Human retroviral sequences associated with extracellular particles in autoimmune diseases: epiphenomenon or possible role in aetiopathogenesis?

The contribution of HERV-E clone 4-1 and other HERV-E members to the pathogenesis of rheumatic autoimmune diseases

Human endogenous retroviruses (HERV)-E consist of a family of more than 1300 elements, stably integrated in the human genome. Some of them are full-length proviruses able to synthesize the viral proteins gag, pol and env. The reactivation of HERV-E elements has been associated to placentation, cancer and autoimmunity. In this narrative review, we aimed to report the status of the art concerning the involvement of HERV-E in rheumatic autoimmune diseases. Following a research on PubMed database, a total of 87 articles were selected. The highest amount of evidence derives from studies on systemic lupus erythematosus (SLE), whereas a few to no data are available on other immune-mediated diseases. In SLE, the hyper-expression of HERV-E clone 4-1 in peripheral blood mononuclear cells or differentiated lymphocytes has been associated with disease activity and autoantibody production. It is likely that HERV-E take part to the pathogenesis of rheumatic autoimmune diseases but additional research is needed.

Retrotransposons shuttling genetic and epigenetic information from the nuclear to the mitochondrial compartment: Do they play a pathogenetic role in scleroderma?

Endogenous retroelements are a class of ancient defective viral insertions contained in the genome of host cells, where they account for up to 40% of all DNA. Centuries of co-existence in host genome have led to the development of immunotolerance to endogenous retroelements, most of which are defective and unable to replicate or transcribe functional proteins. However, given their capacity to move across the nuclear and mitochondrial genome and recombine, they could mix phenotypes and give rise to infections that may trigger innate and adaptive immune responses by sensing receptors capable of recognising foreign nucleic acids and proteins. It has recently been suggested that they play a role in the pathogenesis of autoimmune diseases on the grounds of their partial reactivation or the epigenetic control of host gene transcription. A number of studies have confirmed their contribution to the development of rheumatoid arthritis, multiple sclerosis and systemic lupus erythematosus, but there is still a lack of data concerning systemic sclerosis (SSc). Their role in the pathogenesis of SSc can be hypothesised on the basis of mitochondrial and nuclear chromatinic damage, and hyper-activation of the immune pathway involved in antiviral defense. SSc is characterised by genetic and immunological evidence of a viral infection but, as no viral agent has yet been isolated from SSc patients, the hypothesis that partial reactivation of endogenous retroviruses may trigger the disease cannot be excluded and deserves further investigation.

Infections and multiple sclerosis

Multiple sclerosis is a chronic inflammatory condition of unknown cause. Increasing evidence suggests that the disease develops as a result of interactions between the environment and the immune system in genetically susceptible individuals. It has long been recognized that infections may serve as environmental triggers for the disease, and a large number of pathogens have been proposed to be associated with multiple sclerosis. Here, we detail the historical basis linking infections to multiple sclerosis and review the epidemiology of the disease, which suggests a possible relationship with infectious agents. We also describe pathophysiologic studies in animals and other human demyelinating diseases that have demonstrated a variety of mechanisms by which infectious agents may induce chronic, relapsing central nervous system disease with myelin damage and relative preservation of axons, similar to multiple sclerosis. In addition, we discuss recent studies in individuals with multiple sclerosis indicating enhanced immune responses to infectious antigens, though not consistently demonstrating evidence for ongoing infection. Taken together, these studies suggest a role for infectious agents in the development of multiple sclerosis. Conclusive evidence, however, remains lacking.

Are human endogenous retroviruses pathogenic? An approach to testing the hypothesis

A number of observations have led researchers to postulate that, despite being replication-defective, human endogenous retroviruses (HERVs) may have retained the potential to cause or contribute to disease. However, mechanisms of HERV pathogenicity might differ substantially from those of modern infectious retroviruses or of the infectious precursors of HERVs. Therefore, novel pathways of HERV involvement in disease pathogenesis should be investigated. Recent technological advances in sequencing and bioinformatics are making this task increasingly feasible. The accumulating knowledge of HERV biology may also facilitate the definition and general acceptance of criteria that establish HERV pathogenicity. Here, we explore possible mechanisms whereby HERVs may cause disease and examine the evidence that either has been or should be obtained in order to decisively address the pathogenic potential of HERVs.

Human RNA "rumor" viruses: the search for novel human retroviruses in chronic disease

Retroviruses are an important group of pathogens that cause a variety of diseases in humans and animals. Four human retroviruses are currently known, including human immunodeficiency virus type 1, which causes AIDS, and human T-lymphotropic virus type 1, which causes cancer and inflammatory disease. For many years, there have been sporadic reports of additional human retroviral infections, particularly in cancer and other chronic diseases. Unfortunately, many of these putative viruses remain unproven and controversial, and some retrovirologists have dismissed them as merely "human rumor viruses." Work in this field was last reviewed in depth in 1984, and since then, the molecular techniques available for identifying and characterizing retroviruses have improved enormously in sensitivity. The advent of PCR in particular has dramatically enhanced our ability to detect novel viral sequences in human tissues. However, DNA amplification techniques have also increased the potential for false-positive detection due to contamination. In addition, the presence of many families of human endogenous retroviruses (HERVs) within our DNA can obstruct attempts to identify and validate novel human retroviruses. Here, we aim to bring together the data on "novel" retroviral infections in humans by critically examining the evidence for those putative viruses that have been linked with disease and the likelihood that they represent genuine human infections. We provide a background to the field and a discussion of potential confounding factors along with some technical guidelines. In addition, some of the difficulties associated with obtaining formal proof of causation for common or ubiquitous agents such as HERVs are discussed.

Persistent viral infection in primary Sjogren's syndrome: review and perspectives

Exocrine gland pathology in primary Sjogren's syndrome is characterized by destruction of acinar epithelial cells and chronic lymphocytic infiltrates surrounding ductal epithelial cells. These cells seem to be activated, as it is inferred by their immunophenotype. The cause of this activation and the chronic inflammatory response that targets epithelial cells remain unknown. Here, we will review the evidence pointing to a persistent viral infection as a probable cause of primary Sjogren's syndrome and discuss potential directions for future research.

Oligodendrocytes are a major target of the toxicity of spongiogenic murine retroviruses

The neurovirulent retroviruses FrCasE and Moloney MLV-ts1 cause noninflammatory spongiform neurodegeneration in mice, manifested clinically by progressive spasticity and paralysis. Neurons have been thought to be the primary target of toxicity of these viruses. However the neurons themselves appear not to be infected, and the possible indirect mechanisms driving the neuronal toxicity have remained enigmatic. Here we have re-examined the cells that are damaged by these viruses, using lineage-specific markers. Surprisingly, these cells expressed the basic helix-loop-helix transcription factor Olig2, placing them in the oligodendrocyte lineage. Olig2+ cells were found to be infected, and many of these cells exhibited focal cytoplasmic vacuolation, suggesting that infection by spongiogenic retroviruses is directly toxic to these cells. As cytoplasmic vacuolation progressed, however, signs of viral protein expression appeared to wane, although residual viral RNA was detectable by in situ hybridization. Cells with the most advanced cytoplasmic effacement expressed the C/EBP-homologous protein (CHOP). This protein is up-regulated as a late event in a cellular response termed the integrated stress response. This observation may link the cellular pathology observed in the brain with cellular stress responses known to be induced by these viruses. The relevance of these observations to oligodendropathy in humans is discussed.

Detection of retroviruses in a calvaria of a young mouse

During systematic examination of the development and growth of mouse calvariae, virus particles were detected electron microscopically. Mice, strain NMRI, were obtained from the breeder Harlan-Winkelmann (Borchem, Germany) and mated in the animal house. The area of the sagittal suture from day-18 foetuses and from different stages until day-26 pp as well as from adult mice was studied. In the calvaria of a day-21 pp mouse, type-C virus particles were found in the pericellular space of many, but not all osteogenic cells. The morphology of all viruses found resembled that of retroviruses; they were regular in size, spherical and showed a diameter of 100-120 nm. Budding of viruses from the osteocyte membrane occurred frequently. Also type-A particles were detected intracisternally within the rough endoplasmic reticulum. Budding of type-C virus particles, extracellular deposition of retroviruses and intracisternal formation of type-A particles occurred in the same cell. However, although examinations of 22 different individual calvariae were done, only one obtained from a day-21 pp mice was virus-positive. Most probably, this observation is due to an endogenous virus production. Several mouse strains bear provirus DNA in their genome; the mode of activation of which is unknown. Reports in the literature are rare. Nevertheless, virus-infected material may influence experimental approaches and may be dangerous for the people who work with such material.

Pathogenesis of primary biliary cirrhosis: A unifying model

Primary biliary cirrhosis (PBC) is a disease of unknown etiology leading to progressive destruction of small intrahepatic bile ducts and eventually to liver cirrhosis and failure. It is characterised by female predominance and serum auto-antibodies to mitochondrial antigens targeting the E2 components of the 2-oxoacid dehydrogenase complex. Although they are associated with disease pathogenesis, no concrete evidence has been presented so far. Epidemiological data indicate that a geographical clustering of cases and possible environmental factors are implicated in pathogenesis. A number of genetic factors play a role in determining disease susceptibility or progression, although no definitive conclusion has been reached so far. A key factor to immune pathogenesis is considered to be the breakdown of immune tolerance, either through molecular mimicry or through the so called determinant density model. In this review, the available data regarding the pathogenesis of primary biliary cirrhosis are described and discussed. A new unifying hypothesis based on early endothelin overproduction in primary biliary cirrhosis (PBC) is presented and discussed.

Development of broadly targeted human endogenous gammaretroviral pol-based real time PCRs Quantitation of RNA expression in human tissues

Endogenous retroviral sequences (ERVs) are dynamic genomic components with profound influences on gene expression and genomic structure. Their extent of expression is not well known. Several broadly targeted real-time reverse transcription PCR (QPCRs) systems for surveillance of RNA expression of the major groups of human gammaretroviral ERVs were constructed. The highly conserved reverse transcriptase (RT) and integrase (IN) domains of the pol gene were used as targets for the PCRs, which were both probe-based (TaqMan) and probe-less (SYBR Green). Different levels of primer and probe degeneracy, with or without inosine, were tested. Several of the PCRs had sensitivities of a few HERV nucleic acid copies per PCR reaction. Specificities were approximately as expected from the fit of primers and probes. Gammaretroviral HERV RNA expression was studied in different human tissues. Each HERV group had a specific pattern of expression. HERV-E was highly expressed in testis, HERV-I/T in brain and testis, HERV-H in brain and testis, while HERV-W was highly expressed in placenta. Gammaretroviral RNA was not detected in plasma from 50 blood donors in saliva from 20 persons. In conclusion, a set of tools for investigation of gammaretroviral HERV RNA expression was created.

[Envelope capture by retroviruses]

Retroelement transposition is a major source of diversity in genome evolution. Among the retrotransposable elements, the retroviruses are distinct in that their "transposition" extends from their initial host cells to neighboring cells and organisms. A determining step in the conversion of a retrotransposable element into an infectious retrovirus is the acquisition of an envelope glycoprotein, designated Env. Here, we review some examples of envelope "capture" by mammal retroviruses and provide evidence for such a mechanism by HTLV. This phenomenon may explain the notable conservation of env genes observed between phylogenetically distant retroviruses. Elucidation of these recombination processes should help to clarify retroviral phylogeny, better understand retroviral pathogenesis, and may lead to the identification of new retroelements.

Human endogenous retrovirus (HERV-K) particles in megakaryocytes cultured from essential thrombocythemia peripheral blood stem cells

OBJECTIVE

The aim of this study was to determine the extent of human endogenous retrovirus (HERV) gene translation in megakaryocytes cultured from peripheral blood stem cells of patients with essential thrombocythemia previously reported with platelet-associated HERV sequences and reverse transcriptase activity.

MATERIALS AND METHODS

Terminally differentiated megakaryocytes derived from circulating stem cells in serum-free medium supplemented with stem cell factor and thrombopoietin were processed for electron microscopic immunostaining using a monoclonal antibody against the gag protein of HERV-K10 and an electron dense gold-labeled secondary antibody.

RESULTS

We found that HERV-K gag protein was detected as clusters in the cytoplasm as well as associated with viral particles budding from the cell membrane and into intracellular vacuoles in megakaryocytes from two patients with essential thrombocythemia. None of these structures was observed in megakaryocytes from a normal control or from a patient with chronic myelocytic leukemia.

CONCLUSION

This is the first evidence of HERV-K protein synthesis (gene translation) in human tissue other than seminomas, placenta, or fetal tissue. Translation of the HERV-K gag gene with subsequent packaging of the protein product into viral particles adds a new and important dimension to future studies on the role of HERVs in the myeloproliferative diseases.

Emergence of vertebrate retroviruses and envelope capture

Retroviruses are members of the superfamily of retroelements, mobile genetic elements that transpose via an RNA intermediate. However, retroviruses are distinct from other retroelements in that their "transposition" is not confined to single cells but extends to neighboring cells and organisms. As such, the "transposition" of these elements is defined as infection. It appears that a key step in the conversion of a retrotransposon into a retrovirus is the modular acquisition or capture of an envelope glycoprotein (Env) which facilitates dissemination from its initial host cell. Here we present several examples of retroviruses for which envelope capture has been identified. Indeed, capture may explain the notable conservation of env sequences among otherwise phylogenetically distant retroviruses. In a recent example, sequence homologies reported between the env of the phylogenetically distant murine leukemia viruses (MLV) and human T cell leukemia viruses (HTLV) argue in favor of an env capture by the latter. Env acquisition can provide new adaptive properties to replication-competent viruses in addition to altering their host range. Also, the captured env can alter the spectrum of physiological affects of infection in new host cells and organisms. The elucidation of such envelope exchanges and properties thereof should contribute significantly to the clarification of retroviral phylogeny, insight into retroviral pathogenesis, and to the discovery of new retroviruses.

Intracellular trafficking and surface expression of SS-A (Ro), SS-B (La), poly(ADP-ribose) polymerase and alpha-fodrin autoantigens during apoptosis in human salivary gland cells induced by tumour necrosis factor-alpha

Autoantibodies directed against nucleic acid and protein complexes present in cell nuclei characterize autoimmune diseases and are employed in diagnosis. The mechanisms by which these autoantigens escape immunological tolerance are largely unknown, but a number of recent observations suggest that modified self-protein generated during apoptosis my play an important part in the development of autoimmunity. To investigate the possibility that autoantibodies in patients with Sjögren's syndrome are induced by apoptosis and presented on the surface of the cell, the internal distribution of autoantigens in apoptotic human salivary gland cells was studied in vitro. Salivary gland cells were treated with tumour necrosis factor-alpha, an apoptosis inducer. At increasing times after induction, cells were homogenized and cytoplasmic, cell surface membrane and nuclear compartments were fractionated using a sucrose density-gradient system. Autoantigens alpha-fodrin, SS-A (Ro), SS-B (La), and the enzyme poly(ADP-ribose) polymerase, were detected by conventional immunofluorescence and confirmed by Western immunoblotting. At increasing times after apoptosis, nuclear proteins SS-A (Ro) and SS-B (La), but not poly(ADP-ribose) polymerase were relocated from the cell nucleus to the cell surface membrane. Fodrin, a cytoplasmic protein, was also translocated to the cell membrane after cleavage of alpha-fodrin. These results show that autoantigens fodrin, SS-A (Ro) and SS-B (La) in human salivary gland cells undergo a striking redistribution during apoptosis and relocate to the cell membrane of apoptotic cells. The appearance of autoantigens on the surface of induced cells could form the basis of a mechanism for autoantigen presentation, processing and autoantibody induction.

Assembly and annotation of human chromosome 2q33 sequence containing the CD28, CTLA4, and ICOS gene cluster: analysis by computational, comparative, and microarray approaches

Human chromosome 2q33 is an immunologically important region based on the linkage of numerous autoimmune diseases to the CTLA4 locus. Here, we sequenced and assembled 2q33 bacterial artificial chromosome (BAC) clones, resulting in 381,403 bp of contiguous sequence containing genes encoding a NADH: ubiquinone oxidoreductase, the costimulatory receptors CD28, CTLA4, and ICOS, and a HERV-H type endogenous retrovirus located 366 bp downstream of ICOS in the reverse orientation. Genomic microarray expression analysis using differentially activated T-cell RNA against a subcloned CTLA4/ICOS BAC library revealed upregulation of CTLA4 and ICOS sequences, plus antisense ICOS transcripts generated by the HERV-H, suggesting a potential mechanism for ICOS regulation. We identified four nonlinked, polymorphic, simple repetitive sequence elements in this region, which may be used to delineate genetic effects of ICOS and CTLA4 in disease populations. Comparative genomic analysis of mouse genomic Icos sequences revealed 60% sequence identity in the 5' UTR and regions between exon 2 and the 3' UTR, suggesting the importance of ICOS gene function.

Role of viruses in systemic lupus erythematosus and Sjögren syndrome

Systemic lupus erythematosus and Sjögren syndrome remain elusive in the description of their underlying etiologic causes and pathogenic mechanisms. Although underlying genetic predisposition appears to contribute to both diseases based on twin and other genetic studies, additional factors must play a role. Over the decades additional factors, such as hormonal influence, UV light, environmental exposures (e.g., silica, solvents), and infectious agents have been postulated to play a role. Over the past few years additional information has been published concerning roles of various infectious agents in both lupus and Sjögren syndrome. Although the understanding of this field is still incomplete, significant advances are being made.

Human endogenous retrovirus HERV-W family: chromosomal localization, identification, and phylogeny

A new human endogenous retroviral family (HERV-W) has been described that is related to multiple sclerosis-associated retrovirus sequences that have been identified in particles recovered from monocyte cultures from patients with multiple sclerosis. Using the polymerase chain reaction approach with a human monochromosomal somatic cell hybrid DNA panel, 15 env fragments of the HERV-W family from chromosomes 1, 3, 4, 5, 6, 7, 12, 14, 17, 20, and X were identified and analyzed. These env fragments showed a high degree of nucleotide sequence similarity (91.6-99.6%) to that of HERV-W. Translation of the env fragments showed no frameshift and termination codons by deletion/insertion or point mutation in some clones (W-1-1, W-3-8, W-4-1, W-7-1, W-14-1, W-17-5, W-20-9, and W-X-3). Phylogenetic analysis of the HERV-W family indicates that the HERV-W env fragments divided into five groups through evolutionary divergence in the primate genome. In group IV, a clone (W-12-2) on chromosome 12 shared 100% sequence identity with a clone (W-17-5) on chromosome 17, suggesting either a retrotransposition or a chromosomal translocation in the last 2 to 5 million years.

Infrequency of detection of particle-associated MSRV/HERV-W RNA in the synovial fluid of patients with rheumatoid arthritis

OBJECTIVES

To determine whether the recently identified multiple sclerosis-associated retrovirus, MSRV, is detectable in the serum and synovial fluid of patients with rheumatoid arthritis (RA).

METHODS

A reverse transcription-polymerase chain reaction (RT-PCR) assay was used to seek evidence of particle-associated MSRV/HERV-W RNA in the plasma and synovial fluid of patients with RA and controls. Stringent precautions were taken to avoid detection of contaminating human genomic DNA and cellular RNA sequences.

RESULTS

Thirty-seven plasma samples were tested (20 from RA patients and 17 from controls) but none had detectable MSRV/HERV-W RNA. Synovial fluid samples were available from nine patients with RA and 10 controls. Particle-associated MSRV/HERV-W RNA was reproducibly detected in two of nine synovial fluid samples from RA patients and in one control sample. The identity of RT-PCR products was confirmed by sequencing.

CONCLUSION

MSRV/HERV-W RNA sequences are detectable in the synovial fluid of a small proportion of RA patients, but this phenomenon may not be specific to RA.

Isolation and phylogeny of endogenous retrovirus sequences belonging to the HERV-W family in primates

An investigation was undertaken of primate pol gene sequences from a novel endogenous retrovirus family, ERV-W, related to a new human endogenous retrovirus family (HERV-W) that includes multiple sclerosis-associated retrovirus (MSRV) sequences identified in particles recovered from monocyte cultures from patients with multiple sclerosis. The pol gene sequences of the ERV-W family were detected in hominoids and Old World monkeys, but not in New World monkeys, whereas ERV-W long terminal repeat-like elements were detected in all primates (hominoids, Old World monkeys and New World monkeys). Thirty-two pol gene sequences from hominoids and Old World monkeys showed a high degree of sequence identity to MSRV and other HERV-W sequences. Phylogenetic analysis indicated close relationships of pol gene sequences across primate species. The analysis suggests that the ERV-W family has evolved independently but in constrained patterns ('parallel evolution') in different primate species, including man. The ratio of synonymous to non-synonymous substitutions indicated that negative selective pressure is acting on CHW1-1 from chimpanzee, HBW6-6 from baboon and HWX5 from man, sequences that have no disruption by point mutation or insertions/deletions. Therefore, these pol gene sequences could be associated with an active provirus in primates. The findings indicate that the ERV-W family has continued to evolve in the course of the primate radiation and may include members with a capacity to influence gene function and possibly cause disease.