring of organelle communication within cells is necessary to prepare for cross-presentation, and different endocytic receptors and vesicular traffic patterns customize the emergent cross-presentation compartment to the nature of the peptide source. Three distinct pathways of vesicular traffic converge to form the ideal cross-presentation compartment, each regulated differently to supply a unique component that enables cross-presentation of a diverse repertoire of peptides. Delivery of centerpiece MHC-I molecules is the critical step regulated by microbe-sensitive Toll-like receptors. Defining the subcellular sources of MHC-I and identifying sites of peptide loading during cross-presentation remain key challenges.

Cell Signaling Pathways That Regulate Antigen Presentation

Cell signaling pathways regulate much in the life of a cell: from shuttling cargo through intracellular compartments and onto the cell surface, how it should respond to stress, protecting itself from harm (environmental insults or infections), to ultimately, death by apoptosis. These signaling pathways are important for various aspects of the immune response as well. However, not much is known in terms of the participation of cell signaling pathways in Ag presentation, a necessary first step in the activation of innate and adaptive T cells. In this brief review, I discuss the known signaling molecules (and pathways) that regulate how Ags are presented to T cells and the mechanism(s), if identified. Studies in this area have important implications in vaccine development and new treatment paradigms against infectious diseases, autoimmunity, and cancer.

Intracellular recycling and cross-presentation by MHC class I molecules

Cross-presentation of internalized antigens by dendritic cells requires efficient delivery of Major Histocompatibility Complex (MHC) class I molecules to peptide-loading compartments. Strong evidence suggests that such loading can occur outside of the endoplasmic reticulum; however, the trafficking pathways and sources of class I molecules involved are poorly understood. Examination of non-professional, non-phagocytic cells has revealed a clathrin-independent, Arf6-dependent recycling pathway likely traveled by internalized optimally loaded (closed) class I molecules. Some closed and all open MHC class I molecules travel to late endosomes to be degraded but might also partly be re-loaded with peptides and recycled. Studies of viral interference revealed pathways in which class I molecules are directed to degradation in lysosomes upon ubiquitination at the surface, or upon AP-1 and HIV-nef-dependent misrouting from the Golgi network to lysosomes. While many observations made in non-professional cells remain to be re-examined in dendritic cells, available evidence suggests that both recycling and neo-synthesized class I molecules can be loaded with cross-presented peptides. Recycling molecules can be recruited to phagosomes triggered by innate signals such as TLR4 ligands, and may therefore specialize in loading with phagocytosed antigens. In contrast, AP-1-dependent accumulation at, or trafficking through, a Golgi compartment of newly synthesized molecules appears to be important for cross-presentation of soluble proteins and possibly of long peptides that are processed in the so-called vacuolar pathway. However, significant cell biological work will be required to confirm this or any other model and to integrate knowledge on MHC class I biochemistry and trafficking in models of CD8(+) T-cell priming by dendritic cells.

Properties of the tapasin homologue TAPBPR

The presentation of antigenic peptides by MHC class I molecules plays a vital role in generating T cell responses against infection and cancer. Over the last two decades the central role of tapasin as a peptide editor that influences the loading and optimisation of peptides onto MHC class I molecules has been extensively characterised. Recently, it has become evident that the tapasin-related protein, TAPBPR, functions as a second peptide editor which influences the peptides displayed by MHC class I molecules. Here, we review the discovery of TAPBPR and current understanding of this novel protein in relation to its closest homologue tapasin.

Specific Capture of Peptide-Receptive Major Histocompatibility Complex Class I Molecules by Antibody Micropatterns Allows for a Novel Peptide-Binding Assay in Live Cells

Binding assays with fluorescently labeled ligands and recombinant receptor proteins are commonly performed in 2D arrays. But many cell surface receptors only function in their native membrane environment and/or in a specific conformation, such as they appear on the surface of live cells. Thus, receptors on live cells should be used for ligand binding assays. Here, it is shown that antibodies preprinted on a glass surface can be used to specifically array a peptide receptor of the immune system, i.e., the major histocompatibility complex class I molecule H-2K^b , into a defined pattern on the surface of live cells. Monoclonal antibodies make it feasible to capture a distinct subpopulation of H-2K^b and hold it at the cell surface. This patterned receptor enables a novel peptide-binding assay, in which the specific binding of a fluorescently labeled index peptide is visualized by microscopy. Measurements of ligand binding to captured cell surface receptors in defined confirmations apply to many problems in cell biology and thus represent a promising tool in the field of biosensors.

Genomic Profiling of Patient-Derived Xenografts for Lung Cancer Identifies B2M Inactivation Impairing Immunorecognition

Purpose: We aimed to maximize the performance of detecting genetic alterations in lung cancer using high-throughput sequencing for patient-derived xenografts (PDXs).Experimental Design: We undertook an integrated RNA and whole-exome sequencing of 14 PDXs. We focused on the genetic and functional analysis of β2-microglobulin (B2M), a component of the HLA class-I complex.Results: We identified alterations in genes involved in various functions, such as B2M involved in immunosurveillance. We extended the mutational analysis of B2M to about 230 lung cancers. Five percent of the lung cancers carried somatic mutations, most of which impaired the correct formation of the HLA-I complex. We also report that genes such as CALR, PDIA3, and TAP1, which are involved in the maturation of the HLA-I complex, are altered in lung cancer. By gene expression microarrays, we observed that restitution of B2M in lung cancer cells upregulated targets of IFNα/IFNγ. Furthermore, one third of the lung cancers lacked the HLA-I complex, which was associated with lower cytotoxic CD8⁺ lymphocyte infiltration. The levels of B2M and HLA-I proteins correlated with those of PD-L1. Finally, a deficiency in HLA-I complex and CD8⁺ infiltration tended to correlate with reduced survival of patients with lung cancer treated with anti-PD-1/anti-PD-L1.Conclusions: Here, we report recurrent inactivation of B2M in lung cancer. These observations, coupled with the mutations found at CALR, PDIA3, and TAP1, and the downregulation of the HLA-I complex, indicate that an abnormal immunosurveillance axis contributes to lung cancer development. Finally, our observations suggest that an impaired HLA-I complex affects the response to anti-PD-1/anti-PD-L1 therapies. Clin Cancer Res; 23(12); 3203-13. ©2016 AACR.

Proofreading of Peptide-MHC Complexes through Dynamic Multivalent Interactions

The adaptive immune system is able to detect and destroy cells that are malignantly transformed or infected by intracellular pathogens. Specific immune responses against these cells are elicited by antigenic peptides that are presented on major histocompatibility complex class I (MHC I) molecules and recognized by cytotoxic T lymphocytes at the cell surface. Since these MHC I-presented peptides are generated in the cytosol by proteasomal protein degradation, they can be metaphorically described as a window providing immune cells with insights into the state of the cellular proteome. A crucial element of MHC I antigen presentation is the peptide-loading complex (PLC), a multisubunit machinery, which contains as key constituents the transporter associated with antigen processing (TAP) and the MHC I-specific chaperone tapasin (Tsn). While TAP recognizes and shuttles the cytosolic antigenic peptides into the endoplasmic reticulum (ER), Tsn samples peptides in the ER for their ability to form stable complexes with MHC I, a process called peptide proofreading or peptide editing. Through its selection of peptides that improve MHC I stability, Tsn contributes to the hierarchy of immunodominant peptide epitopes. Despite the fact that it concerns a key event in adaptive immunity, insights into the catalytic mechanism of peptide proofreading carried out by Tsn have only lately been gained via biochemical, biophysical, and structural studies. Furthermore, a Tsn homolog called TAP-binding protein-related (TAPBPR) has only recently been demonstrated to function as a second MHC I-specific chaperone and peptide proofreader. Although TAPBPR is PLC-independent and has a distinct allomorph specificity, it is likely to share a common catalytic mechanism with Tsn. This review focuses on the current knowledge of the multivalent protein–protein interactions and the concomitant dynamic molecular processes underlying peptide-proofreading catalysis. We do not only derive a model that highlights the common mechanistic principles shared by the MHC I editors Tsn and TAPBPR, and the MHC II editor HLA-DM, but also illustrate the distinct quality control strategies employed by these chaperones to sample epitopes. Unraveling the mechanistic underpinnings of catalyzed peptide proofreading will be crucial for a thorough understanding of many aspects of immune recognition, from infection control and tumor immunity to autoimmune diseases and transplant rejection.

The murine cytomegalovirus immunoevasin gp40 binds MHC class I molecules to retain them in the early secretory pathway

In the presence of the murine cytomegalovirus (mCMV) gp40 (m152) protein, murine major histocompatibility complex (MHC) class I molecules do not reach the cell surface but are retained in an early compartment of the secretory pathway. We find that gp40 does not impair folding or high-affinity peptide binding of class I molecules but binds to them to retain them in the endoplasmic reticulum (ER), the ER-Golgi intermediate compartment (ERGIC), and the cis-Golgi, most likely by retrieval from the cis-Golgi to the ER. We identify a sequence in gp40 that is required for both its own retention in the early secretory pathway and for that of class I molecules.