A new human retrovirus: a role in lymphoma?

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.

Single-tube nested quantitative PCR: a rational and sensitive technique for detection of retroviral DNA. Application to RERV-H/HRV-5 and confirmation of its rabbit origin

It was reported earlier that a few patients suffering from non-Hodgkin's lymphoma had low amounts of DNA from the so-called fifth human exogenous retrovirus, HRV-5. A sensitive and rational method for large-scale screening for HRV-5 DNA was therefore developed. It is a single-tube nested quantitative PCR (stnQPCR), which uses two functionally isolated primer pairs and one probe target distinct from related endogenous retroviral sequences, yet encompassing known HRV-5 variation, allowing optimal use of sequence conservation. DNA from lymphoma, myeloma, and follicular dendritic cell lines was tested for HRV-5 positivity, as was DNA from whole blood of blood donors, non-Hodgkin's lymphoma and systemic lupus erythematosus patients, as well as DNA from lymph node biopsies of rheumatoid arthritis patients with lymphoma. One blood donor, one systemic lupus erythematosus patient, two previously known positive non-Hodgkin's lymphoma patients, and one rheumatoid arthritis lymphoma patient, came out positive. They had 24, 2, 148, 480 and 30 proviral copies per microg of DNA from PBMC or lymphoma tissue, respectively. During the completion of this work it was reported that HRV-5 is a rabbit endogenous retrovirus (RERV-H), and that HRV-5 positivity was due to presence of rabbit DNA. DNA from six RERV-H/HRV-5 positive samples was therefore retested. Three also contained rabbit mitochondrial DNA. A search for HRV-5 antibodies using synthetic peptides was negative in sera from three RERV-H/HRV-5 positive individuals, as well as in 144 other sera, according with a noninfectious origin of the RERV-H/HRV-5 DNA in human samples. A search for possible sources of rabbit DNA contamination was negative. Methods for prevention of PCR contamination were strictly adhered to. Three samples from RERV-H/HRV-5 positive individuals positive at the Uppsala laboratory were retested at one or two other laboratories, and all three were positive. Two other samples, which were positive in the Riga laboratory, were tested also in London and also found positive. One non-Hodgkin's lymphoma patient was RERV-H/HRV-5 positive in four consecutive samples, showing that positivity was a property of that patient. It is concluded that the stnQPCR developed to detect and quantify minute amounts of RERV-H/HRV-5 DNA is a principle which can be applied widely and HRV-5 is a RERV-H. Its presence in a few human blood samples could not be explained.

Rabbit endogenous retrovirus-H encodes a functional protease

Recent studies have revealed that 'human retrovirus-5' sequences found in human samples belong to a rabbit endogenous retrovirus family named RERV-H. A part of the gag-pro region of the RERV-H genome was amplified by PCR from DNA in human samples and several forms of RERV-H protease were expressed in bacteria. The RERV-H protease was able to cleave itself from a precursor protein and was also able to cleave the RERV-H Gag polyprotein precursor in vitro whereas a form of the protease with a mutation engineered into the active site was inactive. Potential N- and C-terminal autocleavage sites were characterized. The RERV-H protease was sensitive to pepstatin A, showing it to be an aspartic protease. Moreover, it was strongly inhibited by PYVPheStaAMT, a pseudopeptide inhibitor specific for Mason-Pfizer monkey virus and avian myeloblastosis-associated virus. A structural model of the RERV-H protease was constructed that, together with the activity data, confirms that this is a retroviral aspartic protease.

Novel endogenous retrovirus in rabbits previously reported as human retrovirus 5

Human retrovirus 5 (HRV-5) represented a fragment of a novel retrovirus sequence identified in human RNA and DNA preparations. In this study, the genome of HRV-5 was cloned and sequenced and integration sites were analyzed. Using PCR and Southern hybridization, we showed that HRV-5 is not integrated into human DNA. A survey of other species revealed that HRV-5 is present in the genomic DNA of the European rabbit (Oryctolagus cuniculus) and belongs to an endogenous retrovirus family found in rabbits. The presence of rabbit sequences flanking HRV-5 proviruses in human DNA extracts suggested that rabbit DNA was present in our human extracts, and this was confirmed by PCR analysis that revealed the presence of rabbit mitochondrial DNA sequences in four of five human DNA preparations tested. The origin of the rabbit DNA and HRV-5 in human DNA preparations remains unclear, but laboratory contamination cannot explain the preferential detection of HRV-5 in inflammatory diseases and lymphomas reported previously. This is the first description of a retrovirus genome in rabbits, and sequence analysis shows that it is related to but distinct from A-type retroelements of mice and other rodents. The species distribution of HRV-5 is restricted to rabbits; other species, including other members of the order Lagomorpha, do not contain this sequence. Analysis of HRV-5 expression by Northern hybridization and reverse transcriptase PCR indicates that the virus is transcribed at a low level in many rabbit tissues. In light of these findings we propose that the sequence previously designated HRV-5 should now be denoted RERV-H (for rabbit endogenous retrovirus H).

Human retrovirus type 5 sequences in non-Hodgkin's lymphoma of T cell origin

DNA of a recently described fifth exogenous retrovirus (HRV-5) has been found in blood samples from patients with autoimmune diseases and lymphoma. We analyzed HRV-5 sequence in DNA extracted from whole blood of 17 patients with T cell non-Hodgkin's lymphoma (NHL) and 186 patients with hematological malignancies other than NHL, using a sensitive PCR technique. While all samples of patients with hematological malignancies other than NHL were negative, 2 of the 17 patients with T cell NHL were HRV-5 DNA positive. Both HRV-5-positive patients had T cell NHL of high-grade malignancy (stage IV) and diffuse distribution of the lymphoma, including infiltration of bone marrow or lung and pleura. The difference in HRV-5 DNA detection frequency between NHL and control groups is significant (p value of 0.0004 judged by the Fisher exact test). These data, together with our previous finding of HRV-5 DNA in three B cell NHL cases, are compatible with an association between HRV-5 and NHL, of both T cell and B cell origin.

Retroviruses and psoriasis

Psoriasis occurs with at least undiminished frequency in HIV infected individuals. The behaviour of psoriasis in HIV disease is of interest, both in terms of pathogenesis and therapy, because of the background of profound immunodysregulation. It is paradoxical that, while drugs that target T lymphocytes are effective in psoriasis, the condition should be exacerbated by HIV infection. Antiretroviral therapy may improve psoriasis in tandem with improvement in the overall clinical and virological condition of the patient. The aetiopathogenesis of psoriasis is unknown but genetic and environmental factors are thought to be involved. There are controversial issues regarding the immunological basis of psoriasis and the role of CD4+ versus CD8+ T lymphocytes. Current opinion favours an autoimmune basis for psoriasis, although the precipitating activating signal(s) within psoriatic plaques remains unknown. The immunodysregulation resulting from HIV infection may trigger psoriasis in those genetically predisposed by the Cw*0602 allele. Since CD8+ T cells recognize antigen in the context of class I molecules, the identification of a human leucocyte antigen class I association in HIV-associated psoriasis strengthens the argument for an important role for CD8+ T lymphocytes in the immunopathogenesis of psoriasis. HLA-Cw*0602 could act as a cross-reactive target for cytotoxic T lymphocytes responding to processed peptides from microorganisms. Human retrovirus-5 is a recently described, partially characterized retrovirus and has been implicated in the pathogenesis of psoriatic arthropathy but not psoriasis.

Human retrovirus 5 sequences in peripheral blood cells of patients with B-cell non-Hodgkin's lymphoma

A recently described sequence from a probable 5th human exogenous retrovirus, HRV-5, is related to type A, B and D retroviruses. It was initially detected in a salivary gland biopsy from a patient with Sjögren's syndrome, but it is not consistently associated with this disease. We searched for the HRV-5 sequence in DNA extracted from whole blood of 300 blood donors, 81 patients with hematological malignancy and 21 patients with neurological disease using PCR. While samples from none of the blood donors and the neurological patients became positive, 3 of the 81 patients with hematological malignancy were HRV-5 DNA positive. All 3 had B-cell non-Hodgkin's lymphoma of low grade. The difference in frequency between NHL and controls is statistically significant. HRV-5 DNA was found in DNA from whole blood and in plastic-adherent cells but not in tumor cell DNA. Thus, monocytes/macrophages may be preferred targets for HRV-5. Our result, together with a previous finding of HRV-5 DNA in 2 NHL cases, is compatible with an association between HRV-5 and NHL, whether causal or not.

Retroviruses: ancient and modern

Retroviruses are transmitted in two distinct ways: as infectious virions and as 'endogenous' proviral DNA integrated in the germ line of their hosts. Modern infectious viruses such as HIV recently infected mankind from simian hosts, whereas human endogenous retroviral genomes have been present throughout old world primate evolution. Recently we have characterised novel retroviruses in humans and pigs. Human retrovirus 5 (HRV-5) is detected as an exogenous genome in association with arthritis and systemic lupus erythematosus. Porcine endogenous retroviruses (PERV) are carried in swine DNA but can be activated to produce virions that are infectious for human cells, which has implications for xenotransplantation. A brief account of HRV-5 and PERV is given here.

Human retrovirus-5 in rheumatic disease

It has long been suggested that retroviral infection may play a role in the pathogenesis of autoimmune rheumatic disease. Particles resembling retroviruses have been reported in tissue from patients with Sjögren's syndrome, lupus and rheumatoid arthritis, and molecular mimicry between retroviral antigens and host proteins has been proposed as a mechanism of induction of autoimmunity. Since 1980, four distinct human infectious retroviruses have been discovered, HTLV-I, HTLV-II, HIV-1 and HIV-2. We recently cloned part of a new human retrovirus genome, designated human retrovirus-5 (HRV-5) and demonstrated that this is not endogenous and is therefore a novel infectious retrovirus. Because symptoms resembling arthritis, polymyositis and Sjögren's syndrome occur in individuals infected with HTLV-I and HIV-1, we investigated the possibility that HRV-5 was associated with idiopathic rheumatic disease. Using nested PCR, HRV-5 we demonstrated that proviral DNA was present in approximately 50% of synovial samples of arthritic joints and was also found in over 10% of blood samples of patients with rheumatoid arthritis and systemic lupus erythematosus. HRV-5 proviral DNA was not detectable in affected tissues of autoimmune diseases and was found in only one of over 200 tissues taken at autopsy from non-rheumatoid patients. Sequence analysis of the amplified viral segment showed genetic variation between samples with maintenance of the open reading frame typical of a replicating infectious retrovirus. Thus HRV-5 appears to be a human retrovirus found with a very low genome copy number in most tissues, but which is increased to detectable levels in inflamed joints and blood from patients with rheumatic disease. Whether HRV-5 is aetiologically important in these diseases remains to be determined.

Detection of human retrovirus 5 in patients with arthritis and systemic lupus erythematosus

OBJECTIVE

To examine whether human retrovirus 5 (HRV-5) infection is associated with autoimmune rheumatic disease.

METHODS

DNA from patients with various disorders including inflammatory diseases and from normal subjects was tested by nested polymerase chain reaction (PCR) for HRV-5 proviral DNA. Positive results were confirmed by DNA sequencing.

RESULTS

HRV-5 proviral DNA was detected in 53% of synovial samples from arthritic joints, in 12% of blood samples from patients with rheumatoid arthritis (RA), and in 16% of blood samples from patients with systemic lupus erythematosus. In contrast, it was not detectable by PCR of affected tissues from patients with several other autoimmune diseases and was found in only 1 of >200 tissue specimens obtained at autopsy from non-RA patients. Sequence analysis of the amplified viral segment showed genetic variation between samples with maintenance of the open reading frame, typical of a replicating infectious retrovirus.

CONCLUSION

This is the first report of the frequent detection of HRV-5 in any disease. We propose that the possible involvement of HRV-5 in autoimmune and rheumatic disease should be investigated further.