Immune responses during measles virus infection

Immune amnesia induced by measles and its effects on concurrent epidemics

It has been recently discovered that the measles virus can damage pre-existing immunological memory, destroying B lymphocytes and reducing the diversity of non-specific B cells of the infected host. In particular, this implies that previously acquired immunization from vaccination or direct exposition to other pathogens could be partially erased in a phenomenon named ‘immune amnesia’, whose effects can become particularly worrisome given the actual rise of anti-vaccination movements. Here, we present the first attempt to incorporate immune amnesia into standard models of epidemic spreading by proposing a simple model for the spreading of two concurrent pathogens causing measles and another generic disease. Different analyses confirm that immune amnesia can have important consequences for epidemic spreading, significantly altering the vaccination coverage required to reach herd immunity. We also uncover the existence of novel propagating and endemic phases induced by immune amnesia. Finally, we discuss the meaning and consequences of our results and their relation with, e.g. immunization strategies, together with the possibility that explosive types of transitions may emerge, making immune-amnesia effects particularly dramatic. This work opens the door to further developments and analyses of immune-amnesia effects, contributing also to the theory of interacting epidemics on complex networks.

An antibody against the F glycoprotein inhibits Nipah and Hendra virus infections

Nipah virus (NiV) and Hendra virus (HeV) are zoonotic henipaviruses (HNVs) responsible for outbreaks of encephalitis and respiratory illness with fatality rates of 50–100%. No vaccines or licensed therapeutics currently exist to protect humans against NiV or HeV. HNVs enter host cells by fusing the viral and cellular membranes via the concerted action of the attachment (G) and fusion (F) glycoproteins, the main targets of the humoral immune response. Here, we describe the isolation and humanization of a potent monoclonal antibody cross-neutralizing NiV and HeV. Cryo-electron microscopy, triggering and fusion studies show the antibody binds to a prefusion-specific quaternary epitope, conserved in NiV F and HeV F glycoproteins, and prevents membrane fusion and viral entry. This work supports the importance of the HNV prefusion F conformation for eliciting a robust immune response and paves the way for using this antibody for prophylaxis and post-exposure therapy with NiV- and HeV-infected individuals.

An antibody that recognizes the F glycoproteins from Nipah and Hendra viruses can neutralize both viruses and recognizes a quaternary epitope in the prefusion F trimer, preventing conformational changes required for fusion.

Incidence of varicella in children in Jeju-do, Korea, 2005-2016: age-period-cohort analysis

OBJECTIVES

Although the nationwide inoculation rate of varicella vaccine was approximately 95% in Korean children recently, the number of notified varicella cases is unexpectedly continuously increasing till now. To suggest some hypotheses regarding this discrepancy, an age-period-cohort (APC) analysis as a descriptive epidemiology study was conducted for children residing in Jeju-do, Korea.

METHODS

The raw data were obtained from the nationwide database for insurance claim of healthcare fee provided by the National Health Insurance Service, Korea. The selection criteria were children aged 2-13 years who visited any healthcare center due to varicella from 2005 to 2016 while residing in Jeju-do. After calculating the birth cohort-specific crude incidence rates by age and year, the intrinsic estimator method was used to perform the APC analysis.

RESULTS

As the annual crude incidence rates decreased with increasing age between 2005 and 2016, the age and period effects also decreased. The intrinsic estimator coefficients suggesting the cohort effect shifted from positive to negative in 2011, the starting year of free varicella vaccine program in Jeju-do.

CONCLUSIONS

The results suggested that inoculated varicella vaccines have preventive effects. However, further studies to evaluate waning immunity would be needed.

Measles outbreak propagated by children congregating at water collection points in Mayuge District, eastern Uganda, July - October, 2016

BACKGROUND

On 12 October, 2016 a measles outbreak was reported in Mayuge District, eastern Uganda. We investigated the outbreak to determine its scope, identify risk factors for transmission, evaluate vaccination coverage and vaccine effectiveness, and recommend evidence-based control measures.

METHODS

We defined a probable case as onset of fever (≥3 days) and generalized rash, plus ≥1 of the following: conjunctivitis, cough, and/or runny nose in a Mayuge District resident. A confirmed case was a probable case with measles-specific IgM (+) not explained by vaccination. We reviewed medical records and conducted active community case-finding. In a case-control investigation involving probable case-persons and controls matched by age and village, we evaluated risk factors for transmission for both cases and controls during the case-person's likely exposure period (i.e., 7-21 days prior to rash onset). We estimated vaccine effectiveness (VE) using the formula: VE ≈ (1-ORprotective) × 100. We calculated vaccination coverage using the percentage of controls vaccinated.

RESULTS

We identified 62 probable case-persons (attack rate [AR] = 4.0/10,000), including 3 confirmed. Of all age groups, children < 5 years were the most affected (AR = 14/10,000). The epidemic curve showed a propagated outbreak. Thirty-two percent (13/41) of case-persons and 13% (21/161) of control-persons visited water-collection sites (by themselves or with parents) during the case-persons' likely exposure period (ORM-H = 5.0; 95% CI = 1.5-17). Among children aged 9-59 months, the effectiveness of the single-dose measles vaccine was 75% (95% CI = 25-92); vaccination coverage was 68% (95% CI = 61-76).

CONCLUSIONS

Low vaccine effectiveness, inadequate vaccination coverage and congregation at water collection points facilitated measles transmission in this outbreak. We recommended increasing measles vaccination coverage and restriction of children with signs and symptoms of measles from accessing public gatherings.

Mutational Analysis of Measles Virus Suggests Constraints on Antigenic Variation of the Glycoproteins

Measles virus undergoes error-prone replication like other RNA viruses, but over time, it has remained antigenically monotypic. The constraints on the virus that prevent the emergence of antigenic variants are unclear. As a first step in understanding this question, we subjected the measles virus genome to unbiased insertional mutagenesis, and viruses that could tolerate insertions were rescued. Only insertions in the nucleoprotein, phosphoprotein, matrix protein, as well as intergenic regions were easily recoverable. Insertions in the glycoproteins of measles virus were severely under-represented in our screen. Host immunity depends on developing neutralizing antibodies to the hemagglutinin and fusion glycoproteins; our analysis suggests that these proteins occupy very little evolutionary space and therefore have difficulty changing in the face of selective pressures. We propose that the inelasticity of these proteins prevents the sequence variation required to escape antibody neutralization in the host, allowing for long-lived immunity after infection with the virus.

Peste des petits ruminants virus infection of small ruminants: a comprehensive review

Peste des petits ruminants (PPR) is caused by a Morbillivirus that belongs to the family Paramyxoviridae. PPR is an acute, highly contagious and fatal disease primarily affecting goats and sheep, whereas cattle undergo sub-clinical infection. With morbidity and mortality rates that can be as high as 90%, PPR is classified as an OIE (Office International des Epizooties)-listed disease. Considering the importance of sheep and goats in the livelihood of the poor and marginal farmers in Africa and South Asia, PPR is an important concern for food security and poverty alleviation. PPR virus (PPRV) and rinderpest virus (RPV) are closely related Morbilliviruses. Rinderpest has been globally eradicated by mass vaccination. Though a live attenuated vaccine is available against PPR for immunoprophylaxis, due to its instability in subtropical climate (thermo-sensitivity), unavailability of required doses and insufficient coverage (herd immunity), the disease control program has not been a great success. Further, emerging evidence of poor cross neutralization between vaccine strain and PPRV strains currently circulating in the field has raised concerns about the protective efficacy of the existing PPR vaccines. This review summarizes the recent advancement in PPRV replication, its pathogenesis, immune response to vaccine and disease control. Attempts have also been made to highlight the current trends in understanding the host susceptibility and resistance to PPR.

Epitope dampening monotypic measles virus hemagglutinin glycoprotein results in resistance to cocktail of monoclonal antibodies

The measles virus (MV) is serologically monotypic. Life-long immunity is conferred by a single attack of measles or following vaccination with the MV vaccine. This is contrary to viruses such as influenza, which readily develop resistance to the immune system and recur. A better understanding of factors that restrain MV to one serotype may allow us to predict if MV will remain monotypic in the future and influence the design of novel MV vaccines and therapeutics. MV hemagglutinin (H) glycoprotein, binds to cellular receptors and subsequently triggers the fusion (F) glycoprotein to fuse the virus into the cell. H is also the major target for neutralizing antibodies. To explore if MV remains monotypic due to a lack of plasticity of the H glycoprotein, we used the technology of Immune Dampening to generate viruses with rationally designed N-linked glycosylation sites and mutations in different epitopes and screened for viruses that escaped monoclonal antibodies (mAbs). We then combined rationally designed mutations with naturally selected mutations to generate a virus resistant to a cocktail of neutralizing mAbs targeting four different epitopes simultaneously. Two epitopes were protected by engineered N-linked glycosylations and two epitopes acquired escape mutations via two consecutive rounds of artificial selection in the presence of mAbs. Three of these epitopes were targeted by mAbs known to interfere with receptor binding. Results demonstrate that, within the epitopes analyzed, H can tolerate mutations in different residues and additional N-linked glycosylations to escape mAbs. Understanding the degree of change that H can tolerate is important as we follow its evolution in a host whose immunity is vaccine induced by genotype A strains instead of multiple genetically distinct wild-type MVs.

Host factors and measles virus replication

This review takes a general approach to describing host cell factors that facilitate measles virus (MeV) infection and replication. It relates our current understanding of MeV entry receptors, with emphasis on how these host cell surface proteins contribute to pathogenesis within its host. The roles of SLAM/CD150 lymphocyte receptor and the newly discovered epithelial receptor PVRL4/nectin-4 are highlighted. Host cell factors such as HSP72, Prdx1, tubulin, casein kinase, and actin, which are known to impact viral RNA synthesis and virion assembly, are also discussed. Finally the review describes strategies used by measles virus to circumvent innate immunity and confound the effects of interferon within the host cell. Proteomic studies and genome wide RNAi screens will undoubtedly advance our knowledge in the future.

Replication of associations between cytokine and cytokine receptor single nucleotide polymorphisms and measles-specific adaptive immunophenotypic extremes

Our objective was to replicate previously reported associations between cytokine and cytokine receptor SNPs and humoral and CMI (cell-mediated immune) responses to measles vaccine. All subjects (n=758) received two doses of MMR (measles/mumps/rubella) vaccine. From these subjects, candidate cytokine and cytokine receptor SNPs were genotyped and analyzed in 29-30 subjects falling into one of four "extreme" humoral (Ab(high/low)) and CMI (CMI(high/low)) response quadrants. Associations between seven SNPs (out of 11 in the discovery study) and measles-specific neutralizing antibody levels and IFN-γ ELISPOT responses were evaluated using chi-square tests. We found one replicated association for SNP rs372889 in the IL12RB1 gene (P=0.03 for Ab(high)CMI(high) vs. Ab(low)CMI(low)). Our findings demonstrate the importance of replicating genotypic-phenotypic associations, which can be achieved using immunophenotypic extremes and smaller sample sizes. We speculate that IL12RB1 polymorphisms may affect IL-12 and IL-23 binding and downstream effects, which are critical cytokines in the CMI response to measles vaccine.

Nipah virus infects specific subsets of porcine peripheral blood mononuclear cells

Nipah virus (NiV), a zoonotic paramyxovirus, is highly contagious in swine, and can cause fatal infections in humans following transmission from the swine host. The main viral targets in both species are the respiratory and central nervous systems, with viremia implicated as a mode of dissemination of NiV throughout the host. The presented work focused on the role of peripheral blood mononuclear cells (PBMC) in the viremic spread of the virus in the swine host. B lymphocytes, CD4-CD8-, as well as CD4+CD8- T lymphocytes were not permissive to NiV, and expansion of the CD4+CD8- cells early post infection was consistent with functional humoral response to NiV infection observed in swine. In contrast, significant drop in the CD4+CD8- T cell frequency was observed in piglets which succumbed to the experimental infection, supporting the hypothesis that antibody development is the critical component of the protective immune response. Productive viral replication was detected in monocytes, CD6+CD8+ T lymphocytes and NK cells by recovery of infectious virus in the cell supernatants. Virus replication was supported by detection of the structural N and the non-structural C proteins or by detection of genomic RNA increase in the infected cells. Infection of T cells carrying CD6 marker, a strong ligand for the activated leukocyte cell adhesion molecule ALCAM (CD166) highly expressed on the microvascular endothelial cell of the blood-air and the blood-brain barrier may explain NiV preferential tropism for small blood vessels of the lung and brain.

Immunization strategies against henipaviruses

Hendra virus and Nipah virus are recently discovered and closely related emerging viruses that now comprise the genus henipavirus within the sub-family Paramyxoviridae and are distinguished by their broad species tropism and in addition to bats can infect and cause fatal disease in a wide variety of mammalian hosts including humans. The high mortality associated with human and animal henipavirus infections has highlighted the importance and necessity of developing effective immunization strategies. The development of suitable animal models of henipavirus infection and pathogenesis has been critical for testing the efficacy of potential therapeutic approaches. Several henipavirus challenge models have been used and recent successes in both active and passive immunization strategies against henipaviruses have been reported which have all targeted the viral envelope glycoproteins.

Insights into the regulatory mechanism controlling the inhibition of vaccine-induced seroconversion by maternal antibodies

The inhibition of vaccination by maternal antibodies is a widely observed phenomenon in human and veterinary medicine. Maternal antibodies are known to suppress the B-cell response. This is similar to antibody feedback mechanism studies where passively transferred antibody inhibits the B-cell response against particulate antigens because of epitope masking. In the absence of experimental data addressing the mechanism underlying inhibition by maternal antibodies, it has been suggested that epitope masking explains the inhibition by maternal antibodies, too. Here we report that in the cotton rat model of measles virus (MV) vaccination passively transferred MV-specific immunoglobulin G inhibit B-cell responses through cross-linking of the B-cell receptor with FcγRIIB. The extent of inhibition increases with the number of antibodies engaging FcγRIIB and depends on the Fc region of antibody and its isotype. This inhibition can be partially overcome by injection of MV-specific monoclonal IgM antibody. IgM stimulates the B-cell directly through cross-linking the B-cell receptor via complement protein 3d and antigen to the complement receptor 2 signaling complex. These data demonstrate that maternal antibodies inhibit B-cell responses by interaction with the inhibitory/regulatory FcγRIIB receptor and not through epitope masking.

Induction of neutralizing antibodies to Hendra and Nipah glycoproteins using a Venezuelan equine encephalitis virus in vivo expression system

The emergence of Hendra Virus (HeV) and Nipah Virus (NiV) which can cause fatal infections in both animals and humans has triggered a search for an effective vaccine. Here, we have explored the potential for generating an effective humoral immune response to these zoonotic pathogens using an alphavirus-based vaccine platform. Groups of mice were immunized with Venezuelan equine encephalitis virus replicon particles (VRPs) encoding the attachment or fusion glycoproteins of either HeV or NiV. We demonstrate the induction of highly potent cross-reactive neutralizing antibodies to both viruses using this approach. Preliminary study suggested early enhancement in the antibody response with use of a modified version of VRP. Overall, these data suggest that the use of an alphavirus-derived vaccine platform might serve as a viable approach for the development of an effective vaccine against the henipaviruses.

Interplay between virus-specific effector response and Foxp3 regulatory T cells in measles virus immunopathogenesis

Measles is a highly contagious childhood disease associated with an immunological paradox: although a strong virus-specific immune response results in virus clearance and the establishment of a life-long immunity, measles infection is followed by an acute and profound immunosuppression leading to an increased susceptibility to secondary infections and high infant mortality. In certain cases, measles is followed by fatal neurological complications. To elucidate measles immunopathology, we have analyzed the immune response to measles virus in mice transgenic for the measles virus receptor, human CD150. These animals are highly susceptible to intranasal infection with wild-type measles strains. Similarly to what has been observed in children with measles, infection of suckling transgenic mice leads to a robust activation of both T and B lymphocytes, generation of virus-specific cytotoxic T cells and antibody responses. Interestingly, Foxp3(+)CD25(+)CD4(+) regulatory T cells are highly enriched following infection, both in the periphery and in the brain, where the virus intensively replicates. Although specific anti-viral responses develop in spite of increased frequency of regulatory T cells, the capability of T lymphocytes to respond to virus-unrelated antigens was strongly suppressed. Infected adult CD150 transgenic mice crossed in an interferon receptor type I-deficient background develop generalized immunosuppression with an increased frequency of CD4(+)CD25(+)Foxp3(+) T cells and strong reduction of the hypersensitivity response. These results show that measles virus affects regulatory T-cell homeostasis and suggest that an interplay between virus-specific effector responses and regulatory T cells plays an important role in measles immunopathogenesis. A better understanding of the balance between measles-induced effector and regulatory T cells, both in the periphery and in the brain, may be of critical importance in the design of novel approaches for the prevention and treatment of measles pathology.

Persistent virus infection inhibits type I interferon production by plasmacytoid dendritic cells to facilitate opportunistic infections

Emerging studies indicate an association between virus-induced impairment in type I interferon (IFN-I) production and enhanced susceptibility to opportunistic infections, which represent a major health problem. Here, we provide in vivo evidence that lymphocytic choriomeningitis virus (LCMV) infection of its natural murine host dramatically diminishes the unique capacity of plasmacytoid dendritic cells (pDCs) to secrete high levels of systemic IFN-I. While both acute and persistent LCMV infections suppress pDC IFN-I response, only the persistent virus induces a long-lasting diversion of this innate immune pathway. The consequent reduction in IFN-I production serves to impair natural killer cell responses in LCMV-infected mice challenged subsequently with murine cytomegalovirus (MCMV) as an opportunistic pathogen. This innate defect also compromises the host's ability to counteract early MCMV spread. These findings provide a mechanistic explanation for the occurrence of opportunistic infections following viral insults and have important implications for treating such medical complications.

Acute disseminated encephalomyelitis: clinical and pathogenesis features

Acute disseminated encephalomyelitis (ADEM) is an immune-mediated disorder of the central nervous system (CNS). Disease typically starts with an abrupt onset of neurologic symptoms and signs within days to weeks after a viral infection or immunization. Neuropathological examination of the CNS in ADEM reveals involvement of white matter, with infiltration of monocytoid cells and perivenous demyelination.

Effects of interleukin-12 and interleukin-15 on measles-specific T-cell responses in vaccinated infants

Understanding the infant host response to measles vaccination is important because of their increased mortality from measles and the need to provide effective protection during the first year of life. Measles-specific T and B-cell responses are lower in infants after measles vaccination than in adults. To define potential mechanisms, we investigated age-related differences in measles-specific T-cell proliferation, CD40-L expression, and IFN-gamma production after measles immunization, and the effects of rhIL-12 and rhIL-15 on these responses. Measles-specific T-cell proliferation and mean IFN-gamma release from infant PBMCs were significantly lower when compared with responses of vaccinated children and adults. Infant responses increased to ranges observed in children and adults when both rhIL-12 and rhIL-15 were added to PBMC cultures. Furthermore, a significant rise in T-cell proliferation and IFN-gamma release was observed when infant PBMCs were stimulated with measles antigen in the presence of rhIL-12 and rhIL-15 compared to measles antigen alone. CD40-L expression by infant and adult T cells stimulated with measles antigen was comparable, but fewer infant CD40-L(+) T cells expressed IFN-gamma. These observations suggest that lower measles-specific T-cell immune responses elicited by measles vaccine in infants may be due to diminished levels of key cytokines.

A recombinant subunit vaccine formulation protects against lethal Nipah virus challenge in cats

Nipah virus (NiV) and Hendra virus (HeV) are closely related deadly zoonotic paramyxoviruses that have emerged and re-emerged over the last 10 years. In this study, a subunit vaccine formulation containing only recombinant, soluble, attachment glycoprotein from HeV (sG_HeV) and CpG adjuvant was evaluated as a potential NiV vaccine in the cat model. Different amounts of sG_HeV were employed and sG-induced immunity was examined. Vaccinated animals demonstrated varying levels of NiV-specific Ig systemically and importantly, all vaccinated cats possessed antigen-specific IgA on the mucosa. Upon oronasal challenge with NiV (50,000 TCID₅₀), all vaccinated animals were protected from disease although virus was detected on day 21 post-challenge in one animal. The ability to elicit protective systemic and mucosal immunity in this animal model provides significant progress towards the development of a human subunit vaccine against henipaviruses.

Targeted strategies for henipavirus therapeutics

Hendra and Nipah viruses are related emergent paramyxoviruses that infect and cause disease in animals and humans. Disease manifests as a generalized vasculitis affecting multiple organs, but is the most severe in the respiratory and central nervous systems. The high case fatality and person-to-person transmission associated with the most recent NiV outbreaks, and the recent re-emergence of HeV, emphasize the importance and necessity of effective therapeutics for these novel agents. In recent years henipavirus research has revealed a more complete understanding of pathogenesis and, as a consequence, viable approaches towards vaccines and therapeutics have emerged. All strategies target early steps in viral replication including receptor binding and membrane fusion. Animal models have been developed, some of which may prove more valuable than others for evaluating the efficacy of therapeutic agents and regimes. Assessments of protective host immunity and drug pharmacokinetics will be crucial to the further advancement of therapeutic compounds.

Measles virus-specific CD4 T-cell activity does not correlate with protection against lung infection or viral clearance

Acute measles in children can be prevented by immunization with the live attenuated measles vaccine virus. Although immunization is able to induce CD4 and CD8 T cells as well as neutralizing antibodies, only the latter have been correlated with protective immunity. CD8 T cells, however, have been documented to be important in viral clearance in the respiratory tract, whereas CD4 T cells have been shown to be protective in a mouse encephalitis model. In order to investigate the CD4 T-cell response in infection of the respiratory tract, we have defined a T-cell epitope in the hemagglutinin (H) protein for immunization and developed a monoclonal antibody for depletion of CD4 T cells in the cotton rat model. Although the kinetics of CD4 T-cell development correlated with clearance of virus, the depletion of CD4 T cells during the primary infection did not influence viral titers in lung tissue. Immunization with the H epitope induced a CD4 T-cell response but did not protect against infection. Immunization in the presence of maternal antibodies resulted in the development of a CD4 T-cell response which (in the absence of neutralizing antibodies) did not protect against infection. In summary, CD4 T cells do not seem to protect against infection after immunization and do not participate in clearance of virus infection from lung tissue during measles virus infection. We speculate that the major role of CD4 T cells is to control and clear virus infection from other affected organs like the brain.

Pathobiology of henipavirus entry: insights into therapeutic strategies

The recently emerged paramyxoviruses, Nipah (NiV) and Hendra (HeV), designated as Biosafety Level 4 pathogens, can cause lethal respiratory and neurological disease in both animals and humans. NiV outbreaks have been associated with efficient transmission amongst livestock (pigs) and mortality rates exceeding 70%, with documented cases of human-to-human transmission. Without vaccines or effective therapeutics, NiV and HeV continue to present an impending threat to global health and economies. The current understanding of henipavirus pathobiology has led to the development of small animal models reflecting certain aspects of the human pathology. In this review, we discuss how these animal models have been critical in testing vaccination strategies and in eliciting neutralizing antibodies against the envelope glycoproteins. Additionally, the discovery of the viral receptor and development of pseudotyped-viral systems have allowed us to explore the multiple opportunities for therapeutic intervention existing within the individual steps of the henipavirus entry pathway. Current research shows promise for the future development of effective strategies to limit the impact of these biological threats.

Human leukocyte antigen and interleukin 2, 10 and 12p40 cytokine responses to measles: is there evidence of the HLA effect?

HLA class I and class II associations were examined in relation to measles virus-specific cytokine responses in 339 healthy children who had received two doses of live attenuated measles vaccine. Multivariate linear regression modeling analysis revealed suggestions of associations between the expression of DPA1*0201 (p=0.03) and DPA1*0202 (p=0.09) alleles and interleukin-2 (IL-2) cytokine production (global p-value 0.06). Importantly, cytokine production and DQB1 allele associations (global p-value 0.04) revealed that the alleles with the strongest association with IL-10 secretion were DQB1*0302 (p=0.02), DQB1*0303 (p=0.07) and DQB1*0502 (p=0.06). Measles-specific IL-10 secretion associations approached significance with DRB1 and DQA1 loci (both global p-values 0.08). Specifically, suggestive associations were found between DRB1*0701 (p=0.07), DRB1*1103 (p=0.06), DRB1*1302 (p=0.08), DRB1*1303 (p=0.06), DQA1*0101 (p=0.08), and DQA1*0201 (p=0.04) alleles and measles-induced IL-10 secretion. Further, suggestive association was observed between specific DQA1*0505 (p=0.002) alleles and measles-specific IL-12p40 secretion (global p-value 0.09) indicating that cytokine responses to measles antigens are predominantly influenced by HLA class II genes. We found no associations between any of the alleles of HLA A, B, and Cw loci and cytokine secretion. These novel findings suggest that HLA class II genes may influence the level of cytokine production in the adaptive immune responses to measles vaccine.

Feline model of acute nipah virus infection and protection with a soluble glycoprotein-based subunit vaccine

Nipah virus (NiV) and Hendra virus (HeV) are paramyxoviruses capable of causing considerable morbidity and mortality in a number of mammalian species, including humans. Case reports from outbreaks and previous challenge experiments have suggested that cats were highly susceptible to NiV infection, responding with a severe respiratory disease and systemic infection. Here we have assessed the cat as a model of experimental NiV infection and use it in the evaluation of a subunit vaccine comprised of soluble G glycoprotein (sG). Two groups of two adult cats each were inoculated subcutaneously with either 500 or 5,000 50% tissue culture infective dose(s) (TCID(50)) of NiV. Animals were monitored closely for disease onset, and extensive analysis was conducted on samples and tissues taken during infection and at necropsy to determine viral load and tissue tropism. All animals developed clinical disease 6 to 9 days postinfection, a finding consistent with previous observations. In a subsequent experiment, two cats were immunized with HeV sG and two were immunized with NiV sG. Homologous serum neutralizing titers were greater than 1:20,000, and heterologous titers were greater than 1:20,000 to 16-fold lower. Immunized animals and two additional naive controls were then challenged subcutaneously with 500 TCID(50) of NiV. Naive animals developed clinical disease 6 to 13 days postinfection, whereas none of the immunized animals showed any sign of disease. TaqMan PCR analysis of samples from naive animals revealed considerable levels of NiV genome in a wide range of tissues, whereas the genome was evident in only two immunized cats in only four samples and well below the limit of accurate detection. These results indicate that the cat provides a consistent model for acute NiV infection and associated pathogenesis and an effective subunit vaccine strategy appears achievable.

Gene expression patterns in dendritic cells infected with measles virus compared with other pathogens

Gene expression patterns supply insight into complex biological networks that provide the organization in which viruses and host cells interact. Measles virus (MV) is an important human pathogen that induces transient immunosuppression followed by life-long immunity in infected individuals. Dendritic cells (DCs) are potent antigen-presenting cells that initiate the immune response to pathogens and are postulated to play a role in MV-induced immunosuppression. To better understand the interaction of MV with DCs, we examined the gene expression changes that occur over the first 24 h after infection and compared these changes to those induced by other viral, bacterial, and fungal pathogens. There were 1,553 significantly regulated genes with nearly 60% of them down-regulated. MV-infected DCs up-regulated a core of genes associated with maturation of antigen-presenting function and migration to lymph nodes but also included genes for IFN-regulatory factors 1 and 7, 2'5' oligoadenylate synthetase, Mx, and TNF superfamily proteins 2, 7, 9, and 10 (TNF-related apoptosis-inducing ligand). MV induced genes for IFNs, ILs, chemokines, antiviral proteins, histones, and metallothioneins, many of which were also induced by influenza virus, whereas genes for protein synthesis and oxidative phosphorylation were down-regulated. Unique to MV were the induction of genes for a broad array of IFN-alphas and the failure to up-regulate dsRNA-dependent protein kinase. These results provide a modular view of common and unique DC responses after infection and suggest mechanisms by which MV may modulate the immune response.

Correlations among measles virus-specific antibody, lymphoproliferation and Th1/Th2 cytokine responses following measles-mumps-rubella-II (MMR-II) vaccination

Immunity to measles is conferred by the interplay of humoral and cellular immune responses, the latter being critical in maintaining long-term recall response. Therefore, it is important to evaluate measles-specific humoral and cellular immunity in populations several years after vaccination and understand the correlations among these measures of immunity. We examined measles-specific antibodies, lymphoproliferation and the Th1/Th2 signature cytokines, interferon (IFN)-gamma and interleukin (IL)-4, in a population-based cohort of healthy children from Olmsted County, Minnesota after two doses of measles-mumps-rubella-II (MMR-II) vaccine. We detected positive measures of measles-specific cellular and humoral immunity in the majority of our study population. However, a small proportion of subjects demonstrated an immune response skewed towards the Th2 type, characterized by the presence of either IL-4 and/or measles-specific antibodies and a lack of IFN-gamma production. Further, we observed a significant positive correlation between lymphoproliferation and secretion of IFN-gamma (r = 0.20, P = 0.0002) and IL-4 (r = 0.15, P = 0.005). Measles antibody levels were correlated with lymphoproliferation (r = 0.12, P = 0.03), but lacked correlation to either cytokine type. In conclusion, we demonstrated the presence of both long-term cellular and humoral responses after MMR-II vaccination in a significant proportion of study subjects. Further, a positive correlation between lymphoproliferation and IL-4 and IFN-gamma suggests that immunity to measles may be maintained by both Th1 and Th2 cells. We speculate that the Th2 biased response observed in a subset of our subjects may be insufficient to provide long-term immunity against measles. Further examination of the determinants of Th1 versus Th2 skewing of the immune response and long-term follow-up is needed.

Immunopathogenesis of coronavirus infections: implications for SARS

The severe acute respiratory syndrome (SARS), which was first identified in 2003, is caused by a novel coronavirus: the SARS coronavirus (SARS-CoV).

Many features of the infection indicate that an excessive, but perhaps 'normal', immune response contributes to SARS.

Several coronaviruses cause diseases that result in considerable morbidity and mortality in animals. Some of these diseases are also immune mediated and provide insights into the pathogenesis of SARS.

Feline infectious peritonitis virus (FIPV) causes a fatal, immune-mediated disease of felines. Macrophage infection, lymphocyte depletion and antibody-dependent disease enhancement are hallmarks of this disease.

Infection with the murine coronavirus murine hepatitis virus (MHV) strain JHM results in immune-mediated demyelination. Similar to SARS, macrophage activation is a key component in the pathogenic process.

Another strain of MHV, MHV-3, causes a fatal, fulminant hepatitis. MHV-3 infection of macrophages, with subsequent activation and induction of expression of a novel procoagulant, fibrinogen-like protein 2 (FGL2), is required for severe disease.

Chickens that are infected with avian infectious bronchitis virus (IBV) develop respiratory and renal disease. An excessive innate immune response contributes to the pathogenic process in these animals.

To develop effective therapies for SARS will require understanding of the contributions of direct injury by virus and of the host immune response to pathogenesis. This requires further studies of the interactions of SARS-CoV with its target cells and necessitates the development of an animal model that reproduces the pulmonary infection that is observed in infected humans.

At the end of 2002, the first cases of severe acute respiratory syndrome (SARS) were reported, and in the following year, SARS resulted in considerable mortality and morbidity worldwide. SARS is caused by a novel species of coronavirus (SARS-CoV) and is the most severe coronavirus-mediated human disease that has been described so far. On the basis of similarities with other coronavirus infections, SARS might, in part, be immune mediated. As discussed in this Review, studies of animals that are infected with other coronaviruses indicate that excessive and sometimes dysregulated responses by macrophages and other pro-inflammatory cells might be particularly important in the pathogenesis of disease that is caused by infection with these viruses. It is hoped that lessons from such studies will help us to understand more about the pathogenesis of SARS in humans and to prevent or control outbreaks of SARS in the future.

Receptor binding, fusion inhibition, and induction of cross-reactive neutralizing antibodies by a soluble G glycoprotein of Hendra virus

Hendra virus (HeV) and Nipah virus (NiV) are closely related emerging viruses comprising the Henipavirus genus of the Paramyxovirinae, which are distinguished by their ability to cause fatal disease in both animal and human hosts. These viruses infect cells by a pH-independent membrane fusion event mediated by their attachment (G) and fusion (F) glycoproteins. Previously, we reported on HeV- and NiV-mediated fusion activities and detailed their host-cell tropism characteristics. These studies also suggested that a common cell surface receptor, which could be destroyed by protease, was utilized by both viruses. To further characterize the G glycoprotein and its unknown receptor, soluble forms of HeV G (sG) were constructed by replacing its cytoplasmic tail and transmembrane domains with an immunoglobulin kappa leader sequence coupled to either an S-peptide tag (sG(S-tag)) or myc-epitope tag (sG(myc-tag)) to facilitate purification and detection. Expression of sG was verified in cell lysates and culture supernatants by specific affinity precipitation. Analysis of sG by size exclusion chromatography and sucrose gradient centrifugation demonstrated tetrameric, dimeric, and monomeric species, with the majority of the sG released as a disulfide-linked dimer. Immunofluorescence staining revealed that sG specifically bound to HeV and NiV infection-permissive cells but not to a nonpermissive HeLa cell line clone, suggesting that it binds to virus receptor on host cells. Preincubation of host cells with sG resulted in dose-dependent inhibition of both HeV and NiV cell fusion as well as infection by live virus. Taken together, these data indicate that sG retains important native structural features, and we further demonstrate that administration of sG to rabbits can elicit a potent cross-reactive neutralizing antibody response against infectious HeV and NiV. This HeV sG glycoprotein will be exceedingly useful for structural studies, receptor identification strategies, and vaccine development goals for these important emerging viral agents.

Induction of CD4 T cell proliferation and in vitro Th1-like cytokine responses to measles virus

Mechanisms that lead to induction of life-long immunity to measles virus (MV) are poorly understood. In the present study, we have assessed the activation, proliferation and cytokine secreting function of peripheral blood T cells from MV immune individuals. Expression of cell blastogenesis markers, such as increased forward light scatter and CD38 expression, peaked 5-7 days after infection of peripheral blood mononuclear cells (PBMC) with the live attenuated Edmonston strain of MV. Subset analysis revealed that both CD3- and CD3+ cells expressed activation markers but that the CD3+ T cells predominated late in the culture period corresponding to maximal proliferation and cell recovery. The majority of CD3+ T cells consisted of CD4+CD8- cells. IFN-gamma and IL-4 production similarly showed optimal production late in culture. Depletion of CD4 cells prior to culture and MV stimulation completely abrogated both IFN-gamma and IL-4 production, whereas depletion of CD8 cells did not diminish production, suggesting that CD4+CD8- T cells were principally involved in production of these cytokines. Finally, optimal IFN-gamma production was elicited at high MV doses and IL-4 at much lower doses. These results suggest that among MV immune individuals, in vitro responses to measles are dominated by CD4+ T cells that, depending on antigen dose, primarily produce a Th1-like and, to a lesser extent, a Th1/Th2-mixed pattern of cytokine release.

Differential immune responses to primary measles-mumps-rubella vaccination in Israeli children

Measles remains an important cause of morbidity and mortality worldwide, primarily due to problems associated with delivery of the live attenuated vaccine to susceptible populations. In some developed countries, there is concern about the effects of immunization on the immune system. In this study, we analyzed the responses of 12-month-old Bedouin and Jewish children living in Israel to routine measles-mumps-rubella (MMR) vaccination. Seroconversion to measles was 99% in Bedouin and 79% in Jewish children (P < 0.01), and that to mumps and rubella was 92 to 100% in both groups. Measles neutralizing antibody titers were higher in Bedouin (333 +/- 39 mIU/ml) than Jewish (122 +/- 60 mIU/ml) children (P < 0.002). Immunoglobulin G levels were higher in Bedouin than Jewish children (P = 0.007) and increased after vaccination (P = 0.0009). Leukocyte (P < 0.02) and lymphocyte (P = 0.04) counts were higher and CD4 lymphocyte percentages were lower (P < 0.001) in Bedouin than Jewish children before and after vaccination. Leukocyte counts and natural killer cell numbers did not change after vaccination, but lytic activity increased in Bedouin children (P < 0.005). Spontaneous proliferation of cultured peripheral blood mononuclear cells increased with vaccination, but there were no changes in the proliferative responses to phytohemagglutinin or tetanus toxoid. In summary, no adverse effects of MMR vaccination on immune function were detected. However, there were differences in underlying immunologic parameters and in response to the measles component of the vaccine between Bedouin and Jewish children. It is not known whether genetic differences or environmental exposure accounts for these differences.

Characterization of an in vitro model of alphavirus infection of immature and mature neurons

Terminally differentiated, mature neurons are essential cells that are not easily regenerated. Neurotropic viruses, such as Sindbis virus (SV), cause encephalomyelitis through their ability to replicate in neurons. SV causes the death of immature neurons, while mature neurons can often survive infection. The lack of a reproducible and convenient neuronal cell culture system has hindered a detailed study of the differences in levels of virus replication between immature and mature neurons and the molecular events involved in virus clearance from mature neurons. We have characterized SV replication in immortalized CSM14.1 rat neuronal cells that can be differentiated into neurons. During differentiation, CSM14.1 cells ceased dividing, developed neuronal morphology, and expressed neuron-specific cell markers. SV infection of undifferentiated CSM14.1 cells was efficient and resulted in high levels of virus replication and cell death. SV infection of differentiated CSM14.1 cells was less efficient and resulted in the production of 10- to 100-fold less virus and cell survival. In undifferentiated cells, SV induced a rapid shutdown of cellular protein synthesis and pE2 was efficiently processed to E2 (ratio of E2 to pE2, 2.14). In differentiated cells, the SV-induced shutdown of cellular protein synthesis was transient and pE2 was the primary form of E2 in cells (ratio of E2 to pE2, 0.0426). We conclude that age-dependent restriction of virus replication is an intrinsic property of maturing neurons and that the CSM14.1 cell line is a convenient model system for investigating the interactions of alphaviruses with neurons at various stages of differentiation.

HLA class II alleles and measles virus-specific cytokine immune response following two doses of measles vaccine

Measles virus-specific T cells and the production of cytokines play a critical role in the immune response following measles immunization. To understand the genetic factors that influence variation in IFN-gamma and IL-4 responses following measles immunization and to provide insight into the factors influencing both cellular and humoral immunity to measles, we assessed associations between human leukocyte antigen (HLA) class II genes and measles-specific Th1 and Th2-type cytokine responses in peripheral blood lymphocytes from 339 children previously vaccinated with two doses of measles-mumps-rubella vaccine (MMR-II). Median values for measles-specific IFN-gamma and IL-4 secretion levels were 40.73 and 9.71 pg/ml, respectively. The global tests suggested associations between measles-specific IFN-gamma response and alleles of the DRB1 and DQB1 loci (P=0.07 and P=0.02, respectively). Specifically, DRB1*0301, *0901, and *1501 alleles were significantly associated with IFN-gamma secretion. The alleles that suggested evidence of an HLA association with IL-4 secretion were DRB1*0103, *0701, and *1101. Th1 cytokine responses and DQB1 allele associations revealed that the alleles with the strongest association with IFN-gamma secretion were DQB1*0201, *0303, *0402, and *0602. Specific alleles with a suggestive association with low measles-specific Th2 cytokine responses were DQB1*0202 and *0503. In addition, DPB1*0101, *0201, and *0601 alleles provided suggestive evidence of an HLA association with measles-induced IFN-gamma response, while DPB1*0501 was associated with an IL-4 response. These data suggest that IFN-gamma and IL-4 cytokine responses to measles may be genetically restricted in part by HLA class II genes, which in turn can restrict the cellular immune response to measles vaccine.

Mechanism of CD150 (SLAM) down regulation from the host cell surface by measles virus hemagglutinin protein

Measles virus has been reported to enter host cells via either of two cellular receptors, CD46 and CD150 (SLAM). CD46 is found on most cells of higher primates, while SLAM is expressed on activated B, T, and dendritic cells and is an important regulatory molecule of the immune system. Previous reports have shown that measles virus can down regulate expression of its two cellular receptors on the host cell surface during infection. In this study, the process of down regulation of SLAM by measles virus was investigated. We demonstrated that expression of the hemagglutinin (H) protein of measles virus was sufficient for down regulation. Our studies provided evidence that interactions between H and SLAM in the endoplasmic reticulum (ER) can promote the down regulation of SLAM but not CD46. In addition, we demonstrated that interactions between H and SLAM at the host cell surface can also contribute to SLAM down regulation. These results indicate that two mechanisms involving either intracellular interactions between H and SLAM in the ER or receptor-mediated binding to H at the surfaces of host cells can lead to the down regulation of SLAM during measles virus infection.

Measles virus protein-specific IgM, IgA, and IgG subclass responses during the acute and convalescent phase of infection

The availability of new generation serological assays allowed re-evaluation of the antibody response to measles virus. IgM, IgA, total IgG, and IgG subclass responses were studied to the three major immunogenic measles virus proteins: the fusion protein (F), haemagglutinin (H), and nucleoprotein (N). Plasma samples were obtained from clinically diagnosed measles cases (n = 146) in Khartoum (Sudan) within a week after onset of the rash. Convalescent phase samples were collected from 32 of 117 laboratory-confirmed measles cases at different time points after onset of rash. Glycoprotein-specific IgM, IgG, and IgA antibody levels correlated well to the N-specific response. For IgG and IgA, responses to F were higher than to H. IgA antibody levels were undetectable in about one third of the laboratory-confirmed cases during the acute phase, but positive in all patients tested 1-4 weeks after infection. IgM levels declined rapidly and were lost 3-6 months after infection. IgA levels declined slowly during the first year but did not return to background levels during the subsequent 2 years. IgG avidity maturation was detected during a 3-6 month period after infection. The predominant IgG subclasses during the acute phase were IgG(1) and IgG(3). The latter was lost in the convalescent phase, while the IgG(4) isotype showed a slight rise afterwards. Interestingly, acute phase IgG(3) and IgA responses were associated, and were only detected in samples with high IgG. This study provides a comprehensive perspective on the antibody response to wild-type measles virus infection.

Measles virus (MV) nucleoprotein binds to a novel cell surface receptor distinct from FcgammaRII via its C-terminal domain: role in MV-induced immunosuppression

During acute measles virus (MV) infection, an efficient immune response occurs, followed by a transient but profound immunosuppression. MV nucleoprotein (MV-N) has been reported to induce both cellular and humoral immune responses and paradoxically to account for immunosuppression. Thus far, this latter activity has been attributed to MV-N binding to human and murine FcgammaRII. Here, we show that apoptosis of MV-infected human thymic epithelial cells (TEC) allows the release of MV-N in the extracellular compartment. This extracellular N is then able to bind either to MV-infected or uninfected TEC. We show that recombinant MV-N specifically binds to a membrane protein receptor, different from FcgammaRII, highly expressed on the cell surface of TEC. This new receptor is referred to as nucleoprotein receptor (NR). In addition, different Ns from other MV-related morbilliviruses can also bind to FcgammaRII and/or NR. We show that the region of MV-N responsible for binding to NR maps to the C-terminal fragment (N(TAIL)). Binding of MV-N to NR on TEC triggers sustained calcium influx and inhibits spontaneous cell proliferation by arresting cells in the G(0) and G(1) phases of the cell cycle. Finally, MV-N binds to both constitutively expressed NR on a large spectrum of cells from different species and to human activated T cells, leading to suppression of their proliferation. These results provide evidence that MV-N, after release in the extracellular compartment, binds to NR and thereby plays a role in MV-induced immunosuppression.

Dendritic cells and measles virus infection

Measles is a major cause of childhood mortality in developing countries which is mainly attributed to the ability of measles virus (MV) to suppress general immune responses. Paradoxically, virus-specific immunity is efficiently induced, which leads to viral clearance from the host and confers long-lasting protection against reinfection. As sensitisers of pathogen encounter and instructors of the adaptive immune response, dendritic cells (DCs) may play a decisive role in the induction and quality of the MV-specific immune activation. The ability of MV wild-type strains in particular to infect DCs in vitro is dearly established, and the receptor binding haemagglutinin protein of these viruses essentially determines this particular tropism. DC maturation as induced early after MV infection is likely to be of crucial importance for the induction of MV-specific immunity. DCs may, however, be instrumental in MV-induced immunosuppression. (1) T cell depletion could be brought about by DC-T cell fusion or TRAIL-mediated induction of apoptosis. (2) Inhibition of stimulated IL-12 production from MV-infected DCs might affect T cell responses in qualitative terms in favouring Th2 and suppressing Th1 responses. (3) The viral glycoprotein complex expressed at high levels on infected DCs late in infection is able to directly inhibit T cell proliferation by surface contact-dependent negative signalling. This most likely accounts for the failure of infected DC cultures to stimulate allogeneic and inhibit mitogen-stimulated T cell proliferation in vitro and the pronounced proliferative unresponsiveness of T cell ex vivo to polyclonal and antigen-specific stimulation which is a central finding of MV-induced immunosuppression.

Extent of measles virus spread and immune suppression differentiates between wild-type and vaccine strains in the cotton rat model (Sigmodon hispidus)

Infection of humans with wild-type measles virus leads to strong immune suppression and secondary infections, whereas immunization with an attenuated vaccine strain does not. Using the cotton rat model (Sigmodon hispidus), we investigated whether vaccine and wild-type viruses differ in viral spread and whether this is correlated with inhibition of of proliferation of spleen cells ex vivo after mitogen stimulation. After intranasal infection of cotton rats with wild-type and vaccine strains, it was found that wild-type virus replicates better in lung tissue, spreads to the mediastinal lymph nodes, and induces a more pronounced and longer-lasting inhibition of proliferation of spleen cells ex vivo after mitogen stimulation than does vaccine virus. To induce the same degree of proliferation inhibition, 1,000-fold less wild-type virus was required than vaccine virus. With this system, the virulence of various measles virus isolates and recombinant viruses was tested. Four (in humans and/or monkeys) highly pathogenic virus strains were immunosuppressive, whereas viruses of vaccine virus genotype A were not. Using virus pairs which, due to passage on fibroblasts versus lymphoid cells or due to a point mutation in the hemagglutinin (N481 --> Y), differed in their usage of the two receptor molecules CD46 and CD150 on human cells, it was found that viruses using exclusively CD150 in vitro spread to mediastinal lymph nodes and induced strong immune suppression. These data demonstrate that important parameters of virulence seen in humans, such as viral spread and immune suppression, are reflected in the cotton rat model.

Hemagglutinin protein of wild-type measles virus activates toll-like receptor 2 signaling

Pattern recognition via Toll-like receptors (TLR) by antigen-presenting cells is an important element of innate immunity. We report that wild-type measles virus but not vaccine strains activate cells via both human and murine TLR2, and this is a property of the hemagglutinin (H) protein. The ability to activate cells via TLR2 by wild-type MV H protein is abolished by mutation of a single amino acid, asparagine at position 481 to tyrosine, as is found in attenuated strains, which is important for interaction with CD46, the receptor for these strains. TLR2 activation by MV wild-type H protein stimulates induction of proinflammatory cytokines such as interleukin-6 (IL-6) in human monocytic cells and surface expression of CD150, the receptor for all MV strains. Confirming the specificity of this interaction, wild-type H protein did not induce IL-6 release in macrophages from TLR2-/- mice. Thus, the unique property of MV wild-type strains to activate TLR2-dependent signals might essentially contribute not only to immune activation but also to viral spread and pathogenicity by upregulating the MV receptor on monocytes.

Functional and phenotypic changes in circulating lymphocytes from hospitalized zambian children with measles

Measles is associated with immunosuppression and increased susceptibility to secondary infections and is a particular problem in developing countries. Lymphocyte changes accompanying immune activation and regulation of the immune response may contribute to immunosuppression. To evaluate lymphocyte changes during measles, children (n = 274) hospitalized with measles in Lusaka, Zambia, were evaluated at entry, discharge, and 1-month follow-up and compared to healthy Zambian children (n = 98). Lymphopenia was present on hospital admission and reflected decreased CD4 and CD8 T cells but resolved quickly. Lymphopenia was most marked in girls, in those with temperatures of >38.5 degrees C, and in malnourished children. CD4/CD8 ratios were decreased at all time points and were lower in boys than in girls at discharge and follow-up. Spontaneous death occurred in cultured lymphocytes, and the proportions of freshly isolated cells undergoing apoptosis, based on annexin V and propidium iodide staining, were increased. Surface Fas was increased on both CD4 and CD8 T cells compared to controls, and expression was greater on CD4 T cells and was inversely correlated with lymphocyte viability in culture at study entry. Mitogen stimulation of lymphocytes improved viability, but inhibitors of Fas, tumor necrosis factor (TNF)-related apoptosis-inducing ligand, and TNF did not. Plasma levels of beta(2) microglobulin and soluble Fas, Fas ligand, CD8, CD4, and TNF receptor were increased, and soluble CD8 was higher in boys than in girls. The multiple effects of measles on lymphocytes from Zambian children include decreased numbers in circulation, increased activation, and increased susceptibility to cell death, with substantive differences in the magnitude of these changes between boys and girls.

Interferons mediate terminal differentiation of human cortical thymic epithelial cells

In the thymus, epithelial cells comprise a heterogeneous population required for the generation of functional T lymphocytes, suggesting that thymic epithelium disruption by viruses may compromise T-cell lymphopoiesis in this organ. In a previous report, we demonstrated that in vitro, measles virus induced differentiation of cortical thymic epithelial cells as characterized by (i) cell growth arrest, (ii) morphological and phenotypic changes, and (iii) apoptotis as a final step of this process. In the present report, we have analyzed the mechanisms involved. First, measles virus-induced differentiation of thymic epithelial cells is shown to be strictly dependent on beta interferon (IFN-beta) secretion. In addition, transfection with double-stranded RNA, a common intermediate of replication for a broad spectrum of viruses, is reported to similarly mediate thymic epithelial cell differentiation through IFN-beta induction. Finally, we demonstrated that recombinant IFN-alpha, IFN-beta, or IFN-gamma was sufficient to induce differentiation and apoptosis of uninfected thymic epithelial cells. These observations suggested that interferon secretion by either infected cells or activated leukocytes, such as plasmacytoid dendritic cells or lymphocytes, may induce thymic epithelium disruption in a pathological context. Thus, we have identified a new mechanism that may contribute to thymic atrophy and altered T-cell lymphopoiesis associated with many infections.

Respiratory syncytial virus fusion protein mediates inhibition of mitogen-induced T-cell proliferation by contact

Human respiratory syncytial virus (HRSV) and bovine respiratory syncytial virus (BRSV) are major pathogens in infants and calves, respectively. Experimental BRSV infection of calves and lambs is associated with lymphopenia and a reduction in responsiveness of peripheral blood lymphocytes (PBLs) to mitogens ex vivo. In this report, we show that in vitro mitogen-induced proliferation of PBLs is inhibited after contact with RSV-infected and UV-inactivated cells or with cells expressing RSV envelope proteins on the cell surface. The protein responsible was identified as the RSV fusion protein (F), as cells infected with a recombinant RSV expressing F as the single envelope protein or cells transfected with a plasmid encoding F were able to induce this effect. Thus, direct contact with RSV F is necessary and sufficient to inhibit proliferation of PBLs. Interestingly, F derived from HRSV was more efficient in inhibiting human PBL proliferation, while F from BRSV was more efficient in inhibiting bovine PBLs. Since various T-cell activation markers were upregulated after presenter cell contact, T lymphocytes are viable and may still be activated by mitogen. However, a significant fraction of PBLs were delayed or defective in G0/G1 to S-phase transit.

Neutralizing human Fab fragments against measles virus recovered by phage display

Five human recombinant Fab fragments (Fabs) specific for measles virus (MV) proteins were isolated from three antibody phage display libraries generated from RNAs derived from bone marrow or splenic lymphocytes from three MV-immune individuals. All Fabs reacted in an enzyme-linked immunosorbent assay with MV antigens. In radioimmunoprecipitation assays two of the Fabs, MV12 and MT14, precipitated an approximately equal 80-kDa protein band corresponding to the hemagglutinin (H) protein from MV-infected Vero cell cultures, while two other Fabs, MT64 and GL29, precipitated an approximately equal 60-kDa protein corresponding the nucleocapsid (N) protein. In competition studies with MV fusion, H- and N protein-specific monoclonal antibodies (MAbs), the H-specific Fabs predominantly blocked the binding of H-specific MAbs, while the N-specific Fabs blocked MAbs to N. In addition, N-specific Fabs bound to denatured MV N protein in Western blotting. The specificity of the fifth Fab, MV4, could not be determined. By plaque reduction assays, three of the five Fabs, MV4, MV12, and MT14, exhibited neutralizing activity (80% cutoff) against MV (LEC-KI strain) at concentrations ranging between approximately 2 and 7 microg x ml(-1). Neutralization capacity against MV strains Edmonston and Schwarz was also detected, albeit at somewhat higher Fab concentrations. In conclusion, three neutralizing Fabs were isolated, two of them reactive against the H glycoprotein of MV and another reactive against an undefined epitope. This is the first study in which MV-neutralizing human recombinant Fab antibodies have been isolated from phage display libraries.

The haemagglutinin protein is an important determinant of measles virus tropism for dendritic cells in vitro

Recombinant measles viruses (MV) in which the authentic glycoprotein genes encoding the fusion and the haemagglutinin (H) proteins of the Edmonston (ED) vaccine strains were swapped singly or doubly for the corresponding genes of a lymphotropic MV wild-type virus (strain WTF) were used previously to investigate MV tropism in cell lines in tissue culture. When these recombinants and their parental strains, the molecular ED-based clone (ED-tag) and WTF, were used to infect cotton rats, only viruses expressing the MV WTF H protein replicated in secondary lymphatic tissues and caused significant immunosuppression. In vitro, viruses containing the ED H protein revealed a tropism for human peripheral blood lymphocytes as documented by enhanced binding and virus production, whereas those containing the WTF H protein replicated well in monocyte-derived dendritic cells (Mo-DC). This did not correlate with more efficient binding of these viruses to DC, but with an enhancement of uptake, virus spread, accumulation of viral antigens and virus production. Thus, replacement of the ED H protein with WTF H protein was sufficient to confer the DC tropism of WTF to ED-tag in vitro. This study suggests that the MV H protein plays an important role in determining cell tropism to immune cells and this may play an important role in the induction of immunosuppression in vivo.

Disruption of Akt kinase activation is important for immunosuppression induced by measles virus

Surface-contact-mediated signaling induced by the measles virus (MV) fusion and hemagglutinin glycoproteins is necessary and sufficient to induce T-cell unresponsiveness in vitro and in vivo. To define the intracellular pathways involved, we analyzed interleukin (IL)-2R signaling in primary human T cells and in Kit-225 cells. Unlike IL-2-dependent activation of JAK/STAT pathways, activation of Akt kinase was impaired after MV contact both in vitro and in vivo. MV interference with Akt activation was important for immunosuppression, as expression of a catalytically active Akt prevented negative signaling by the MV glycoproteins. Thus, we show here that MV exploits a novel strategy to interfere with T-cell activation during immunosuppression.

Recombinant measles viruses expressing heterologous antigens of mumps and simian immunodeficiency viruses

We have genetically engineered a panel of recombinant measles viruses (rMVs) that express from various positions within the MV genome either the HN or F surface glycoproteins of mumps virus (MuV) or the env, gag or pol proteins from simian immunodeficiency virus (SIV). All rMVs were rescued from the respective antigenomic plasmid constructs; progeny viruses replicated comparably to the progenitor Edmonston B MV, but showed slight propagation retardation, which was dependent on the size and nature of the expressed proteins and on the genomic position of the inserts. All transgenes except that encoding mumps F glycoprotein were faithfully maintained and expressed even after virus amplification by 10(20). Our results suggest possible applications of rMVs as live-attenuated, multivalent vaccines against retroviruses such as SIV and HIV as well as other pathogens more distantly related to MV than MuV.

Resistance of recent measles virus wild-type isolates to antibody-mediated neutralization by vaccinees with antibody

The neutralization capacity of sera from Luxembourgian adolescent vaccinees and from Nigerian women with measles-induced immunity to a number of measles virus strains was compared. Although both cohorts were matched for their hemagglutination inhibition and standard neutralization titers, 12 of the 22 late convalescent sera, and only 6 of 24 vaccinees neutralized all viruses. Similarly, only 2 of 20 viruses were not neutralized by at least 75% of late convalescent sera, in comparison to 10 of 20 viruses that resisted neutralization by at least 75% of the vaccinees. The more resistant viruses were not limited to a certain clade. One Nigerian virus was resistant to neutralization by 30% of the late convalescent women and by 75% of vaccinees. These results suggest that qualitative differences in neutralizing antibodies may reduce further protection of infants by passively acquired immunity against wild-type viruses when vaccinated girls become mothers.