Human herpesvirus 7 infection increases the expression levels of CD46 and CD59 in target cells

Relating GPI-Anchored Ly6 Proteins uPAR and CD59 to Viral Infection

The Ly6 (lymphocyte antigen-6)/uPAR (urokinase-type plasminogen activator receptor) superfamily protein is a group of molecules that share limited sequence homology but conserved three-fingered structures. Despite diverse cellular functions, such as in regulating host immunity, cell adhesion, and migration, the physiological roles of these factors in vivo remain poorly characterized. Notably, increasing research has focused on the interplays between Ly6/uPAR proteins and viral pathogens, the results of which have provided new insight into viral entry and virus-host interactions. While LY6E (lymphocyte antigen 6 family member E), one key member of the Ly6E/uPAR-family proteins, has been extensively studied, other members have not been well characterized. Here, we summarize current knowledge of Ly6/uPAR proteins related to viral infection, with a focus on uPAR and CD59. Our goal is to provide an up-to-date view of the Ly6/uPAR-family proteins and associated virus-host interaction and viral pathogenesis.

Relative Contribution of Cellular Complement Inhibitors CD59, CD46, and CD55 to Parainfluenza Virus 5 Inhibition of Complement-Mediated Neutralization

The complement system is a part of the innate immune system that viruses need to face during infections. Many viruses incorporate cellular regulators of complement activation (RCA) to block complement pathways and our prior work has shown that Parainfluenza virus 5 (PIV5) incorporates CD55 and CD46 to delay complement-mediated neutralization. In this paper, we tested the role of a third individual RCA inhibitor CD59 in PIV5 interactions with complement pathways. Using a cell line engineered to express CD59, we show that small levels of functional CD59 are associated with progeny PIV5, which is capable of blocking assembly of the C5b-C9 membrane attack complex (MAC). PIV5 containing CD59 (PIV5-CD59) showed increased resistance to complement-mediated neutralization in vitro comparing to PIV5 lacking regulators. Infection of A549 cells with PIV5 and RSV upregulated CD59 expression. TGF-beta treatment of PIV5-infected cells also increased cell surface CD59 expression and progeny virions were more resistant to complement-mediated neutralization. A comparison of individual viruses containing only CD55, CD46, or CD59 showed a potency of inhibiting complement-mediated neutralization, which followed a pattern of CD55 > CD46 > CD59.

Complement Evasion Strategies of Viruses: An Overview

Being a major first line of immune defense, the complement system keeps a constant vigil against viruses. Its ability to recognize large panoply of viruses and virus-infected cells, and trigger the effector pathways, results in neutralization of viruses and killing of the infected cells. This selection pressure exerted by complement on viruses has made them evolve a multitude of countermeasures. These include targeting the recognition molecules for the avoidance of detection, targeting key enzymes and complexes of the complement pathways like C3 convertases and C5b-9 formation - either by encoding complement regulators or by recruiting membrane-bound and soluble host complement regulators, cleaving complement proteins by encoding protease, and inhibiting the synthesis of complement proteins. Additionally, viruses also exploit the complement system for their own benefit. For example, they use complement receptors as well as membrane regulators for cellular entry as well as their spread. Here, we provide an overview on the complement subversion mechanisms adopted by the members of various viral families including Poxviridae, Herpesviridae, Adenoviridae, Flaviviridae, Retroviridae, Picornaviridae, Astroviridae, Togaviridae, Orthomyxoviridae and Paramyxoviridae.

Modulation of host CD59 expression by varicella-zoster virus in human xenografts in vivo

Varicella-zoster virus (VZV) is the causative agent of both chickenpox (varicella) and shingles (zoster). VZV survives host defenses, even with an intact immune system, and disseminates in the host before causing disease. To date, several diverse immunomodulatory strategies used by VZV to undermine host immunity have been identified; however, few studies have addressed the complement evasion strategies used by this virus. Here, we show that expression of CD59, which is a key member of host regulators of complement activation (RCA), is significantly upregulated in response to VZV infection in human T cells and dorsal root ganglia (DRG) but not in human skin xenografts in SCID-hu mice in vivo. This is the first report demonstrating that VZV infection upregulates host CD59 expression in a tissue-specific manner in vivo, which may aid VZV in complement evasion and pathogenesis.

Viral regulators of complement activation: structure, function and evolution

The complement system surveillance in the host is effective in controlling viral propagation. Consequently, to subvert this effector mechanism, viruses have developed a series of adaptations. One among these is encoding mimics of host regulators of complement activation (RCA) which help viruses to avoid being labeled as 'foreign' and protect them from complement-mediated neutralization and complement-enhanced antiviral adaptive immunity. In this review, we provide an overview on the structure, function and evolution of viral RCA proteins.

Human herpesvirus 6 glycoprotein complex formation is required for folding and trafficking of the gH/gL/gQ1/gQ2 complex and its cellular receptor binding

Human herpesvirus 6 (HHV-6) is a T-cell-tropic betaherpesvirus. A glycoprotein (g) complex that is unique to HHV-6, gH/gL/gQ1/gQ2, is a viral ligand for its cellular receptor, human CD46. However, whether complex formation or one component of the complex is required for CD46 binding and how the complex is transported in cells are open questions. Furthermore, in HHV-6-infected cells the gQ1 protein modified with N-linked glycans is expressed in two forms with different molecular masses: an 80-kDa form (gQ1-80K) and a 74-kDa form (gQ1-74K). Only gQ1-80K, but not gQ1-74K, forms the complex with gQ2, gH, and gL, and this four-component complex is incorporated into mature virions. Here, we characterized the molecular context leading to the maturation of gQ1 by expressing combinations of the individual gH/gL/gQ1/gQ2 components in 293T cells. Surprisingly, only when all four molecules were expressed was a substantial amount of gQ1-80K detected, indicating that all three of the other molecules (gQ2, gH, and gL) were necessary and sufficient for gQ1 maturation. We also found that only the tetrameric complex, and not its subsets, binds to CD46. Finally, a gQ2-null virus constructed in the BAC (bacterial artificial chromosome) system could not be reconstituted, indicating that gQ2 is essential for virus growth. These results show that gH, gL, gQ1, and gQ2 are all essential for the trafficking and proper folding of the gH/gL/gQ1/gQ2 complex and, thus, for HHV-6 infection.

Dynamic control of the complement system by modulated expression of regulatory proteins

The complement system serves many biological functions, including the eradication of invasive pathogens and the removal of damaged cells and immune-complexes. Uncontrolled complement activation causes injury to host cells, however, so adequate regulation of the system is essential. Control of the complement system is maintained by a group of cell surface and circulating proteins referred to as complement regulatory proteins. The expression of the cell surface complement regulatory proteins varies from tissue to tissue. Furthermore, specific cell types can upregulate or downregulate the expression of these proteins in response to a variety of signals or insults. Altered regulation of the complement regulatory proteins can have important effects on local complement activation. In some circumstances this can be beneficial, such as in the setting of certain infections. In other circumstances, however, this can be a cause of complement-mediated injury of the tissue. A full understanding of the mechanisms by which the complement system is modulated at the local level can have important implications for how we diagnose and treat a wide range of inflammatory diseases.

Virus–complement interactions: an assiduous struggle for dominance

The complement system is a major component of the innate immune system that recognizes invading pathogens and eliminates them by means of an array of effector mechanisms, in addition to using direct lytic destruction. Viruses, in spite of their small size and simple composition, are also deftly recognized and neutralized by the complement system. In turn, as a result of years of coevolution with the host, viruses have developed multiple mechanisms to evade the host complement. These complex interactions between the complement system and viruses have been an area of focus for over three decades. In this article, we provide a broad overview of the field using key examples and up-to-date information on the complement-evasion strategies of viruses.

Human herpesvirus 7

Az emberi 7-es herpeszvírus 1990 óta ismert, közeli rokonságban áll a 6-os herpeszvírussal, annak B változatával. Csak emberi sejtekben szaporodik, receptora a CD4 molekula. A fertőzött sejtek egy részében élethossziglan lappang, gyakran reaktiválódik és a nyálban tünetmentesen ürül. Gyermekek egy része 3–4 éves korára tünetmentesen fertőződik, de minden életkorban találhatók szeronegatív egyének, akik fogékonyak a fertőzés iránt. Gyermekekben ritkán exanthema subitum, múló lázas-görcsös állapotok, fiatal felnőttekben rózsahámlás, immunszuppresszált egyénekben a reaktiválódott 6-os B herpeszvírussal és cytomegalovirussal egyetemben halálos szövődmények alakulhatnak ki. Egyéb vírusokat is aktiválhat más kórképekben. A vírus patogenezisében a legfontosabb, hogy megváltozik a fertőzött lymphocytákból kiáramló citokinek és növekedési faktorok egyensúlya, amely láncreakcióként hat az immunrendszer és egyéb szervek sejtjeire. A vírusellenes antitestek kimutatása kereskedelmi forgalomban kapható készletekkel (immunfluoreszcencia, ELISA, immunoblot), a nukleinsav kimutatása fészkes polimeráz láncreakcióval lehetséges. A fertőzés következtében kialakult betegségek legtöbbje nem igényel kezelést, de súlyos szövődmények esetén ganciclovir és származékai, valamint foscarnet és cidofovir alkalmazható.

Hepatitis B virus sensitizes hepatocytes to complement-dependent cytotoxicity through downregulating CD59

Hepatitis B virus (HBV) infection afflicts over 350 million people worldwide and is a leading cause of hepatitis, cirrhosis and hepatocellular carcinoma. HBV replicates noncytopathically in hepatocytes, and most of the hepatic injury is caused by the immune response to the virus. While most studies focused on the adaptive immune response, the role of the innate immune response, especially the complement activation, in HBV infection remains obscure. To identify proteins that are involved in the pathogenesis of HBV infection, we carried out gene microarray analysis to compare the gene expression profile of HBV transgenic BALB/c mice with that of control mice. CD59 mRNA, which encodes an important complement regulatory protein (CRP) expressed on cell surface, was found to be significantly downregulated in HBV transgenic liver, a result that was further confirmed by RT-PCR and real-time PCR. To explore the relationship between CD59 and HBV infection, we examined the effect of HBV on CD59 expression and complement-dependent cytolysis in two hepatocyte cell lines. We found that HBV could significantly downregulate CD59 expression and sensitize cells to complement-dependent lysis. Blocking CD59 function using a CD59-specific antibody greatly diminished the HBV effect. Similar CD59 downregulation was also observed in the livers of patients with chronic HBV infection. These results demonstrate that HBV can sensitize hepatocytes to complement-dependent cytotoxicity (CDC) through downregulating CD59, which may lead to the activation of complement system and cause liver inflammation.

Complement and its role in protection and pathogenesis of flavivirus infections

The complement system is a family of serum and cell surface proteins that recognize pathogen-associated molecular patterns, altered-self ligands, and immune complexes. Activation of the complement cascade triggers several antiviral functions including pathogen opsonization and/or lysis, and priming of adaptive immune responses. In this review, we will examine the role of complement activation in protection and/or pathogenesis against infection by Flaviviruses, with an emphasis on experiments with West Nile and Dengue viruses.

T-cell regulation by CD46 and its relevance in multiple sclerosis

CD46 is a complement regulatory molecule expressed on every cell type, except for erythrocytes. While initially described as a regulator of complement activity, it later became a 'magnet for pathogens', binding to several viruses and bacteria. More recently, an alternative role for such complement molecules has emerged: they do regulate T-cell immunity, affecting T-cell proliferation and differentiation. In particular, CD46 stimulation induces Tr1 cells, regulatory T cells characterized by massive production of interleukin-10 (IL-10), a potent anti-inflammatory cytokine. Hence, CD46 is likely to control inflammation. Indeed, data from CD46 transgenic mice highlight a role for CD46 in inflammation, with antagonist roles depending on the cytoplasmic tail being expressed. Furthermore, recent data have shown that CD46 is defective in multiple sclerosis, IL-10 production being severely impaired in these patients. This lack of IL-10 production probably participates in the inflammation observed in patients with multiple sclerosis. This review will summarize the data on CD46 and T cells, and how CD46 is likely involved in multiple sclerosis.

Role of complement in immune regulation and its exploitation by virus

Complement is activated during the early phase of viral infection and promotes destruction of virus particles as well as the initiation of inflammatory responses. Recently, complement and complement receptors have been reported to play an important role in the regulation of innate as well as adaptive immune responses during infection. The regulation of host immune responses by complement involves modulation of dendritic cell activity in addition to direct effects on T-cell function. Intriguingly, many viruses encode homologs of complement regulatory molecules or proteins that interact with complement receptors on antigen-presenting cells and lymphocytes. The evolution of viral mechanisms to alter complement function may augment pathogen persistence and limit immune-mediated tissue destruction. These observations suggest that complement may play an important role in both innate and adaptive immune responses to infection as well as virus-mediated modulation of host immunity.