Immune checkpoints in viral latency

Gingival tissue as a reservoir for human immunodeficiency virus type 1: Preliminary results of a cross-sectional observational study

BACKGROUND

Despite combined antiretroviral therapy (cART), total cure of immunodeficiency virus type 1 (HIV-1) infection remains elusive. Chronic periodontitis (CP) is strongly associated with HIV-1 infection. This condition is characterized by an intense inflammatory infiltrate mainly constituted of immune cells which in turn may be a valuable source of HIV-1 reactivation. This study aimed to determine if gingival tissue could act as a reservoir for HIV-1.

METHODS

Twelve patients with HIV-1 and CP and 12 controls (no HIV-1-infection and no CP) were evaluated in a cross-sectional study. RNA viral load and interleukin (IL) levels were determined in blood plasma and saliva. Histological sections of gingival tissue were stained with fluorescent antibodies against p24 antigen and different cellular biomarkers.

RESULTS

In six of the 12 patients, HIV RNA load was detected, despite cART; in three of them, expression of viral RNA was also detected in saliva. The levels of IL-2, IL-6, and IL-12 were higher in blood and saliva of patients with HIVand CP than controls. HIV-1 p24 antigen was detected by immunostaining in gingival biopsies of 10 of the 12 patients but in no controls. Immune markers for T cells and antigen-presenting cells were also identified in most patients and some controls.

CONCLUSION

These preliminary data showing the detection of HIV-1 p24 antigen in the gingival biopsies of a significant part of patients with HIV-1 and CP under cART together with the presence of immune cells, plead for the existence of a HIV-1 reservoir in the gingival tissue of this population.

Critical Role of Regulatory T Cells in the Latency and Stress-Induced Reactivation of HSV-1

Herpes simplex virus 1 (HSV-1) spreads in populations through a latency entry and reactivation cycle. The role of host immune-suppressive factor regulatory T cells (Treg cells) in controlling latency establishment and reactivation is not completely understood. Here, using an HSV-1 ocular infection murine model, we observe a positive correlation between the level of Treg cells and viral infectivity and demonstrate the requirement for Treg cells in latency establishment. Furthermore, we show that host stress leads to HSV-1 reactivation via increased Treg cell control of CD8⁺ T cells, permitting viral replication under diminished immune surveillance. Together, we propose that Treg cell regulation may serve as a key target for controlling HSV infection.

The Diverse Roles of microRNAs at the Host⁻Virus Interface

MicroRNAs (miRNAs) are small, non-coding RNAs that regulate gene expression at the post-transcriptional level. Through this activity, they are implicated in almost every cellular process investigated to date. Hence, it is not surprising that miRNAs play diverse roles in regulation of viral infections and antiviral responses. Diverse families of DNA and RNA viruses have been shown to take advantage of cellular miRNAs or produce virally encoded miRNAs that alter host or viral gene expression. MiRNA-mediated changes in gene expression have been demonstrated to modulate viral replication, antiviral immune responses, viral latency, and pathogenesis. Interestingly, viruses mediate both canonical and non-canonical interactions with miRNAs to downregulate specific targets or to promote viral genome stability, translation, and/or RNA accumulation. In this review, we focus on recent findings elucidating several key mechanisms employed by diverse virus families, with a focus on miRNAs at the host–virus interface during herpesvirus, polyomavirus, retroviruses, pestivirus, and hepacivirus infections.

Characterisation of putative immunomodulatory gene knockouts of lumpy skin disease virus in cattle towards an improved vaccine

Lumpy skin disease virus (LSDV) is responsible for causing severe economic losses to cattle farmers throughout Africa, the Middle East, and more recently, South-Eastern Europe and Russia. It belongs to the Capripoxvirus genus of the Poxviridae family, with closely related sheeppox and goatpox viruses. Like other poxviruses, the viral genome codes for a number of genes with putative immunomodulatory capabilities. Current vaccines for protecting cattle against lumpy skin disease (LSD) based on live-attenuated strains of field isolates passaged by cell culture, resulting in random mutations. Although generally effective, these vaccines can have drawbacks, including injection site reactions and/or limited immunogenicity. A pilot study was conducted using a more targeted approach where two putative immunomodulatory genes were deleted separately from the genome of a virulent LSDV field isolate. These were open reading frame (ORF) 005 and ORF008, coding for homologues of an interleukin 10-like and interferon-gamma receptor-like gene, respectively. The resulting knockout constructs were evaluated in cattle for safety, immunogenicity and protection. Severe post-vaccinal reactions and febrile responses were observed for both constructs. Two calves inoculated with the ORF008 knockout construct developed multiple lesions and were euthanised. Following challenge, none of the animals inoculated with the knockout constructs showed any external clinical signs of LSD, compared to the negative controls. Improved cellular and humoral immune responses were recorded in both of these groups compared to the positive control. The results indicate that at the high inoculation doses used, the degree of attenuation achieved was insufficient for further use in cattle due to the adverse reactions observed.

Infection homeostasis: implications for therapeutic and immune programming of metabolism in controlling infection

Homeostasis underpins at a systems level the regulatory control of immunity and metabolism. While physiologically these systems are often viewed as independent, there is increasing evidence showing a tight coupling between immune and metabolic functions. Critically upon infection, the homeostatic regulation for both immune and metabolic pathways is altered yet these changes are often investigated in isolation. Here, we summarise our current understanding of these processes in the context of a clinically relevant pathogen, cytomegalovirus. We synthesise from the literature an integrative view of a coupled immune-metabolic infection process, centred on sugar and lipid metabolism. We put forward the notion that understanding immune control of key metabolic enzymatic steps in infection will promote the future development of novel therapeutic modalities based on metabolic modifiers that either enhance protection or inhibit infection.

The same major histocompatibility complex polymorphism involved in control of HIV influences peptide binding in the mouse H-2Ld system

Single-site polymorphisms in human class I major histocompatibility complex (MHC) products (HLA-B) have recently been shown to correlate with HIV disease progression or control. An identical single-site polymorphism (at residue 97) in the mouse class I product H-2L(d) influences stability of the complex. To gain insight into the human polymorphisms, here we examined peptide binding, stability, and structures of the corresponding L(d) polymorphisms, Trp(97) and Arg(97). Expression of L(d)W97 and L(d)R97 genes in a cell line that is antigen-processing competent showed that L(d)R97 was expressed at higher levels than L(d)W97, consistent with enhanced stability of self-peptide·L(d)R97 complexes. To further examine peptide-binding capacities of these two allelic variants, we used a high affinity pep-L(d) specific probe to quantitatively examine a collection of self- and foreign peptides that bind to L(d). L(d)R97 bound more effectively than L(d)W97 to most peptides, although L(d)W97 bound more effectively to two peptides. The results support the view that many self-peptides in the L(d) system (or the HLA-B system) would exhibit enhanced binding to Arg(97) alleles compared with Trp(97) alleles. Accordingly, the self-peptide·MHC-Arg(97) complexes would influence T-cell selection behavior, impacting the T-cell repertoire of these individuals, and could also impact peripheral T cell activity through effects of self-peptide·L(d) interacting with TCR and/or CD8. The structures of several peptide·L(d)R97 and peptide·L(d)W97 complexes provided a framework of how this single polymorphism could impact peptide binding.

Hepatitis B virus (HBV) induces the expression of interleukin-8 that in turn reduces HBV sensitivity to interferon-alpha

High levels of serum interleukin-8 (IL-8) have been detected in chronic hepatitis B (CHB) patients during episodes of hepatitis flares. We investigated whether hepatitis B virus (HBV) may directly induce IL-8 production and whether IL-8 may antagonize interferon-alpha (IFN-α) antiviral activity against HBV. We showed that CHB patients had significantly higher IL-8 levels both in serum and in liver tissue than controls. In HBV-replicating HepG2 cells, IL-8 transcription was significantly activated. AP-1, C/EBP and NF-kB transcription factors were concurrently necessary for maximum IL-8 induction. Moreover, HBx viral protein was recruited onto the IL-8 promoter and this was paralleled by IL8-bound histone hyperacetylation and by active recruitment of transcriptional coactivators. Inhibition of IL-8 increases the antiviral activity of IFN-α against HBV. Our results indicate that HBV activates IL-8 gene expression by targeting the epigenetic regulation of the IL-8 promoter and that IL-8 may contribute to reduce HBV sensitivity to IFN-α.

Virus manipulation of cell cycle

Viruses depend on host cell resources for replication and access to those resources may be limited to a particular phase of the cell cycle. Thus manipulation of cell cycle is a commonly employed strategy of viruses for achieving a favorable cellular environment. For example, viruses capable of infecting nondividing cells induce S phase in order to activate the host DNA replication machinery and provide the nucleotide triphosphates necessary for viral DNA replication (Flemington in J Virol 75:4475-4481, 2001; Sullivan and Pipas in Microbiol Mol Biol Rev 66:179-202, 2002). Viruses have developed several strategies to subvert the cell cycle by association with cyclin and cyclin-dependent kinase complexes and molecules that regulate their activity. Viruses tend to act on cellular proteins involved in a network of interactions in a way that minimal protein-protein interactions lead to a major effect. The complex and interactive nature of intracellular signaling pathways controlling cell division affords many opportunities for virus manipulation strategies. Taking the maxim "Set a thief to catch a thief" as a counter strategy, however, provides us with the very same virus evasion strategies as "ready-made tools" for the development of novel antivirus therapeutics. The most obvious are attenuated virus vaccines with critical evasion genes deleted. Similarly, vaccines against viruses causing cancer are now being successfully developed. Finally, as viruses have been playing chess with our cell biology and immune responses for millions of years, the study of their evasion strategies will also undoubtedly reveal new control mechanisms and their corresponding cellular intracellular signaling pathways.

Mechanism of T cell exhaustion in a chronic environment

T cell exhaustion develops under conditions of antigen-persistence caused by infection with various chronic pathogens, such as human immunodeficiency virus (HIV) and mycobacterium tuberculosis (TB), or by the development of cancer. T cell exhaustion is characterized by stepwise and progressive loss of T cell function, which is probably the main reason for the failed immunological control of chronic pathogens and cancers. Recent observations have detailed some of the intrinsic and extrinsic factors that influence the severity of T cell exhaustion. Duration and magnitude of antigenic activation of T cells might be associated with up-regulation of inhibitory receptors, which is a major intrinsic factor of T cell exhaustion. Extrinsic factors might include the production of suppressive cytokines, T cell priming by either non-professional antigen-presenting cells (APCs) or tolerogenic dendritic cells (DCs), and alteration of regulatory T (Treg) cells. Further investigation of the cellular and molecular processes behind the development of T cell exhaustion can reveal therapeutic targets and strategies for the treatment of chronic infections and cancers. Here, we report the properties and the mechanisms of T cell exhaustion in a chronic environment.

Modulation of host innate and adaptive immune defenses by cytomegalovirus: timing is everything

Human cytomegalovirus (HCMV) (HHV-5, a beta-herpesvirus) causes the vast majority of infection-related congenital birth defects, and can trigger severe disease in immune suppressed individuals. The high prevalence of societal infection, the establishment of lifelong persistence and the growing number of immune-related diseases where HCMV is touted as a potential promoter is slowly heightening public awareness to this virus. The millions of years of co-evolution between CMV and the immune system of its host provides for a unique opportunity to study immune defense strategies, and pathogen counterstrategies. Dissecting the timing of the cellular and molecular processes that regulate innate and adaptive immunity to this persistent virus has revealed a complex defense network that is shaped by CMV immune modulation, resulting in a finely tuned host-pathogen relationship.

Oral infectious diseases: a potential risk factor for HIV virus recrudescence?

As the highly active antiretroviral therapy (HAART) has transitioned human immunodeficiency virus (HIV) infection into a 'chronic disease' management strategy, there is growing evidence that infection with non-HIV pathogens in HIV+ patients may have important public health implications in undermining HAART success and acquired immunodeficiency syndrome progression. Several bacterial and host cell products during infections with non-HIV pathogens have shown the capacity to regulate HIV replication in latently infected cells. A high prevalence of oral infections caused by bacteria, viruses and fungi has been described in HIV+ patients, including periodontal disease. The oral cavity appears to be a site of HIV pathogenesis and potential reservoir for the disease as HIV RNA and DNA forms are present in saliva as well as in gingival crevicular fluid, and oral epithelial cells are susceptible to either cell free or cell-associated HIV infection. The clinical and biological bases of potential associations between chronic oral inflammatory disorders, such as periodontal disease, and exacerbation of HIV viraemia have received little attention. This review attempts to evaluate the current understanding of HIV reactivation as a result of co-infection and/or inflammation induced by non-HIV pathogens in HIV-infected patients, and presents a hypothetic model about the potential role of periodontitis as a global oral infection that potentially contributes to HIV recrudescence.

Manipulating both the inhibitory and stimulatory immune system towards the success of therapeutic vaccination against chronic viral infections

SUMMARY

One potentially promising strategy to control chronic infections such as human immunodeficiency virus, hepatitis B virus, and hepatitis C virus is therapeutic vaccination, which aims to reduce persisting virus by stimulating a patient's own antiviral immune responses. However, this approach has fallen short of expectations, because antiviral T cells generated during chronic infections often become functionally exhausted and thus do not respond properly to therapeutic vaccination. Therefore, it is necessary to develop a therapeutic vaccine strategy to more effectively boost endogenous T-cell responses to control persistent viral infections. Studies to elucidate the cause of impaired T-cell function have pointed to sustained inhibitory receptor signaling through T-cell expression of programmed death 1 (PD-1). Recently, another inhibitory molecule, cytotoxic T lymphocyte antigen 4 (CTLA-4), and also an immunosuppressive cytokine, interleukin 10 (IL-10), have been reported to be potential factors of establishing immune suppression and viral persistence. Blocking these negative signaling pathways could restore the host immune system, enabling it to respond to further stimulation. Indeed, combining therapeutic vaccination along with the blockade of inhibitory signals could synergistically enhance functional CD8(+) T-cell responses and improve viral control in chronically infected mice, providing a promising strategy for the treatment of chronic viral infections. Furthermore, not only the ablation of negative signals but also the addition of stimulatory signals, such as interleukin 2 (IL-2), might prove to be a potentially promising strategy to augment the efficacy of therapeutic vaccination against chronic viral infections.

Apoptosis resistance in HIV-1 persistently-infected cells is independent of active viral replication and involves modulation of the apoptotic mitochondrial pathway

Background

HIV triggers the decline of CD4⁺ T cells and leads to progressive dysfunction of cell-mediated immunity. Although an increased susceptibility to cell death occurs during the acute phase of HIV infection, persistently-infected macrophages and quiescent T-cells seem to be resistant to cell death, representing a potential reservoir for virus production.

Results

Lymphoid (H9/HTLVIII_B and J1.1) and pro-monocytic (U1) HIV-1 persistently-infected cell lines were treated with hydrogen peroxide (H₂O₂) and staurosporine (STS) for 24 h, and susceptibility to apoptosis was evaluated and compared with uninfected counterparts (H9, Jurkat and U937 respectively). When exposed to different pro-apoptotic stimuli, all persistently-infected cell lines showed a dramatic reduction in the frequency of apoptotic cells in comparison with uninfected cells. This effect was independent of the magnitude of viral replication, since the induction of viral production in lymphoid or pro-monocytic cells by exposure to TNF-α or PMA did not significantly change their susceptibility to H₂O₂- or STS-induced cell death. A mechanistic analysis revealed significant diferences in mitochondrial membrane potential (MMP) and caspase-3 activation between uninfected and persistently-infected cells. In addition, Western blot assays showed a dramatic reduction of the levels of pro-apototic Bax in mitochondria of persistently-infected cells treated with H₂O₂ or STS, but not in uninfected cells.

Conclusion

This study represents the first evidence showing that resistance to apoptosis in persistently-infected lymphoid and monocytic cells is independent of active viral production and involves modulation of the mitochondrial pathway. Understanding this effect is critical to specifically target the persistence of viral reservoirs, and provide insights for future therapeutic strategies in order to promote complete viral eradication.

Dynamics of recurrent viral infection

In chronic viral infection, low levels of viral replication and infectious particle production are maintained over long periods, punctuated by brief bursts of high viral production and release. We apply well-established principles of modelling virus dynamics to the study of chronic viral infection, demonstrating that a model which incorporates the distinct contributions of cytotoxic T lymphocytes (CTLs) and antibodies exhibits long periods of quiescence followed by brief bursts of viral production. This suggests that for recurrent viral infections, no special mechanism or exogenous trigger is necessary to provoke an episode of reactivation; rather, the system may naturally cycle through recurrent episodes at intervals which can be many years long. We also find that exogenous factors which cause small fluctuations in the natural course of the infection can trigger a recurrent episode. Our model predicts that longer periods between recurrences are associated with more severe viral episodes. Four factors move the system towards less frequent, more severe episodes: decreased viral infectivity, decreased CTL efficacy, decreased memory T cell response and increased antibody efficacy.

Autoimmunity to nucleosomes related to viral infection: a focus on hapten-carrier complex formation

Systemic lupus erythematosus (SLE) is an autoimmune disorder with unknown aetiology. The major hallmark of this disease is the presence of antibodies against nuclear components, including double-stranded (ds)DNA and histones. The disease affects different organs, particularly the skin, kidneys and the nervous system. Although the exact molecular mechanisms underlying the pathophysiological processes in SLE remain unknown, several inherent and environmental factors seem to be involved in the ethiopathogenesis of this disorder. Viruses may be one of the factors that induce the production of autoreactive antibodies although the involved mechanisms are still incompletely understood. One proposed mechanism for virus-induced production of autoantibodies is molecular mimicry. Another mechanism derives from studies with the human polyomavirus BK. In these studies, in vivo binding of the polyomaviruses large T-antigen to chromatin of infected cells may render chromatin immunogenic. The large T-antigen-chromatin complex may thus function as a hapten-carrier model with subsequent production of anti-chromatin antibodies, including anti-dsDNA and anti-histones antibodies. This review focuses on the recent findings suggesting that this model may be applicable for other human viruses associated with SLE.

Viral mimicry of cytokines, chemokines and their receptors

Viruses have evolved elegant mechanisms to evade detection and destruction by the host immune system. One of the evasion strategies that have been adopted by large DNA viruses is to encode homologues of cytokines, chemokines and their receptors--molecules that have a crucial role in control of the immune response. Viruses have captured host genes or evolved genes to target specific immune pathways, and so viral genomes can be regarded as repositories of important information about immune processes, offering us a viral view of the host immune system. The study of viral immunomodulatory proteins might help us to uncover new human genes that control immunity, and their characterization will increase our understanding of not only viral pathogenesis, but also normal immune mechanisms. Moreover, viral proteins indicate strategies of immune modulation that might have therapeutic potential.

Epstein-Barr virus encoded interleukin-10 inhibits HLA-class I, ICAM-1, and B7 expression on human monocytes: implications for immune evasion by EBV

Monocytes and macrophages play a central role in viral infections, as a target for viruses and in activation of both innate and adaptive immune responses. Epstein-Barr virus (EBV) has evolved elaborate strategies to dampen the immune system and to persist within the host. There is evidence that the product of the BCRF-1 open reading frame of EBV, viral interleukin-10 (vIL-10), inhibits the capacity of monocytes/macrophages to induce T cell activation, but the full mechanism of this effect is unknown. To determine whether this effect might involve modulation of the expression of accessory molecules known to be important in T cell activation, we analyzed by flow cytometry the influence of vIL-10 on the basal as well as on IFN-gamma-induced up-regulation of HLA molecules, ICAM-1, and two members of the B7 family B7.1 (CD80) and B7.2 (CD86) at the surface of human monocytes. Viral IL-10 in a concentration-dependent manner inhibited both basal- and IFN-gamma-induced HLA-class II, ICAM-1 (basal levels of ICAM-2 and ICAM-3 is unaffected), CD80, and CD86 up-regulation when added simultaneously with IFN-gamma. In contrast, complete inhibition of HLA-class I expression on monocytes/macrophages occurred only when vIL-10 was present 2 h prior to the addition of IFN-gamma, implying that vIL-10 affects an early step in the IFN-gamma signaling pathway. As both monocytes and macrophages can be infected by EBV, we propose that vIL-10-mediated impairment of monocyte/macrophage antigen-presenting function could help the virus-infected cells to avoid detection by the host's T cells on one hand and contribute to its immunosuppressive properties on the other.

Hijacking and exploitation of IL-10 by intracellular pathogens

Macrophages play a central role in infections, as a target for pathogens and in activation of the immune system. Interleukin-10 (IL-10), a cytokine produced by macrophages, is a potent immunosuppressive factor. Some intracellular pathogens specifically target macrophages for infection and use IL-10 to dampen the host immune response and stall their elimination from the host. Certain viruses induce production of cellular IL-10 by macrophages, whereas other viruses encode their own viral IL-10 homologs. Additionally, specific bacteria, including several Mycobacteria spp. and Listeria monocytogenes, can survive and replicate in macrophages while inducing cellular IL-10, highlighting a potential role for IL-10 of macrophage origin in the immunosuppressive etiology of these pathogens. Thus, the exploitation of IL-10 appears to be a common mechanism of immunosuppression by a diverse group of intracellular pathogens that can infect macrophages.