Infection of cynomolgus macaques (Macaca fascicularis) and rhesus macaques (Macaca mulatta) with different wild-type measles viruses

What's going on with measles?

Measles is a highly transmissible systemic viral infection associated with substantial mortality primarily due to secondary infections. Measles induces lifelong immunity to reinfection but loss of immunity to other pathogens. An attenuated live virus vaccine is highly effective, but lapses in delivery have resulted in increasing cases worldwide. Although the primary cause of failure to control measles is failure to vaccinate, waning vaccine-induced immunity and the possible emergence of more virulent virus strains may also contribute.

Measles Infection Dose Responses: Insights from Mathematical Modeling

How viral infections develop can change based on the number of viruses initially entering the body. The understanding of the impacts of infection doses remains incomplete, in part due to challenging constraints, and a lack of research. Gaining more insights is crucial regarding the measles virus (MV). The higher the MV infection dose, the earlier the peak of acute viremia, but the magnitude of the peak viremia remains almost constant. Measles is highly contagious, causes immunosuppression such as lymphopenia, and contributes substantially to childhood morbidity and mortality. This work investigated mechanisms underlying the observed wild-type measles infection dose responses in cynomolgus monkeys. We fitted longitudinal data on viremia using maximum likelihood estimation, and used the Akaike Information Criterion (AIC) to evaluate relevant biological hypotheses and their respective model parameterizations. The lowest AIC indicates a linear relationship between the infection dose, the initial viral load, and the initial number of activated MV-specific T cells. Early peak viremia is associated with high initial number of activated MV-specific T cells. Thus, when MV infection dose increases, the initial viremia and associated immune cell stimulation increase, and reduce the time it takes for T cell killing to be sufficient, thereby allowing dose-independent peaks for viremia, MV-specific T cells, and lymphocyte depletion. Together, these results suggest that the development of measles depends on virus-host interactions at the start and the efficiency of viral control by cellular immunity. These relationships are additional motivations for prevention, vaccination, and early treatment for measles.

Canine Distemper Virus Pathogenesis in the Ferret Model

Canine distemper virus (CDV) is a highly contagious pathogen within the morbillivirus genus infecting a wide range of different carnivore species. The virus shares most biological features with other closely related morbilliviruses, including clinical signs, tissue tropism, and replication cycle in the respective host organisms.In the laboratory environment, experimental infections of ferrets with CDV were established as a potent surrogate model for the analysis of several aspects of the biology of the human morbillivirus, measles virus (MeV). The animals are naturally susceptible to CDV and display severe clinical signs resembling the disease seen in patients infected with MeV. As seen with MeV, CDV infects immune cells and is thus associated with a strong transient immunosuppression. Here we describe several methods to evaluate viral load and parameters of immunosuppression in blood-circulating immune cells isolated from CDV-infected animals.

Inoculation of raccoons with a wild-type-based recombinant canine distemper virus results in viremia, lymphopenia, fever, and widespread histological lesions

Raccoons are naturally susceptible to canine distemper virus (CDV) infection and can be a potential source of spill-over events. CDV is a highly contagious morbillivirus that infects multiple species of carnivores and omnivores, resulting in severe and often fatal disease. Here, we used a recombinant CDV (rCDV) based on a full-genome sequence detected in a naturally infected raccoon to perform pathogenesis studies in raccoons. Five raccoons were inoculated intratracheally with a recombinant virus engineered to express a fluorescent reporter protein, and extensive virological, serological, histological, and immunohistochemical assessments were performed at different time points post inoculation. rCDV-infected white blood cells were detected as early as 4 days post inoculation (dpi). Raccoon necropsies at 6 and 8 dpi revealed replication in the lymphoid tissues, preceding spread into peripheral tissues observed during necropsies at 21 dpi. Whereas lymphocytes, and to a lesser extent myeloid cells, were the main target cells of CDV at early time points, CDV additionally targeted epithelia at 21 dpi. At this later time point, CDV-infected cells were observed throughout the host. We observed lymphopenia and lymphocyte depletion from lymphoid tissues after CDV infection, in the absence of detectable CDV neutralizing antibodies and an impaired ability to clear CDV, indicating that the animals were severely immunosuppressed. The use of a wild-type-based recombinant virus in a natural host species infection study allowed systematic and sensitive assessment of antigen detection by immunohistochemistry, enabling further comparative pathology studies of CDV infection in different species. IMPORTANCE Expansion of the human interface supports increased interactions between humans and peridomestic species like raccoons. Raccoons are highly susceptible to canine distemper virus (CDV) and are considered an important target species. Spill-over events are increasingly likely, potentially resulting in fatal CDV infections in domestic and free ranging carnivores. CDV also poses a threat for (non-human) primates, as massive outbreaks in macaque colonies were reported. CDV pathogenesis was studied by experimental inoculation of several species, but pathogenesis in raccoons was not properly studied. Recently, we generated a recombinant virus based on a full-genome sequence detected in a naturally infected raccoon. Here, we studied CDV pathogenesis in its natural host species and show that distemper completely overwhelms the immune system and spreads to virtually all tissues, including the central nervous system. Despite this, raccoons survived up to 21 d post inoculation with long-term shedding, supporting an important role of raccoons as host species for CDV.

Measles in non-human primates

It is six decades since the measles vaccine was first introduced, and yet we continue to see frequent outbreaks of this disease occurring all over the world. Many non-human primate (NHP) species, including apes, are susceptible to the measles virus. Spontaneous measles outbreaks have been described in a number of zoos and primate centers worldwide. Research into the spontaneous and experimental infection of laboratory primates with measles represents an invaluable source of information regarding the biology and pathogenesis of this virus and continues to be an irreplaceable and unique tool for testing vaccines and treatments. The purpose of this literature review is to summarize and analyze published data on the circulation of the measles virus among free-living synanthropic and captive primate populations, as well as the results of experiments that have modeled this infection in NHPs.

Nebulized fusion inhibitory peptide protects cynomolgus macaques from measles virus infection

Measles is the most contagious airborne viral infection and the leading cause of child death among vaccine-preventable diseases. We show here that aerosolized lipopeptide fusion inhibitor, derived from heptad-repeat regions of the measles virus (MeV) fusion protein, blocks respiratory MeV infection in a non-human primate model, the cynomolgus macaque. We use a custom-designed mesh nebulizer to ensure efficient aerosol delivery of peptide to the respiratory tract and demonstrate the absence of adverse effects and lung pathology in macaques. The nebulized peptide efficiently prevents MeV infection, resulting in the complete absence of MeV RNA, MeV-infected cells, and MeV-specific humoral responses in treated animals. This strategy provides an additional means to fight against respiratory infection in non-vaccinated people, that can be readily translated to human trials. It presents a proof-of-concept for the aerosol delivery of fusion inhibitory peptides to protect against measles and other airborne viruses, including SARS-CoV-2, in case of high-risk exposure.

FeMV is a cathepsin-dependent unique morbillivirus infecting the kidneys of domestic cats

Feline morbillivirus (FeMV) is a recently discovered pathogen of domestic cats and has been classified as a morbillivirus in the Paramyxovirus family. We determined the complete sequence of FeMV^US5 directly from an FeMV-positive urine sample without virus isolation or cell passage. Sequence analysis of the viral genome revealed potential divergence from characteristics of archetypal morbilliviruses. First, the virus lacks the canonical polybasic furin cleavage signal in the fusion (F) glycoprotein. Second, conserved amino acids in the hemagglutinin (H) glycoprotein used by all other morbilliviruses for binding and/or fusion activation with the cellular receptor CD150 (signaling lymphocyte activation molecule [SLAM]/F1) are absent. We show that, despite this sequence divergence, FeMV H glycoprotein uses feline CD150 as a receptor and cannot use human CD150. We demonstrate that the protease responsible for cleaving the FeMV F glycoprotein is a cathepsin, making FeMV a unique morbillivirus and more similar to the closely related zoonotic Nipah and Hendra viruses. We developed a reverse genetics system for FeMV^US5 and generated recombinant viruses expressing Venus fluorescent protein from an additional transcription unit located either between the phospho-protein (P) and matrix (M) genes or the H and large (L) genes of the genome. We used these recombinant FeMVs to establish a natural infection and demonstrate that FeMV causes an acute morbillivirus-like disease in the cat. Virus was shed in the urine and detectable in the kidneys at later time points. This opens the door for long-term studies to address the postulated role of this morbillivirus in the development of chronic kidney disease.

Dynamic Control Balancing Cell Proliferation and Inflammation is Crucial for an Effective Immune Response to Malaria

Malaria has a complex pathology with varying manifestations and symptoms, effects on host tissues, and different degrees of severity and ultimate outcome, depending on the causative Plasmodium pathogen and host species. Previously, we compared the peripheral blood transcriptomes of two macaque species (Macaca mulatta and Macaca fascicularis) in response to acute primary infection by Plasmodium knowlesi. Although these two species are very closely related, the infection in M. mulatta is fatal, unless aggressively treated, whereas M. fascicularis develops a chronic, but tolerable infection in the blood. As a reason for this stark difference, our analysis suggests delayed pathogen detection in M. mulatta followed by extended inflammation that eventually overwhelms this monkey’s immune response. By contrast, the natural host M. fascicularis detects the pathogen earlier and controls the inflammation. Additionally, M. fascicularis limits cell proliferation pathways during the log phase of infection, presumably in an attempt to control inflammation. Subsequent cell proliferation suggests a cell-mediated adaptive immune response. Here, we focus on molecular mechanisms underlying the key differences in the host and parasite responses and their coordination. SICAvar Type 1 surface antigens are highly correlated with pattern recognition receptor signaling and important inflammatory genes for both hosts. Analysis of pathogen detection pathways reveals a similar signaling mechanism, but with important differences in the glutamate G-protein coupled receptor (GPCR) signaling pathway. Furthermore, differences in inflammasome assembly processes suggests an important role of S100 proteins in balancing inflammation and cell proliferation. Both differences point to the importance of Ca²⁺ homeostasis in inflammation. Additionally, the kynurenine-to-tryptophan ratio, a known inflammatory biomarker, emphasizes higher inflammation in M. mulatta during log phase. Transcriptomics-aided metabolic modeling provides a functional method for evaluating these changes and understanding downstream changes in NAD metabolism and aryl hydrocarbon receptor (AhR) signaling, with enhanced NAD metabolism in M. fascicularis and stronger AhR signaling in M. mulatta. AhR signaling controls important immune genes like IL6, IFNγ and IDO1. However, direct changes due to AhR signaling could not be established due to complicated regulatory feedback mechanisms associated with the AhR repressor (AhRR). A complete understanding of the exact dynamics of the immune response is difficult to achieve. Nonetheless, our comparative analysis provides clear suggestions of processes that underlie an effective immune response. Thus, our study identifies multiple points of intervention that are apparently responsible for a balanced and effective immune response and thereby paves the way toward future immune strategies for treating malaria.

Molecular Epidemiology of Measles in California, United States-2019

In 2019, the United States (US) experienced the highest number of measles importations and cases in the postelimination era. More than a quarter of imported cases entered the US through California. Measles surveillance efforts in California resulted in the identification of 26 importations, 6 outbreaks, and 72 cases in 2019. Only genotype B3 and D8 measles strains were detected. Genotype-specific differences were noted in the incidence of vaccine failures, hospitalizations, and severe complications among cases. A targeted whole genome sequencing approach provided higher-resolution discrimination between epidemiologically linked and sporadically introduced strains than conventional N450 sequencing. Our report underscores the importance of ensuring appropriate measles vaccination status, especially prior to international travel to measles-endemic regions, and highlights the value of a strong measles surveillance system in minimizing outbreaks and preserving measles elimination status in the US.

Measles pathogenesis, immune suppression and animal models

Measles virus causes a disease with seemingly innocent symptoms, such as fever and rash. However, measles immune suppression causes increased susceptibility to opportunistic infections that are responsible for the majority of over 100000 yearly fatalities. The pathogenesis of measles is complex, because measles virus uses multiple receptors to infect different cell types in different phases of the disease. Experimental morbillivirus infections with wild-type viruses in natural host species have demonstrated that direct infection and depletion of memory immune cells causes immune amnesia. This was confirmed in studies of a measles outbreak in unvaccinated children and provides an explanation for epidemiological observations of long-term increases in morbidity and mortality after measles.

Measles virus persistence and its consequences

Clearance of measles virus is complex. Infectious virus is cleared by the adaptive immune response manifested by the characteristic maculopapular rash. CD8+ T cells are major effectors of infectious virus clearance, a process that may fail in individuals with compromised cellular immune responses leading to progressive giant cell pneumonia and/or measles inclusion body encephalitis. In contrast to the usual rapid clearance of infectious virus, clearance of viral RNA is slow with persistence in lymphoid tissue for many months. Persistence of MeV RNA may contribute to the late development of the slowly progressive disease subacute sclerosing panencephalitis in children infected at a young age and to measles-associated immune suppression but also to maturation of the immune response and development of life-long immunity.

mSphere of Influence: Understanding Virus-Host Interactions Requires a Multifaceted Approach

Rory de Vries works in the field of viral pathogenesis and focuses on interactions between respiratory viruses (or corresponding vaccines) and the host immune system. In this mSphere of Influence article, he reflects on how the articles “Predominant infection of CD150⁺ lymphocytes and dendritic cells during measles virus infection of macaques” by R. L. de Swart et al. (R. L. de Swart, M. Ludlow, L. de Witte, Y. Yanagi, et al., PLoS Pathog 3:e178, 2007, https://doi.org/10.1371/journal.ppat.0030178) and “Long-term measles-induced immunomodulation increases overall childhood infectious disease mortality” by M.

Rory de Vries works in the field of viral pathogenesis and focuses on interactions between respiratory viruses (or corresponding vaccines) and the host immune system. In this mSphere of Influence article, he reflects on how the articles “Predominant infection of CD150⁺ lymphocytes and dendritic cells during measles virus infection of macaques” by R. L. de Swart et al. (R. L. de Swart, M. Ludlow, L. de Witte, Y. Yanagi, et al., PLoS Pathog 3:e178, 2007, https://doi.org/10.1371/journal.ppat.0030178) and “Long-term measles-induced immunomodulation increases overall childhood infectious disease mortality” by M. J. Mina et al. (M. J. Mina, C. J. Metcalf, R. L. de Swart, A. D. M. E. Osterhaus, and B. T. Grenfell, Science 348:694–699, 2015, https://doi.org/10.1126/science.aaa3662) made an impact on him. These articles studied interactions between measles virus and the host and influenced him by making two important points. (i) It is crucial to use nonadapted (recombinant) viruses in disease-relevant model systems when studying virus-host interactions. (ii) Studying viral pathogenesis requires a combination of in vitro, ex vivo, and in vivo studies, and a group of researchers with multiple expertises. He learned that only when all these aspects are combined, can one truly answer the question: “How does a virus cause disease?”

Measles Virus Infection and Immunity in a Suboptimal Vaccination Coverage Setting

Despite efforts to improve surveillance and vaccination coverage, measles virus (MeV) continues to cause outbreaks also in high-income countries. As the reference laboratory of the Veneto Region, Italy, we analyzed changes in population immunity, described measles outbreaks, investigated MeV genetic diversity, and evaluated cross-protection of measles vaccination against MeV epidemic strains. Like most European areas, the Veneto Region has suboptimal measles vaccination coverage and is facing a growing public mistrust of vaccination. A progressive decline of measles vaccine uptake was observed during the last decade in the Veneto Region, leading to immunity gaps in children and young adults. Measles outbreaks were caused by the same MeV genotype B3, D4, and D8 strains that were circulating in other European countries. Eleven cases of measles were observed in immunized subjects. These cases were not associated with particular MeV genotypes nor with mutations in epitopes recognized by neutralizing antibodies. Accordingly, sera from fully vaccinated subjects cross-neutralized epidemic MeV strains, including the genotypes B3, D4, and D8, with the same high efficiency demonstrated against the vaccine strain. In fully vaccinated subjects, high MeV IgG antibody titers persisted up to 30 years following vaccination. These results support the use of the current measles-containing vaccines and strategies to strengthen vaccination.

Measles Encephalitis: Towards New Therapeutics

Measles remains a major cause of morbidity and mortality worldwide among vaccine preventable diseases. Recent decline in vaccination coverage resulted in re-emergence of measles outbreaks. Measles virus (MeV) infection causes an acute systemic disease, associated in certain cases with central nervous system (CNS) infection leading to lethal neurological disease. Early following MeV infection some patients develop acute post-infectious measles encephalitis (APME), which is not associated with direct infection of the brain. MeV can also infect the CNS and cause sub-acute sclerosing panencephalitis (SSPE) in immunocompetent people or measles inclusion-body encephalitis (MIBE) in immunocompromised patients. To date, cellular and molecular mechanisms governing CNS invasion are still poorly understood. Moreover, the known MeV entry receptors are not expressed in the CNS and how MeV enters and spreads in the brain is not fully understood. Different antiviral treatments have been tested and validated in vitro, ex vivo and in vivo, mainly in small animal models. Most treatments have high efficacy at preventing infection but their effectiveness after CNS manifestations remains to be evaluated. This review describes MeV neural infection and current most advanced therapeutic approaches potentially applicable to treat MeV CNS infection.

Genotype-Specific Measles Transmissibility: A Branching Process Analysis

Background

Substantial heterogeneity in measles outbreak sizes may be due to genotype-specific transmissibility. Using a branching process analysis, we characterize differences in measles transmission by estimating the association between genotype and the reproduction number R among postelimination California measles cases during 2000-2015 (400 cases, 165 outbreaks).

Methods

Assuming a negative binomial secondary case distribution, we fit a branching process model to the distribution of outbreak sizes using maximum likelihood and estimated the reproduction number R for a multigenotype model.

Results

Genotype B3 is found to be significantly more transmissible than other genotypes (P = .01) with an R of 0.64 (95% confidence interval [CI], .48-.71), while the R for all other genotypes combined is 0.43 (95% CI, .28-.54). This result is robust to excluding the 2014-2015 outbreak linked to Disneyland theme parks (referred to as "outbreak A" for conciseness and clarity) (P = .04) and modeling genotype as a random effect (P = .004 including outbreak A and P = .02 excluding outbreak A). This result was not accounted for by season of introduction, age of index case, or vaccination of the index case. The R for outbreaks with a school-aged index case is 0.69 (95% CI, .52-.78), while the R for outbreaks with a non-school-aged index case is 0.28 (95% CI, .19-.35), but this cannot account for differences between genotypes.

Conclusions

Variability in measles transmissibility may have important implications for measles control; the vaccination threshold required for elimination may not be the same for all genotypes or age groups.

Case-based surveillance of measles in Sicily during 2012-2017: The changing molecular epidemiology and implications for vaccine strategies

Following the indication of the World Health Organization, a national plan for the elimination of measles was approved in Italy and this included the improvement of the molecular surveillance of measles viruses and the interruption of indigenous transmission of the disease. Nevertheless, large outbreaks continue to occur in almost all regions of the country, including Sicily. Here we describe the epidemiology and molecular dynamics of measles viruses as a result of the measles surveillance activity carried out by the "Reference Laboratory for Measles and Rubella" in Sicily over a 5-year period. Biological samples of 259 suspected measles cases were tested for viral RNA detection and a total of 223 (86.1%) were classified as laboratory confirmed. The median age of confirmed measles cases was 21.0 years and about half of them were adults aged 19 years and older. Overall, one-third of the patients showed clinical complications and these latter were more common among adults than children (44.9% vs. 25.7%). The vast majority of measles cases were unvaccinated (94.2%, n = 210). The phylogenetic analysis of 221 measles virus nucleotide sequences revealed sporadic detections of genotypes D4 and H1, while endemic circulation of genotypes D8 and B3 was documented. Genotype D8 was associated with epidemics occurred between 2013 and 2016, whereas genotype B3 was more recently introduced into Sicily characterizing the current measles outbreak. The results of this study confirm the autochthonous co-circulation of viral variants belonging to different genotypes during the study period, and emphasizes the need of measles surveillance programmes in order to investigate the viral dynamics, the pathways of disease transmission, and to eventually adapt the development of successfull vaccine formulations.

Hemagglutinin-specific neutralization of subacute sclerosing panencephalitis viruses

Subacute sclerosing panencephalitis (SSPE) is a progressive, lethal complication of measles caused by particular mutants of measles virus (MeV) that persist in the brain despite high levels of neutralizing antibodies. We addressed the hypothesis that antigenic drift is involved in the pathogenetic mechanism of SSPE by analyzing antigenic alterations in the MeV envelope hemagglutinin protein (MeV-H) found in patients with SSPE in relation to major circulating MeV genotypes. To this aim, we obtained cDNA for the MeV-H gene from tissue taken at brain autopsy from 3 deceased persons with SSPE who had short (3-4 months, SMa79), average (3.5 years, SMa84), and long (18 years, SMa94) disease courses. Recombinant MeVs with a substituted MeV-H gene were generated by a reverse genetic system. Virus neutralization assays with a panel of anti-MeV-H murine monoclonal antibodies (mAbs) or vaccine-immunized mouse anti-MeV-H polyclonal sera were performed to determine the antigenic relatedness. Functional and receptor-binding analysis of the SSPE MeV-H showed activity in a SLAM/nectin-4-dependent manner. Similar to our panel of wild-type viruses, our SSPE viruses showed an altered antigenic profile. Genotypes A, G3, and F (SSPE case SMa79) were the exception, with an intact antigenic structure. Genotypes D7 and F (SSPE SMa79) showed enhanced neutralization by mAbs targeting antigenic site IIa. Genotypes H1 and the recently reported D4.2 were the most antigenically altered genotypes. Epitope mapping of neutralizing mAbs BH015 and BH130 reveal a new antigenic site on MeV-H, which we designated Φ for its intermediate position between previously defined antigenic sites Ia and Ib. We conclude that SSPE-causing viruses show similar antigenic properties to currently circulating MeV genotypes. The absence of a direct correlation between antigenic changes and predisposition of a certain genotype to cause SSPE does not lend support to the proposed antigenic drift as a pathogenetic mechanism in SSPE.

Needle-free delivery of measles virus vaccine to the lower respiratory tract of non-human primates elicits optimal immunity and protection

Needle-free measles virus vaccination by aerosol inhalation has many potential benefits. The current standard route of vaccination is subcutaneous injection, whereas measles virus is an airborne pathogen. However, the target cells that support replication of live-attenuated measles virus vaccines in the respiratory tract are largely unknown. The aims of this study were to assess the in vivo tropism of live-attenuated measles virus and determine whether respiratory measles virus vaccination should target the upper or lower respiratory tract. Four groups of twelve cynomolgus macaques were immunized with 10⁴ TCID₅₀ of recombinant measles virus vaccine strain Edmonston-Zagreb expressing enhanced green fluorescent protein. The vaccine virus was grown in MRC-5 cells and formulated with identical stabilizers and excipients as used in the commercial MV^EZ vaccine produced by the Serum Institute of India. Animals were immunized by hypodermic injection, intra-tracheal inoculation, intra-nasal instillation, or aerosol inhalation. In each group six animals were euthanized at early time points post-vaccination, whereas the other six were followed for 14 months to assess immunogenicity and protection from challenge infection with wild-type measles virus. At early time-points, enhanced green fluorescent protein-positive measles virus-infected cells were detected locally in the muscle, nasal tissues, lungs, and draining lymph nodes. Systemic vaccine virus replication and viremia were virtually absent. Infected macrophages, dendritic cells and tissue-resident lymphocytes predominated. Exclusive delivery of vaccine virus to the lower respiratory tract resulted in highest immunogenicity and protection. This study sheds light on the tropism of a live-attenuated measles virus vaccine and identifies the alveolar spaces as the optimal site for respiratory delivery of measles virus vaccine.

Delineating morbillivirus entry, dissemination and airborne transmission by studying in vivo competition of multicolor canine distemper viruses in ferrets

Identification of cellular receptors and characterization of viral tropism in animal models have vastly improved our understanding of morbillivirus pathogenesis. However, specific aspects of viral entry, dissemination and transmission remain difficult to recapitulate in animal models. Here, we used three virologically identical but phenotypically distinct recombinant (r) canine distemper viruses (CDV) expressing different fluorescent reporter proteins for in vivo competition and airborne transmission studies in ferrets (Mustela putorius furo). Six donor ferrets simultaneously received three rCDVs expressing green, red or blue fluorescent proteins via conjunctival (ocular, Oc), intra-nasal (IN) or intra-tracheal (IT) inoculation. Two days post-inoculation sentinel ferrets were placed in physically separated adjacent cages to assess airborne transmission. All donor ferrets developed lymphopenia, fever and lethargy, showed progressively increasing systemic viral loads and were euthanized 14 to 16 days post-inoculation. Systemic replication of virus inoculated via the Oc, IN and IT routes was detected in 2/6, 5/6 and 6/6 ferrets, respectively. In five donor ferrets the IT delivered virus dominated, although replication of two or three different viruses was detected in 5/6 animals. Single lymphocytes expressing multiple fluorescent proteins were abundant in peripheral blood and lymphoid tissues, demonstrating the occurrence of double and triple virus infections. Transmission occurred efficiently and all recipient ferrets showed evidence of infection between 18 and 22 days post-inoculation of the donor ferrets. In all cases, airborne transmission resulted in replication of a single-colored virus, which was the dominant virus in the donor ferret. This study demonstrates that morbilliviruses can use multiple entry routes in parallel, and co-infection of cells during viral dissemination in the host is common. Airborne transmission was efficient, although transmission of viruses expressing a single color suggested a bottleneck event. The identity of the transmitted virus was not determined by the site of inoculation but by the viral dominance during dissemination.

Comparison of neutralizing antibody titers against outbreak-associated measles genotypes (D4, H1 and B3) in Iran

Despite the accessibility of a promising vaccine, outbreaks of the measles virus (MV) take place even in well-vaccinated populations. D4, H1 and B3 genotypes have been detected regularly in different regions of Iran. These observations highlight the necessity of evaluating the protective efficacy of the vaccine against currently circulating MV genotypes during the elimination phase. A focus reduction neutralization test has been developed to measure the neutralizing antibodies against different genotypes of MV, such as H1, D4, B3 and vaccine strain (A), in children after second doses of measles vaccine. The geometric mean titer (GMT) rates of the sera against D4, H1, B3 and A genotypes were 95.9, 90.5, 32.0 and 76.1, respectively. Low GMTs of antibody against the B3 genotype compared with the other genotypes were indicated. Based on the current study results, the MV antibody titers in the sera of vaccinated cases are sufficient to neutralize all circulating genotypes in Iran; however, neutralizing antibody titers were lower for the B3 genotype than for the H1, D4 and A genotypes. The heterogeneous nature of MV, for instance the nucleotide sequence diversity between different strains, necessitates the evaluation of the protective efficacy of the vaccine against measles B3 genotype in countries where this virus has been the most commonly identified circulating genotype.

The phosphoprotein genes of measles viruses from subacute sclerosing panencephalitis cases encode functional as well as non-functional proteins and display reduced editing

Products expressed from the second (P/V/C) gene are important in replication and abrogating innate immune responses during acute measles virus (MV) infection. Thirteen clone sets were derived from the P/V/C genes of measles virus (MV) RNA extracted from brains of a unique collection of seven cases of subacute sclerosing panencephalitis (SSPE) caused by persistent MV in the central nervous system (CNS). Whether these functions are fully maintained when MV replicates in the CNS has not been previously determined. Co-transcriptional editing of the P mRNAs by non-template insertion of guanine (G) nucleotides, which generates mRNAs encoding the viral V protein, occurs much less frequently (9%) in the SSPE derived samples than during the acute infection (30-50%). Thus it is likely that less V protein, which is involved in combatting the innate immune response, is produced. The P genes in MV from SSPE cases were not altered by biased hypermutation but exhibited a high degree of variation within each case. Most but not all SSPE derived phospho-(P) proteins were functional in mini genome replication/transcription assays. An eight amino acid truncation of the carboxyl-terminus made the P protein non-functional while the insertion of an additional glycine residue by insertion of G nucleotides at the editing site had no effect on protein function.

Limited in vivo production of type I or type III interferon after infection of macaques with vaccine or wild-type strains of measles virus

The innate immune response to viral infections often includes induction of types I and III interferons (IFNs) and production of antiviral proteins. Measles is a severe virus-induced rash disease, but in vitro studies suggest that in the absence of defective interfering RNAs, neither wild-type (WT) nor vaccine strains of measles virus (MeV) induce IFN. To determine whether IFN is produced in vivo, we studied tissues from macaques infected with vaccine or WT strains of MeV using quantitative reverse transcriptase-polymerase chain reaction to assess levels of IFN and IFN-stimulated gene (ISG) mRNAs and a flow cytometry-based bioassay to assess levels of biologically active IFN. There was little to no induction of type I IFN, type III IFN, Mx, or ISG56 mRNAs in monkeys infected with vaccine or WT MeV and no IFN detection by bioassay. Therefore, the innate responses to infection with vaccine or WT strains of MeV are not dependent on IFN production.

Live-attenuated measles virus vaccine targets dendritic cells and macrophages in muscle of nonhuman primates

Although live-attenuated measles virus (MV) vaccines have been used successfully for over 50 years, the target cells that sustain virus replication in vivo are still unknown. We generated a reverse genetics system for the live-attenuated MV vaccine strain Edmonston-Zagreb (EZ), allowing recovery of recombinant (r)MV(EZ). Three recombinant viruses were generated that contained the open reading frame encoding enhanced green fluorescent protein (EGFP) within an additional transcriptional unit (ATU) at various positions within the genome. rMV(EZ)EGFP(1), rMV(EZ)EGFP(3), and rMV(EZ)EGFP(6) contained the ATU upstream of the N gene, following the P gene, and following the H gene, respectively. The viruses were compared in vitro by growth curves, which indicated that rMV(EZ)EGFP(1) was overattenuated. Intratracheal infection of cynomolgus macaques with these recombinant viruses revealed differences in immunogenicity. rMV(EZ)EGFP(1) and rMV(EZ)EGFP(6) did not induce satisfactory serum antibody responses, whereas both in vitro and in vivo rMV(EZ)EGFP(3) was functionally equivalent to the commercial MV(EZ)-containing vaccine. Intramuscular vaccination of macaques with rMV(EZ)EGFP(3) resulted in the identification of EGFP(+) cells in the muscle at days 3, 5, and 7 postvaccination. Phenotypic characterization of these cells demonstrated that muscle cells were not infected and that dendritic cells and macrophages were the predominant target cells of live-attenuated MV.

IMPORTANCE

Even though MV strain Edmonston-Zagreb has long been used as a live-attenuated vaccine (LAV) to protect against measles, nothing is known about the primary cells in which the virus replicates in vivo. This is vital information given the push to move toward needle-free routes of vaccination, since vaccine virus replication is essential for vaccination efficacy. We have generated a number of recombinant MV strains expressing enhanced green fluorescent protein. The virus that best mimicked the nonrecombinant vaccine virus was formulated according to protocols for production of commercial vaccine virus batches, and was subsequently used to assess viral tropism in nonhuman primates. The virus primarily replicated in professional antigen-presenting cells, which may explain why this LAV is so immunogenic and efficacious.

Measles vaccination of nonhuman primates provides partial protection against infection with canine distemper virus

Measles virus (MV) is being considered for global eradication, which would likely reduce compliance with MV vaccination. As a result, children will grow up without MV-specific immunity, creating a potential niche for closely related animal morbilliviruses such as canine distemper virus (CDV). Natural CDV infection causing clinical signs has never been reported in humans, but recent outbreaks in captive macaques have shown that CDV can cause disease in primates. We studied the virulence and tropism of recombinant CDV expressing enhanced green fluorescent protein in naive and measles-vaccinated cynomolgus macaques. In naive animals CDV caused viremia and fever and predominantly infected CD150(+) lymphocytes and dendritic cells. Virus was reisolated from the upper and lower respiratory tracts, but infection of epithelial or neuronal cells was not detectable at the time points examined, and the infections were self-limiting. This demonstrates that CDV readily infects nonhuman primates but suggests that additional mutations are necessary to achieve full virulence in nonnatural hosts. Partial protection against CDV was observed in measles-vaccinated macaques, as demonstrated by accelerated control of virus replication and limited shedding from the upper respiratory tract. While neither CDV infection nor MV vaccination induced detectable cross-reactive neutralizing antibodies, MV-specific neutralizing antibody levels of MV-vaccinated macaques were boosted by CDV challenge infection, suggesting that cross-reactive VN epitopes exist. Rapid increases in white blood cell counts in MV-vaccinated macaques following CDV challenge suggested that cross-reactive cellular immune responses were also present. This study demonstrates that zoonotic morbillivirus infections can be controlled by measles vaccination.

IMPORTANCE

Throughout history viral zoonoses have had a substantial impact on human health. Given the drive toward global eradication of measles, it is essential to understand the zoonotic potential of animal morbilliviruses. Morbilliviruses are thought to have evolved from a common ancestral virus that jumped species and adapted to new hosts. Recently, canine distemper virus (CDV), a morbillivirus normally restricted to carnivores, caused disease outbreaks in nonhuman primates. Here, we report that experimental CDV infection of monkeys resulted in fever and leukopenia. The virus replicated to high levels in lymphocytes but did not spread to epithelial cells or the central nervous system. Importantly, like measles virus in macaques, the infections were self-limiting. In measles-vaccinated macaques CDV was cleared more rapidly, resulting in limited virus shedding from the upper respiratory tract. These studies demonstrate that although CDV can readily infect primates, measles immunity is protective, and CDV infection is self-limiting.

Immunogenicity of a recombinant measles HIV-1 subtype C vaccine

The HIV epidemic is greatest in Sub-Saharan Africa and India where HIV-1 subtype C is predominant. To control the spread of HIV in these parts of the world a preventive HIV-1 subtype C vaccine is urgently required. Here we report the immunogenicity of a candidate HIV-1 subtype C vaccine delivered by a recombinant measles vector carrying an insert encoding HIV-1 subtype C Gag, RT and Nef (MV1-F4), in MHC-typed non-human primates. HIV-1 specific cytokine secreting CD4+ and CD8+ T cell responses were detected in 15 out of 16 vaccinees. These HIV-specific T cell responses persisted in lymphoid tissues. Anti-HIV-1 antibody responses were detected in 15 out of 16 vaccinees and titres were boosted by a second immunisation carried out 84 days later. These findings support further exploration of the MV1-F4 vector as a candidate HIV-1 subtype C vaccine or as part of a wider vaccine strategy.

Poor immune responses of newborn rhesus macaques to measles virus DNA vaccines expressing the hemagglutinin and fusion glycoproteins

A vaccine that would protect young infants against measles could facilitate elimination efforts and decrease morbidity and mortality in developing countries. However, immaturity of the immune system is an important obstacle to the development of such a vaccine. In this study, DNA vaccines expressing the measles virus (MeV) hemagglutinin (H) protein or H and fusion (F) proteins, previously shown to protect juvenile macaques, were used to immunize groups of 4 newborn rhesus macaques. Monkeys were inoculated intradermally with 200 μg of each DNA at birth and at 10 months of age. As controls, 2 newborn macaques were similarly vaccinated with DNA encoding the influenza virus H5, and 4 received one dose of the current live attenuated MeV vaccine (LAV) intramuscularly. All monkeys were monitored for development of MeV-specific neutralizing and binding IgG antibody and cytotoxic T lymphocyte (CTL) responses. These responses were poor compared to the responses induced by LAV. At 18 months of age, all monkeys were challenged intratracheally with a wild-type strain of MeV. Monkeys that received the DNA vaccine encoding H and F, but not H alone, were primed for an MeV-specific CD8(+) CTL response but not for production of antibody. LAV-vaccinated monkeys were protected from rash and viremia, while DNA-vaccinated monkeys developed rashes, similar to control monkeys, but had 10-fold lower levels of viremia. We conclude that vaccination of infant macaques with DNA encoding MeV H and F provided only partial protection from MeV infection.

Immunogenicity of a recombinant measles-HIV-1 clade B candidate vaccine

Live attenuated measles virus is one of the most efficient and safest vaccines available, making it an attractive candidate vector for a HIV/AIDS vaccine aimed at eliciting cell-mediated immune responses (CMI). Here we have characterized the potency of CMI responses generated in mice and non-human primates after intramuscular immunisation with a candidate recombinant measles vaccine carrying an HIV-1 insert encoding Clade B Gag, RT and Nef (MV1-F4). Eight Mauritian derived, MHC-typed cynomolgus macaques were immunised with 10⁵ TCID₅₀ of MV1-F4, four of which were boosted 28 days later with the same vaccine. F4 and measles virus (MV)-specific cytokine producing T cell responses were detected in 6 and 7 out of 8 vaccinees, respectively. Vaccinees with either M6 or recombinant MHC haplotypes demonstrated the strongest cytokine responses to F4 peptides. Polyfunctional analysis revealed a pattern of TNFα and IL-2 responses by CD4+ T cells and TNFα and IFNγ responses by CD8+ T cells to F4 peptides. HIV-specific CD4+ and CD8+ T cells expressing cytokines waned in peripheral blood lymphocytes by day 84, but CD8+ T cell responses to F4 peptides could still be detected in lymphoid tissues more than 3 months after vaccination. Anti-F4 and anti-MV antibody responses were detected in 6 and 8 out of 8 vaccinees, respectively. Titres of anti-F4 and MV antibodies were boosted in vaccinees that received a second immunisation. MV1-F4 carrying HIV-1 Clade B inserts induces robust boostable immunity in non-human primates. These results support further exploration of the MV1-F4 vector modality in vaccination strategies that may limit HIV-1 infectivity.

Measles virus selectively blind to signaling lymphocyte activation molecule as a novel oncolytic virus for breast cancer treatment

Oncolytic viruses hold much promise as novel therapeutic agents that can be combined with conventional therapeutic modalities. Measles virus (MV) is known to enter cells using the signaling lymphocyte activation molecule (SLAM), which is expressed on cells of the immune system. Although human breast cancer cell lines do not express SLAM, we found that a wild-type MV (HL strain) efficiently infected various breast cancer cell lines, causing cell death. Based on this finding, we used reverse genetics to generate a recombinant MV selectively unable to use SLAM (rMV-SLAMblind). The rMV-SLAMblind lacked infectivity for SLAM-positive lymphoid cells, while retaining oncolytic activity against breast cancer cells. We showed that, unlike the MV vaccine strains, rMV-SLAMblind used PVRL4 (polio virus receptor-related 4) as a receptor to infect breast cancer cells and not the ubiquitously expressed CD46. Consistent with this, rMV-SLAMblind infected CD46-positive primary normal human cells at a much-reduced level, whereas a vaccine strain of the Edmonston lineage (rMV-Edmonston) efficiently infected and killed them. The rMV-SLAMblind showed antitumor activity against human breast cancer xenografts in immunodeficient mice. The oncolytic activity of rMV-SLAMblind was significantly greater than that of rMV-Edmonston. To assess the in vivo safety, three monkeys seronegative for MV were inoculated with rMV-SLAMblind, and no clinical symptoms were documented. On the basis of these results, rMV-SLAMblind could be a promising candidate as a novel oncolytic virus for breast cancer treatment.

Morbillivirus receptors and tropism: multiple pathways for infection

Morbilliviruses, which include measles virus (MeV), canine distemper virus, and rinderpest virus, are among the most important pathogens in their respective hosts and cause severe syndromes. Morbilliviruses are enveloped viruses with two envelope proteins, one of which is hemagglutinin (H) protein, which plays a role in binding to cellular receptors. During morbillivirus infection, the virus initially targets lymphoid cells and replicates efficiently in the lymph nodes. The principal cellular receptor for morbillivirus is signaling lymphocyte activation molecule (SLAM, also called CD150), which is exclusively expressed on immune cells. This feature reflects the strong lymphoid cell tropism and viral spread in the infected body. Morbillivirus infection, however, affects various tissues in the body, including the lung, kidney, gastrointestinal tract, vascular endothelium, and brain. Thus, other receptors for morbilliviruses in addition to SLAM might exist. Recently, nectin-4 has been identified as a novel epithelial cell receptor for MeV. The expression of nectin-4 is localized to polarized epithelial cells, and this localization supports the notion of cell tropism since MeV also grows well in the epithelial cells of the respiratory tract. Although two major receptors for lymphoid and epithelial cells in natural infection have been identified, morbillivirus can still infect many other types of cells with low infectivity, suggesting the existence of inefficient but ubiquitously expressed receptors. We have identified other molecules that are implicated in morbillivirus infection of SLAM-negative cells by alternative mechanisms. These findings indicate that morbillivirus utilizes multiple pathways for establishment of infection. These studies will advance our understanding of morbillivirus tropism and pathogenesis.

Evaluation of Measles Vaccine Virus as a Vector to Deliver Respiratory Syncytial Virus Fusion Protein or Epstein-Barr Virus Glycoprotein gp350

Live attenuated recombinant measles vaccine virus (MV) Edmonston-Zagreb (EZ) strain was evaluated as a viral vector to express the ectodomains of fusion protein of respiratory syncytial virus (RSV F) or glycoprotein 350 of Epstein-Barr virus (EBV gp350) as candidate vaccines for prophylaxis of RSV and EBV. The glycoprotein gene was inserted at the 1^st or the 3^rd position of the measles virus genome and the recombinant viruses were generated. Insertion of the foreign gene at the 3^rd position had a minimal impact on viral replication in vitro. RSV F or EBV gp350 protein was secreted from infected cells. In cotton rats, EZ-RSV F and EZ-EBV gp350 induced MV- and insert-specific antibody responses. In addition, both vaccines also induced insert specific interferon gamma (IFN-γ) secreting T cell response. EZ-RSV F protected cotton rats from pulmonary replication of RSV A2 challenge infection. In rhesus macaques, although both EZ-RSV F and EZ-EBV gp350 induced MV specific neutralizing antibody responses, only RSV F specific antibody response was detected. Thus, the immunogenicity of the foreign antigens delivered by measles vaccine virus is dependent on the nature of the insert and the animal models used for vaccine evaluation.

Cell tropism and pathogenesis of measles virus in monkeys

Measles virus (MV) is an enveloped negative strand RNA virus belonging to the family of Paramyxoviridae, genus Morbillivirus, and causes one of the most contagious diseases in humans. Experimentally infected non-human primates are used as animal models for studies of the pathogenesis of human measles. We established a reverse genetics system based on a highly pathogenic wild-type MV. Infection of monkeys with recombinant MV strains generated by reverse genetics enabled analysis of the molecular basis of MV pathogenesis. The essential in vivo function of accessory genes was indicated by infecting monkeys with recombinant MV strains deficient in the expression of accessory genes. Furthermore, recombinant wild-type MV strains expressing enhanced green fluorescent protein enabled visual tracking of MV-infected cells in vitro and in vivo. To date, three different molecules have been identified as receptors for MV. Signaling lymphocyte activation molecule (SLAM, also called CD150), expressed on immune cells, is a major receptor for MV. CD46, ubiquitously expressed in all nucleated cells in humans and monkeys, is a receptor for vaccine and laboratory-adapted strains of MV. The newly identified nectin-4 (also called poliovirus-receptor-like-4) is an epithelial cell receptor for MV. However, recent findings have indicated that CD46 acts as an MV receptor in vitro but not in vivo. The impact of the receptor usage of MV in vivo on the disease outcome is now under investigation.

Risk analysis for measles reintroduction after global certification of eradication

BACKGROUND

Measles virus will continue to exist after certification of global eradication as virus stocks and infectious materials held in laboratories, in persistently and chronically infected individuals, and possibly in undetected foci of transmission. A literature search was undertaken to identify and evaluate the main risks for reintroduction of measles transmission in the absence of universal measles immunization.

METHODS

A qualitative risk assessment was conducted following a series of literature searches using the PubMed database.

RESULTS

If the criteria for global certification of eradication are stringent and require rigorous validation, then the risk of undetected measles transmission after certification is very low. Risk of unintentional reintroduction from any source, including persistent infections and laboratory materials is low to very low but depends on the extent of measles vaccine use. If immunization levels decrease, measles will become a credible agent for bioterrorism through intentional release.

CONCLUSIONS

Posteradication risks are low and should not deter any attempt at measles eradication. More information on measles transmission dynamics and the role of atypical infections is required to determine requirements for global certification of eradication. Posteradication risks would be minimized through development and implementation of an international risk management strategy, including requirements for a posteradication vaccine stockpile.

The correlation between infectivity and incubation period of measles, estimated from households with two cases

The generation time of an infectious disease is the time between infection of a primary case and infection of a secondary case by the primary case. Its distribution plays a key role in understanding the dynamics of infectious diseases in populations, e.g. in estimating the basic reproduction number. Moreover, the generation time and incubation period distributions together characterize the effectiveness of control by isolation and quarantine. In modelling studies, a relation between the two is often not made specific, but a correlation is biologically plausible. However, it is difficult to establish such correlation, because of the unobservable nature of infection events. We have quantified a joint distribution of generation time and incubation period by a novel estimation method for household data with two susceptible individuals, consisting of time intervals between disease onsets of two measles cases. We used two such datasets, and a separate incubation period dataset. Results indicate that the mean incubation period and the generation time of measles are positively correlated, and that both lie in the range of 11-12 days, suggesting that infectiousness of measles cases increases significantly around the time of symptom onset. The correlation between times from infection to secondary transmission and to symptom onset could critically affect the predicted effectiveness of isolation and quarantine.

Early target cells of measles virus after aerosol infection of non-human primates

Measles virus (MV) is highly infectious, and has long been thought to enter the host by infecting epithelial cells of the respiratory tract. However, epithelial cells do not express signaling lymphocyte activation molecule (CD150), which is the high-affinity cellular receptor for wild-type MV strains. We have generated a new recombinant MV strain expressing enhanced green fluorescent protein (EGFP), based on a wild-type genotype B3 virus isolate from Khartoum, Sudan (KS). Cynomolgus macaques were infected with a high dose of rMV^KSEGFP by aerosol inhalation to ensure that the virus could reach the full range of potential target cells throughout the entire respiratory tract. Animals were euthanized 2, 3, 4 or 5 days post-infection (d.p.i., n = 3 per time point) and infected (EGFP⁺) cells were identified at all four time points, albeit at low levels 2 and 3 d.p.i. At these earliest time points, MV-infected cells were exclusively detected in the lungs by fluorescence microscopy, histopathology and/or virus isolation from broncho-alveolar lavage cells. On 2 d.p.i., EGFP⁺ cells were phenotypically typed as large mononuclear cells present in the alveolar lumen or lining the alveolar epithelium. One to two days later, larger clusters of MV-infected cells were detected in bronchus-associated lymphoid tissue (BALT) and in the tracheo-bronchial lymph nodes. From 4 d.p.i. onward, MV-infected cells were detected in peripheral blood and various lymphoid tissues. In spite of the possibility for the aerosolized virus to infect cells and lymphoid tissues of the upper respiratory tract, MV-infected cells were not detected in either the tonsils or the adenoids until after onset of viremia. These data strongly suggest that in our model MV entered the host at the alveolar level by infecting macrophages or dendritic cells, which traffic the virus to BALT or regional lymph nodes, resulting in local amplification and subsequent systemic dissemination by viremia.

Measles remains an important vaccine-preventable cause of morbidity and mortality in developing countries. The causative agent, measles virus (MV), is one of the most contagious viruses known. Measles has an incubation time of approximately two weeks, and surprisingly little is known about the early events after MV infection. Epithelial cells in the upper respiratory tract have long been considered as early target cells, but more recently alveolar macrophages (AM) and dendritic cells (DC) have been proposed as alternatives. We have infected cynomolgus macaques with a high dose of a recombinant EGFP-expressing MV strain via aerosol inhalation, to ensure that the virus had access to the entire respiratory tract. At 2 days post-infection, MV-infected mononuclear cells were detectable in the alveolar lumen but not in the upper respiratory tract. These infected cells migrated through the bronchus-associated lymphoid tissue to the draining tracheo-bronchial lymph node at 3 days post-infection. Systemic infection was initiated from this point, as observed in macaques euthanized 4 or 5 days post-infection. Thus, even though the aerosolized virus had access to epithelial cells and lymphoid tissues along the entire respiratory tract, AM and DC in the lungs were the first cells that sustained MV replication.

Interplay of measles virus with early induced cytokines reveals different wild type phenotypes

Differential effects of measles virus (MV) on the innate immune response may influence virus spread and severity of disease. Using a representative panel of 22 MV strains including 14 different genotypes, we found that wild-type (wt) differ considerably in their sensitivity to type I interferon (IFN). The wt virus production was 2-47-fold lower in IFN-alpha treated Vero/hSLAM cells, whereas vaccine virus production was reduced only 2-3-fold. Sequence analysis of the MV-P/C/V gene, revealed no obvious amino acid mutations that correlated with the different phenotypes. Strains also widely differed in their ability to induce type I IFN, tumor necrosis factor (TNF) alpha and other cytokines in human A549/hSLAM cells. Some wt strains that were highly sensitive to type I IFN induced only low levels of these and other cytokines. In vitro wt strains that produced the 5' copy-back defective interfering RNAs (5'cb-diRNA) characterized by Shingai et al. (2007), induced high levels of cytokines that otherwise were only reached by vaccine strains. These 5'cb-diRNAs emerged only in virus cultures during multiple passaging and were not detectable in clinical samples of measles patients. These subgenomic RNAs are an important confounding parameter in passaged wt viruses which must be carefully assessed in all in vitro studies. The present data show that MV wt strains differ in their sensitivity and their ability to temper with the innate immune response, which may result in differences in virulence.

Emerging and reemerging infectious diseases of nonhuman primates in the laboratory setting

Despite numerous advances in the diagnosis and control of infectious diseases of nonhuman primates in the laboratory setting, a number of infectious agents continue to plague colonies. Some, such as measles virus and Mycobacterium tuberculosis, cause sporadic outbreaks despite well-established biosecurity protocols, whereas others, such as retroperitoneal fibromatosis-associated herpesvirus, have only recently been discovered, often as a result of immunosuppressive experimental manipulation. Owing to the unique social housing requirements of nonhuman primates, importation of foreign-bred animals, and lack of antemortem diagnostic assays for many new diseases, elimination of these agents is often difficult or impractical. Recognition of these diseases is therefore essential because of their confounding effects on experimental data, impact on colony health, and potential for zoonotic transmission. This review summarizes the relevant pathology and pathogenesis of emerging and reemerging infectious diseases of laboratory nonhuman primates.

In vivo tropism of attenuated and pathogenic measles virus expressing green fluorescent protein in macaques

The global increase in measles vaccination has resulted in a significant reduction of measles mortality. The standard route of administration for the live-attenuated measles virus (MV) vaccine is subcutaneous injection, although alternative needle-free routes, including aerosol delivery, are under investigation. In vitro, attenuated MV has a much wider tropism than clinical isolates, as it can use both CD46 and CD150 as cellular receptors. To compare the in vivo tropism of attenuated and pathogenic MV, we infected cynomolgus macaques with pathogenic or attenuated recombinant MV expressing enhanced green fluorescent protein (GFP) (strains IC323 and Edmonston, respectively) via the intratracheal or aerosol route. Surprisingly, viral loads and cellular tropism in the lungs were similar for the two viruses regardless of the route of administration, and CD11c-positive cells were identified as the major target population. However, only the pathogenic MV caused significant viremia, which resulted in massive virus replication in B and T lymphocytes in lymphoid tissues and viral dissemination to the skin and the submucosa of respiratory epithelia. Attenuated MV was rarely detected in lymphoid tissues, and when it was, only in isolated infected cells. Following aerosol inhalation, attenuated MV was detected at early time points in the upper respiratory tract, suggesting local virus replication. This contrasts with pathogenic MV, which invaded the upper respiratory tract only after the onset of viremia. This study shows that despite in vitro differences, attenuated and pathogenic MV show highly similar in vivo tropism in the lungs. However, systemic spread of attenuated MV is restricted.

Extraction and characterization of the rhesus macaque T-cell receptor beta-chain genes

Rhesus macaque models have been instrumental in the development and testing of vaccines before human studies and have provided fundamental insights into the determinants of immune efficacy in a variety of infectious diseases. However, the characterization of antigen-specific T-cell receptor (TCR) repertoires during adaptive immune responses in these models has earlier relied on human TCR gene assignments. Here, we extracted and characterized TCR beta-chain (TRB) genes from the recently sequenced rhesus macaque genome that are homologous to the human TRB genes. Comparison of the rhesus macaque TRB genes with the human TRB genes showed an average best match similarity of 92.9%. Furthermore, we confirmed the usage of most rhesus macaque TRB genes by expressed TCRbeta sequences within epitope-specific TCR repertoires. This primary description of the rhesus macaque TRB genes will provide a standardized nomenclature and enable better characterization of TCR usage in studies that use this species.

Measles studies in the macaque model

Much of our current understanding of measles has come from experiments in non-human primates. In 1911, Goldberger and Anderson showed that macaques inoculated with filtered secretions from measles patients developed measles, thus demonstrating that the causative agent of this disease was a virus. Since then, different monkey species have been used for experimental measles virus infections. Moreover, infection studies in macaques demonstrated that serial passage of the virus in vivo and in vitro resulted in virus attenuation, providing the basis for all current live-attenuated measles vaccines. This chapter will review the macaque model for measles, with a focus on vaccination and immunopathogenesis studies conducted over the last 15 years. In addition, recent data are highlighted demonstrating that the application of a recombinant measles virus strain expressing enhanced green fluorescent protein dramatically increased the sensitivity of virus detection, both in living and sacrificed animals, allowing new approaches to old questions on measles vaccination and pathogenesis.

Measles: old vaccines, new vaccines

Isolation of measles virus in tissue culture by Enders and colleagues in the 1960s led to the development of the first measles vaccines. An inactivated vaccine provided only short-term protection and induced poor T cell responses and antibody that did not undergo affinity maturation. The response to this vaccine primed for atypical measles, a more severe form of measles, and was withdrawn. A live attenuated virus vaccine has been highly successful in protection from measles and in elimination of endemic measles virus transmission with the use of two doses. This vaccine is administered by injection between 9 and 15 months of age. Measles control would be facilitated if infants could be immunized at a younger age, if the vaccine were thermostable, and if delivery did not require a needle and syringe. To these ends, new vaccines are under development using macaques as an animal model and various combinations of the H, F, and N viral proteins. Promising studies have been reported using DNA vaccines, subunit vaccines, and virus-vectored vaccines.

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.

Predominant infection of CD150+ lymphocytes and dendritic cells during measles virus infection of macaques

Measles virus (MV) is hypothesized to enter the host by infecting epithelial cells of the respiratory tract, followed by viremia mediated by infected monocytes. However, neither of these cell types express signaling lymphocyte activation molecule (CD150), which has been identified as the receptor for wild-type MV. We have infected rhesus and cynomolgus macaques with a recombinant MV strain expressing enhanced green fluorescent protein (EGFP); thus bringing together the optimal animal model for measles and a virus that can be detected with unprecedented sensitivity. Blood samples and broncho-alveolar lavages were collected every 3 d, and necropsies were performed upon euthanasia 9 or 15 d after infection. EGFP production by MV-infected cells was visualized macroscopically, in both living and sacrificed animals, and microscopically by confocal microscopy and FACS analysis. At the peak of viremia, EGFP fluorescence was detected in skin, respiratory and digestive tract, but most intensely in all lymphoid tissues. B- and T-lymphocytes expressing CD150 were the major target cells for MV infection. Highest percentages (up to 30%) of infected lymphocytes were detected in lymphoid tissues, and the virus preferentially targeted cells with a memory phenotype. Unexpectedly, circulating monocytes did not sustain productive MV infection. In peripheral tissues, large numbers of MV-infected CD11c+ MHC class-II+ myeloid dendritic cells were detected in conjunction with infected T-lymphocytes, suggesting transmission of MV between these cell types. Fluorescent imaging of MV infection in non-human primates demonstrated a crucial role for lymphocytes and dendritic cells in the pathogenesis of measles and measles-associated immunosuppression.

Heterologous prime-boost strategy to immunize very young infants against measles: pre-clinical studies in rhesus macaques

Infants in developing countries are at high risk of developing severe clinical measles if they become infected during the "window of vulnerability" (age 4-9 months), when declining maternal antibodies do not protect against wild virus, yet impede successful immunization by attenuated measles vaccine. We developed two Sindbis replicon-based DNA vaccines expressing measles virus hemagglutinin and fusion protein with the goal of priming young infants to respond safely and effectively to subsequent boosting with attenuated measles vaccine. Intradermal prime with DNA vaccines by needle-free injection followed by aerosol or parenteral boost with licensed measles vaccine was well tolerated by juvenile and young infant rhesus macaques, and protected against clinical measles and viremia on wild-type virus challenge. A proteosome-measles vaccine administered alone (three doses) or as a boost following DNA vaccine priming was also safe and protective. These promising results pave the way for clinical trials to assess this prime-boost strategy.