Enhanced MHC class II-restricted presentation of measles virus (MV) hemagglutinin in transgenic mice expressing human MV receptor CD46

Induction of Proinflammatory Multiple Sclerosis-Associated Retrovirus Envelope Protein by Human Herpesvirus-6A and CD46 Receptor Engagement

The aberrant expression of human endogenous retrovirus (HERV) elements of the HERV-W family has been associated with different diseases, including multiple sclerosis (MS). In particular, the expression of the envelope protein (Env) from the multiple sclerosis-associated retrovirus (MSRV), a member of HERV-W family and known for its potent proinflammatory activity, is repeatedly detected in the brain lesions and blood of MS patients. Furthermore, human herpesvirus 6 (HHV-6) infection has long been suspected to play a role in the pathogenesis of MS and neuroinflammation. We show here that both HHV-6A and stimulation of its receptor, transmembrane glycoprotein CD46, induce the expression of MSRV-Env. The engagement of extracellular domains SCR3 and SCR4 of CD46-Cyt1 isoform was required for MSRV-env transactivation, limiting thus the MSRV-Env induction to the CD46 ligands binding these domains, including C3b component of complement, specific monoclonal antibodies, and both infectious and UV-inactivated HHV-6A, but neither HHV-6B nor measles virus vaccine strain. Induction of MSRV-Env required CD46 Cyt-1 singling and was abolished by the inhibitors of protein kinase C. Finally, both membrane-expressed and secreted MSRV-Env trigger TLR4 signaling, displaying thus a proinflammatory potential, characteristic for this viral protein. These data expand the specter of HHV-6A effects in the modulation of the immune response and support the hypothesis that cross-talks between exogenous and endogenous viruses may contribute to inflammatory diseases and participate in neuroinflammation. Furthermore, they reveal a new function of CD46, known as an inhibitor of complement activation and receptor for several pathogens, in transactivation of HERV env genes, which may play an important role in the pathogenesis of inflammatory diseases.

Host-Pathogen Interactions in Measles Virus Replication and Anti-Viral Immunity

The measles virus (MeV) is a contagious pathogenic RNA virus of the family Paramyxoviridae, genus Morbillivirus, that can cause serious symptoms and even fetal complications. Here, we summarize current molecular advances in MeV research, and emphasize the connection between host cells and MeV replication. Although measles has reemerged recently, the potential for its eradication is promising with significant progress in our understanding of the molecular mechanisms of its replication and host-pathogen interactions.

Human herpesvirus 6A infection in CD46 transgenic mice: viral persistence in the brain and increased production of proinflammatory chemokines via Toll-like receptor 9

Human herpesvirus 6 (HHV-6) is widely spread in the human population and has been associated with several neuroinflammatory diseases, including multiple sclerosis. To develop a small-animal model of HHV-6 infection, we analyzed the susceptibility of several lines of transgenic mice expressing human CD46, identified as a receptor for HHV-6. We showed that HHV-6A (GS) infection results in the expression of viral transcripts in primary brain glial cultures from CD46-expressing mice, while HHV-6B (Z29) infection was inefficient. HHV-6A DNA persisted for up to 9 months in the brain of CD46-expressing mice but not in the nontransgenic littermates, whereas HHV-6B DNA levels decreased rapidly after infection in all mice. Persistence in the brain was observed with infectious but not heat-inactivated HHV-6A. Immunohistological studies revealed the presence of infiltrating lymphocytes in periventricular areas of the brain of HHV-6A-infected mice. Furthermore, HHV-6A stimulated the production of a panel of proinflammatory chemokines in primary brain glial cultures, including CCL2, CCL5, and CXCL10, and induced the expression of CCL5 in the brains of HHV-6A-infected mice. HHV-6A-induced production of chemokines in the primary glial cultures was dependent on the stimulation of toll-like receptor 9 (TLR9). Finally, HHV-6A induced signaling through human TLR9 as well, extending observations from the murine model to human infection. Altogether, this study presents a first murine model for HHV-6A-induced brain infection and suggests a role for TLR9 in the HHV-6A-initiated production of proinflammatory chemokines in the brain, opening novel perspectives for the study of virus-associated neuropathology.

IMPORTANCE

HHV-6 infection has been related to neuroinflammatory diseases; however, the lack of a suitable small-animal infection model has considerably hampered further studies of HHV-6-induced neuropathogenesis. In this study, we have characterized a new model for HHV-6 infection in mice expressing the human CD46 protein. Infection of CD46 transgenic mice with HHV-6A resulted in long-term persistence of viral DNA in the brains of infected animals and was followed by lymphocyte infiltration and upregulation of the CCL5 chemokine in the absence of clinical signs of disease. The secretion of a panel of chemokines was increased after infection in primary murine brain glial cultures, and the HHV-6-induced chemokine expression was inhibited when TLR9 signaling was blocked. These results describe the first murine model for HHV-6A-induced brain infection and suggest the importance of the TLR9 pathway in HHV-6A-initiated neuroinflammation.

CD46: the 'multitasker' of complement proteins

Complement is undeniably quintessential for innate immunity by detecting and eliminating infectious microorganisms. Recent work, however, highlights an equally profound impact of complement on the induction and regulation of a wide range of immune cells. In particular, the complement regulator CD46 emerges as a key sensor of immune activation and a vital modulator of adaptive immunity. In this review, we summarize the current knowledge of CD46-mediated signalling events and their functional consequences on immune-competent cells with a specific focus on those in CD4(+) T cells. We will also discuss the promises and challenges that potential therapeutic modulation of CD46 may hold and pose.

CD46 measles virus receptor polymorphisms influence receptor protein expression and primary measles vaccine responses in naive Australian children

Despite the availability of measles vaccines, infants continue to die from measles. Measles vaccine responses vary between individuals, and poor immunogenicity is likely to preclude protection against measles. CD46 is a ubiquitously expressed specific receptor for vaccine strains of measles virus. CD46 polymorphisms have not been functionally investigated but may affect CD46 protein expression, which in turn may mediate primary measles antibody responses in infants. In a cohort of children aged 12 to 14 months from Perth, Australia (n = 137), after their first dose of measles-mumps-rubella (MMR) vaccine, CD46 polymorphisms were genotyped, and postvaccination measles IgG and CD46 protein expression before and after measles lysate stimulation of cells were measured. Three CD46 variants (rs7144, rs11118580, and rs2724384) were significantly associated with measles virus-specific IgG levels (P = 0.008, P = 0.026, and P = 0.018, respectively). There were significant differences between CD46 rs7144 genotypes and CD46 protein expression on T cells, as well as the downregulation of CD46 and T-cell frequency after measles lysate stimulation. We show that CD46 polymorphisms were associated with primary measles antibody responses in naive infants. We also report the first association of a measles virus receptor polymorphism with functional effects on the receptor, suggesting a possible mechanism through which antibody responses are altered. Elucidating all of the interconnecting genetic factors that alter primary measles vaccine responses may be important for identifying children at risk of poor immunogenicity or vaccine failure and for the future design of vaccine strategies to help these children.

Comparison of the immune responses induced by chimeric alphavirus-vectored and formalin-inactivated alum-precipitated measles vaccines in mice

A variety of vaccine platforms are under study for development of new vaccines for measles. Problems with past measles vaccines are incompletely understood and underscore the need to understand the types of immune responses induced by different types of vaccines. Detailed immune response evaluation is most easily performed in mice. Although mice are not susceptible to infection with wild type or vaccine strains of measles virus, they can be used for comparative evaluation of the immune responses to measles vaccines of other types. In this study we compared the immune responses in mice to a new protective alphavirus replicon particle vaccine expressing the measles virus hemagglutinin (VEE/SIN-H) with a non-protective formalin-inactivated, alum-precipitated measles vaccine (FI-MV). MV-specific IgG levels were similar, but VEE/SIN-H antibody was high avidity IgG2a with neutralizing activity while FI-MV antibody was low-avidity IgG1 without neutralizing activity. FI-MV antibody was primarily against the nucleoprotein with no priming to H. Germinal centers appeared, peaked and resolved later for FI-MV. Lymph node MV antibody-secreting cells were more numerous after FI-MV than VEE/SIN-H, but were similar in the bone marrow. VEE/SIN-H-induced T cells produced IFN-gamma and IL-4 both spontaneously ex vivo and after stimulation, while FI-MV-induced T cells produced IL-4 only after stimulation. In summary, VEE/SIN-H induced a balanced T cell response and high avidity neutralizing IgG2a while FI-MV induced a type 2 T cell response, abundant plasmablasts, late germinal centers and low avidity non-neutralizing IgG1 against the nucleoprotein.

Measles virus and CD46

Measles virus (MV) was isolated in 1954 (Enders and Peeble 1954). It is among the most contagious of viruses and a leading cause of mortality in children in developing countries (Murray and Lopez 1997; Griffin 2001; Bryce et al. 2005). Despite intense research over decades on the biology and pathogenesis of the virus and the successful development in 1963 of an effective MV vaccine (Cutts and Markowitz 1994), cell entry receptor(s) for MV remained unidentified until 1993. Two independent studies showed that transfection of nonsusceptible rodent cells with human CD46 renders these cells permissive to infection with the Edmonston and Halle vaccine strains of measles virus (Dorig et al. 1993; Naniche et al. 1993). A key finding in these investigations was that MV binding and infection was inhibited by monoclonal and polyclonal antibodies to CD46. These reports established CD46 as a MV cell entry receptor. This chapter summarizes the role of CD46 in measles virus infection.

Variations in measles vaccine–specific humoral immunity by polymorphisms in SLAM and CD46 measles virus receptors

BACKGROUND

Measles infection requires 2 cellular receptors, signaling lymphocyte activation molecule (SLAM) and CD46. Known and novel single nucleotide polymorphisms (SNPs) in SLAM and CD46 genes might influence the immune response to measles vaccine.

OBJECTIVE

We sought to identify SNP associations in SLAM and CD46 genes with variations in measles antibody response.

METHODS

We genotyped known SNPs in SLAM and CD46 genes in 339 subjects vaccinated with 2 doses of measles-mumps-rubella vaccine. We also sequenced the measles virus-binding domains of SLAM and CD46 to identify novel SNPs.

RESULTS

Increased representation of minor alleles for rs3796504 and rs164288 in the SLAM gene was associated with an allele dose-related decrease (4-fold) in measles-specific antibodies. Heterozygous genotype TC for rs12076998 located in the untranslated region 33 bp upstream of the measles virus-binding domain of the SLAM gene was associated with higher median antibody levels (1991 vs 1467 IU/L, P = .01) compared with wild-type TT. Within the CD46 gene, the minor allele C for intronic SNP (rs11118580) was associated with an allele dose-related decrease in measles antibodies (1072 vs 1795 IU/L, P < .01). Decreases in minor allele counts for rs3796504, rs164288, and rs1118580 demonstrated a significant (P < .001) additive effect on measles-specific antibodies.

CONCLUSION

Our data suggest that specific SNPs present in both the SLAM and CD46 genes are associated with measurable and significant variations in antibody response after measles vaccination.

CLINICAL IMPLICATIONS

Understanding the immunogenetics of measles vaccine receptors is important to better understand variations in immune responses to vaccines and to design better vaccines.

Monoclonal antibody detection of CD46 clustering beneath Neisseria gonorrhoeae microcolonies

CD46 (membrane cofactor protein), a complement-regulatory protein that participates in innate and acquired immunity, also serves as a receptor for viral and bacterial pathogens. CD46 isoforms terminate in one of two cytoplasmic tails, Cyt1 or Cyt2, which differ in signaling and trafficking properties. Dissecting the functions of the two cytoplasmic tails in these cellular processes has been hampered by the absence of specific reagents. Here we report the construction of Cyt1- and Cyt2-specific monoclonal antibodies (MAbs). These MAbs recognize unique epitopes within the tails and can be used for immunofluorescence microscopy, immunoblotting, and immunoprecipitation. Studies of Neisseria gonorrhoeae-infected cells with the CD46 tail MAbs demonstrate the differential recruitment of Cyt1 and Cyt2 to the cortical plaque.

Immunosuppression caused by measles virus: role of viral proteins

Measles virus (MV) causes transient but profound immunosuppression resulting in increased susceptibility to secondary bacterial and viral infections. Due to the development of these opportunistic infections, measles remains the leading vaccine-preventable cause of child death worldwide. Different immune abnormalities have been associated with measles, including disappearance of delayed-type hypersensitivity reactions, impaired lymphocyte and antigen-presenting cell functions, down-regulation of pro-inflammatory interleukin 12 production and altered interferon alpha/beta signalling pathways. Several MV proteins have been suggested to hinder immune functions: hemagglutinin, fusion protein, nucleoprotein and the non-structural V and C proteins. This review will focus on the novel functions attributed to MV proteins in the immunosuppression associated with measles. Here, we highlight new advances in the field, emphasising the interaction between MV proteins and their cellular targets, in particular the cell membrane receptors, CD46, CD150, TLR2 and FcgammaRII in the induction of immunological abnormalities associated with measles.

Emerging roles and new functions of CD46

In the past 20 years, our understanding of the workings of complement regulatory protein, CD46 (membrane cofactor protein), has grown as has the impressive list of pathogens interacting with this membrane-bound complement inhibitor. Referred to as a "pathogen magnet," CD46 serves as a receptor for seven human pathogens. Initially discovered as a widely expressed C3b- and C4b-binding protein, it was subsequently shown to be a cofactor for the serine protease factor I to inactivate by limited proteolysis these two opsonins and components of the convertases. The involvement of CD46 in reproductive processes continues to be an emerging story. It is a protector of placental tissue, but it may also play a more direct role in reproduction through its expression on the inner acrosomal membrane of spermatozoa. Cross-linking CD46 with antibodies or natural or pathogenic ligands induces rapid turnover and signaling events. In this regard, much attention is currently focused on generating human T lymphocyte regulatory cells by cross-linking CD46. Finally, highlighting its importance in protecting cells against excessive complement activation is the discovery that even a heterozygous deficiency of CD46 predisposes to hemolytic uremic syndrome.

Detection of T cell memory to measles virus in experimentally infected rhesus macaques by cytokine flow cytometry

A low, average frequency (0.61%) of measles virus (MV)-specific CD4 and CD8+ T cells was detected in rhesus monkeys experimentally infected with or vaccinated against MV. Both IFN-gamma and TNF-alpha positive T cells were visualized by flow cytometry. However, the conditions of short-term culture and stimulation to detect MV-specific T cells required significant modifications from a previously established method that reliably detects T cells in rhesus monkeys persistently infected with SIV. Both whole viral antigen and short synthetic peptide pools were an adequate antigenic stimulus. MV-specific T cells were detectable up to 11 years after exposure to the virus, although we cannot rule out possible subclinical re-exposure of the monkeys to vaccine virus during this time. Thus, flow cytometric methods can permit mechanistic studies of antigen-specific memory T cell dynamics following an acute viral infection in a primate model.

CD46: A complement regulator and pathogen receptor that mediates links between innate and acquired immune function

In the last 10 years, the human cell-surface molecule, CD46, has evolved from 'just another complement regulator' to a receptor for a striking array of pathogens. CD46 not only protects cells from complement-mediated attack and facilitates infection by a large number of pathogens, but also exerts complex effects on cellular immune function. It has been proposed that CD46 links innate and adaptive immunity by affecting cellular immune function in response to complement binding, and the role of CD46 in the pathogenesis of many infectious pathogens is now the subject of intense investigation. So far, the flood of information that implicates CD46 in modifying a host response to measles, Neisseria, human herpes virus 6, and pathogens that activate complement has not yet been matched with a comprehensive understanding of the molecular mechanisms by which CD46 affects immune function. This review summarizes the evidence that points to a significant role for CD46 in a range of pathological processes and describes how CD46 might exert its effects by altering signal transduction and antigen presentation pathways.

B cell receptors and complement receptors target the antigen to distinct intracellular compartments

The processing of exogenous Ags is an essential step for the generation of immunogenic peptides that will be presented to T cells. This processing relies on the efficient intracellular targeting of Ags, because it depends on the content of the compartments in which Ags are delivered in APCs. Opsonization of Ags by the complement component C3 strongly enhances their presentation by B cells and increases their immunogenicity in vivo. To investigate the role of C3 in the targeting of Ags, we compared the intracellular traffic of proteins internalized by complement receptor (CR) and B cell receptor (BCR) in B lymphocytes. Whereas both receptors are able to induce efficient Ag presentation, their intracellular pathways are different. CR ligand is delivered to compartments containing MHC class II molecules (MHC-II) but devoid of transferrin receptor and Lamp-2, whereas BCR rapidly targets its ligand toward Lamp-2-positive, late endosomal MHC-II-enriched compartments through intracellular vesicles containing transferrin receptor. CR and BCR are delivered to distinct endocytic pathways, and the kinetic evolution of the protein content of these pathways is very different. Both types of compartments contain MHC-II, but CR-targeted compartments receive less neosynthesized MHC-II than do BCR-targeted compartments. The targeting induced by CR toward compartments that are distinct from BCR-targeted compartments probably participates in C3 modulation of Ag presentation.

Ligand binding determines whether CD46 is internalized by clathrin-coated pits or macropinocytosis

CD46 is a ubiquitous human cell surface receptor for the complement components C3b and C4b and for various pathogens, including the measles virus and human herpes virus 6. Ligand binding to CD46 affects (i) protection of autologous cells from complement attack by breakdown of complement components, (ii) intracellular signals that affect the regulation of immune cell function, (iii) antigen presentation, and (iv) down-regulation of cell surface CD46. Recent evidence indicates that CD46 signaling can link innate and acquired immune function. The molecular mechanisms for these processes and the importance of intracellular trafficking of the receptor have not yet been elucidated. We demonstrate here that, in nonlymphoid cells, CD46 is constitutively internalized via clathrin-coated pits, traffics to multivesicular bodies, and is recycled to the cell surface. However, cross-linking of CD46 at the cell surface, by either multivalent antibody or by measles virus, induces pseudopodia that engulf the ligand in a process similar to macropinocytosis, and leads to the degradation of cell surface CD46. Thus, we have elucidated two pathways for CD46 internalization, which are regulated by the valence of cross-linking of CD46 and which utilize either clathrin-coated pits or pseudopodial extension. This has important implications for CD46 signaling, antigen presentation, CD46 down-regulation, and engulfment of pathogens.

T cell responses in acute measles

Immune containment of measles virus (MV) infection has long been a focus of interest for investigators. An emerging theme is that MV immunity is conferred by appropriately polarized antiviral CD4+ and CD8+ T cell populations. Recent technological advances permit the analysis of the composition and dynamics of these CD4+ and CD8+ T cell responses at the single cell level, and of the molecular events responsible for their induction. Novel insights into these issues for measles are discussed in the light of their importance for the development of an improved vaccine.

Immune and artificial selection in the haemagglutinin (H) glycoprotein of measles virus

We present a maximum likelihood (ML) analysis of the selection pressures that have shaped the evolution of the large (L) protein and the haemagglutinin (H) glycoprotein of measles virus (MV). A number of amino acid sites that have potentially been subject to adaptive evolution were identified in the H protein using sequences from every known genotype of MV. All but one of these putative positively selected sites reside within the ectodomain of the H protein, where they often show an association with positions of potential B-cell epitopes and sites known to interact with the CD46 receptor. This suggests that MV may be under pressure from the immune system, albeit relatively weakly, to alter sites within epitopes and hence evade the humoral immune response. The positive selection identified at amino acid 546 was shown to correlate with the passage history of MV isolates in Vero cells. We reveal that Vero cell passaging has the potential to introduce an artificial signal of adaptive evolution through selection for changes that increase affinity for the CD46 receptor.

The role of dendritic cells in the innate immune system

Dendritic cells (DCs) are bone-marrow-derived leucocytes that are specialised antigen-presenting cells capable of stimulating a primary T-lymphocyte response to specific antigen. In this chapter we discuss the role DCs play in the innate response acting as a critical link with the adaptive response and the influence of the innate response on dendritic cells.

Productive measles virus brain infection and apoptosis in CD46 transgenic mice

Measles virus (MV) infection causes acute childhood disease, associated in certain cases with infection of the central nervous system (CNS) and development of neurological disease. To develop a murine model of MV-induced pathology, we generated several lines of transgenic mice ubiquitously expressing as the MV receptor a human CD46 molecule with either a Cyt1 or Cyt2 cytoplasmic tail. All transgenic lines expressed CD46 protein in the brain. Newborn transgenic mice, in contrast to nontransgenic controls, were highly sensitive to intracerebral infection by the MV Edmonston strain. Signs of clinical illness (lack of mobility, tremors, and weight loss) appeared within 5 to 7 days after infection, followed by seizures, paralysis, and death of the infected animals. Virus replication was detected in neurons from infected mice, and virus was reproducibly isolated from transgenic brain tissue. MV-induced apoptosis observed in different brain regions preceded the death of infected animals. Similar results were obtained with mice expressing either a Cyt1 or Cyt2 cytoplasmic tail, demonstrating the ability of different isoforms of CD46 to function as MV receptors in vivo. In addition, maternally transferred immunity delayed death of offspring given a lethal dose of MV. These results document a novel CD46 transgenic murine model where MV neuronal infection is associated with the production of infectious virus, similarly to progressive infectious measles encephalitis seen in immunocompromised patients, and provide a new means to study pathogenesis of MV infection in the CNS.