Studying experimental measles virus vaccines in the presence of maternal antibodies in the cotton rat model (Sigmodon hispidus)

Sendai virus-based RSV vaccine protects against RSV challenge in an in vivo maternal antibody model

Respiratory syncytial virus (RSV) is the cause of significant morbidity and mortality among infants, and despite decades of research there remains no licensed vaccine. SeVRSV is a Sendai virus (SeV)-based live intranasal vaccine that expresses the full length RSV fusion (F) gene. SeV is the murine counterpart of human parainfluenza virus type 1. Given that the target population of SeVRSV is young infants, we questioned whether maternal antibodies typical of this age group would inhibit SeVRSV vaccine efficacy. After measuring SeV- and RSV-specific serum neutralizing antibody titers in human infants, we matched these defined titers in cotton rats by the passive transfer of polyclonal or monoclonal antibody products. Animals were then vaccinated with SeVRSV followed by a 3 month rest period to allow passively transferred antibodies to wane. Animals were finally challenged with RSV to measure the de novo vaccine-induced immune responses. Despite the presence of passively-transferred serum neutralizing antibodies at the time of vaccination, SeVRSV induced immune responses that were protective against RSV challenge. The data encourage advancement of SeVRSV as a candidate vaccine for the protection of children from morbidity and mortality caused by RSV.

The pathogenesis of measles revisited

Measles continues to be an important cause of childhood mortality in developing countries. The causative agent, measles virus (MV), is a member of the family Paramyxoviridae, genus Morbillivirus, and is spread via the respiratory route. MV was originally thought 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 (SLAM, CD150), which has been identified as the main receptor for wild-type MV. Measles has a relatively long incubation time, which makes it difficult to study the early stages of MV infection in humans. The animal models that best reflect the pathogenesis of measles are based on nonhuman primates. The use of recombinant MV strains expressing fluorescent proteins has greatly facilitated studies on viral tropism in macaques. These studies indicate that dendritic cells and lymphocytes expressing CD150 are the primary target cells for MV infection. At late stages of the infection MV also infects epithelial cells, despite the fact that these do not express CD150. Whether these cells express an as yet unidentified additional MV receptor remains unclear. On basis of these data it could be envisaged that dendritic cells are the first target cells for MV infection. These antigen-presenting cells may traffic the virus to the regional lymph nodes where they can transmit the virus to lymphocytes, which during viremia disseminate the virus throughout the body.

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

Both rhesus and cynomolgus macaques have been used as animal models for measles vaccination and immunopathogenesis studies. A number of studies have suggested that experimental measles virus (MV) infection induces more-characteristic clinical features in rhesus than in cynomolgus monkeys. In the present study, both macaque species were infected with two different wild-type MV strains and clinical, virological and immunological parameters were compared. The viruses used were a genotype C2 virus isolated in The Netherlands in 1991 (MV-Bil) and a genotype B3 virus isolated from a severe measles case in Sudan in 1997 (MV-Sudan). Following infection, all rhesus monkeys developed a skin rash and conjunctivitis, which were less obvious in cynomolgus monkeys. Fever was either mild or absent in both species. Virus reisolation profiles from peripheral blood mononuclear cells and broncho-alveolar lavage cells and the kinetics of MV-specific IgM and IgG responses were largely identical in the two animal species. However, in animals infected with MV-Sudan, viraemia appeared earlier and lasted longer than in animals infected with MV-Bil. This was also reflected by the earlier appearance of MV-specific serum IgM antibodies after infection with MV-Sudan. Collectively, these data show that cynomolgus and rhesus macaques are equally susceptible to wild-type MV infection, although infection in the skin seems to follow a different course in rhesus macaques. MV-Sudan proved more pathogenic for non-human primates than MV-Bil, which may render it more suitable for use in future pathogenesis studies.

Interferon-inducible Mx gene expression in cotton rats: cloning, characterization, and expression during influenza viral infection

Mx proteins belong to the superfamily of large GTPases with antiviral activity against a wide range of RNA viruses. In vivo, the expression of Mx genes is tightly regulated by the presence of type I interferons (IFNs), and their induction has been described during several viral infections. However, because of the absence of functional Mx genes in most common laboratory strains of mice, in vivo studies of the expression of these genes during viral infection have been hampered. We have cloned the cDNAs for the cotton rat homologs of Mx1 and Mx2 genes that encode full-length proteins. Mx1 localized in the nucleus, whereas Mx2, as its human homolog MxA, localized in the cytoplasm. The expression of Mx genes in cotton rat cells was induced by type I IFNs (IFN-alpha and IFN-beta) but induced only marginally with type II IFN (IFN-gamma). In vivo, the expression of Mx genes was dramatically augmented in lungs of cotton rats infected with influenza virus. The expression of Mx genes and protein(s) was dependent on the dose of virus and the time postinfection for the analysis. Our data present for the first time a complete analysis of the kinetics of expression of these influenza resistant genes in vivo and underscore the fidelity and sensitivity of the cotton rat model for the study of influenza viral infection.

Placental transfer of measles antibodies: effect of gestational age and maternal vaccination status

Despite nationwide measles vaccination coverage in Israel of over 90%, repeated outbreaks of measles have spread from isolated communities with poor immunization uptake. Some severely affected individuals were children under 1 year of age, including premature infants. We evaluated the serological status of 195 newborn infants and their 161 mothers divided into four groups: vaccinated mothers (VMs) and premature infants, VM and full term infants, naturally immunized mothers (NIMs) and premature infants, NIM and full term infants. Maternal and cord blood measles antibody titers were determined by haemagglutination inhibition (HI) test and microneutralization test (mNT). Fewer than 40% of preterm infants of VM and less then 70% of preterm infants of NIM had protective titers at birth. The results of this study may aid in formulating new measles vaccination recommendations for preterm infants.

Experimental vaccines against measles in a world of changing epidemiology

Vaccination with the current live attenuated measles vaccine is one of the most successful and cost-effective medical interventions. However, as a result of persisting maternal antibodies and immaturity of the infant immune system, this vaccine is poorly immunogenic in children <9 months old. Immunity against the live vaccine is less robust than natural immunity and protection less durable. There may also be some concern about (vaccine) virus spread during the final stage of an eventual measles eradication program. Opinions may differ with respect to the potential threat that some of these concerns may be to the World Health Organisation goal of measles elimination, but there is a consensus that the development of new measles vaccines cannot wait. Candidate vaccines are based on viral or bacterial vectors expressing recombinant viral proteins, naked DNA, immune stimulating complexes or synthetic peptides mimicking neutralising epitopes. While some of these candidate vaccines have proven their efficacy in monkey studies, aerosol formulated live attenuated measles vaccine are evaluated in clinical trials.

Vaccination against measles: a neverending story

Measles, a highly contagious viral disease, is a major childhood killer in developing countries, accounting for almost 1 million deaths every year globally. Measles virus normally does not cause a persistent infection, no animal reservoir for measles virus exists, no vector is involved in its spread, only one serotype exists, the virus is antigenically stable and vaccination with the currently used live attenuated vaccines proved to be highly effective in preventing disease. Therefore, theoretically measles should be considered eradicable. This article provides a review of past and current measles vaccination efforts and development and need of new generation experimental measles vaccines.