Involvement of the chaperone tapasin in HLA-B44 allelic losses in colorectal tumors

A diversity of novel type-2 innate lymphoid cell subpopulations revealed during tumour expansion

Type 2 innate lymphoid cells (ILC2s) perform vital functions in orchestrating humoral immune responses, facilitating tissue remodelling, and ensuring tissue homeostasis. Additionally, in a role that has garnered considerably less attention, ILC2s can also enhance Th1-related cytolytic T lymphocyte immune responses against tumours. Studies have thus far generally failed to address the mystery of how one ILC2 cell-type can participate in a multiplicity of functions. Here we utilized single cell RNA sequencing analysis to create the first comprehensive atlas of naïve and tumour-associated lung ILC2s and discover multiple unique subtypes of ILC2s equipped with developmental gene programs that become skewed during tumour expansion favouring inflammation, antigen processing, immunological memory and Th1-related anti-tumour CTL responses. The discovery of these new subtypes of ILC2s challenges current paradigms of ILC2 biology and provides an explanation for their diversity of function.

A novel cell-based screen identifies chemical entities that reverse the immune-escape phenotype of metastatic tumours

Genetic and epigenetic events have been implicated in the downregulation of the cellular antigen processing and presentation machinery (APM), which in turn, has been associated with cancer evasion of the immune system. When these essential components are lacking, cancers develop the ability to subvert host immune surveillance allowing cancer cells to become invisible to the immune system and, in turn, promote cancer metastasis. Here we describe and validate the first high-throughput cell-based screening assay to identify chemical extracts and unique chemical entities that reverse the downregulation of APM components in cell lines derived from metastatic tumours. Through the screening of a library of 480 marine invertebrate extracts followed by bioassay-guided fractionation, curcuphenol, a common sesquiterpene phenol derived from turmeric, was identified as the active compound of one of the extracts. We demonstrate that curcuphenol induces the expression of the APM components, TAP-1 and MHC-I molecules, in cell lines derived from both metastatic prostate and lung carcinomas. Turmeric and curcumins that contain curcuphenol have long been utilized not only as a spice in the preparation of food, but also in traditional medicines for treating cancers. The remarkable discovery that a common component of spices can increase the expression of APM components in metastatic tumour cells and, therefore reverse immune-escape mechanisms, provides a rationale for the development of foods and advanced nutraceuticals as therapeutic candidates for harnessing the power of the immune system to recognize and destroy metastatic cancers.

Specific cannabinoids revive adaptive immunity by reversing immune evasion mechanisms in metastatic tumours

Emerging cancers are sculpted by neo-Darwinian selection for superior growth and survival but minimal immunogenicity; consequently, metastatic cancers often evolve common genetic and epigenetic signatures to elude immune surveillance. Immune subversion by metastatic tumours can be achieved through several mechanisms; one of the most frequently observed involves the loss of expression or mutation of genes composing the MHC-I antigen presentation machinery (APM) that yields tumours invisible to Cytotoxic T lymphocytes, the key component of the adaptive cellular immune response. Fascinating ethnographic and experimental findings indicate that cannabinoids inhibit the growth and progression of several categories of cancer; however, the mechanisms underlying these observations remain clouded in uncertainty. Here, we screened a library of cannabinoid compounds and found molecular selectivity amongst specific cannabinoids, where related molecules such as Δ9-tetrahydrocannabinol, cannabidiol, and cannabigerol can reverse the metastatic immune escape phenotype in vitro by inducing MHC-I cell surface expression in a wide variety of metastatic tumours that subsequently sensitizing tumours to T lymphocyte recognition. Remarkably, H3K27Ac ChIPseq analysis established that cannabigerol and gamma interferon induce overlapping epigenetic signatures and key gene pathways in metastatic tumours related to cellular senescence, as well as APM genes involved in revealing metastatic tumours to the adaptive immune response. Overall, the data suggest that specific cannabinoids may have utility in cancer immunotherapy regimens by overcoming immune escape and augmenting cancer immune surveillance in metastatic disease. Finally, the fundamental discovery of the ability of cannabinoids to alter epigenetic programs may help elucidate many of the pleiotropic medicinal effects of cannabinoids on human physiology.

TAPBPR promotes antigen loading on MHC-I molecules using a peptide trap

Chaperones Tapasin and TAP-binding protein related (TAPBPR) perform the important functions of stabilizing nascent MHC-I molecules (chaperoning) and selecting high-affinity peptides in the MHC-I groove (editing). While X-ray and cryo-EM snapshots of MHC-I in complex with TAPBPR and Tapasin, respectively, have provided important insights into the peptide-deficient MHC-I groove structure, the molecular mechanism through which these chaperones influence the selection of specific amino acid sequences remains incompletely characterized. Based on structural and functional data, a loop sequence of variable lengths has been proposed to stabilize empty MHC-I molecules through direct interactions with the floor of the groove. Using deep mutagenesis on two complementary expression systems, we find that important residues for the Tapasin/TAPBPR chaperoning activity are located on a large scaffolding surface, excluding the loop. Conversely, loop mutations influence TAPBPR interactions with properly conformed MHC-I molecules, relevant for peptide editing. Detailed biophysical characterization by solution NMR, ITC and FP-based assays shows that the loop hovers above the MHC-I groove to promote the capture of incoming peptides. Our results suggest that the longer loop of TAPBPR lowers the affinity requirements for peptide selection to facilitate peptide loading under conditions and subcellular compartments of reduced ligand concentration, and to prevent disassembly of high-affinity peptide-MHC-I complexes that are transiently interrogated by TAPBPR during editing.

HLA class I loss in colorectal cancer: implications for immune escape and immunotherapy

T cell-mediated immune therapies have emerged as a promising treatment modality in different malignancies including colorectal cancer (CRC). However, only a fraction of patients currently respond to treatment. Understanding the lack of responses and finding biomarkers with predictive value is of great importance. There is evidence that CRC is a heterogeneous disease and several classification systems have been proposed that are based on genomic instability, immune cell infiltration, stromal content and molecular subtypes of gene expression. Human leukocyte antigen class I (HLA-I) plays a pivotal role in presenting processed antigens to T lymphocytes, including tumour antigens. These molecules are frequently lost in different types of cancers, including CRC, resulting in tumour immune escape from cytotoxic T lymphocytes during the natural history of cancer development. The aim of this review is to (i) summarize the prevalence and molecular mechanisms behind HLA-I loss in CRC, (ii) discuss HLA-I expression/loss in the context of the newly identified CRC molecular subtypes, (iii) analyze the HLA-I phenotypes of CRC metastases disseminated via blood or the lymphatic system, (iv) discuss strategies to recover/circumvent HLA-I expression/loss and finally (v) review the role of HLA class II (HLA-II) in CRC prognosis.

Novel insights into the molecular mechanisms of HLA class I abnormalities

Alterations in the MHC class I surface antigens represent one mechanism of tumor cells to escape from natural or immunotherapy-induced antitumor immune responses. In order to restore MHC class I expression, knowledge about the underlying molecular mechanisms of MHC class I defects in different tumor types is required. In most cases, abnormalities of MHC class I expression are reversible by cytokines suggesting a deregulation rather than structural abnormalities of members of the antigen-processing and presentation machinery (APM). The impaired expression of APM components could be controlled at the epigenetic, transcriptional and/or posttranscriptional level. Furthermore, a direct link between altered transcription factor binding, interferon signal transduction and MHC class I APM component expression has been shown, which might be further associated with cell cycle progression. This information will not only give novel insights into the (patho) physiology of the antigen-processing and presenting pathway, but will help in the future to design effective T cell-based immunotherapies.

Identification of E2F1 as an important transcription factor for the regulation of tapasin expression

HER-2/neu overexpression in tumor cells caused abnormalities of MHC class I surface expression due to impaired expression of components of the antigen-processing machinery (APM) including the low molecular weight proteins, the transporter associated with antigen processing (TAP), and the chaperone tapasin, whereas the expression of MHC class I heavy chain as well as β(2)-microglobulin was only marginally affected. This oncogene-mediated deficient APM component expression could be reverted by interferon-γ treatment, suggesting a deregulation rather than structural alterations as underlying molecular mechanisms. To determine the level of regulation, the transcriptional activity of APM components was analyzed in HER-2/neu(-) and HER-2/neu(+) cells. All major APM components were transcriptionally down-regulated in HER-2/neu(+) when compared with HER-2/neu(-) cells, which was accompanied by a reduced binding of RNA polymerase II to the APM promoters. Site-directed mutagenesis of the p300- and E2F-binding sites in the APM promoters did not reconstitute the oncogene-mediated decreased transcription rate with the exception of tapasin, which was restored in HER-2/neu(+) cells to levels of wild type tapasin promoter activity in HER-2/neu(-) fibroblasts. The E2F-directed control of tapasin expression was further confirmed by chromatin immunoprecipitation analyses showing that E2F1 and p300 bind to the tapasin and APM promoters in both cell lines. Moreover, siRNA-mediated silencing of E2F1 was associated with an increased tapasin expression, whereas transient overexpression of E2F1 launch a reduced tapasin transcription, suggesting that E2F1 is an essential transcription factor for tapasin.

Analysis of HLA class I expression in progressing and regressing metastatic melanoma lesions after immunotherapy

Despite the potential efficacy of cancer immunotherapy in preclinical studies, it did not show yet significant positive clinical results in humans with only a small number of cancer patients demonstrating objective tumor regression. This poor clinical outcome can be explained by the generation of sophisticated tumor immune escape mechanism, in particular, abnormalities in the expression of HLA class I antigens. We have studied the expression of HLA class I antigens in ten metastatic lesions obtained from a melanoma patient undergoing immunotherapy. Five lesions were obtained after Interferon-alpha-2b treatment and five after autologous vaccination plus BCG (M-VAX). Eight metastases were regressing after immunotherapy while two were progressing. The eight regressing metastases showed high level of HLA class I expression, whereas the two progressing lesions had low levels as measured by real time PCR and immunohistological techniques. These results indicate a strong association between HLA class I expression and progression or regression of the metastatic lesions. Our data support the hypothesis that the level of HLA class I expression is an important parameter of tumor immune escape that needs to be monitored.

Differential contribution of TAP and tapasin to HLA class I antigen expression

Expression of class I human leucocyte antigens (HLA) on the surface of malignant cells is critical for their recognition and destruction by cytotoxic T lymphocytes. Surface expression requires assembly and folding of HLA class I molecules in the endoplasmic reticulum with the assistance of proteins such as Transporter associated with Antigen Processing (TAP) and tapasin. Interferon-gamma induces both TAP and tapasin so dissection of which protein contributes more to HLA class I expression has not been possible previously. In this study, we take advantage of a human melanoma cell line in which TAP can be induced, but tapasin cannot. Interferon-gamma increases TAP protein levels dramatically but HLA class I expression at the cell surface does not increase substantially, indicating that a large increase in peptide supply is not sufficient to increase HLA class I expression. On the other hand, transfection of either allelic form of tapasin (R240 or T240) enhances HLA-B*5001 and HLA-B*5701 antigen expression considerably with only a modest increase in TAP. Together, these data indicate that in the presence of minimal TAP activity, tapasin can promote substantial HLA class I expression at the cell surface.

The Double Role of the Endoplasmic Reticulum Chaperone Tapasin in Peptide Optimization of HLA Class I Molecules

During the assembly of the HLA class I molecules with peptides in the peptide-loading complex, a series of transient interactions are made with ER-resident chaperones. These interactions culminate in the trafficking of the HLA class I molecules to the cell surface and presentation of peptides to CD8(+) T lymphocytes. Within the peptide-loading complex, the glycoprotein tapasin exhibits a relevant function. This immunoglobulin (Ig) superfamily member in the endoplasmic reticulum membrane tethers empty HLA class I molecules to the transporter associated with antigen-processing (TAP) proteins. This review will address the current concepts regarding the double role that tapasin plays in the peptide optimization and surface expression of the HLA class I molecules.

HLA and MICA associations with head and neck squamous cell carcinoma

Head and neck squamous cell carcinoma (HNSCC) is a very aggressive tumour arising from the epithelial lining of the upper aerodigestive tract. The precise mechanisms involved in the pathogenesis of HNSCC have not been elucidated. Previous studies observed aberrant HLA expression patterns on HNSCC tumour cells and this study focused on the allelic polymorphism of HLA genes and the MHC class I chain related gene A (MICA) and HNSCC. We investigated whether associations with HLA and/or MIC alleles or haplotypes are involved in the pathogenesis of HNSCC and could explain the observed HLA expression patterns. Patients and controls were typed for HLA-A, HLA-B, HLA-C, HLA-DRB1 and HLA-DQB1 with sequence specific priming (SSP), supplemented with sequencing based typing (SBT). MICA allelic polymorphism was included and MICA allele assignment was based upon the combination of high resolution SBT of exons 2-4 in combination with repeat analysis and nucleotide polymorphism of exon 5. HLA-B *35 (p=0.014, OR=0.31) and HLA-B *40 (p=0.013, OR=2.9) were significantly associated in respectively the metastasized patients and the oral cavity patients. In addition, the HLA-B *40-DRB1 *13 haplotype (p=0.016, OR=4.1) was more often observed in the oral cavity patient group. The biological significance of the prevalence of specific HLA haplotypes in patients with oral cavity HNSCC and metastasizing HNSCC requires further investigation.

MHC Class I Antigens and Immune Surveillance in Transformed Cells

MHC class I antigens play a crucial role in the interaction of tumor cells with the host immune system, in particular, in the presentation of peptides as tumor-associated antigens to cytotoxic lymphocytes (CTLs) and in the regulation of cytolytic activity of natural killer (NK) cells. In this review we discuss the role of MHC class I antigens in the recognition and elimination of transformed cells and in the generation of tumor immune escape routes when MHC class I losses occur in tumors. The different altered MHC class I phenotypes and their distribution in different human tumors are the main topic of this review. In addition, molecular defects that underlie MHC alterations in transformed cells are also described in detail. Future research directions in this field are also discussed, including the laboratory analysis of tumor MHC class I-negative variants and the possible restoration of MHC class I expression.

TAP1 and TAP2 polymorphisms and their linkage disequilibrium with HLA-DR, -DP, and -DQ in an eastern Andalusian population

Transporter associated with antigen processing (TAP) molecules are involved in the processing of endogenous peptides that bind to major histocompatibility complex (MHC) class I molecules. The possible functional significance of TAP polymorphisms for antigenic peptide transport is an unresolved issue. Population genetics is a tool for investigating the evolutionary and functional significance of genetic polymorphisms. We studied 105 unrelated individuals from Eastern Andalusia in Southern Spain for TAP1 and TAP2 polymorphisms and to detect linkage disequilibrium between TAP1 and TAP2 and between TAP1/TAP2 and human lymphocyte antigen (HLA) DR, DP, and DQ genes. HLA-DR, -DQ, -DP, and TAP1 loci were genotyped with the polymerase chain reaction (PCR)-sequence-specific oligonucleotide method, and TAP2 genes were typed by using the amplification-refractory mutation system-PCR technique. The alleles TAP1*D (3.3%), TAP2*D (2.4%), and TAP2*E (2.9%) were present in the Eastern Andalusian population but not in the general Spanish population. No evidence of linkage disequilibrium was found between TAP1 and TAP2 or between the TAP genes and HLA-DR, -DP, and -DQ in the Eastern Andalusian population. These results are consistent with the absence of coevolution between TAP and MHC class II genes and the hypothesis of selective neutrality.