Measles virus receptor SLAM (CD150)

Infectious and lifestyle modifiers of immunity and host resilience

The interindividual heterogeneity of the immune system likely determines the personal risk for acquiring infections and developing diseases with inflammatory components. In addition to genetic factors, the immune system's heterogeneity is driven by diverging exposures of leukocytes and their progenitors to infections, vaccinations, and health behavior, including lifestyle-related stimuli such as diet, physical inactivity, and psychosocial stress. We review how such experiences alter immune cell responses to concurrent and subsequent challenges, leading to either improved host resilience or disease susceptibility due to a muted or overzealous immune system, with a primary focus on the contribution of innate immune cells. We explore the involvement of diverse mechanisms, including trained immunity, and their relevance for infections and cardiovascular disease, as these prevalent conditions are heavily influenced by immune cell abundance and phenotypic adaptions. Understanding the mechanistic bases of immune modulations by prior or co-exposures may lead to new therapies targeting dysfunctional inflammation.

A durable protective immune response to wild-type measles virus infection of macaques is due to viral replication and spread in lymphoid tissues

Infection with wild-type (WT) measles virus (MeV) is an important cause of childhood mortality that leads to lifelong protective immunity in survivors. WT MeV and the live-attenuated MeV used in the measles vaccine (LAMV) are antigenically similar, but the determinants of attenuation are unknown, and protective immunity induced by LAMV is less robust than that induced by WT MeV. To identify factors that contribute to these differences, we compared virologic and immunologic responses after respiratory infection of rhesus macaques with WT MeV or LAMV. In infected macaques, WT MeV replicated efficiently in B and T lymphocytes with spreading throughout lymphoid tissues resulting in prolonged persistence of viral RNA. In contrast, LAMV replicated efficiently in the respiratory tract but displayed limited spread to lymphoid tissue or peripheral blood mononuclear cells. In vitro, WT MeV and LAMV replicated similarly in macaque primary respiratory epithelial cells and human lymphocytes, but LAMV-infected lymphocytes produced little virus. Plasma concentrations of interleukin-1β (IL-1β), IL-12, interferon-γ (IFN-γ), CCL2, CCL11, CXCL9, and CXCL11 increased in macaques after WT MeV but not LAMV infection. WT MeV infection induced more protective neutralizing, hemagglutinin-specific antibodies and bone marrow plasma cells than did LAMV infection, although numbers of MeV-specific IFN-γ- and IL-4-producing T cells were comparable. Therefore, MeV attenuation may involve altered viral replication in lymphoid tissue that limited spread and decreased the host antibody response, suggesting a link between lifelong protective immunity and the ability of WT MeV, but not LAMV, to spread in lymphocytes.

Measles Vaccine

Measles remains an important cause of child morbidity and mortality worldwide despite the availability of a safe and efficacious vaccine. The current measles virus (MeV) vaccine was developed empirically by attenuation of wild-type (WT) MeV by in vitro passage in human and chicken cells and licensed in 1963. Additional passages led to further attenuation and the successful vaccine strains in widespread use today. Attenuation is associated with decreased replication in lymphoid tissue, but the molecular basis for this restriction has not been identified. The immune response is age dependent, inhibited by maternal antibody (Ab) and involves induction of both Ab and T cell responses that resemble the responses to WT MeV infection, but are lower in magnitude. Protective immunity is correlated with levels of neutralizing Ab, but the actual immunologic determinants of protection are not known. Because measles is highly transmissible, control requires high levels of population immunity. Delivery of the two doses of vaccine needed to achieve >90% immunity is accomplished by routine immunization of infants at 9-15 months of age followed by a second dose delivered before school entry or by periodic mass vaccination campaigns. Because delivery by injection creates hurdles to sustained high coverage, there are efforts to deliver MeV vaccine by inhalation. In addition, the safety record for the vaccine combined with advances in reverse genetics for negative strand viruses has expanded proposed uses for recombinant versions of measles vaccine as vectors for immunization against other infections and as oncolytic agents for a variety of tumors.

Biophysical Properties and Antiviral Activities of Measles Fusion Protein Derived Peptide Conjugated with 25-Hydroxycholesterol

Measles virus (MV) infection is re-emerging, despite the availability of an effective vaccine. The mechanism of MV entry into a target cell relies on coordinated action between the MV hemagglutinin (H) receptor binding protein and the fusion envelope glycoprotein (F) which mediates fusion between the viral and cell membranes. Peptides derived from the C-terminal heptad repeat (HRC) of F can interfere with this process, blocking MV infection. As previously described, biophysical properties of HRC-derived peptides modulate their antiviral potency. In this work, we characterized a MV peptide fusion inhibitor conjugated to 25-hydroxycholesterol (25HC), a cholesterol derivative with intrinsic antiviral activity, and evaluated its interaction with membrane model systems and human blood cells. The peptide (MV.

In Vivo Efficacy of Measles Virus Fusion Protein-Derived Peptides Is Modulated by the Properties of Self-Assembly and Membrane Residence

Measles virus (MV) infection is undergoing resurgence and remains one of the leading causes of death among young children worldwide despite the availability of an effective measles vaccine. MV infects its target cells by coordinated action of the MV hemagglutinin (H) and fusion (F) envelope glycoproteins; upon receptor engagement by H, the prefusion F undergoes a structural transition, extending and inserting into the target cell membrane and then refolding into a postfusion structure that fuses the viral and cell membranes. By interfering with this structural transition of F, peptides derived from the heptad repeat (HR) regions of F can inhibit MV infection at the entry stage. In previous work, we have generated potent MV fusion inhibitors by dimerizing the F-derived peptides and conjugating them to cholesterol. We have shown that prophylactic intranasal administration of our lead fusion inhibitor efficiently protects from MV infection in vivo We show here that peptides tagged with lipophilic moieties self-assemble into nanoparticles until they reach the target cells, where they are integrated into cell membranes. The self-assembly feature enhances biodistribution and the half-life of the peptides, while integration into the target cell membrane increases fusion inhibitor potency. These factors together modulate in vivo efficacy. The results suggest a new framework for developing effective fusion inhibitory peptides.

IMPORTANCE

Measles virus (MV) infection causes an acute illness that may be associated with infection of the central nervous system (CNS) and severe neurological disease. No specific treatment is available. We have shown that fusion-inhibitory peptides delivered intranasally provide effective prophylaxis against MV infection. We show here that specific biophysical properties regulate the in vivo efficacy of MV F-derived peptides.

Flexibility of the Head-Stalk Linker Domain of Paramyxovirus HN Glycoprotein Is Essential for Triggering Virus Fusion

The Paramyxoviridae comprise a large family of enveloped, negative-sense, single-stranded RNA viruses with significant economic and public health implications. For nearly all paramyxoviruses, infection is initiated by fusion of the viral and host cell plasma membranes in a pH-independent fashion. Fusion is orchestrated by the receptor binding protein hemagglutinin-neuraminidase (HN; also called H or G depending on the virus type) protein and a fusion (F) protein, the latter undergoing a major refolding process to merge the two membranes. Mechanistic details regarding the coupling of receptor binding to F activation are not fully understood. Here, we have identified the flexible loop region connecting the bulky enzymatically active head and the four-helix bundle stalk to be essential for fusion promotion. Proline substitution in this region of HN of parainfluenza virus 5 (PIV5) and Newcastle disease virus HN abolishes cell-cell fusion, whereas HN retains receptor binding and neuraminidase activity. By using reverse genetics, we engineered recombinant PIV5-EGFP viruses with mutations in the head-stalk linker region of HN. Mutations in this region abolished virus recovery and infectivity. In sum, our data suggest that the loop region acts as a "hinge" around which the bulky head of HN swings to-and-fro to facilitate timely HN-mediate F-triggering, a notion consistent with the stalk-mediated activation model of paramyxovirus fusion.

IMPORTANCE

Paramyxovirus fusion with the host cell plasma membrane is essential for virus infection. Membrane fusion is orchestrated via interaction of the receptor binding protein (HN, H, or G) with the viral fusion glycoprotein (F). Two distinct models have been suggested to describe the mechanism of fusion: these include "the clamp" and the "provocateur" model of activation. By using biochemical and reverse genetics tools, we have obtained strong evidence in favor of the HN stalk-mediated activation of paramyxovirus fusion. Specifically, our data strongly support the notion that the short linker between the head and stalk plays a role in "conformational switching" of the head group to facilitate F-HN interaction and triggering.

Biosafety considerations for attenuated measles virus vectors used in virotherapy and vaccination

Attenuated measles virus (MV) is one of the most effective and safe vaccines available, making it attractive candidate vector to prevent infectious diseases. Attenuated MV have acquired the ability to use the complement regulator CD46 as a major receptor to mediate virus entry and intercellular fusion. Therefore, attenuated MV strains preferentially infect and destroy a wide variety of cancer cells making them also attractive oncolytic vectors. The use of recombinant MV vector has to comply with various regulatory requirements, particularly relating to the assessment of potential risks for human health and the environment. The present article highlights the main characteristics of MV and recombinant MV vectors used for vaccination and virotherapy and discusses these features from a biosafety point of view.

A chicken homologue of nectin-4 functions as a measles virus receptor

Measles virus (MV) vaccine strains use CD46, signaling lymphocyte activation molecule, and nectin-4 in human cells as receptors. Meanwhile, many of them are propagated in primary chicken embryonic fibroblasts (CEFs). Our data revealed that CEFs express a nectin-4 homologous molecule (CEF nectin-4) containing well-conserved motifs in the FG and BC loops, but not in the C'C″ loop. MV infected CHO cells expressing CEF nectin-4 and induced syncytia in these cells, confirming that CEF nectin-4 functions as an MV receptor and that the C'C″ loop is not critical for this function. Nectin-4-blind mutations in MV H protein reduced the infectivity of MV in CEF nectin-4-expressing cells. Infection of CEFs with the MV vaccine AIK-C strain was partially blocked by an anti-nectin-4 antibody, indicating that CEF nectin-4 plays a role for propagation of MV vaccines in CEFs.

Prevention of measles virus infection by intranasal delivery of fusion inhibitor peptides

Measles virus (MV) infection is undergoing resurgence and remains one of the leading causes of death among young children worldwide despite the availability of an effective measles vaccine. MV infects its target cells by coordinated action of the MV H and the fusion (F) envelope glycoprotein; upon receptor engagement by H, the prefusion F undergoes a structural transition, extending and inserting into the target cell membrane and then refolding into a postfusion structure that fuses the viral and cell membranes. By interfering with this structural transition of F, peptides derived from the heptad-repeat (HR) regions of F can potently inhibit MV infection at the entry stage. We show here that specific features of H's interaction with its receptors modulate the susceptibility of MV F to peptide fusion inhibitors. A higher concentration of inhibitory peptides is required to inhibit F-mediated fusion when H is engaged to its nectin-4 receptor than when H is engaged to its CD150 receptor. Peptide inhibition of F may be subverted by continued engagement of receptor by H, a finding that highlights the ongoing role of H-receptor interaction after F has been activated and that helps guide the design of more potent inhibitory peptides. Intranasal administration of these peptides results in peptide accumulation in the airway epithelium with minimal systemic levels of peptide and efficiently prevents MV infection in vivo in animal models. The results suggest an antiviral strategy for prophylaxis in vulnerable and/or immunocompromised hosts.

IMPORTANCE

Measles virus (MV) infection causes an acute illness that may be associated with infection of the central nervous system (CNS) and severe neurological disease. No specific treatment is available. We have shown that parenterally delivered fusion-inhibitory peptides protect mice from lethal CNS MV disease. Here we show, using established small-animal models of MV infection, that fusion-inhibitory peptides delivered intranasally provide effective prophylaxis against MV infection. Since the fusion inhibitors are stable at room temperature, this intranasal strategy is feasible even outside health care settings, could be used to protect individuals and communities in case of MV outbreaks, and could complement global efforts to control measles.

Probing the paramyxovirus fusion (F) protein-refolding event from pre- to postfusion by oxidative footprinting

To infect a cell, the Paramyxoviridae family of enveloped viruses relies on the coordinated action of a receptor-binding protein (variably HN, H, or G) and a more conserved metastable fusion protein (F) to effect membrane fusion and allow genomic transfer. Upon receptor binding, HN (H or G) triggers F to undergo an extensive refolding event to form a stable postfusion state. Little is known about the intermediate states of the F refolding process. Here, a soluble form of parainfluenza virus 5 F was triggered to refold using temperature and was footprinted along the refolding pathway using fast photochemical oxidation of proteins (FPOP). Localization of the oxidative label to solvent-exposed side chains was determined by high-resolution MS/MS. Globally, metastable prefusion F is oxidized more extensively than postfusion F, indicating that the prefusion state is more exposed to solvent and is more flexible. Among the first peptides to be oxidatively labeled after temperature-induced triggering is the hydrophobic fusion peptide. A comparison of peptide oxidation levels with the values of solvent-accessible surface area calculated from molecular dynamics simulations of available structural data reveals regions of the F protein that lie at the heart of its prefusion metastability. The strong correlation between the regions of F that experience greater-than-expected oxidative labeling and epitopes for neutralizing antibodies suggests that FPOP has a role in guiding the development of targeted therapeutics. Analysis of the residue levels of labeled F intermediates provides detailed insights into the mechanics of this critical refolding event.

Vaxfectin adjuvant improves antibody responses of juvenile rhesus macaques to a DNA vaccine encoding the measles virus hemagglutinin and fusion proteins

DNA vaccines formulated with the cationic lipid-based adjuvant Vaxfectin induce protective immunity in macaques after intradermal (i.d.) or intramuscular (i.m.) delivery of 0.5 to 1 mg of codon-optimized DNA encoding the hemagglutinin (H) and fusion (F) proteins of measles virus (MeV). To characterize the effect of Vaxfectin at lower doses of H+F DNA, rhesus macaques were vaccinated twice with 20 μg of DNA plus Vaxfectin i.d., 100 μg of DNA plus Vaxfectin i.d., 100 μg of DNA plus Vaxfectin i.m. or 100 μg of DNA plus phosphate-buffered saline (PBS) i.m. using a needleless Biojector device. The levels of neutralizing (P = 0.036) and binding (P = 0.0001) antibodies were higher after 20 or 100 μg of DNA plus Vaxfectin than after 100 μg of DNA plus PBS. Gamma interferon (IFN-γ)-producing T cells were induced more rapidly than antibody, but were not improved with Vaxfectin. At 18 months after vaccination, monkeys were challenged with wild-type MeV. None developed rash or viremia, but all showed evidence of infection. Antibody levels increased, and IFN-γ- and interleukin-17-producing T cells, including cells specific for the nucleoprotein absent from the vaccine, were induced. At 3 months after challenge, MeV RNA was detected in the leukocytes of two monkeys. The levels of antibody peaked 2 to 4 weeks after challenge and then declined in vaccinated animals reflecting low numbers of bone marrow-resident plasma cells. Therefore, Vaxfectin was dose sparing and substantially improved the antibody response to the H+F DNA vaccine. This immune response led to protection from disease (rash/viremia) but not from infection. Antibody responses after challenge were more transient in vaccinated animals than in an unvaccinated animal.

Measles virus, immune control, and persistence

Measles remains one of the most important causes of child morbidity and mortality worldwide with the greatest burden in the youngest children. Most acute measles deaths are owing to secondary infections that result from a poorly understood measles-induced suppression of immune responses. Young children are also vulnerable to late development of subacute sclerosing panencephalitis, a progressive, uniformly fatal neurologic disease caused by persistent measles virus (MeV) infection. During acute infection, the rash marks the appearance of the adaptive immune response and CD8(+) T cell-mediated clearance of infectious virus. However, after clearance of infectious virus, MeV RNA persists and can be detected in blood, respiratory secretions, urine, and lymphoid tissue for many weeks to months. This prolonged period of virus clearance may help to explain measles immunosuppression and the development of lifelong immunity to re-infection, as well as occasional infection of the nervous system. Once MeV infects neurons, the virus can spread trans-synaptically and the envelope proteins needed to form infectious virus are unnecessary, accumulate mutations, and can establish persistent infection. Identification of the immune mechanisms required for the clearance of MeV RNA from multiple sites will enlighten our understanding of the development of disease owing to persistent infection.

Measles virus glycoprotein-pseudotyped lentiviral vectors are highly superior to vesicular stomatitis virus G pseudotypes for genetic modification of monocyte-derived dendritic cells

Dendritic cells (DCs) are potent antigen-presenting cells capable of promoting or regulating innate and adaptive immune responses against non-self antigens. To better understand the DC biology or to use them for immune intervention, a tremendous effort has been made to improve gene transfer in these cells. Lentiviral vectors (LVs) have conferred a huge advantage in that they can transduce nondividing cells such as human monocyte-derived DCs (MDDCs) but required high amounts of viral particles and/or accessory proteins such as Vpx or Vpr to achieve sufficient transduction rates. As a consequence, these LVs have been shown to cause dramatic functional modifications, such as the activation or maturation of transduced MDDCs. Taking advantage of new pseudotyped LVs, i.e., with envelope glycoproteins from the measles virus (MV), we demonstrate that MDDCs are transduced very efficiently with these new LVs compared to the classically used vesicular stomatitis virus G-pseudotyped LVs and thus allowed to achieve high transduction rates at relatively low multiplicities of infection. Moreover, in this experimental setting, no activation or maturation markers were upregulated, while MV-LV-transduced cells remained able to mature after an appropriate Toll-like receptor stimulation. We then demonstrate that our MV-pseudotyped LVs use DC-SIGN, CD46, and CD150/SLAM as receptors to transduce MDDCs. Altogether, our results show that MV-pseudotyped LVs provide the most accurate and simple viral method for efficiently transferring genes into MDDCs without affecting their activation and/or maturation status.

The association of CD46, SLAM and CD209 cellular receptor gene SNPs with variations in measles vaccine-induced immune responses: a replication study and examination of novel polymorphisms

BACKGROUND

The measles virus (MV) interacts with two known cellular receptors: CD46 and SLAM. The transmembrane receptor CD209 interacts with MV and augments dendritic cell infection.

METHODS

764 subjects previously immunized with measles-mumps-rubella vaccine were genotyped for 66 candidate SNPs in the CD46, SLAM and CD209 genes as part of a larger study.

RESULTS

A previously detected association of the CD46 SNP rs2724384 with measles-specific antibodies was successfully replicated in this study. Increased representation of the minor allele G for an intronic CD46 SNP was associated with an allele dose-related decrease (978 vs. 522 mIU/ml, p = 0.0007) in antibody levels. This polymorphism rs2724384 also demonstrated associations with IL-6 (p = 0.02), IFN-α (p = 0.007) and TNF-α (p = 0.0007) responses. Two polymorphisms (coding rs164288 and intronic rs11265452) in the SLAM gene that were associated with measles antibody levels in our previous study were associated with IFN-γ Elispot (p = 0.04) and IL-10 responses (p = 0.0008), respectively, in this study. We found associations between haplotypes, AACGGAATGGAAAG (p = 0.009) and GGCCGAGAGGAGAG (p < 0.001), in the CD46 gene and TNF-α secretion.

CONCLUSION

Understanding the functional and mechanistic consequences of these genetic polymorphisms on immune response variations could assist in directing new measles and potentially other viral vaccine design, and in better understanding measles immunogenetics.

Canine distemper virus utilizes different receptors to infect chicken embryo fibroblasts and vero cells

Inducing animal viruses to adapt to chicken embryos or chicken embryo fibroblasts (CEF) is a common method to develop attenuated live vaccines with full security. Canine distemper virus (CDV) also does this, but the mechanisms and particular receptors remain unclear. Virus overlay protein blot assays were carried out on CEF membrane proteins, which were extracted respectively with a Mem-PER™ kit, a radioimmunoprecipitation assay buffer or a modified co-immunoprecipitation method, and revealed a common 57 kDa positive band that differed from the 42-kDa positive band in Vero cells and also from those receptors reported in lymphocytes and 293 cells, indicating a receptor diversity of CDV and the possibility of the 57-kDa protein acting as a receptor that is involved in adaptive infection of CDV Kunming strain to CEF.

[Measles vaccine]

Further attenuated live measles vaccine strains were developed through passages in chick embryo cells or other non-permissive cells from the Edmonston strain. The number of measles patients has reduced through worldwide acceptance of measles vaccine. Measles elimination was achieved in American continents and the goal of measles elimination in Western Pacific region was aimed by 2012. Recent development of molecular techniques facilitates the reverse genetics to recover the infectious virus from the cDNA clone constructed from measles RNA genome. Using this technology, characteristics of attenuated measles vaccine strain were investigated and new approach has started to develop the recombinant measles vaccine expressing foreign virus antigen(s) against the infectious diseases for which no effective vaccine is available. Besides infectious diseases, the oncolytic measles virus based on measles vaccine strains was developed for targeting cancer cells.

Adaptation of wild-type measles virus to cotton rat lung cells: E89K mutation in matrix protein contributes to its fitness

Wild-type measles virus (wtMeV) adapted well to cotton rat lung (CRL) cells after serial passages. In order to evaluate the contributions of the individual genes of wtMeV for adaptation, whole genome sequences of the adapted and original viruses were determined and analyzed. The results showed that there were two mutations in the whole genome of the adapted virus. One mutation was located at the 265th nucleotide in the open reading frame (ORF) of the M gene, resulting in the substitution of the 89th amino acid from E (glutamate) to K (lysine). The other was a silent mutation located at the 4182nd nucleotide in the ORF of the L gene. It was demonstrated that the E89K mutation in the M protein is responsible for the adaptation of wtMeV MV99Y in CRL cells. Cotton rats were infected with adapted virus and the original strain via intranasal inoculation. Virus titer results showed that adapted strain replicated better than the original strain in cotton rat lungs. It is suggested that the E89K mutation also contributes to the enhancement of wtMeV replication in a cotton rat model infected intranasally. The results revealed that the E89K mutation in the M protein plays a key role in wtMeV adaptation in cotton rat and CRL cells.

Contribution of matrix, fusion, hemagglutinin, and large protein genes of the CAM-70 measles virus vaccine strain to efficient growth in chicken embryonic fibroblasts

Attenuated live vaccines of measles virus (MV) have been developed from clinical isolates by serial propagation in heterologous cells, mainly chicken embryonic cells. The safety and effectiveness of these vaccines have been well established. However, the molecular mechanism of their attenuation remains a subject of investigation. The CAM-70 MV vaccine strain was developed from the Tanabe strain by serial propagation in chicken embryonic cells. In the present study, we assessed the contribution of each gene in the CAM-70 strain to efficient growth in chicken embryonic fibroblasts (CEF). We used a cloned MV IC323 based on the wild-type IC-B strain and generated a series of IC323s that possess one or more of the CAM-70 genes. Then, we examined the infection of CEF and CEF expressing human signaling lymphocyte activation molecule with the recombinant MVs. Our results demonstrated that MV needs to adapt to CEF at both the entry and postentry steps and that the CAM-70 matrix protein gene plays an important role in adaptation to CEF at the early stage of the virus replication cycle. The CAM-70 large protein gene was responsible for the efficient transcription and replication in CEF, and the CAM-70 hemagglutinin and fusion protein genes were responsible for efficient entry. Investigations focusing on these genes might elucidate unknown molecular mechanisms underlying the attenuation of MV.

Sequence analysis of morbillivirus CD150 receptor-Signaling Lymphocyte Activation Molecule (SLAM) of different animal species

Signaling Lymphocyte Activation Molecule-SLAM (CD150) molecule has been reported as a putative receptor for most morbilliviruses for their respective host species. In this study, we determined the complete nucleotide sequence of the gene coding for the morbillivirus receptor-SLAM from the four species, namely, goat (Capra hircus), sheep (Ovis aries), Indian cattle (Bos indicus), and buffalo (Bubalus bubalis). The nucleotide (nt) open reading frame sequence of SLAM gene in all the four species studied was 1017 nucleotides in length encoding a polypeptide of 339 amino acids (aa), similar to Bos taurus, but different from canine, human, marmoset, and mouse SLAM, which were 1029, 1008, 1011, and 1032 nts, respectively, in length, and coding for 343, 336, 337, and 344 aa, respectively. Sequence analysis revealed 96.3-98.5% and 92.9-96.8% identities among the four species at the nt and aa level, respectively. Sequence diversity at aa level between various species revealed that the critical functional region of SLAM protein among different species is relatively conserved, thereby facilitating this molecule to act as a receptor for morbillivirus. Phylogenetic relationship based on the aa sequences of SLAM protein revealed that caprine, ovine, cattle, and buffalo fall under a defined cluster but caprine SLAM is more closely related to ovine, followed by bovine.

Measles virus-induced immunosuppression

Immunosuppression is the major cause of infant death associated with acute measles and therefore of substantial clinical importance. Major hallmarks of this generalized modulation of immune functions are (1) lymphopenia, (2) a prolonged cytokine imbalance consistent with suppression of cellular immunity to secondary infections, and (3) silencing of peripheral blood lymphocytes, which cannot expand in response to ex vivo stimulation. Lymphopenia results from depletion, which can occur basically at any stage of lymphocyte development, and evidently, expression of the major MV receptor CD150 plays an important role in targeting these cells. Virus transfer to T cells is thought to be mediated by dendritic cells (DCs), which are considered central to the induction of T cell silencing and functional skewing. As a consequence of MV interaction, viability and functional differentiation of DCs and thereby their expression pattern of co-stimulatory molecules and soluble mediators are modulated. Moreover, MV proteins expressed by these cells actively silence T cells by interfering with signaling pathways essential for T cell activation.

Adaptation to cell culture induces functional differences in measles virus proteins

Background

Live, attenuated measles virus (MeV) vaccine strains were generated by adaptation to cell culture. The genetic basis for the attenuation of the vaccine strains is unknown. We previously reported that adaptation of a pathogenic, wild-type MeV to Vero cells or primary chicken embryo fibroblasts (CEFs) resulted in a loss of pathogenicity in rhesus macaques. The CEF-adapted virus (D-CEF) contained single amino acid changes in the C and matrix (M) proteins and two substitutions in the shared amino terminal domain of the phosphoprotein (P) and V protein. The Vero-adapted virus (D-VI) had a mutation in the cytoplasmic tail of the hemagglutinin (H) protein.

Results

In vitro assays were used to test the functions of the wild-type and mutant proteins. The substitution in the C protein of D-CEF decreased its ability to inhibit mini-genome replication, while the wild-type and mutant M proteins inhibited replication to the same extent. The substitution in the cytoplasmic tail of the D-VI H protein resulted in reduced fusion in a quantitative fusion assay. Co-expression of M proteins with wild-type fusion and H proteins decreased fusion activity, but the mutation in the M protein of D-CEF did not affect this function. Both mutations in the P and V proteins of D-CEF reduced the ability of these proteins to inhibit type I and II interferon signaling.

Conclusion

Adaptation of a wild-type MeV to cell culture selected for genetic changes that caused measurable functional differences in viral proteins.

Stable transduction of quiescent T cells without induction of cycle progression by a novel lentiviral vector pseudotyped with measles virus glycoproteins

A major limitation of current lentiviral vectors (LVs) is their inability to govern efficient gene transfer into quiescent cells such as primary T cells, which hampers their application for gene therapy. Here we generated high-titer LVs incorporating Edmonston measles virus (MV) glycoproteins H and F on their surface. They allowed efficient transduction through the MV receptors, SLAM and CD46, both present on blood T cells. Indeed, these H/F-displaying vectors outperformed by far VSV-G-LVs for the transduction of IL-7-prestimulated T cells. More importantly, a single exposure to these H/F-LVs allowed efficient gene transfer in quiescent T cells, which are not permissive for VSV-G-LVs that need cell-cycle entry into the G1b phase for efficient transduction. High-level transduction of resting memory (50%) and naive (11%) T cells with H/F-LVs, which seemed to occur mainly through SLAM, was not at cost of cell-cycle entry or of target T-cell activation. Finally, the naive or memory phenotypes of transduced resting T cells were maintained and no changes in cytokine profiles were detected, suggesting that T-cell populations were not skewed. Thus, H/F-LV transduction of resting T cells overcomes the limitation of current lentiviral vectors and may improve the efficacy of T cell-based gene therapy.

Genetic changes that affect the virulence of measles virus in a rhesus macaque model

To identify genetic changes that lead to the attenuation of measles virus (MV), a strain of MV that is pathogenic in rhesus macaques was adapted to grow in Vero cells, Vero/hSLAM cells and, to simulate the process used to derive live attenuated vaccines, in primary chicken embryo fibroblasts (CEF). Comparison of the complete genomic sequences of the pathogenic wild-type (Davis87-wt) and four cell culture-adapted strains derived from it showed complete conservation of sequence in the Vero/hSLAM-passaged virus. Viruses adapted to Vero cells and CEF had predicted amino acid changes in the nucleocapsid protein, phosphoprotein, V protein, C protein, matrix protein, and the cytoplasmic tail of the hemagglutinin protein. All four cell culture-adapted strains, including the Vero/hSLAM cell-passaged virus, were able to productively infect Vero cells, but the peak viral titers differed. The Vero cell-adapted strains were unable to replicate in Chinese Hamster Ovary cells expressing CD46, indicating that they had not adapted to use the CD46 receptor. The Vero/hSLAM cell-passaged virus retained pathogenicity in rhesus macaques as measured by the appearance of a skin rash while the Vero cell-adapted and CEF-adapted strains had lost the ability to cause a rash. There were no significant differences in viral titers in peripheral blood mononuclear cells among monkeys infected with any of the viral stocks tested. These results identify a limited number of genetic changes in the genome of MV that lead to attenuation in vivo.

Structural basis of viral invasion: lessons from paramyxovirus F

The structures of glycoproteins that mediate enveloped virus entry into cells have revealed dramatic structural changes that accompany membrane fusion and provided mechanistic insights into this process. The group of class I viral fusion proteins includes the influenza hemagglutinin, paramyxovirus F, HIV env, and other mechanistically related fusogens, but these proteins are unrelated in sequence and exhibit clearly distinct structural features. Recently determined crystal structures of the paramyxovirus F protein in two conformations, representing pre-fusion and post-fusion states, reveal a novel protein architecture that undergoes large-scale, irreversible refolding during membrane fusion, extending our understanding of this diverse group of membrane fusion machines.

Regulation of cellular and humoral immune responses by the SLAM and SAP families of molecules

SAP (SLAM-associated protein) was identified in 1998 as an adaptor molecule involved in the intracellular signaling pathways elicited through the cell surface receptor SLAM and as the protein defective in the human immunodeficiency X-linked lymphoproliferative disease (XLP). During the past eight years, it has been established that the SLAM family of cell surface receptors (SLAM, 2B4, NTB-A, Ly9, CD84) and the SAP family of adaptors (SAP, EAT-2, ERT) play critical roles in lymphocyte development, differentiation, and acquisition of effector functions. Studies of these proteins have shown unexpected roles in cytokine production by T cells and myeloid cells, T cell-dependent humoral immune responses, NK cell-mediated cytotoxicity, and NKT cell development. This review highlights recent findings that have improved our understanding of the roles of the SLAM and SAP families of molecules in immune regulation and discusses how perturbations in the signaling pathways involving these proteins can result in different disease states.

Measles virus M and F proteins associate with detergent-resistant membrane fractions and promote formation of virus-like particles

Assembly and release of particles comprise a late step in virus-host cell interactions. Though it may share major biological properties with its orthologues in related viruses, trafficking and oligomerization of the matrix (M) protein of Measles virus (MV) and its relative contribution to assembly and budding of particles from particular host cells have not been addressed in more detail. Plasmid-driven expression of authentic and mutant M proteins revealed that the amino acid at position 89, an important adaptation determinant for growth of attenuated strains in Vero cells, influences the electrophoretic mobility but not the intracellular distribution of M proteins, nor their ability to oligomerize or migrate as a doublet band in SDS-PAGE. M proteins were found to co-float with detergent-resistant membrane fractions (DRM) and this was enhanced upon co-expression of the F protein. In contrast to their DRM association, the ability of M proteins to promote release of virus-like particles (VLPs) was not affected by the presence of F proteins, which on their own also efficiently promoted VLP production. Thus, DRM recruitment of MV F and M proteins and their ability to drive particle formation are not correlated.

SAP regulation of follicular helper CD4 T cell development and humoral immunity is independent of SLAM and Fyn kinase

Mutations in SH2D1A resulting in lack of SLAM-associated protein (SAP) expression cause the human genetic immunodeficiency X-linked lymphoproliferative disease. A severe block in germinal center development and lack of long-term humoral immunity is one of the most prominent phenotypes of SAP(-) mice. We show, in this study, that the germinal center block is due to an essential requirement for SAP expression in Ag-specific CD4 T cells to develop appropriate follicular helper T cell functions. It is unknown what signaling molecules are involved in regulation of SAP-dependent CD4 T cell help functions. SAP binds to the cytoplasmic tail of SLAM, and we show that SLAM is expressed on resting and activated CD4 T cells, as well as germinal center B cells. In addition, SAP can recruit Fyn kinase to SLAM. We have now examined the role(s) of the SLAM-SAP-Fyn signaling axis in in vivo CD4 T cell function and germinal center development. We observed normal germinal center development, long-lived plasma cell development, and Ab responses in SLAM(-/-) mice after a viral infection (lymphocytic choriomeningitis virus). In a separate series of experiments, we show that SAP is absolutely required in CD4 T cells to drive germinal center development, and that requirement does not depend on SAP-Fyn interactions, because CD4 T cells expressing SAP R78A are capable of supporting normal germinal center development. Therefore, a distinct SAP signaling pathway regulates follicular helper CD4 T cell differentiation, separate from the SLAM-SAP-Fyn signaling pathway regulating Th1/Th2 differentiation.

Multiple amino acid substitutions in hemagglutinin are necessary for wild-type measles virus to acquire the ability to use receptor CD46 efficiently

Measles virus (MV) possesses two envelope glycoproteins, namely, the receptor-binding hemagglutinin (H) and fusion proteins. Wild-type MV strains isolated in B-lymphoid cell lines use signaling lymphocyte activation molecule (SLAM), but not CD46, as a cellular receptor, whereas MV vaccine strains of the Edmonston lineage use both SLAM and CD46 as receptors. Studies have shown that the residue at position 481 of the H protein is critical in determining the use of CD46 as a receptor. However, the wild-type IC-B strain with a single N481Y substitution in the H protein utilizes CD46 rather inefficiently. In this study, a number of chimeric and mutant H proteins, and recombinant viruses harboring them, were generated to determine which residues of the Edmonston H protein are responsible for its efficient use of CD46. Our results show that three substitutions (N390I and E492G plus N416D or T446S), in addition to N481Y, are necessary for the IC-B H protein to use CD46 efficiently as a receptor. The N390I, N416D, and T446S substitutions are present in the H proteins of all strains of the Edmonston lineage, whereas the E492G substitution is found only in the H protein of the Edmonston tag strain generated from cDNAs. The T484N substitution, found in some of the Edmonston-lineage strains, resulted in a similar effect on the use of CD46 to that caused by the E492G substitution. Thus, multiple residues in the H protein that have not previously been implicated have important roles in the interaction with CD46.

Refolding of a paramyxovirus F protein from prefusion to postfusion conformations observed by liposome binding and electron microscopy

For paramyxoviruses, two viral glycoproteins are key to the entry process: an attachment protein (HN, H, or G) and the fusion protein (F). The F protein folds to a metastable state that can be triggered to undergo large conformational rearrangements to a fusogenic intermediate and a more stable postfusion state. The triggering mechanism that controls paramyxovirus fusion has not been elucidated. To correlate the molecular structure of a soluble form of the prefusion F (PIV5 F-GCNt) with the biological function of F, soluble F protein was triggered to refold. In the absence of HN, heat was found to function as a surrogate F trigger, and F associated with liposomes and aggregated on sucrose density gradients. Electron microscopy data showed that triggered F formed rosettes. Taken together these data suggest that release and membrane insertion of the hydrophobic fusion peptide require both cleavage of F and heat. Heating of cleaved F causes conversion to a postfusion form as judged by its "golf tee" morphology in the electron microscope. Heating of uncleaved F also causes conversion of F to a morphologically similar form. The reactivity of the F protein with conformation-specific mAbs and peptide binding suggest that soluble F-GCNt and membrane-bound F proteins refold through a comparable pathway.

Adaptation of canine distemper virus to canine footpad keratinocytes modifies polymerase activity and fusogenicity through amino acid substitutions in the P/V/C and H proteins

The wild-type canine distemper virus (CDV) strain A75/17 induces a non-cytocidal infection in cultures of canine footpad keratinocytes (CFKs) but produces very little progeny virus. After only three passages in CFKs, the virus produced 100-fold more progeny and induced a limited cytopathic effect. Sequence analysis of the CFK-adapted virus revealed only three amino acid differences, of which one was located in each the P/V/C, M and H proteins. In order to assess which amino acid changes were responsible for the increase of infectious virus production and altered phenotype of infection, we generated a series of recombinant viruses. Their analysis showed that the altered P/V/C proteins were responsible for the higher levels of virus progeny formation and that the amino acid change in the cytoplasmic tail of the H protein was the major determinant of cytopathogenicity.

Measles virus: cellular receptors, tropism and pathogenesis

Measles virus(MV), a member of the genusMorbillivirusin the familyParamyxoviridae, is an enveloped virus with a non-segmented, negative-strand RNA genome. It has two envelope glycoproteins, the haemagglutinin (H) and fusion proteins, which are responsible for attachment and membrane fusion, respectively. Human signalling lymphocyte activation molecule (SLAM; also called CD150), a membrane glycoprotein of the immunoglobulin superfamily, acts as a cellular receptor for MV. SLAM is expressed on immature thymocytes, activated lymphocytes, macrophages and dendritic cells and regulates production of interleukin (IL)-4 and IL-13 by CD4+T cells, as well as production of IL-12, tumour necrosis factor alpha and nitric oxide by macrophages. The distribution of SLAM is in accord with the lymphotropism and immunosuppressive nature of MV.Canine distemper virusandRinderpest virus, other members of the genusMorbillivirus, also use canine and bovine SLAM as receptors, respectively. Laboratory-adapted MV strains may use the ubiquitously expressed CD46, a complement-regulatory molecule, as an alternative receptor through amino acid substitutions in the H protein. Furthermore, MV can infect SLAM−cells, albeit inefficiently, via the SLAM- and CD46-independent pathway, which may account for MV infection of epithelial, endothelial and neuronal cellsin vivo. MV infection, however, is not determined entirely by the H protein–receptor interaction, and other MV proteins can also contribute to its efficient growth by facilitating virus replication at post-entry steps. Identification of SLAM as the principal receptor for MV has provided us with an important clue for better understanding of MV tropism and pathogenesis.

Measles virus induces expression of SIP110, a constitutively membrane clustered lipid phosphatase, which inhibits T cell proliferation

Interference of measles virus (MV) with phosphatidyl-inositol-3-kinase (PI3K) activation in response to T cell receptor ligation was identified as important for the induction of T cell paralysis. We now show that MV exposure of unstimulated T cells induces expression of SIP110, an isoform of the lipid phosphatase SHIP145, which is translated from an intron-derived sequences containing mRNA. We found that MV contact can regulate stimulated exon inclusion into pre-mRNAs by targeting PI3K or MAPK-dependent nuclear translocation and activation of splicing regulatory serine-arginine rich (SR) and Sam68 proteins. Induction of SIP110 in resting T cells relied on MV-dependent interference with basal activity of the PI3K. SIP110 was cloned from MV-exposed T cells, and, when transiently expressed in primary or Jurkat T cells, localized into membrane clusters independently of T cell activation. Confirming that SIP110 is a catalytically active lipid phosphatase, its transgenic expression abolished basal and impaired PMA/ionomycin-stimulated phosphorylation of the Akt kinase which is important for T cell proliferation. Thus MV causes induction of SIP110 expression, which constitutively depletes the cellular phosphoinositol-3,4,5-phosphate pool suggesting that thereby the threshold for activation signals necessary for the induction of T cell proliferation is raised.

Receptor-independent infection of murine coronavirus: analysis by spinoculation

A highly neurovirulent murine coronavirus JHMV (wild-type [wt] JHMV) is known to spread from cells infected via the murine coronavirus mouse hepatitis virus receptor (MHVR) to cells without MHVR (MHVR-independent infection), whereas a mutant virus isolated from wt JHMV, srr7, spread only in an MHVR-dependent fashion. These observations were obtained by the overlay of JHMV-infected cells onto receptor-negative cells that are otherwise resistant to wt JHMV infection. MHVR-independent infection is hypothetically thought to be attributed to a naturally occurring fusion activation of the wt JHMV S protein, which did not occur in the case of srr7. Attachment of S protein on cells without MHVR during the S-protein activation process seems to be a key condition. Thus, in the present study, we tried to see whether wt JHMV virions that are attached on MHVR-negative cells are able to infect those cells. In order to make virions attach to the cell surface without MHVR, we have used spinoculation, namely, the centrifugation of cells together with inoculated virus at 3,000 rpm for 2 h. This procedure forces viruses to attach to the cell surface, as revealed by quantitative estimation of attached virions by real-time PCR and also facilitated wt JHMV infection to MHVR-negative cells, but failed to do so for srr7. Virions of both wt and srr7 attached on MHVR-negative cells by spinoculation were facilitated for infection in the presence of a soluble form of MHVR that induces conformational changes of both wt and srr7. It was further revealed that wt JHMV S1, but not srr7, was released from the cell surface when S protein was expressed on cells. These observations support the hypothesis that attachment of the virion to MHVR-negative cells is a critical step and that a unique feature of wt JHMV S1 to be released from S2 in a naturally occurring event is involved in an MHVR-independent infection.

Silencing T cells or T-cell silencing: concepts in virus-induced immunosuppression

The ability to evade or suppress the host's immune response is a property of many viruses, indicating that this provides an advantage for the pathogen to spread efficiently or even to establish a persistent infection. The type and complexity of its genome and cell tropism but also its preferred type of host interaction are important parameters which define the strategy of a given virus to modulate the immune system in an optimal manner. Because they take a central position in any antiviral defence, the activation and function of T cells are the predominant target of many viral immunosuppressive regimens. In this review, two different strategies whereby this could be achieved are summarized. Retroviruses can infect professional antigen-presenting cells and impair their maturation and functional properties. This coincides with differentiation and expansion of silencing T cells referred to as regulatory T cells with suppressive activity, mainly to CD8+ effector T cells. The second concept, outlined for measles virus, is a direct, contact-mediated silencing of T cells which acquire a transient paralytic state.

Recombinant wild-type measles virus containing a single N481Y substitution in its haemagglutinin cannot use receptor CD46 as efficiently as that having the haemagglutinin of the Edmonston laboratory strain

Signalling lymphocyte activation molecule (SLAM) acts as a cellular receptor for Measles virus (MV). The recombinant MV, based on a SLAM-using clinical isolate in which asparagine at position 481 of the haemagglutinin was replaced with tyrosine, was generated. Characterization of this recombinant virus revealed that the N481Y substitution in the haemagglutinin allowed it to utilize CD46 as an alternative receptor, but that its ability to use CD46 was rather low in CD46+ SLAM− cell lines compared with that of the recombinant virus possessing the haemagglutinin of the Edmonston laboratory strain. Thus, an N481Y substitution alone may not be sufficient to make SLAM-using MVs use CD46 efficiently, suggesting that further substitutions in the haemagglutinin are required for them to grow efficiently in CD46+ cells like the Edmonston strain. This may be a reason why few CD46-using MVs are detected in vivo.

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.

Contributions of matrix and large protein genes of the measles virus edmonston strain to growth in cultured cells as revealed by recombinant viruses

The Edmonston strain of measles virus (MV) was obtained by sequential passages of the original isolate in various cultured cells. Although attenuated in vivo, it grows efficiently in most primate cell lines. Previous studies have revealed that MV tropism cannot be solely explained by the use of CD150 and/or CD46 as a cellular receptor. In order to evaluate the contributions of individual genes of the Edmonston strain to growth in cultured cells, we generated a series of recombinant viruses in which part of the genome of the clinical isolate IC-B (which uses CD150 as a receptor) was replaced with the corresponding sequences of the Edmonston strain. The recombinant virus possessing the Edmonston hemagglutinin (H) gene (encoding the receptor-binding protein) grew as efficiently in Vero cells as the Edmonston strain. Those viruses having either the matrix (M) or large (L) protein gene from the Edmonston strain could also replicate well in Vero cells, although they entered them at low efficiencies. P64S and E89K substitutions were responsible for the ability of the M protein to make virus grow efficiently in Vero cells, while the first half of the Edmonston L gene was important for better replication. Despite efficient growth in Vero cells, the recombinant viruses with these mutations had growth disadvantage in CD150-positive lymphoid B95a cells. Thus, not only the H gene but also the M and L genes contribute to efficient replication of the Edmonston strain in some cultured cells.

Role of sialic acid-containing molecules in paramyxovirus entry into the host cell: A minireview

Sialic acid-containing compounds play a key role in the initial steps of the paramyxovirus life cycle. As enveloped viruses, their entry into the host cell consists of two main events: binding to the host cell and membrane fusion. Virus adsorption occurs at the surface of the host cell with the recognition of specific receptor molecules located at the cell membrane by specific viral attachment proteins. The viral attachment protein present in some paramyxoviruses (Respirovirus, Rubulavirus and Avulavirus) is the HN glycoprotein, which binds to cellular sialic acid-containing molecules and exhibits sialidase and fusion promotion activities. Gangliosides of the gangliotetraose series bearing the sialic acid N-acetylneuraminic (Neu5Ac) on the terminal galactose attached in alpha2-3 linkage, such as GD1a, GT1b, and GQ1b, and neolacto-series gangliosides are the major receptors for Sendai virus. Much less is known about the receptors for other paramyxoviruses than for Sendai virus. Human parainfluenza viruses 1 and 3 preferentially recognize oligosaccharides containing N-acetyllactosaminoglycan branches with terminal Neu5Acalpha2-3Gal. In the case of Newcastle disease virus, has been reported the absence of a specific pattern of the gangliosides that interact with the virus. Additionally, several works have described the use of sialylated glycoproteins as paramyxovirus receptors. Accordingly, the design of specific sialic acid analogs to inhibit the sialidase and/or receptor binding activity of viral attachment proteins is an important antiviral strategy. In spite of all these data, the exact nature of paramyxovirus receptors, apart from their sialylated nature, and the mechanism(s) of viral attachment to the cell surface are poorly understood.

Long untranslated regions of the measles virus M and F genes control virus replication and cytopathogenicity

Measles is still a major cause of mortality mainly in developing countries. The causative agent, measles virus (MeV), is an enveloped virus having a nonsegmented negative-sense RNA genome, and belongs to the genus Morbillivirus of the family Paramyxoviridae. One feature of the moribillivirus genomes is that the M and F genes have long untranslated regions (UTRs). The M and F mRNAs of MeV have 426-nucleotide-long 3' and 583-nucleotide-long 5' UTRs, respectively. Though these long UTRs occupy as much as approximately 6.4% of the virus genome, their function remains unknown. To elucidate the role of the long UTRs in the context of virus infection, we used the reverse genetics based on the virulent strain of MeV, and generated a series of recombinant viruses having alterations or deletions in the long UTRs. Our results showed that these long UTRs per se were not essential for MeV replication, but that they regulated MeV replication and cytopathogenicity by modulating the productions of the M and F proteins. The long 3' UTR of the M mRNA was shown to have the ability to increase the M protein production, promoting virus replication. On the other hand, the long 5' UTR of the F mRNA was found to possess the capacity to decrease the F protein production, inhibiting virus replication and yet greatly reducing cytopathogenicity. We speculate that the reduction in cytopathogenicity may be advantageous for MeV fitness and survival in nature.

Structure of the parainfluenza virus 5 F protein in its metastable, prefusion conformation

Enveloped viruses have evolved complex glycoprotein machinery that drives the fusion of viral and cellular membranes, permitting entry of the viral genome into the cell. For the paramyxoviruses, the fusion (F) protein catalyses this membrane merger and entry step, and it has been postulated that the F protein undergoes complex refolding during this process. Here we report the crystal structure of the parainfluenza virus 5 F protein in its prefusion conformation, stabilized by the addition of a carboxy-terminal trimerization domain. The structure of the F protein shows that there are profound conformational differences between the pre- and postfusion states, involving transformations in secondary and tertiary structure. The positions and structural transitions of key parts of the fusion machinery, including the hydrophobic fusion peptide and two helical heptad repeat regions, clarify the mechanism of membrane fusion mediated by the F protein.

Efficient rescue of measles virus from cloned cDNA using SLAM-expressing Chinese hamster ovary cells

We here report a highly efficient reverse genetics system for measles virus (MeV), using Chinese hamster ovary cells constitutively expressing a MeV receptor human signaling lymphocyte activation molecule (CHO/hSLAM cells). The recombinant vaccinia virus vTF7-3 that encodes the T7 RNA polymerase under the control of the early/late promoter was used in the system. Replication of vTF7-3 was highly restricted in CHO/hSLAM cells, but the virus could still drive the T7 promoter, allowing us to recover MeV from the transfected cDNA efficiently. With this system the number of infectious centers, in which MeV replication cycles are initiated from transfected cDNAs, was approximately 100 times higher than that with the previous system (. J. Virol. 74, 6643-6647), and the recovery rate was 100%. The wild-type MeV that encodes the lac-Z gene of approximately 3.2kb in length, was easily generated with this CHO/hSLAM system, while such virus could not be recovered with the previous system. Since SLAM acts as a cellular receptor for both MeV vaccine and wild-type strains, the Edmonston vaccine strain was also recovered with this system more efficiently than with any other systems reported previously. Thus, the CHO/hSLAM-based system would expand applications of the MeV reverse genetics by allowing productions of mutant MeVs that have been difficult to generate with less efficient systems.

Viral entry

Virus entry is initiated by recognition by receptors present on the surface of host cells. Receptors can be major mediators of virus tropism, and in many cases receptor interactions occur in an apparently programmed series of events utilizing multiple receptors. After receptor interaction, both enveloped and nonenveloped viruses must deliver their genome across either the endosomal or plasma membrane for infection to proceed. Genome delivery occurs either by membrane fusion (in the case of enveloped viruses) or by pore formation or other means of permeabilizing the lipid bilayer (in the case of nonenveloped viruses). For those viruses that enter cells via endosomes, specific receptor interactions (and the signaling events that ensue) may control the particular route of endocytosis and/or the ultimate destination of the incoming virus particles. Our conception of virus entry is increasingly becoming more complex; however, the specificity involved in entry processes, once ascertained, may ultimately lead to the production of effective antiviral agents.

New insights into the molecular mechanism of interleukin-10-mediated immunosuppression

Interleukin-10 (IL-10) is an important immunomodulatory cytokine, which has attracted much attention because of its anti-inflammatory properties. It reduces antigen presentation and inhibits T cell activation. IL-10-treated myeloid cells lose their ability to respond toward the endotoxin lipopolysaccharide (LPS) with the production of several proinflammatory mediators. Thereby, IL-10 limits excessive inflammatory reactions in response to endotoxin as it occurs in colitis or endotoxin shock. Mice can be tolerized toward endotoxin shock when pretreated with a sublethal dose of LPS. This can be mimicked in vitro as LPS desensitization, resulting in a similar LPS hyporesponsiveness as observed with IL-10 pretreatment. However, an early block in LPS signaling characterizes LPS desensitization, whereas IL-10 seems to target late events. Controversial reports have been published where IL-10 would interfere with the induction of proinflammatory mediators, and little is known about the molecular mechanisms behind the anti-inflammatory activities of IL-10. Some recent publications have tried to gain more insight into the molecular mechanism of IL-10 by gene-expression profiling and functional studies in myeloid-derived cells. These results are reviewed here and compared with the progress that has been made to understand the induction of endotoxin tolerance by LPS itself.

CD209L (L-SIGN) is a receptor for severe acute respiratory syndrome coronavirus

Angiotensin-converting enzyme 2 (ACE2) is a receptor for SARS-CoV, the novel coronavirus that causes severe acute respiratory syndrome [Li, W. Moore, M. J., Vasilieva, N., Sui, J., Wong, S. K., Berne, M. A., Somasundaran, M., Sullivan, J. L., Luzuriaga, K., Greenough, T. C., et al. (2003) Nature 426, 450-454]. We have identified a different human cellular glycoprotein that can serve as an alternative receptor for SARS-CoV. A human lung cDNA library in vesicular stomatitis virus G pseudotyped retrovirus was transduced into Chinese hamster ovary cells, and the cells were sorted for binding of soluble SARS-CoV spike (S) glycoproteins, S(590) and S(1180). Clones of transduced cells that bound SARS-CoV S glycoprotein were inoculated with SARS-CoV, and increases in subgenomic viral RNA from 1-16 h or more were detected by multiplex RT-PCR in four cloned cell lines. Sequencing of the human lung cDNA inserts showed that each of the cloned cell lines contained cDNA that encoded human CD209L, a C-type lectin (also called L-SIGN). When the cDNA encoding CD209L from clone 2.27 was cloned and transfected into Chinese hamster ovary cells, the cells expressed human CD209L glycoprotein and became susceptible to infection with SARS-CoV. Immunohistochemistry showed that CD209L is expressed in human lung in type II alveolar cells and endothelial cells, both potential targets for SARS-CoV. Several other enveloped viruses including Ebola and Sindbis also use CD209L as a portal of entry, and HIV and hepatitis C virus can bind to CD209L on cell membranes but do not use it to mediate virus entry. Our data suggest that the large S glycoprotein of SARS-CoV may use both ACE2 and CD209L in virus infection and pathogenesis.

Dissection of measles virus V protein in relation to its ability to block alpha/beta interferon signal transduction

Interferon (IFN)-alpha and -beta are the main cytokines for innate immune responses against viral infections. To replicate efficiently in the hosts, viruses have evolved various countermeasures to the IFN response. The V protein of measles virus (MV) has been shown to block IFN-alpha/beta signalling. Here, the wild-type IC-B strain of MV was shown to grow comparably in the presence and absence of IFN-alpha, whereas replication of the Edmonston tag strain recovered from cloned DNA was strongly suppressed in its presence. The V protein of the IC-B strain, but not the Edmonston tag strain, blocked IFN-alpha signalling. The V protein of the Edmonston strain from the ATCC also inhibited IFN-alpha signalling. There were three amino acid differences between the V proteins of the Edmonston ATCC and tag strains, and substitutions of both residues at positions 110 and 272 were required for the Edmonston ATCC V protein to lose IFN-antagonist activity. The P protein of the IC-B strain, which shares the N-terminal 231 aa residues with the V protein, also inhibited IFN-alpha signalling. Indeed, fragments comprising only those 231 residues of the IC-B and Edmonston ATCC V proteins, but not the Edmonston tag V protein, were able to block IFN-alpha signalling. However, the N-terminal region of the Edmonston tag V protein, when attached to the C-terminal region of the Edmonston ATCC V protein, inhibited IFN-alpha signalling. Taken together, our results indicate that both the N- and C-terminal regions contribute to the IFN-antagonist activity of the MV V protein.

Differential expression of SAP and EAT-2-binding leukocyte cell-surface molecules CD84, CD150 (SLAM), CD229 (Ly9) and CD244 (2B4)

The CD150 (SLAM) family consists of nine leukocyte cell-surface proteins involved in lymphocyte activation that belong to the immunoglobulin (Ig) superfamily. Six members of this family--CD84, CD150 (SLAM), CD229 (Ly9), CD244 (2B4), NTB-A, and CS1--associate with adapter proteins--SLAM-associated protein (SAP) and EAT-2. SAP is a short intracellular molecule that is mutated in humans with X-linked lymphoproliferative disease. Flow cytometric analysis of the expression of CD84, CD150, CD229, and CD244 cell-surface receptors on several leukocyte and lymphocyte subsets was performed. CD84 and CD150 were present on thymocytes, mature T cells and antigen-presenting cells. The expression of CD84 and CD150 was high on memory T cells. CD150 expression was strongly up-regulated after cell activation. In contrast to CD84, CD150 was absent on resting monocytes and immature dendritic cells (DCs). CD229 presented a pattern of expression restricted to lymphocytes. CD244 was preferentially expressed on natural killer cells, CD8(+) effector cells, resting monocytes, basophils, and eosinophils. We describe a broader distribution of CD84, CD150, CD229, and CD244 than previously reported and show that they are differentially expressed on hematopoietic cells. The heterogeneous expression of these receptors indicates that these molecules may play non-redundant functions in the regulation of both innate and adaptive immune responses.

Metabolic effects of acute measles in chronically malnourished Nigerian children

We hypothesized that acute measles infection imposes severe metabolic demands on malnourished children. Nigerian rural communities, characterized by severe poverty and extensive malnutrition, served as site for this study. Sixty-five children (mean [+/-SD] age 2.67 +/- 1.96 years) with measles and a randomly selected equal number of children (age 2.83 +/- 1.23 years) from the same communities but measles-free were studied. Both groups were serologically negative for human immunodeficiency virus. The percentages of nonmeasles group who were underweight and wasted as exemplified by weight for age (WAZ) and weight for height (WHZ) scores less than -2.0 SD were 43% and 23%, respectively. Comparative values for the measles group (66% and 54% respectively) were significantly (P < 0.01 or 0.001) different. Compared to the controls, measles-infected children had significantly (P < 0.001) higher plasma cortisol level, marked hyporetinemia (plasma retinol 0.62 +/- 0.24 micromol/L) and prominent reduction (P < 0.002) in the sum of serum essential amino acids. Measles promoted a TH(1) to TH(2) cytokine shift, with severe depletion of plasma interleukin (IL)-12, a key cytokine in the development of cell mediated immunity. IL-6, a key stimulator of hepatic acute phase protein response, was prominently (P < 0.002) increased in plasma in measles-infected children. Glucocorticoids exert effects on cytokine expression, as well as on cytokine receptor expression and cytokine-regulated biological responses. They enhance synergistically, the effects of IL-1 and IL-6 type cytokines on many acute phase proteins. Because of the prominent increase in circulating level of cortisol in acute measles, glucocorticoid treatment for associated sepsis may pose serious problems. Additionally, glucocorticoids antagonize several effects of retinoids at cellular and transcriptional levels, thus suggesting that hypercortisolemia may increase the requirement for retinoids.