Epstein-Barr virus transactivates the human endogenous retrovirus HERV-K18 that encodes a superantigen

High prevalence of an IgG response against murine leukemia virus (MLV) in patients with psoriasis

Increasing evidence suggests that human endogenous retroviruses (HERV) could participate in the pathogenesis of autoimmune diseases such as multiple sclerosis and lupus erythematosus. To assess a possible association of murine leukemia virus (MLV)-like group of HERVs with psoriasis we searched for antibodies against MLV proteins in the sera of patients. We showed that anti-MLV antibodies (total) were detected in both psoriatic and control sera. However, they were detected with a higher frequency in psoriasis when compared with controls (91 vs. 53%, respectively, P=0.001). In addition, the IgG response was dramatically increased in psoriasis (86 vs. 8%, respectively, P<0.0001). This immunoreactivity was observed against the products of both the gag and env genes, and the most antigenic proteins were the gp65-70. Moreover, we observed that anti-p30 MLV antibodies reacted with an epidermal protein with a molecular weight of 50 kDa in protein extracts from both normal and psoriatic skin cultures. These observations suggest that HERVs of the MLV-like group could contribute to the immunopathogenesis of psoriasis.

Differential kinetics and specificity of EBV-specific CD4+ and CD8+ T cells during primary infection

The generation and maintenance of virus-specific CD4(+) T cells in humans are not well understood. We used short in vitro stimulation assays followed by intracellular cytokine staining to characterize the timing, magnitude, and Ag specificity of CD4(+) T cells over the course of primary EBV infection. Lytic and latent protein-specific CD4(+) T cells were readily detected at presentation with acute infectious mononucleosis and declined rapidly thereafter. Responses to BZLF-1, BMLF-1, and Epstein-Barr nuclear Ag-3A were more commonly detected than responses to Epstein-Barr nuclear Ag-1. Concurrent analyses of BZLF-1-specific CD4(+) and CD8(+) T cells revealed differences in the expansion, specificity, and stability of CD4(+) and CD8(+) T cell-mediated responses over time. Peripheral blood EBV load directly correlated with the frequency of EBV-specific CD4(+) T cell responses at presentation and over time, suggesting that EBV-specific CD4(+) T cell responses are Ag-driven.

Role of endogenous retroviruses in autoimmune diseases

Endogenous retroviruses (ERVs) correspond to the integrated proviral form of infectious retroviruses that are trapped within the genome by mutations. Endogenous retroviruses represent a key molecular link between the host genome and infectious viral particles. Proteins encoded by ERVs are recognized by antiviral immune responses and become targets of autoreactivity. Activation of ERVs, such as human ERV-K or a human T-cell lymphotropic virus-related endogenous sequence, may also mediate pathogenicity of Epstein-Barr virus. Endogenous retrovirus peptides can directly regulate immune responses. Thus, molecular mimicry and immunomodulation by ERVs may account for self-reactivity and abnormal T- and B-cell functions in autoimmune disorders.

Demystified. Human endogenous retroviruses

Human endogenous retroviruses (HERVs) are a family of viruses within our genome with similarities to present day exogenous retroviruses. HERVs have been inherited by successive generations and it is possible that some have conferred biological benefits. However, several HERVs have been implicated in certain cancers and autoimmune diseases. This article demystifies these retroviruses by providing an insight into HERVs, their means of classification, and a synopsis of HERVs implicated in cancer and autoimmunity. Furthermore, the biological roles of HERVs are explored.

The role of neutralizing antibodies for mouse mammary tumor virus transmission and mammary cancer development

Mouse mammary tumor virus (MMTV) infection establishes chronic germinal centers and a lifelong neutralizing Ab response. We show that removal of the draining lymph node after establishment of the germinal center reaction led to complete loss of neutralizing Abs despite comparable infection levels in peripheral lymphocytes. Importantly, in the absence of neutralization, only the exocrine organs mammary gland, salivary gland, pancreas, and skin showed strikingly increased infection, resulting in accelerated mammary tumor development. Induction of stronger neutralization did not influence chronic infection levels of peripheral lymphoid organs but strongly inhibited mammary gland infection and virus transmission to the next generation. Taken together, we provide evidence that a tight equilibrium in virus neutralization allows limited infection of exocrine organs and controls cancer development in susceptible mouse strains. These experiments show that a strong neutralizing Ab response induced after infection is not able to control lymphoid MMTV infection. Strong neutralization, however, is capable of blocking amplification of mammary gland infection, tumor development, and virus transmission to the next generation. The results also indicate a role of neutralization in natural resistance to MMTV infection.

Characterization of a novel human endogenous retrovirus, HERV-H/F, expressed in human leukemia cell lines

We have identified and characterized a human endogenous retrovirus (HERV) gag transcript in the human pre-B cell leukemia line Reh. The transcript was found to be a splice product of a structurally intact HERV element located on chromosome 6q13. Its primer binding site is complementary to phenylalanine (F) tRNA, common for the HERV-F family, but the overall genome sequence is closely related to the HERV-H family. The retroviral sequence was therefore designated HERV-H/F. The HERV element shows a distinct mRNA expression pattern among hematopoietic cancer cell lines with expression in some leukemia-derived cell lines of B-lymphoid and myeloid origin. No expression was observed in normal human tissues, indicating a cancer-specific expression pattern. The 5' long terminal repeat (LTR) was tested for promoter activity in HERV-H/F expressing and nonexpressing cell lines. The cell specificity of the LTR-mediated reporter gene expression did not conclusively correlate with endogenous virus expression, indicating that the transcription regulation of this gene is not alone dependent on cell-specific activity of transcription factors.

Manipulation of immune responses by Epstein–Barr virus

Epstein-Barr virus (EBV) infects and persists for life in the majority of the human population. Persistence is achieved through a combination of strictly regulated programs of latent infection in B-cells and chronic reactivation of virus replication in lymphoid tissue and mucosal surfaces. The resulting multiple patterns of virus-host interaction have selected unique strategies of immune escape. T-cell mediated immunity plays a central role in the control of EBV latency and several immune escape mechanism that protect the virus at this stage of its life circle have been characterized in details. In contrast, the contribution of innate immunity and the immune regulation of productive infection are largely unexplored areas that may yield important clues on the establishment and maintenance of EBV persistence. This review summarizes well known and emerging mechanisms of EBV immune escape that may reveal new strategies of immunoregulation and promote new approaches to the prophylaxis and treatment of EBV associated diseases.

A battle for survival: immune control and immune evasion in murine gamma-herpesvirus-68 infection

CD8(+) T cells are generally considered a key defence against herpesviruses. The murine gamma-herpesvirus-68 encodes two proteins that limit their efficacy. M3 neutralizes chemokines, while K3 downregulates MHC class I glycoproteins. The consequence of this evasion is that CD4(+) T cells are essential to the control of persistent infection.

Synergism between HIV and other viruses in the mouth

The HIV family replicate in and are shed from the mouth. Oral sexual practices potentially contribute to the overall extent of HIV transmission, particularly if high-risk practices are not restricted. Herpesviruses and papillomaviruses that appear in the oral cavity can determine oral HIV replication. The mechanisms probably include heterologous transactivation, enhanced expression of HIV receptors and co-receptors in target cells, release of cytokines and chemokines, and production of superantigens. Oral diseases peculiar to, or more common in, the HIV-infected patient further predispose to heightened oral HIV replication and trafficking. Defining the mechanisms by which oral viruses interact with HIV in the co-infected host should permit intervention measures against oral HIV transmission to be more precisely targeted.

Detection of viral DNA and immune responses to the human herpesvirus 6 101-kilodalton virion protein in patients with multiple sclerosis and in controls

Human herpesvirus 6 (HHV-6), a latent lymphotropic and neurotropic virus, has been suspected as an etiologic agent in multiple sclerosis (MS). The study was undertaken to correlate virologic evidence for HHV-6 activity with the state of host immunity to HHV-6 in MS patients and control subjects. The study revealed that cell-free DNA of HHV-6 was detected more frequently in both serum and cerebrospinal fluid of MS patients than in those of control subjects. T cells recognizing the recombinant 101-kDa protein (101K) corresponding to the major immunoreactive region unique to HHV-6 occurred at significantly lower precursor frequency in MS patients than in control subjects. The resulting HHV-6-specific T-cell lines obtained from MS patients exhibited skewed cytokine profiles characterized by the inability to produce interleukin-4 (IL-4) and IL-10. The decreased T-cell responses to HHV-6 and the altered cytokine profile were consistent with significantly declined serum immunoglobulin G (IgG) titers for HHV-6 of MS patients compared to those of control subjects. In contrast, elevated serum IgM titers for HHV-6 were detected in the majority of MS patients, which may reflect frequent exposure of B cells to HHV-6. The findings suggest that the decreased immune responses to HHV-6 may be responsible for ineffective clearance of HHV-6 in MS patients.

Expression of transcription factor AML-2 (RUNX3, CBF(alpha)-3) is induced by Epstein-Barr virus EBNA-2 and correlates with the B-cell activation phenotype

To identify cell proteins regulated by the Epstein-Barr virus (EBV) transcription factor EBNA-2, we analyzed a cell line with conditional EBNA-2 activity by using microarray expression profiling. This led to the identification of two novel target genes induced by EBNA-2. The first of these, interleukin-16, is an immunomodulatory cytokine involved in the regulation of CD4 T cells. The second, AML-2, is a member of the Runt domain family of transcription factors. Quiescent B cells initially expressed AML-1 but, 48 h after virus infection, the levels of AML-1 decreased dramatically, whereas the amount of AML-2 protein increased. Analysis of a panel of B-cell lines indicated that AML-2 expression is normally predominant in EBV latency III, whereas AML-1 is associated with EBV latency I or EBV-negative cells. The AML genes are the first example of cell transcription factors whose expression correlates with the latency I/III phenotype.

Human endogenous retrovirus IDDMK(1,2)22 and mouse mammary tumor virus superantigens differ in their ability to stimulate murine T cell hybridomas

Recently, a newly identified human HERV-K18 like endogenous retrovirus (IDDMK(1,2)22) has been associated to the etiology of type I diabetes (IDDM). Although the exact mechanism remains unclear, it was postulated that the 3' end ORF product of the env gene of IDDMK(1,2)22 would trigger a V beta 7-specific human T cell expansion leading to their infiltration in the pancreas of afflicted patients and to the autoimmune destruction of the insulin-producing beta cells. Since then, such superantigen (SAg)-like activity as well as the association between the IDDMK(1,2)22 virus and IDDM pathogenesis have been challenged. To further characterize functionally the putative IDDMK(1,2)22-encoded SAg, we have cloned from human DNA the identical 462bp ORF sequence originally described. The IDDMK(1,2)22 ORF fragment was transfected in the same human B cell line (Raji) originally used as APC to demonstrate the V beta 7 specificity. The immunostimulatory potential of IDDM ORF was tested on murine T cell hybridomas and compared to the well-characterized mouse mammary tumor virus Mtv7 SAg transfected in the same conditions. A panel of 16 T cell hybridomas encompassing 14 different V betas was analyzed. We have failed to detect IDDMK(1,2)22-induced IL-2 production from any of these hybridomas, even those bearing the murine V beta 1 mV beta 1, V beta 4 or V beta 10 TcR beta chains which are most closely related to the human V beta 7 (hV beta 7). Our results suggest that IDDMK(1,2)22 ORF is devoid of superantigenic activity as defined by classical criteria.

Superantigens: microbial agents that corrupt immunity

Microbial superantigens are a family of protein exotoxins that share the ability to trigger excessive and aberrant activation of T cells. The best characterised are the staphylococcal enterotoxins and the streptococcal pyrogenic exotoxins that trigger the staphylococcal and streptococcal toxic shock syndromes. It is now apparent that superantigens have a wider role in the pathology of infectious diseases than has previously been appreciated. Staphylococcus aureus and Streptococcus pyogenes together produce 19 different superantigens. The range of microorganisms known to produce superantigens has expanded to include Gram negative bacteria, mycoplasma, and viruses. Research is beginning to shed light on the more subtle parts these molecules play in causing disease and to produce some real possibilities for specific treatment of superantigen-induced toxicity. We aim to highlight these new developments and review the science behind these fascinating molecules.

So many ways of getting in the way: diversity in the molecular architecture of superantigen-dependent T-cell signaling complexes

Superantigens (SAGs) elicit massive T-cell proliferation through simultaneous interaction with MHC and TCR molecules. SAGs have been implicated in toxic shock syndrome and food poisoning, and they may also play a pathogenic role in autoimmune diseases. The best-characterized group of SAGs are the pyrogenic bacterial SAGs, which utilize a high degree of genetic variation on a common structural scaffold to achieve a wide range of MHC-binding and T-cell-stimulating effects while assisting pathogen evasion of the adaptive immune response. Several new structures of SAG-MHC and SAG-TCR complexes have significantly increased understanding of the molecular bases for high-affinity peptide/MHC binding by SAGs and for TCR Vbeta domain specificity of SAGs. Using the currently available SAG-MHC and SAG-TCR complex structures, models of various trimolecular MHC-SAG-TCR complexes may be constructed that reveal wide diversity in the architecture of SAG-dependent T-cell signaling complexes, which nevertheless may result in similar signaling outcomes.

Sleeping with the enemy--endogenous superantigens in humans

We usually think of superantigens (SAg) as dangerous toxins that may cause toxic shock syndrome and death. Now, based on two papers in this issue of Immunity, it seems that we all have SAg genes within us, lying dormant and waiting to be activated under special circumstances.

Interferon-alpha-induced endogenous superantigen. a model linking environment and autoimmunity

We earlier proposed that a human endogenous retroviral (HERV) superantigen (SAg) IDDMK(1,2)22 may cause type I diabetes by activating autoreactive T cells. Viral infections and induction of interferon-alpha (IFN-alpha) are tightly associated with the onset of autoimmunity. Here we establish a link between viral infections and IFN-alpha-regulated SAg expression of the polymorphic and defective HERV-K18 provirus. HERV-K18 has three alleles, IDDMK(1,2)22 and two full-length envelope genes, that all encode SAgs. Expression of HERV-K18 SAgs is inducible by IFN-alpha and this is sufficient to stimulate V beta 7 T cells to levels comparable to transfectants constitutively expressing HERV-K18 SAgs. Endogenous SAgs induced via IFN-alpha by viral infections is a novel mechanism through which environmental factors may cause disease in genetically susceptible individuals.