Sequence and insertion sites of murine melanoma-associated retrovirus

Antibodies against endogenous retroviruses

The human genome harbors hundreds of thousands of integrations of ancient retroviruses, amassed over millions of years of evolution. To reduce further amplification in the genome, the host prevents transcription of these now endogenous retroviruses (ERVs) through epigenetic repression and, with evolutionary time, ERVs are incapacitated by accumulating mutations and deletions. However, several members of recently endogenized ERV groups still retain the capacity to produce viral RNA, retroviral proteins, and higher order structures, including virions. The retention of viral characteristics, combined with the reversible nature of epigenetic repression, particularly as seen in cancer, allow for immunologically unanticipated ERV expression, perceived by the adaptive immune system as a genuine retroviral infection, to which it has to respond. Accordingly, antibodies reactive with ERV antigens have been detected in diverse disorders and, occasionally, in healthy individuals. Although they are part of self, the retroviral legacy of ERV antigens, and association with and, possibly, causation of disease states may set them apart from typical self-antigens. Consequently, the pathogenic or, indeed, host-protective capacity of antibodies targeting ERV antigens is likely to be context-dependent. Here, we review the immunogenicity of typical ERV proteins, with emphasis on the antibody response and its potential disease implications.

The Immunological Conundrum of Endogenous Retroelements

Our defenses against infection rely on the ability of the immune system to distinguish invading pathogens from self. This task is exceptionally challenging, if not seemingly impossible, in the case of retroviruses that have integrated almost seamlessly into the host. This review examines the limits of innate and adaptive immune responses elicited by endogenous retroviruses and other retroelements, the targets of immune recognition, and the consequences for host health and disease. Contrary to theoretical expectation, endogenous retroelements retain substantial immunogenicity, which manifests most profoundly when their epigenetic repression is compromised, contributing to autoinflammatory and autoimmune disease and age-related inflammation. Nevertheless, recent evidence suggests that regulated immune reactivity to endogenous retroelements is integral to immune system development and function, underpinning cancer immunosurveillance, resistance to infection, and responses to the microbiota. Elucidation of the interaction points with endogenous retroelements will therefore deepen our understanding of immune system function and contribution to disease.

An Endogenous Retrovirus Vaccine Encoding an Envelope with a Mutated Immunosuppressive Domain in Combination with Anti-PD1 Treatment Eradicates Established Tumours in Mice

Endogenous retroviruses (ERVs) account for 8% of our genome, and, although they are usually silent in healthy tissues, they become reactivated and expressed in pathological conditions such as cancer. Several studies support a functional role of ERVs in tumour development and progression, specifically through their envelope (Env) protein, which contains a region described as an immunosuppressive domain (ISD). We have previously shown that targeting of the murine ERV (MelARV) Env using virus-like vaccine (VLV) technology, consisting of an adenoviral vector encoding virus-like particles (VLPs), induces protection against small tumours in mice. Here, we investigate the potency and efficacy of a novel MelARV VLV with a mutated ISD (ISDmut) that can modify the properties of the adenoviral vaccine-encoded Env protein. We show that the modification of the vaccine's ISD significantly enhanced T-cell immunogenicity in both prime and prime-boost vaccination regimens. The modified VLV in combination with an α-PD1 checkpoint inhibitor (CPI) exhibited excellent curative efficacy against large established colorectal CT26 tumours in mice. Furthermore, only ISDmut-vaccinated mice that survived CT26 challenge were additionally protected against rechallenge with a triple-negative breast cancer cell line (4T1), showing that our modified VLV provides cross-protection against different tumour types expressing ERV-derived antigens. We envision that translating these findings and technology into human ERVs (HERVs) could provide new treatment opportunities for cancer patients with unmet medical needs.

Antibodies against endogenous retroviruses promote lung cancer immunotherapy

B cells are frequently found in the margins of solid tumours as organized follicles in ectopic lymphoid organs called tertiary lymphoid structures (TLS)1,2. Although TLS have been found to correlate with improved patient survival and response to immune checkpoint blockade (ICB), the underlying mechanisms of this association remain elusive1,2. Here we investigate lung-resident B cell responses in patients from the TRACERx 421 (Tracking Non-Small-Cell Lung Cancer Evolution Through Therapy) and other lung cancer cohorts, and in a recently established immunogenic mouse model for lung adenocarcinoma3. We find that both human and mouse lung adenocarcinomas elicit local germinal centre responses and tumour-binding antibodies, and further identify endogenous retrovirus (ERV) envelope glycoproteins as a dominant anti-tumour antibody target. ERV-targeting B cell responses are amplified by ICB in both humans and mice, and by targeted inhibition of KRAS(G12C) in the mouse model. ERV-reactive antibodies exert anti-tumour activity that extends survival in the mouse model, and ERV expression predicts the outcome of ICB in human lung adenocarcinoma. Finally, we find that effective immunotherapy in the mouse model requires CXCL13-dependent TLS formation. Conversely, therapeutic CXCL13 treatment potentiates anti-tumour immunity and synergizes with ICB. Our findings provide a possible mechanistic basis for the association of TLS with immunotherapy response.

Identification of Highly Cross-Reactive Mimotopes for a Public T Cell Response in Murine Melanoma

While immune checkpoint blockade results in durable responses for some patients, many others have not experienced such benefits. These treatments rely upon reinvigorating specific T cell-antigen interactions. However, it is often unknown what antigens are being recognized by T cells or how to potently induce antigen-specific responses in a broadly applicable manner. Here, we characterized the CD8⁺ T cell response to a murine model of melanoma following combination immunotherapy to determine the basis of tumor recognition. Sequencing of tumor-infiltrating T cells revealed a repertoire of highly homologous TCR sequences that were particularly expanded in treated mice and which recognized an antigen from an endogenous retrovirus. While vaccination against this peptide failed to raise a protective T cell response in vivo, engineered antigen mimotopes induced a significant expansion of CD8⁺ T cells cross-reactive to the original antigen. Vaccination with mimotopes resulted in killing of antigen-loaded cells in vivo yet showed modest survival benefit in a prophylactic vaccine paradigm. Together, this work demonstrates the identification of a dominant tumor-associated antigen and generation of mimotopes which can induce robust functional T cell responses that are cross-reactive to the endogenous antigen across multiple individuals.

Immunotherapy-induced antibodies to endogenous retroviral envelope glycoprotein confer tumor protection in mice

Following curative immunotherapy of B16F10 tumors, ~60% of mice develop a strong antibody response against cell-surface tumor antigens. Their antisera confer prophylactic protection against intravenous challenge with B16F10 cells, and also cross-react with syngeneic and allogeneic tumor cell lines MC38, EL.4, 4T1, and CT26. We identified the envelope glycoprotein (env) of a murine endogenous retrovirus (ERV) as the antigen accounting for the majority of this humoral response. A systemically administered anti-env monoclonal antibody cloned from such a response protects against tumor challenge, and prophylactic vaccination against the env protein protects a majority of naive mice from tumor establishment following subcutaneous inoculation with B16F10 cells. These results suggest the potential for effective prophylactic vaccination against analogous HERV-K env expressed in numerous human cancers.

Adenovirus based virus-like-vaccines targeting endogenous retroviruses can eliminate growing colorectal cancers in mice

Endogenous retroviruses (ERVs) that make up 8% of the human genome have been associated with the development and progression of cancer. The murine model system of the melanoma associated retrovirus (MelARV), which is expressed in different murine cancer cell lines, can be used to study mechanisms and therapeutic approaches against ERVs in cancer. We designed a vaccine strategy (Ad5-MelARV) of adenoviruses encoding the MelARV proteins Gag and Env that assemble in vivo into virus-like particles displaying the cancer-associated MelARV Env to the immune system. The novel vaccine was designed to induce both humoral as well as cellular immune responses in order to attack ERV expressing tumor cells. Despite a lack of antibody induction, we found that T cell responses were strong enough to prevent colorectal CT26 tumor growth and progression in BALB/c mice after a single vaccination before or after tumor challenge. A combination with the checkpoint inhibitor anti-PD-1 further increased the efficacy of the vaccination leading to complete tumor regression. Furthermore, immune responses in vaccinated mice were not restricted to only one cancer cell line but vaccinated animals were also protected from a rechallenge with the distinct breast cancer cell line 4T1. Thus, the developed vaccine strategy could represent a novel tool to successfully target diverse ERV-bearing tumors in cancer patients.

Restoration of Endogenous Retrovirus Infectivity Impacts Mouse Cancer Models

Mouse models have been instrumental in establishing fundamental principles of cancer initiation and progression and continue to be invaluable in the discovery and further development of cancer therapies. Nevertheless, important aspects of human disease are imperfectly approximated in mouse models, notably the involvement of endogenous retroviruses (ERVs). Replication-defective ERVs, present in both humans and mice, may affect tumor development and antitumor immunity through mechanisms not involving infection. Here, we revealed an adverse effect of murine ERVs with restored infectivity on the behavior of mouse cancer models. In contrast to human cancer, where infectious ERVs have never been detected, we found that ERV infectivity was frequently restored in transplantable, as well as genetic, mouse cancer models. Such replication-competent, ERV-derived retroviruses were responsible for unusually high expression of retroviral nucleic acids and proteins in mouse cancers. Infectious ERV-derived retroviruses produced by mouse cancer cells could directly infect tumor-infiltrating host immune cells and fundamentally modified the host's immune defenses to cancer, as well as the outcome of immunotherapy. Therefore, infectious retroviruses, variably arising in mouse cancer models, but not in human cancer, have the potential to confound many immunologic studies and should be considered as a variable, if not altogether avoided. Cancer Immunol Res; 6(11); 1292-300. ©2018 AACR.

Making a virtue of necessity: the pleiotropic role of human endogenous retroviruses in cancer

Like all other mammals, humans harbour an astonishing number of endogenous retroviruses (ERVs), as well as other retroelements, embedded in their genome. These remnants of ancestral germline infection with distinct exogenous retroviruses display various degrees of open reading frame integrity and replication capability. Modern day exogenous retroviruses, as well as the infectious predecessors of ERVs, are demonstrably oncogenic. Further, replication-competent ERVs continue to cause cancers in many other species of mammal. Moreover, human cancers are characterized by transcriptional activation of human endogenous retroviruses (HERVs). These observations conspire to incriminate HERVs as causative agents of human cancer. However, exhaustive investigation of cancer genomes suggests that HERVs have entirely lost the ability for re-infection and thus the potential for insertional mutagenic activity. Although there may be non-insertional mechanisms by which HERVs contribute to cancer development, recent evidence also uncovers potent anti-tumour activities exerted by HERV replication intermediates or protein products. On balance, it appears that HERVs, despite their oncogenic past, now represent potential targets for immune-mediated anti-tumour mechanisms.

This article is part of the themed issue ‘Human oncogenic viruses’.

Physiological and Pathological Transcriptional Activation of Endogenous Retroelements Assessed by RNA-Sequencing of B Lymphocytes

In addition to evolutionarily-accrued sequence mutation or deletion, endogenous retroelements (EREs) in eukaryotic genomes are subject to epigenetic silencing, preventing or reducing their transcription, particularly in the germplasm. Nevertheless, transcriptional activation of EREs, including endogenous retroviruses (ERVs) and long interspersed nuclear elements (LINEs), is observed in somatic cells, variably upon cellular differentiation and frequently upon cellular transformation. ERE transcription is modulated during physiological and pathological immune cell activation, as well as in immune cell cancers. However, our understanding of the potential consequences of such modulation remains incomplete, partly due to the relative scarcity of information regarding genome-wide ERE transcriptional patterns in immune cells. Here, we describe a methodology that allows probing RNA-sequencing (RNA-seq) data for genome-wide expression of EREs in murine and human cells. Our analysis of B cells reveals that their transcriptional response during immune activation is dominated by induction of gene transcription, and that EREs respond to a much lesser extent. The transcriptional activity of the majority of EREs is either unaffected or reduced by B cell activation both in mice and humans, albeit LINEs appear considerably more responsive in the latter host. Nevertheless, a small number of highly distinct ERVs are strongly and consistently induced during B cell activation. Importantly, this pattern contrasts starkly with B cell transformation, which exhibits widespread induction of EREs, including ERVs that minimally overlap with those responsive to immune stimulation. The distinctive patterns of ERE induction suggest different underlying mechanisms and will help separate physiological from pathological expression.

The YUMM lines: a series of congenic mouse melanoma cell lines with defined genetic alterations

The remarkable success of immune therapies emphasizes the need for immune-competent cancer models. Elegant genetically engineered mouse models of a variety of cancers have been established, but their effective use is limited by cost and difficulties in rapidly generating experimental data. Some mouse cancer cell lines are transplantable to immunocompetent host mice and have been utilized extensively to study cancer immunology. Here, we describe the Yale University Mouse Melanoma (YUMM) lines, a comprehensive system of mouse melanoma cell lines that are syngeneic to C57BL/6, have well-defined human-relevant driver mutations, and are genomically stable. This will be a useful tool for the study of tumor immunology and genotype-specific cancer biology.

Endogenous retroviruses and the development of cancer

Mammalian genomes include a considerable number of endogenous retroviruses (ERVs), relics of ancestral infectious retroviruses, whose proviruses have invaded the germ-line. The documented ability of infectious retroviruses to cause cancer has greatly contributed to the discovery of ERVs. It also reinforced the concept that ERVs are causative agents of many cancers, a notion that historically has not always stood up to experimental scrutiny. The recent greater appreciation of the complexity of ERV biology and the identification of dedicated host mechanisms controlling ERV activity have revealed novel interactions between ERVs and their hosts, with the potential to cause or contribute to disease. In this review, the involvement of ERVs in cancer initiation and progression is discussed, as well as their contribution to our understanding of the process of transformation and to the invention of innovative preventive and therapeutic cancer treatments.

Detection and identification of cancerous murine fibroblasts, transformed by murine sarcoma virus in culture, using Raman spectroscopy and advanced statistical methods

BACKGROUND

Cancer is one of the leading worldwide causes of death. It may be induced by a variety of factors, including carcinogens, radiation, genetic factors, or DNA and RNA viruses. The early detection of cancer is critical for its successful therapy, which can result in complete recovery from some types of cancer.

METHODS

Raman spectroscopy has been widely used in medicine and biology. It is a noninvasive, nondestructive, and water-insensitive technique that can detect changes in cells and tissues that are caused by different disorders, such as cancer. In this study, Raman spectroscopy was used for the identification and characterization of murine fibroblast cell lines (NIH/3T3) and malignant fibroblast cells transformed by murine sarcoma virus (NIH-MuSV) cells.

RESULTS

Using principal component analysis and LDA it was possible to differentiate between the NIH/3T3 and NIH-MuSV cells with an 80-85% success rate based on their Raman shift spectra.

CONCLUSIONS

The best results for differentiation were achieved from spectra that were obtained from the rich membrane sites.

GENERAL SIGNIFICANCE

Because of its homogeneity and complete control of most factors affecting its growth, cell culture is a preferred model for the detection and identification of specific biomarkers related to cancer transformation or other cellular modifications.

Nucleic acid-sensing Toll-like receptors are essential for the control of endogenous retrovirus viremia and ERV-induced tumors

The genome of vertebrates contains endogenous retroviruses (ERVs) that are largely nonfunctional relicts of ancestral germline infection by exogenous retroviruses. However, in some mouse strains ERVs are actively involved in disease. Here we report that nucleic acid-recognizing Toll-like receptors 3, 7, and 9 (TLR 3, TLR7, and TLR9) are essential for the control of ERVs. Loss of TLR7 function caused spontaneous retroviral viremia that coincided with the absence of ERV-specific antibodies. Importantly, additional TLR3 and TLR9 deficiency led to acute T cell lymphoblastic leukemia, underscoring a prominent role for TLR3 and TLR9 in surveillance of ERV-induced tumors. Experimental ERV infection induced a TLR3-, TLR7-, and TLR9-dependent group of "acute-phase" genes previously described in HIV and SIV infections. Our study suggests that in addition to their role in innate immunity against exogenous pathogens, nucleic acid-recognizing TLRs contribute to the immune control of activated ERVs and ERV-induced tumors.

Resurrection of endogenous retroviruses in antibody-deficient mice

The mammalian host has developed a long-standing symbiotic relationship with a considerable number of microbial species. These include the microbiota on environmental surfaces, such as the respiratory and gastrointestinal tracks¹, and also endogenous retroviruses (ERVs), comprising a substantial fraction of the mammalian genome^2,3. The long-term consequences for the host of interaction with these microbial species can range from mutualism to parasitism and are not always completely understood. The potential impact of one microbial symbiont on another is even less clear. We have studied the control of ERVs in the commonly-used C57BL/6 (B6) mouse strain, which lacks endogenous murine leukaemia viruses (MLVs) able to replicate in murine cells. We demonstrate the spontaneous emergence of fully infectious ecotropic⁴ MLV (eMLV) in B6 mice with a range of distinct immune deficiencies affecting antibody production. These recombinant retroviruses establish infection of immunodeficient mouse colonies, and ultimately result in retrovirus-induced lymphomas. Notably, ERV activation in immune-deficient mice is prevented in husbandry conditions associated with reduced or absent intestinal microbiota. Our results shed light onto a previously unappreciated role for immunity in the control of ERVs and provide a potential mechanistic link between immune activation by microbial triggers and a range of pathologies associated with ERVs, including cancer.

Emv2, the only endogenous ecotropic murine leukemia virus of C57BL/6J mice

With the proliferation of sequence data, great challenges are posed in the correct annotation of endogenous retroviruses, which together comprise up to ten per cent of the genomes of many organisms. It is therefore essential that all sources of information are carefully considered before drawing conclusions concerning the phylogeny, distribution and biological properties of endogenous retroviruses. We suggest that such due diligence has not been applied in the description of an endogenous ecotropic retrovirus that recently appeared in Retrovirology.

Differential expression of human endogenous retrovirus K transcripts in primary human melanocytes and melanoma cell lines after UV irradiation

Human endogenous retroviruses (HERVs) are discussed as causative agents of various diseases including cancers. Expression of endogenous retroviral sequences can be induced by ultraviolet (UV) light, which is also considered as a cofactor in the development of cutaneous melanoma. Therefore, we investigated whether ultraviolet C (UVC) induces HERV-K rec and np9 expression in normal human epidermal melanocytes (NHEM) and in melanoma cell lines. NHEM and four different melanoma cell lines were irradiated with 10 and 30 mJ/cm(2) UVC, respectively. Expression of the HERV-K transcripts rec and np9 was measured 0, 6, 12, and 24 h after UV exposure by quantitative real-time PCR. In NHEM, HERV-K rec expression was significantly induced 24 h after UV exposure with 10 mJ/cm(2) UVC, whereas np9 expression transiently increased 6 and 12 h after irradiation with both UV doses. In contrast, in melanoma cell lines MEWO, G-361 and GR-M rec, and np9 expression was downregulated or remained unchanged after UV treatment. UVC irradiation induced HERV-K rec and np9 expression in NHEM, which might be an indicator of a functional role of Rec and/or Np9 in melanoma formation.

B16 melanoma secretomes and in vitro invasiveness: syntenin as an invasion modulator

To characterize proteins involved in melanoma dissemination, protein profiles from B16F10 and B16Bl6 cells were compared, as only B16Bl6 cells give pulmonary metastases after subcutaneous graft. As B16F10 and B16Bl6 cells had the same invasive capacities in vitro, we wondered whether their extracellular content could be different and correlate with their metastatic properties. We have shown that B16F10 and B16Bl6 culture cell supernatants have different modulatory effects on HT1080 fibrosarcoma cell invasion in Matrigel-coated chambers. B16Bl6 supernatants significantly enhanced HT1080 in vitro invasion as compared with B16F10 ones, suggesting differences in their protein profiles. Indeed, proteomic analysis allowed the identification of 18 differential proteins. Among the proteins with a higher concentration in B16Bl6 supernanants, lactate dehydrogenase B, M2 pyruvate kinase, cathepsin D, and galectin 1 were involved in the melanoma aggressiveness signature. Interestingly, several Gag retroviral proteins, as well as syntenin, were found mainly in the B16F10 secretome. Although its intracellular form is known as an aggressive melanoma marker, we show for the first time that syntenin was actively secreted and could reduce the invasion process, probably by protein interactions in the B16 model.

The role of human endogenous retroviruses in melanoma

Sequencing of the human genome has established that our DNA harbours many endogenous retrovirus (ERV) sequences, remnants of ancestral exogenous retroviral infections fixed in the germline DNA. In recent years, human ERVs (HERVs) have been implicated in melanomagenesis. Retrovirus-like particles and the expression of HERV mRNA and proteins have been demonstrated in melanoma tissue. In addition, antibodies to HERV proteins have been observed in patients with melanoma. In vitro and mouse models have provided fascinating insights into the potential mechanisms of HERVs in melanomagenesis. This review considers the evidence associating HERVs with melanoma.

Mobility and integration sites of a murine C57BL/6 melanoma endogenous retrovirus involved in tumor progression in vivo

Tumor development is a multistep process in which both genetic and epigenetic events cooperate for the emergence of a malignant clone with metastatic properties. The possibility that endogenous retroviruses promote the expansion of a neoplastic clone by subverting immunosurveillance has been proposed and recently demonstrated in the case of the B16 murine melanoma, which spontaneously express the melanoma-associated retrovirus (MelARV). Indeed, knocking down, by RNA interference, this endogenous retrovirus resulted in the rejection of the tumor cells in immunocompetent mice, without any alteration of their transformed phenotype. Here, we characterize the MelARV proviruses present in the B16 melanoma. Complete sequencing of the viral genomic RNA and characterization of the integration sites within both the B16 tumor cells and a subline selected in vivo for increased metastatic activity disclosed mobility of the element with new proviral insertions targeting critical genes and altering their transcriptional profile. The results show that MelARV can act both at the genetic level, inducing mutations by insertion, and at the epigenetic level, promoting immunosuppression of the host. These properties may as well be relevant to human tumors, such as germline tumors and melanoma, where endogenous retroviruses are active.

Expression analysis of androgen-responsive genes in the prostate of veal calves treated with anabolic hormones

In order to identify indirect molecular biomarkers of anabolic treatments in veal calves, an animal experiment was performed using two combinations of growth promoters (consisting of boldenone undecylenate and estradiol benzoate, and of testosterone enantate and estradiol benzoate). We selected a set of 12 genes that are known to be androgen responsive in other mammalian species. The expression profile of this set of genes was analysed on prostate samples of veal calves using a real-time RT-PCR approach. For each selected gene the corresponding bovine sequence was obtained and a gene specific real-time assay was optimised and validated. The amplification was shown to be highly specific, linear and efficient. High reproducibility (<1%) and low-test variability (<2.5%) were also been achieved. Messenger RNA levels were quantified in prostate samples, non-parametric analysis of variance showed significant up-regulation of three genes (MAF, ESR1 and AR) and significant down-regulation of four genes (HMGCS1, HPGD, DBI, and LIM) in treated samples when compared with untreated controls. To assess the possibility of identifying hormone-treated animals by molecular means we performed a discriminant analysis that was effective in classifying treated and non-treated samples with an accuracy of 93%. Our results indicate that identification of treatment with steroid hormones in veal calves by means of gene expression analysis is a feasible approach and could be improved increasing both the number of genes and the number of controls analysed.

A recombinant endogenous retrovirus amplified in a mouse neuroblastoma is involved in tumor growthin vivo

The theory of immunoediting postulates that tumor cells exhibit a reduced immunogenicity to escape eradication by the host immune system. It has been proposed that endogenous retroviruses--provided that they are active--could play a role in this process, via the immunosuppressive domain carried by their envelope protein. Here, we demonstrate that the Neuro-2a tumor cell line--originating from a spontaneous A/J mouse neuroblastoma--produces an infectious retrovirus that most probably results from a recombination event between 2 mouse endogenous retroviral elements. This Neuro-2a-associated recombinant retrovirus derives from the unique ecotropic provirus located at the Emv-1 locus, but with a gag sequence conferring B-tropism, thus allowing its high-level amplification in Neuro-2a cells. We show that knocking down -by RNA interference- this endogenous retrovirus in Neuro-2a cells has no effect on the transformed phenotype of the cells, but results in delayed tumor growth and prolonged animal survival, following engraftment of the cells into immunocompetent mice. Recombination between endogenous retroviruses, amplification of the resulting element and high-level expression of its immunosuppressive activity are therefore likely steps of an immunoediting process, leading to an invading tumor.

Expression of human endogenous retrovirus K in melanomas and melanoma cell lines

The human endogenous retrovirus K family (HERV-K) comprises 30 to 50 closely related proviruses, most of which are defective. In contrast to all other human endogenous retroviruses, some HERV-K proviruses have maintained open reading frames for all viral proteins. In addition to the structural proteins Gag and Env and the reverse transcriptase, two regulatory proteins (Rec and Np9) have been described. Malignant melanoma has the highest mortality among skin cancers and is particularly aggressive. To study the expression of HERV-K, a set of seven primers was developed that allows discrimination between full-length and spliced mRNA and mRNA from deleted and undeleted proviruses. Expression of full-length mRNA from deleted and undeleted proviruses was detected in all human cells investigated. Expression of spliced env and rec was detected in a teratocarcinoma cell line, in 45% of the metastatic melanoma biopsies, and in 44% of the melanoma cell lines. In normal neonatal melanocytes, spliced rec was detected but not spliced env. Viral proteins were shown to be expressed in primary melanomas, metastases, and melanoma cell lines by immunohistochemistry, immunofluorescence, and Western blot analyses using specific antisera. For the first time, antibodies against HERV-K were found in melanoma patients. Melanomas are, in addition to teratocarcinomas and human breast cancer, the third tumor type with enhanced expression of HERV-K.

A mutation in the Icsbp1 gene causes susceptibility to infection and a chronic myeloid leukemia-like syndrome in BXH-2 mice

BXH-2 mice develop a fatal myeloid leukemia by a two-step mutagenic process. First, a BXH-2-specific recessive mutation causes a myeloproliferative syndrome. Second, retroviral insertions alter oncogenes or tumor suppressors, resulting in clonal expansion of leukemic cells. We have identified a recessive locus on chromosome 8 (Myls) that is responsible for myeloproliferation in BXH-2. This Myls interval has been narrowed down to 2 Mb and found to contain several positional candidates, including the interferon consensus sequence-binding protein 1 gene (Icsbp, also known as interferon regulatory factor 8 [IRF8]). We show that BXH-2 mice carry a mutation (915 C to T) resulting in an arginine-to-cysteine substitution at position 294 within the predicted IRF association domain of the protein. Although expression of Icsbp1 mRNA transcripts is normal in BXH-2 splenocytes, these cells are unable to produce interleukin 12 and interferon-gamma in response to activating stimuli, confirming that R294C behaves as a loss-of-function mutation. Myeloproliferation in BXH-2 mice is concomitant to increased susceptibility to Mycobacterium bovis (BCG) despite the presence of resistance alleles at the Nramp1 locus. These results suggest a two-step model for chronic myeloid leukemia in BXH-2, in which inactivation of Icsbp1 predisposes to myeloproliferation and immunodeficiency. This event is required for retroviral replication, and subsequent insertional mutagenesis that causes leukemia in BXH-2 mice.

Endogenous Retrovirus Expression Is Required for Murine Melanoma Tumor Growth In vivo

Tumor development is a multistep process in which both genetic and epigenetic events cooperate for the emergence of a malignant clone. The possibility that endogenous retroviruses promote the expansion of a neoplastic clone by subverting immune surveillance has been proposed, but remained elusive. Here we show that knocking down-by RNA interference-an endogenous retrovirus spontaneously induced in the B16 murine melanoma results in the rejection of the tumor cells in immunocompetent mice, under conditions where control melanoma cells grow into lethal tumors. The knockdown does not modify the transformed phenotype of the cells, as measured both in vitro by a soft agar assay and in vivo by tumor cell proliferation in immunoincompetent (X-irradiated and severe combined immunodeficiency) mice. Tumor rejection can be reverted upon adoptive transfer of regulatory T cells from control melanoma-engrafted mice, as well as upon reexpression of the sole envelope gene of the endogenous retrovirus in the knocked down cells. These results show that endogenous retroviruses can be essential for a regulatory T-cell-mediated subversion of immune surveillance and could be relevant to human tumors where such elements-and especially their envelope gene-are induced.

Detection of reverse transcriptase activity in human melanoma cell lines and identification of a murine leukemia virus contaminant

BACKGROUND

Stimulated by earlier reports on the presence of retroviruses in mouse and hamster melanoma cell lines, we addressed the question as to whether human melanoma cell lines might also harbour a retrovirus.

METHODS AND RESULTS

The melanoma cell lines SK-MEL-25, SK-MEL-28, MEL-JUSO, MML-I, MeWo, A-375, Colo-38, BS-780 were confirmed to be human by human leucocyte antigen (HLA) typing, and supernatants were tested by the product-enhanced reverse transcriptase (PERT) assay for reverse transcriptase (RT) activity. Cell lines SK-MEL-25, SK-MEL-28, MEL-JUSO and MML-I were positive, whereas cell lines MeWo, A-375, Colo-38 and BS-780 were negative. The RT activity peaked at a buoyant density in sucrose typical for retroviruses. From this peak fraction an R-U5 sequence indistinguishable from murine leukemia virus (MLV) was identified by particle-associated retrovirus RNA amplification (PARRA). Semiquantitative MLV-specific RNA-PCR demonstrated colocalization of the MLV-like RNA and RT activity on the sucrose gradient of SK-Mel-25. MLV RNA and DNA were also detectable in culture supernatants of SK-MEL-28, MEL-JUSO and MML-I, but not of MeWo, A-375, Colo-38 and BS-780 by semiquantitative polymerase chain reaction (PCR). Sequence comparison revealed highest homology with the RET sequence previously identified in mouse myeloma SP2/0-AG14 cells. Taken together, our data strongly suggest that certain human melanoma cell lines are productively infected by a MLV which was probably introduced during tumour passage in mice or by laboratory contamination many years ago and subsequently spread to other lines.

CONCLUSION

We recommend mandatory testing of melanoma and other human cell lines for contamination with infectious MLV or other animal retroviruses, similar to mycoplasma screening, in order to avoid artificial experimental data.

Retroviral insertional mutagenesis: tagging cancer pathways

Slow transforming retroviruses, such as the Moloney murine leukemia virus (M-MuLV), induce tumors upon infection of a host after a relatively long latency period. The underlying mechanism leading to cell transformation is the activation of proto-oncogenes or inactivation of tumor suppressor genes as a consequence of proviral insertions into the host genome. Cells carrying proviral insertions that confer a selective advantage will preferentially grow out. This means that proviral insertions mark genes contributing to tumorigenesis, as was demonstrated by the identification of numerous proto-oncogenes in retrovirally induced tumors in the past. Since cancer is a complex multistep process, the proviral insertions in one clone of tumor cells also represent oncogenic events that cooperate in tumorigenesis. Novel advances, such as the launch of the complete mouse genome, high-throughput isolation of proviral flanking sequences, and genetically modified animals have revolutionized proviral tagging into an elegant and efficient approach to identify signaling pathways that collaborate in cancer.

Analysis of the helper virus in murine retrovirus-induced immunodeficiency syndrome: evidence for immunoselection of the dominant and subdominant CTL epitopes of the BM5 ecotropic virus

In genetically susceptible strains, such as C57BL/6 (B6) mice, LP-BM5 causes murine AIDS (MAIDS). LP-BM5 is a complex mixture of murine leukemia viruses (MuLV) that includes replication competent ecotropic (BM5eco) and mink cell focus inducing (MCF), and replication defective (BM5d) MuLV. At present, for the BM5eco virus, sequence information on only the gag region is available. In this paper, we describe for the first time the sequencing of the entire BM5eco viral genome as well as analysis of homology with two other previously sequenced and well-characterized MuLVs, Emv-11 and Emv-2, the latter constituting the parental virus for BM5eco. We propose that the detailed sequence comparisons herein provide cogent evidence that BM5eco utilizes variations in cytotoxic T lymphocytes (CTL) epitopes as an immune escape mechanism. This CTL evasion mechanism may contribute substantially to the underlying prototypic susceptibility of B6 mice to LP-BM5-induced MAIDS.

A new one-step RT-PCR method for virus quantitation in murine AIDS

The causative agent of murine AIDS (MAIDS) in C57BL/6 mice, is a defective murine leukemia virus (BM5d) that requires the replication-competent helper virus (BM5e). Since this animal model of immunodeficiency, which shows many similarities to human AIDS, is also used to test the efficacy and toxicity of antiretroviral drugs, a method that allows the quantitative detection of both viruses would be very useful also if hampered potentially by endogenous viral sequences usually present in mice. While BM5d alone could induce the disease, the effect of BM5e on the immune system of diseased mice is unclear. A specific and reliable one-step RT-PCR method was developed for the co-amplification, with the same efficiency, of BM5d or BM5e with ss-actin used as an internal standard. The standard curves produced with cloned cDNA sequences (ss-actin and BM5d or BM5e) assure that all samples are analyzed during the exponential phase of the reaction. Using this new assay which provided a dynamic range of at least four-log-unit, the ratio of initial absolute amounts of the virus and ss-actin RNA was determined, obtaining quantitative information on virus-specific cellular-transcript in the lymph nodes and spleen during the natural history of the disease and during therapeutic regimens.

Downregulation of mafB expression in T-helper cells during early differentiation in vitro

We have studied the expression of a human homologue of mafB (maf-1), a member of the family of large maf transcription factors. In support of the suggested key role that mafB expression plays in differentiating macrophages, we found mafB to be expressed at a very high level in monocytic U937 and THP-1 cell lines. However, we show here that mafB transcription is not restricted to myeloid cells but can also be detected in lymphoid cells, indicating transcriptional plasticity during haematopoiesis. In conclusion, strong proliferative signals mediated by T-cell activation and interleukins (IL-4 and IL-12) downregulate the mafB messenger RNA transcript level when resting naïve CD4+ T-helper cells enter the differentiation pathway in vitro.

Spectroscopic characterization of human and mouse primary cells, cell lines and malignant cells

Fourier transform infrared (FTIR) spectroscopy is currently being developed as a new optical approach to the diagnosis and characterization of cell or tissue pathology. The advantage of FTIR microspectroscopy over conventional FTIR spectroscopy in the diagnosis of malignancies is that it facilitates inspection of restricted regions of the cell culture or tissue. In this study, we set out to evaluate FTIR microspectroscopy as a diagnostic tool for identifying retrovirus-induced malignancies. Our study showed significant and consistent differences between cultures of different types of cells of both mouse and human origin, i.e. primary fibroblast cells (one to two passages in cell culture), fibroblast cell lines and malignant cells transformed by murine sarcoma virus. An impressive decrease in the levels of phosphate and other metabolites was seen in malignant cells compared with primary cells. The levels of these metabolites in the cell lines were significantly lower than in the primary cells but higher than in the malignant cells. In addition, the peak attributed to the PO2- symmetric stretching mode at 1082 cm(-1) in primary cells shifted significantly to 1085 cm(-1) for the cell line and to 1087 cm(-1) for the malignant cells. These differences taken together with differences in the shapes of various bands throughout the spectrum strongly support the possibility of developing FTIR microspectroscopy for the detection and study of malignant--and possibly premalignant--cells.

The program of androgen-responsive genes in neoplastic prostate epithelium

The human prostate gland is an important target organ of androgenic hormones. Testosterone and dihydrotestosterone interact with the androgen receptor to regulate vital aspects of prostate growth and function including cellular proliferation, differentiation, apoptosis, metabolism, and secretory activity. Our objective in this study was to characterize the temporal program of transcription that reflects the cellular response to androgens and to identify specific androgen-regulated genes (ARGs) or gene networks that participate in these responses. We used cDNA microarrays representing about 20,000 distinct human genes to profile androgen-responsive transcripts in the LNCaP adenocarcinoma cell line and identified 146 genes with transcript alterations more than 3-fold. Of these, 103 encode proteins with described functional roles, and 43 represent transcripts that have yet to be characterized. Temporal gene expression profiles grouped the ARGs into four distinct cohorts. Five uncharacterized ARGs demonstrated exclusive or high expression levels in the prostate relative to other tissues studied. A search of available DNA sequence upstream of 28 ARGs identified 25 with homology to the androgen response-element consensus-binding motif. These results identify previously uncharacterized and unsuspected genes whose expression levels are directly or indirectly regulated by androgens; further, they provide a comprehensive temporal view of the transcriptional program of human androgen-responsive cells.

Anti-viral activity of red microalgal polysaccharides against retroviruses

Red microalgal polysaccharides significantly inhibited the production of retroviruses (murine leukemia virus- MuLV) and cell transformation by murine sarcoma virus(MuSV-124) in cell culture. The most effective inhibitory effect of these polysaccharides against both cell transformation and virus production was obtained when the polysaccharide was added 2 h before or at the time of infection. Although, addition of the polysaccharide post-infection significantly reduced the number of transformed cells, but its effect was less marked than that obtained when the polysaccharide was added before or at the time of infection.The finding that the inhibition of cell transformation by MuSV-124 was reversible after removal of the polysaccharide suggested that microalgal polysaccharides inhibited a late step after provirus integration into the host genome. In conclusion, our findings could support the possibility that the polysaccharide may affect early steps in the virus replication cycle, such as virus absorption into the host cells, in addition to its effect on a late step after provirus integration.

Novel spectral method for the study of viral carcinogenesis in vitro

Fourier transform infrared (FTIR) spectroscopy is a unique technique for the laboratory diagnosis of cellular variations based on the characteristic molecular vibrational spectra of the cells. Microscopic FTIR was used to investigate spectral differences between normal and malignant fibroblasts transformed by retrovirus infection. A detailed analysis showed significant differences between cancerous and normal cells. The contents of vital cellular metabolites were significantly lower in the transformed cells than in the normal cells. In an attempt to identify the cellular components responsible for the observed spectral differences between normal and cancerous cells, we found significant differences between DNA of normal and cancerous cells.

Maf and Jun nuclear oncoproteins share downstream target genes for inducing cell transformation

The Maf oncoprotein is a basic leucine zipper (bZip)-bearing transcriptional activator that recognizes the Maf recognition element (MARE) DNA sequence. In this study, we investigated the role of Maf's transactivation function in cell transformation. Replacement of the conserved amino terminus transactivator domain of Maf by a heterologous and stronger transactivator domain (the acidic transactivator domain of VP16) resulted in enhanced transformation of chicken embryo fibroblast cells. In contrast, the fusing of a transcriptional repressor domain (Sin3 interaction domain of Mxi1) with the whole Maf protein masked the transactivator function of Maf, which in turn inhibited its transforming activity. Furthermore, the leucine zipper domain of Maf, which defines its dimer-forming specificity, was exchangeable with that of GCN4 yeast protein in terms of its transactivating and cell transforming activities. Thus, heterodimer formation with other bZip factors is not required for Maf's ability to transform. These results together suggest that transactivation through MARE is necessary for Maf-induced transformation and that there exist downstream target gene(s) for transformation. Since the MARE sequence overlaps with the recognition element of another bZip oncoprotein Jun, we assessed whether Jun and Maf induce cell transformation through activating the same genes. We thus constructed a mutated version of Jun that has a GCN4 leucine zipper and lacks the transactivator domain. This mutant repressed the cell transformation not only by Jun but also by Maf. Thus, Maf and Jun share downstream target gene(s) that are involved in cell transformation.

Nitric oxide (NO), methylation and TIMP-1 expression in BL6 melanoma cells transfected with MHC class I genes

We have previously found that transfection of BL6-8 melanoma cells with the H-2K, but not H-2D/L genes resulted in loss of their metastatic ability that was associated with decrease in their invasiveness and up-regulation of TIMP-1 expression. In the present study using the methylation-specific PCR (MSP) we found that lack of TIMP-1 expression in BL6-8 is associated with methylation in the TIMP-1 5' regulatory area. In the H-2Kb transfected CL8-1 melanoma cells up-regulation of TIMP-1 was in parallel with loss of TIMP-1 gene methylation. Treatment of BL6-8 with 5-azacytidine or with an inhibitor of histone deacetylase trichostatin A resulted in up-regulation of TIMP-1 expression. These results indicate that methylation and histone deacetylation play an important role in transcription repression of TIMP-1 in BL6 melanoma cells. Some data showed that nitric oxide (NO) could affect methylation and expression of various gene. Therefore we analyzed NO production in B16 melanoma cell lines with different expression of TIMP-1. We have found that B16F10 and BL6-8 melanoma cells do not express TIMP-1 and do not produce nitric oxide (NO) even after stimulation with IFN-gamma and LPS. However, BL6-8 cells transfected with H-2Kb or H-2Kd, but not H-2Dd or H-2Ld gene expressed TIMP-1 and produced NO constitutevely. NO production in these cells was further stimulated by IFN-gamma and LPS. Northern blot analysis showed that expression of iNOS was paralleled with TIMP-1 expression in the tested melanoma cells. However, NO produced by SNAP or inhibition of NO production by NMA did not affect TIMP-1 expression in the tested melanoma cells. Thus, TIMP-1 expression and NO production in BL6 melanoma cells transfected with MHC class I gene coincides but it remains unclear whether NO is responsible for the change in TIMP-1 methylation and expression.

A set of Hox proteins interact with the Maf oncoprotein to inhibit its DNA binding, transactivation, and transforming activities

Maf oncoprotein is a basic-leucine zipper (bZip) type of transcriptional activator. Since many transcription factors are known to form functional complexes, we searched for proteins that interact with the DNA-binding domain of Maf using the phage display method and identified two homeodomain-containing proteins, Hoxd12 and MHox/Prx1/Phox1/Pmx1. Studies with mutants of Hox and Maf proteins showed that they associate through their DNA-binding domains; the homeodomain of Hox and the bZip domain of Maf, respectively. Reflecting the high similarity of the bZip domain, all other Maf family members tested (c-/v-Maf, MafB, MafK, MafF, and MafG) also associated with the Hox proteins. Pax6, whose homeodomain is relatively similar to MHox, also could interact with Maf. However, two other bZip oncoproteins, Fos and Jun, failed to associate with the Hox proteins, while a distantly related Hox family member, Meis1, could not interact with Maf. Through interactions with the bZip domain, the Hox proteins inhibited the DNA binding activity of Maf, whereas the binding of Hox proteins to their recognition sequences was not abrogated by Maf. We further showed that coexpression of the Hox proteins repressed transcriptional activation and transforming activity of Maf. These results suggested that the interaction of a set of Hox proteins with Maf family members may interfere not only with their oncogenicity but also with their physiological roles.