C-myc represses transiently transfected HLA class I promoter sequences not locus-specifically

HLA class I antigen processing machinery defects in antitumor immunity and immunotherapy

Human leukocyte antigen (HLA) class I antigen-processing machinery (APM) plays a crucial role in the synthesis and expression of HLA class I tumor antigen-derived peptide complexes; the latter mediate the recognition and elimination of malignant cells by cognate T cells. Defects in HLA class I APM component expression and/or function are frequently found in cancer cells, providing them with an immune escape mechanism that has relevance in the clinical course of the disease and in the response to T-cell-based immunotherapy. The majority of HLA class I APM defects (>75%) are caused by epigenetic mechanisms or dysregulated signaling and therefore can be corrected by strategies that counteract the underlying mechanisms. Their application in oncology is likely to improve responses to T-cell-based immunotherapies, including checkpoint inhibition.

The MYC oncogene is a global regulator of the immune response

The MYC proto-oncogene is a gene product that coordinates the transcriptional regulation of a multitude of genes that are essential to cellular programs required for normal as well as neoplastic cellular growth and proliferation, including cell cycle, self-renewal, survival, cell growth, metabolism, protein and ribosomal biogenesis, and differentiation. Here, we propose that MYC regulates these programs in a manner that is coordinated with a global influence on the host immune response. MYC had been presumed to contribute to tumorigenesis through tumor cell-intrinsic influences. More recently, MYC expression in tumor cells has been shown to regulate the tumor microenvironment through effects on both innate and adaptive immune effector cells and immune regulatory cytokines. Then, MYC was shown to regulate the expression of the immune checkpoint gene products CD47 and programmed death-ligand 1. Similarly, other oncogenes, which are known to modulate MYC, have been shown to regulate immune checkpoints. Hence, MYC may generally prevent highly proliferative cells from eliciting an immune response. MYC-driven neoplastic cells have coopted this mechanism to bypass immune detection. Thus, MYC inactivation can restore the immune response against a tumor. MYC-induced tumors may be particularly sensitive to immuno-oncology therapeutic interventions.

The two sides of HER2/neu: immune escape versus surveillance

The oncogene HER2 is one of the prototypes for targeted immunotherapy of cancer using both monoclonal antibodies as well as T cell based immunotherapies. Effective humoral and cellular immune responses against HER2 can be induced, but these responses can be influenced by the effects of this oncogene on the target tumor cells. The processes involved in HER2-mediated adaptive and innate immunity and the molecular mechanisms underlying the escape of HER2-expressing tumor cells from immune surveillance, particularly from cytotoxic T cells, are discussed. Implementing this knowledge in clinical trials to revert immune evasion may help optimize immunotherapies directed against HER2-expressing tumors.

MHC class I antigen processing and presenting machinery: organization, function, and defects in tumor cells

The surface presentation of peptides by major histocompatibility complex (MHC) class I molecules is critical to all CD8(+) T-cell adaptive immune responses, including those against tumors. The generation of peptides and their loading on MHC class I molecules is a multistep process involving multiple molecular species that constitute the so-called antigen processing and presenting machinery (APM). The majority of class I peptides begin as proteasome degradation products of cytosolic proteins. Once transported into the endoplasmic reticulum by TAP (transporter associated with antigen processing), peptides are not bound randomly by class I molecules but are chosen by length and sequence, with peptidases editing the raw peptide pool. Aberrations in APM genes and proteins have frequently been observed in human tumors and found to correlate with relevant clinical variables, including tumor grade, tumor stage, disease recurrence, and survival. These findings support the idea that APM defects are immune escape mechanisms that disrupt the tumor cells' ability to be recognized and killed by tumor antigen-specific cytotoxic CD8(+) T cells. Detailed knowledge of APM is crucial for the optimization of T cell-based immunotherapy protocols.

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.

Inverse correlation of HER2 with MHC class I expression on oesophageal squamous cell carcinoma

BACKGROUND

As HER2 is expressed in 30% of oesophageal squamous cell carcinomas (ESCCs), T-cell-based immunotherapy and monoclonal antibodies targeted against HER2 are attractive, novel approaches for ESCCs. However, it was shown that there is an inverse correlation between HER2 and MHC class I expression on tumours. Thus, the correlation between HER2 and MHC class I expressions on ESCC was evaluated.

METHODS

Expressions of MHC class I and HER2 in ESCC tissues (n=80) and cell lines were assessed by immunohistochemistry, fluorescence in situ hybridisation (FISH), and flow cytometry. We investigated whether HER2 downregulation with small interfering RNA (siRNA) in ESCC cell lines could upregulate the expression of MHC class I and the antigen presentation machinery components, and could increase their sensitivity for tumour antigen-specific CTLs.

RESULTS

There was an inverse correlation between HER2 and MHC class I expressions in both tumour tissues and cell lines. Downregulation of HER2 with siRNA resulted in the upregulation of MHC class I expression, leading to increased CTL recognition by tumour antigen-specific CTLs.

CONCLUSION

HER2-overexpressing ESCC tumour cells showed a reduced sensitivity for CTLs through the downregulation of MHC class I.

Expression of c-myc and bcl-2 in primary and advanced cutaneous melanoma

Apoptosis is an important co-factor in the pathogenesis of a plethora of malignancies. Enhanced c-myc activation can result either in proliferation or apoptosis. Coexpression with antiapoptotic bcl-2, which abrogates the apoptotic function of c-myc might lead to an enormous growth advantage of cells. In order to elucidate the role of c-myc and bcl-2 as well as the coexpression of both genes in human melanoma, their expression was studied in four samples of normal skin (SK), 15 surgical margins (SM), 20 benign melanocytic nevi (MN), 20 primary melanomas (MM), and 30 melanoma metastases (MMET) by RT-PCR. These results were compared with immunohistochemistry (IH) in 7 SK, 7 SM, 26 MN, 50 MM, and 34 MMET. Similar results were found with both methods. However, MMET expressed c-myc (PCR 28/30, IH 23/34) as well as bcl-2 (PCR 27/30, IH 24/34) more frequently. Primary melanomas showed a similar expression pattern as SM and nevi. Moreover, in contrast to SK, SM, MN, and MM coexpression of bcl-2 and c-myc was found more frequently in MMET (PCR 25/30, p < 0.01, IH 19/34, p < 0.01). These results indicate that coexpression of c-myc and bcl-2 appears to be associated with advanced melanoma and contributes to the malignant phenotype.

Loss of single HLA class I allospecificities in melanoma cells due to selective genomic abbreviations

Expression of human leucocyte antigen (HLA) Class I molecules is essential for the recognition of malignant melanoma (MM) cells by CD8(+) T lymphocytes. A complete or partial loss of HLA Class I molecules is a potent strategy for MM cells to escape from immunosurveillance. In 2 out of 55 melanoma cell cultures we identified a complete phenotypic loss of HLA allospecificities. Both patients have been treated unsuccessfully with HLA-A2 peptides. To identify the reasons underlying the loss of single HLA-A allospecificities, we searched for genomic alterations at the locus for HLA Class I alpha-chain on chromosome 6 in melanoma cell cultures established from 2 selected patients with MM in advanced stage. This deficiency was associated with alterations of HLA-A2 gene sequences as determined by polymerase chain reaction-sequence specific primers (PCR-SSP). Karyotyping revealed a chromosomal loss in Patient 1, whereas melanoma cell cultures established from Patient 2 displayed 2 copies of chromosome 6. Loss of heterozygosity (LOH) using markers located around position 6p21 was detected in both cases. By applying group-specific primer-mixes spanning the 5'-flanking region of the HLA-A2 gene locus the relevant region was amplified by PCR and subsequent sequencing allowed alignment with the known HLA Class I reference sequences. Functional assays using HLA-A2-restricted cytotoxic T-cell clones were performed in HLA-A2 deficient MM cultures and revealed a drastically reduced susceptibility to CTL lysis in HLA-A2 negative cells. We could document the occurrence of selective HLA-A2 deficiencies in cultured advanced-stage melanoma metastases and identify their molecular causes as genomic alterations within the HLA-A gene locus.

Influence of increased c-Myc expression on the growth characteristics of human melanoma

Overexpression of the proto-oncogene c-myc has been associated with neoplastic transformation in a variety of tumors. For human melanoma high c-myc expression has been found in the vertical growth phase and higher positivity reported in metastases than primary tumors. The principle aim of this study was to determine, whether c-Myc expression influences the metastatic behavior of human melanoma in the absence of lymphocyte-mediated immune phenomena. The growth characteristics and tumor biology of two c-myc transfectants of the human melanoma cell line IGR39D, expressing c-Myc 1.7 and three times over baseline and the respective vector control were analyzed both in vitro and in a severe combined immunodeficient mouse model in vivo. Both c-myc transfectants showed increased growth rates, anchorage independent growth and directed cell movement in culture. Subcutaneously implanted IGR39D melanomas highly overexpressing c-Myc spontaneously formed macroscopic metastases (lymph nodes and lung) in severe combined immunodeficient mice in all cases (n = 7 per group), whereas less prominent c-Myc overexpression caused the development of only lung micrometastases. During the time period leading to terminal disease in animals injected with c-myc transfected human melanoma cells, melanoma development was not seen in vector controls. These findings suggest that constitutive high c-Myc expression in human melanoma results in a more aggressive growth behavior both in vitro and in vivo and favors metastasis in severe combined immunodeficient mice by factors unrelated to immune phenomena such as class I human leukocyte antigen downregulation known to be associated with c-Myc expression.

HLA-B down-regulation in human melanoma is mediated by sequences located downstream of the transcription-initiation site

Major histocompatibility complex (MHC, HLA in humans) class I molecules play an important role in cellular immunology by presenting viral, tumor-associated or minor histocompatibility antigen-derived peptides to T cells. Tumor cells frequently fail to express one or more of the different MHC class I loci (HLA-A, -B and -C), thereby avoiding elimination by T cells. In primary human melanomas as well as melanoma cell lines, HLA class I expression is frequently down-regulated in a B locus-specific manner. The HLA class I promoter contains a number of cis-regulatory elements located upstream of the transcription-initiation site, among them enhancer A and an interferon-stimulated response element. In the present study, we show that novel sequences located 13 to 33 bp downstream of the transcription-initiation site mediate HLA-B locus-specific down-regulation in human melanoma cell lines. Furthermore, involvement of the +13 to +33-bp region in HLA-B locus-specific down-regulation in vivo is supported by in vitro experiments showing locus-specific binding of protein complexes to the +13 to +33-bp region.

Higher frequency of selective losses of HLA-A and -B allospecificities in metastasis than in primary melanoma lesions

Expression of HLA class I molecules is essential for the recognition of tumor cells by CD8+ T cells. In this study, 48 bioptic samples of 42 patients in all stages of melanoma were investigated after short-time cultivation of tumor cells. To confirm melanocytic origin of cultured cells, samples were screened for mRNA expression of melanoma markers gp100, tyrosinase, MAGE-3, MelanA, and MUC18 by reverse transcriptase-polymerase chain reaction. Surface expression of specific HLA-A and -B allospecificities on melanoma cells were analyzed with a standard microcytotoxicity assay after stimulation with interferon (IFN)-alpha and compared with the background found in peripheral blood mononuclear cells from the corresponding patients. Immunohistochemistry and flow cytometry confirmed specific losses in cases where the appropriate monoclonal antibodies were available. The level of expression of HLA-I, HLA-II, and intercellular adhesion molecule 1 antigens on melanoma cells cultured in the presence or absence of IFN-alpha and IFN-gamma was determined cytofluorometrically. All cell cultures tested were found to be positive for one or more melanocytic markers by reverse transcriptase-polymerase chain reaction. The specific HLA-I alleles on the cultured cells were detectable in 45 of 48 samples. In 11 cases a specific loss of one HLA-I allele was observed (2 x A2, B7, B8, B18, 4XB44, B47, B49). Ten of these samples were derived from locoregional lymphnode metastases or from distant metastatic tumors. Only one sample from a primary melanoma showed a specific loss of HLA-I (B47). IFN-alpha upregulated expression of HLA-I up to 4-fold. IFN-gamma enhanced the appearance of HLA-II up to 35-fold and the expression of intercellular adhesion molecule 1 up to 40-fold. Selective loss of HLA-I allospecificities might be a major step in tumor progression.