Expression of an endogenous retroviral transcript is associated with murine lupus

Emerging roles of TLR7 and TLR9 in murine SLE

Systemic lupus erythematosus (SLE) is an autoimmune disorder characterized by B cell hyperactivity leading to the production of various autoantibodies and subsequent development of glomerulonephritis, i.e. lupus nephritis. Among the principal targets of autoantibodies produced in murine SLE are nucleic acid-protein complexes, such as chromatin and ribonucleoproteins, and the envelope glycoprotein gp70 of endogenous retroviruses. The preferential production of these autoantibodies is apparently promoted by the presence of genetic abnormalities leading to defects in the elimination of apoptotic cells and to an enhanced expression of endogenous retroviruses. Moreover, recent studies revealed that the innate receptors TLR7 and TLR9 are critically involved in the activation of dendritic cells and autoreactive B cells through the recognition of endogenous DNA- or RNA-containing antigens and subsequent development of autoimmune responses against nuclear autoantigens. Furthermore, the regulation of autoimmune responses against endogenous retroviral gp70 by TLR7 suggested the implication of endogenous retroviruses in this autoimmune response. Clearly, further elucidation of the precise molecular role of TLR7 and TLR9 in the development of autoimmune responses will help to develop novel therapeutic strategies and targets for SLE.

Role of endogenous retroviruses in autoimmune diseases

Molecular epidemiologic proof that HERVs and other retroelements are involved in autoimmunity or other disorders is complicated by their large numbers in the human genome. As discussed, most HERVs are no longer functional or active because of the accumulation of mutations, frameshifts, and deletions. Detection or quantification of HERV transcripts that may be pathologically involved in a particular autoimmune disease thus is often compromised by the presence in great excess of related, but nonfunctional, RNA. This phenomenon should not deter active work in the field, although it will require development of improved methods to discriminate accurately between closely related RNA transcripts. Development of improved immunologic methods to precisely identify epitopes on autoantigens or rare self-reactive T-cell clones may further implicate HERVs and the other repetitive elements in regulation of the immune system in health and disease.

The potential roles of endogenous retroviruses in autoimmunity

Endogenous retroviruses (ERVs) are estimated to comprise up to 1% of human DNA. While the genome of many ERVs is interrupted by termination codons, deletions or frame shift mutations, some ERVs are transcriptionally active and recent studies reveal protein expression or particle formation by human ERVs. ERVs have been implicated as aetiological agents of autoimmune disease, because of their structural and sequence similarities to exogenous retroviruses associated with immune dysregulation and their tissue-specific or differentiation-dependent expression. In fact, retrovirus-like particles distinct from those of known exogenous retroviruses and immune responses to ERV proteins have been observed in autoimmune disease. Quantitatively or structurally aberrant expression of normally cryptic ERVs, induced by environmental or endogenous factors, could initiate autoimmunity through direct or indirect mechanisms. ERVs may lead to immune dysregulation as insertional mutagens or cis-regulatory elements of cellular genes involved in immune function. ERVs may also encode elements like tax in human T-lymphotrophic virus type I (HTLV-I) or tat in human immunodeficiency virus-I (HIV-I) that are capable of transactivating cellular genes. More directly, human ERV gene products themselves may be immunologically active, by analogy with the superantigen activity in the long terminal repeat (LTR) of mouse mammary tumour viruses (MMTV) and the non-specific immunosuppressive activity in mammalian type C retrovirus env protein. Alternatively, increased expression of an ERV protein, or expression of a novel ERV protein not expressed in the thymus during acquisition of immune tolerance, may lead to its perception as a neoantigen. Paraneoplastic syndromes raise the possibility that novel ERV-encoded epitopes expressed by a tumour elicit immunity to cross-reactive epitopes in normal tissues. Recombination events between different but related ERVs, to whose products the host is immunologically tolerant, may also generate new antigenic determinants. Frequently reported humoral immunity to exogenous retrovirus proteins in autoimmune disease could be elicited by cross-reactive ERV proteins. A review of the evidence implicating ERVs in immune dysfunction leads to the conclusion that direct molecular studies are likely to establish a pathogenic role for ERVs in autoimmune disease.

Ro/SS-A and La/SS-B: autoantigens in Sjögren's syndrome?

Sjögren's syndrome is characterized serologically by the presence of autoantibodies against Ro/SS-A and La/SS-B. The mechanisms by which these autoantibodies arise is not clear at this moment. B cells capable of producing antibodies to Ro/SS-A and La/SS-B seem to be present in every individual; whether or not an individual indeed makes these antibodies is governed by T cells. Recent experiments from us and from others indicate that T cell tolerance towards Ro/SS-A and La/SS-B can be broken by immunization of normal mice with recombinant human Ro/SS-A or La/SS-B. The specificity of the T cells directing the anti-Ro/SS-A and anti-La/SS-B autoantibody response in these animals has not yet been elucidated. T cells may either be directed against foreign epitopes present on the human immunogen or they may be truly autoreactive. In patients with Sjögren's syndrome, a comparable immunization route might encompass product of viral origin directing the T cell response via RNA/protein complexes. Putative candidates comprise viruses that make use of RNAse polymerase-III, such as Epstein Barr virus, adenovirus, vesicular stomatitis virus en rabies virus.

Lack of relationship between human immunodeficiency virus infection and systemic lupus erythematosus

The objective of this work was to determine whether HIV-1 and HIV-2 could be involved in the pathogenesis of systemic lupus erythematosus (SLE). Seventy-five consecutive Caucasian patients with SLE presenting at one institution over a 2-year period were studied. Serum samples were surveyed for anti-HIV-1 antibodies by a commercial ELISA coated with HIV-1-p24. For confirmation, conventional immunoblots were performed with the following antigens: HIV-1-gp41, p31, p24 and p17 (recombinant) and HIV-2-gp36 (synthetic peptide). Additionally, Western blots with HIV-1-gp160, gp120, gp41, p65, p51, p24 and p18 bands were applied. Seventeen (23%) patients exhibited reactivity with HIV-1-p24 in the ELISA, but in the immunoblots and Western blots these sera samples were negative. Patients with SLE may exhibit a reactivity with HIV-1-p24 in the ELISA for HIV infection screening but not in the confirmatory blots. This false-positive reactivity is probably due to molecular mimicry between autoantigens and retroviruses or a contaminant or artefacts in the antigen preparation procedure.

Are endogenous retroviruses involved in human autoimmune disease?

A role for viruses in the etiopathogenesis of human autoimmune diseases has long been suspected but has not yet been proven. In Sjögren's syndrome (SS), there is continuing experimental support for the possible involvement of Epstein-Barr virus. Since the advent of AIDS, there is also great interest in retroviruses and autoimmune disease. We previously reported that 30% of SS patients and 36% of systemic lupus erythematosus (SLE) patients have serum antibodies to the p24 gag protein of HIV-1. We now report that two mechanisms classic for retroviruses (molecular mimicry and immunosuppression) may be operative in SS and SLE. The p24 gag protein shares a proline-rich epitope with the Sm nucleoprotein to which many SLE patients have antibodies. The impaired lymphocyte activation seen in peripheral blood T cells in SS patients is also seen in a human T cell line infected with an A-type retroviral particle linked to SS. Many studies suggest that endogenous retroviral sequences are important in immunoregulation. We now suggest that endogenous retroviral sequences may also be important in the etiology and pathogenesis of SS and SLE.

Antibodies against p24 of HIV-1 in patients with systemic lupus erythematosus?

The involvement of retroviruses in the etiopathogenesis of autoimmunity is discussed. Recently antibodies against p24 of HIV-1 were described in patients from Texas with SLE. Therefore we investigated the presence of these antibodies in our SLE collective (German caucasians and blacks from Michigan) employing ELISA and Western blot. No anti-p24 reactivity was observed in our SLE patients in Western blots, suggesting that there may be ethnological or regional differences between our patients those from Texas.

Multiple overlapping homologies between two rheumatoid antigens and immunosuppressive viruses

Amino acid (aa) sequence homologies between viruses and autoimmune nuclear antigens are suggestive of viral involvement in disorders such as systemic lupus erythematosus (SLE) and scleroderma. We analyzed the frequency of exact homologies of greater than or equal to 5 aa between 61 viral proteins (19,827 aa), 8 nuclear antigens (3813 aa), and 41 control proteins (11,743 aa). Both pentamer and hexamer homologies between control proteins and viruses are unexpectedly abundant, with hexamer matches occurring in 1 of 3 control proteins (or once every 769 aa). However, 2 nuclear antigens, the SLE-associated 70-kDa antigen and the scleroderma-associated CENP-B protein, are highly unusual in containing multiple homologies to a group of synergizing immunosuppressive viruses. Two viruses, herpes simplex virus 1 (HSV-1) and human immunodeficiency virus 1 (HIV-1), contain sequences exactly duplicated at 15 sites in the 70-kDa antigen and at 10 sites in CENP-B protein. The immediate-early (IE) protein of HSV-1, which activates HIV-1 regulatory functions, contains three homologies to the 70-kDa antigen (two hexamers and a pentamer) and two to CENP-B (a hexamer and pentamer). There are four homologies (including a hexamer) common to the 70-kDa antigen and Epstein-Barr virus, and three homologies (including two hexamers) common to CENP-B and cytomegalovirus. The majority of homologies in both nuclear antigens are clustered in highly charged C-terminal domains containing epitopes for human autoantibodies. Furthermore, most homologies have a contiguous or overlapping distribution, thereby creating a high density of potential epitopes. In addition to the exact homologies tabulated, motifs of matching sequences are repeated frequently in these domains. Our analysis suggests that coexpression of heterologous viruses having common immunosuppressive functions may generate autoantibodies cross-reacting with certain nuclear proteins.

Analysis of thymic endogenous retroviral expression in murine lupus. Genetic and immune studies

Inbred mouse genomes contain two subclasses of proviruses related to mink cell focus-forming (MCF) retroviruses: polytropic (Pmv), and modified polytropic (Mpmv). To determine whether one of these subclasses is associated with murine lupus, oligonucleotide probes specific for Pmv or Mpmv sequences were used in Northern analyses. Thymus 8.4 kb Mpmv RNA was expressed in five of five lupus-prone strains and crosses and this expression was not affected by genes that retard or accelerate development of lupus. Two of four leukemia-prone strains expressed low levels of such thymic transcripts, but none of 11 control strains did. 8.4 kb Mpmv RNA expression was not induced in thymuses of control mice by the lpr/lpr or gld/gld genotypes (which cause polyclonal immune activation) nor by treatment with mitogens. In contrast to Mpmv, thymic 8.4 kb Pmv expression was poorly associated with autoimmunity: it was easily detected in nearly all strains, and was increased by polyclonal activation in control mice. These studies indicate that the organ-specific thymic 8.4 kb Mpmv expression (a) is characteristic of several genetic backgrounds which predispose to murine lupus, (b) precedes and does not correlate with disease development, (c) is not due to polyclonal activation, and (d) is regulated independently of 8.4 kb Pmv expression.

Preferential nuclear compartmentalization of endogenous mink cell focus-forming-related retroviral transcripts

Endogenous mink cell focus-forming (MCF)-like retroviral sequences in the murine genome are stable, inherited sequences analogous to other chromosomal genes. As such, it is thought that they are transcribed and translated in a manner analogous to other genes. However, when the SL12.4 CD4-, CD8- thymoma cell line was studied for nuclear/cytoplasmic distribution of endogenous MCF-related transcripts, there was a nuclear predominance. The great majority of full-length 8.4-kb endogenous MCF-related transcripts were nuclear. Even the smaller, spliced 3.0-kb transcripts were at least as prominent in the nucleus as the cytoplasm, whereas cellular RNA was 80% cytoplasmic and other cellular transcripts were represented in the cytoplasm to a much greater extent than the nucleus. Size cannot fully account for the nuclear presence of MCF-related endogenous transcripts, because the 3.0-kb MCF transcripts occurred in the nucleus to a much greater relative extent than 3.8-kb c-myb transcripts. These studies point to retroviral-like structures of these transcripts as influencing their intracellular compartmentalization.