Attenuated temperature-sensitive respiratory syncytial virus mutants generated by cold adaptation

The Cold-Adapted, Temperature-Sensitive SARS-CoV-2 Strain TS11 Is Attenuated in Syrian Hamsters and a Candidate Attenuated Vaccine

Live attenuated vaccines (LAVs) replicate in the respiratory/oral mucosa, mimic natural infection, and can induce mucosal and systemic immune responses to the full repertoire of SARS-CoV-2 structural/nonstructural proteins. Generally, LAVs produce broader and more durable protection than current COVID-19 vaccines. We generated a temperature-sensitive (TS) SARS-CoV-2 mutant TS11 via cold-adaptation of the WA1 strain in Vero E6 cells. TS11 replicated at >4 Log10-higher titers at 32 °C than at 39 °C. TS11 has multiple mutations, including those in nsp3, a 12-amino acid-deletion spanning the furin cleavage site of the S protein and a 371-nucleotide-deletion spanning the ORF7b-ORF8 genes. We tested the pathogenicity and protective efficacy of TS11 against challenge with a heterologous virulent SARS-CoV-2 D614G strain 14B in Syrian hamsters. Hamsters were randomly assigned to mock immunization-challenge (Mock-C) and TS11 immunization-challenge (TS11-C) groups. Like the mock group, TS11-vaccinated hamsters did not show any clinical signs and continuously gained body weight. TS11 replicated well in the nasal cavity but poorly in the lungs and caused only mild lesions in the lungs. After challenge, hamsters in the Mock-C group lost weight. In contrast, the animals in the TS11-C group continued gaining weight. The virus titers in the nasal turbinates and lungs of the TS11-C group were significantly lower than those in the Mock-C group, confirming the protective effects of TS11 immunization of hamsters. Histopathological examination demonstrated that animals in the Mock-C group had severe pulmonary lesions and large amounts of viral antigens in the lungs post-challenge; however, the TS11-C group had minimal pathological changes and few viral antigen-positive cells. In summary, the TS11 mutant was attenuated and induced protection against disease after a heterologous SARS-CoV-2 challenge in Syrian hamsters.

Engineering of Live Chimeric Vaccines against Human Metapneumovirus

Human metapneumovirus (HMPV) is an important human pathogen that, along with respiratory syncytial virus (RSV), is a major cause of respiratory tract infections in young infants. Development of an effective vaccine against Pneumoviruses has proven to be particularly difficult; despite over 50 years of research in this field, no vaccine against HMPV or RSV is currently available. Recombinant chimeric viruses expressing antigens of other viruses can be generated by reverse genetics and used for simultaneous immunization against more than one pathogen. This approach can result in the development of promising vaccine candidates against HMPV, and several studies have indeed validated viral vectors expressing HMPV antigens. In this review, we summarize current efforts in generating recombinant chimeric vaccines against HMPV, and we discuss their potential optimization based on the correspondence with RSV studies.

Effect of genetic background and delivery route on the preclinical properties of a live attenuated RSV vaccine

A safe and effective vaccine against RSV remains an important unmet public health need. Intranasally (IN) delivered live-attenuated vaccines represent the most extensively studied approach for immunization of RSV-naïve infants and children, however, achieving an effective balance of attenuation and immunogenicity has proven challenging. Here we report pre-clinical immunogenicity and efficacy data utilizing a live-attenuated vaccine candidate, RGΔM2-2, which was obtained by deleting the M2-2 open reading frame from the genome of the MSA1 clinical isolate. Intramuscular (IM) administration of RGΔM2-2 in cotton rats induced immunity and protective efficacy that was comparable to that induced by intranasal (IN) immunization. In contrast, the protective efficacy of RGΔM2-2 delivered by the IM route to African green monkeys was substantially reduced as compared to the efficacy following IN administration, despite comparable levels of serum neutralizing antibodies. This result suggests that mucosal immunity may play an important role in RSV protection. The RGΔM2-2 vaccine also demonstrated different attenuation profiles when tested in cotton rats, non-human primates, and a human airway epithelial (HAE) cell model. The data suggest RGΔM2-2 is less attenuated than a similarly designed vaccine candidate constructed on the A2 genetic background. These findings have important implications with regard to both the design and the preclinical safety testing of live-attenuated vaccines.

Vaccines for neonatal viral infections: Towards a live respiratory syncytial virus vaccine: a study in risk

There is risk attached to the development of respiratory syncytial virus vaccines, live attenuated or otherwise, but without the acceptance of this risk by manufacturers, health providers and the public, the conclusion of successful Phase III trials may lie in the distant future.

Cold adapted avian pneumovirus for use as live, attenuated vaccine in turkeys

We report the development of a cold adapted strain of avian pneumovirus (APV) and its evaluation as a live vaccine candidate in 2-week-old turkey poults. A US isolate of APV (APV/MN/turkey/1-a/97) was serially passaged in Vero cells for 41 passages and then adapted to grow at sub-optimal temperatures by growing successively at 35, 33 and 31 degrees C for eight passages at each temperature. The virus thus adapted to grow at 31 degrees C was used as a candidate vaccine. The birds were vaccinated with two different doses of cold adapted virus and challenged with virulent virus 2 weeks after vaccination. No clinical signs were observed post-vaccination. Upon challenge, no clinical signs were seen in vaccinated birds but severe clinical signs were seen in non-vaccinated, challenged birds. The signs included unilateral or bilateral mucoid nasal discharge, watery eyes and swelling of infraorbital sinuses. The antibody levels in vaccinated birds were not very high. None of the vaccinated birds were found to shed virus after challenge in their choanal secretions whereas all of the non-vaccinated, challenged birds shed the virus. The absence of clinical signs and virus shedding in vaccinated birds as compared to that in non-vaccinated birds suggests that the cold adapted strain of APV is a viable candidate for use as a live, attenuated vaccine in turkeys.

Current concepts on active immunization against respiratory syncytial virus for infants and young children

Respiratory syncytial virus (RSV) is the most important causative agent of viral respiratory tract infections in infants and young children. Passive immunization against RSV became available recently, but this does not apply to an effective vaccine as a result of dramatic adverse results of immunization with a RSV candidate vaccine in the 1960s and the lack of full knowledge of the immune response induced by RSV. Nonetheless intensive research during the past two decades has resulted in several interesting candidate vaccines, of which some have gone through testing in humans. These include the subunit vaccines PFP-1, PFP-2, BBG2Na and cold-passaged/temperature-sensitive mutants. The development of candidate vaccines against RSV is discussed. Because of questions, uncertainties and difficulties with the development of effective vaccines against RSV, it will probably be at least another 5 to 10 years before routine immunization against RSV becomes available.

The live attenuated subgroup B respiratory syncytial virus vaccine candidate RSV 2B33F is attenuated and immunogenic in chimpanzees, but exhibits partial loss of the ts phenotype following replication in vivo

The cold-adapted (ca), temperature-sensitive (ts) respiratory syncytial virus (RSV) subgroup B vaccine candidate, designated RSV 2B33F, was found previously to be restricted in replication, immunogenic, and protective against wild-type (wt) virus challenge in rodents and African green monkeys. We sought to investigate the level of attenuation, immunogenicity and genetic stability of this vaccine candidate in seronegative chimpanzees. The 2B33F vaccine candidate was attenuated in chimpanzees and manifested a ten- and 1000-fold restriction in replication in the upper and lower respiratory tracts respectively, compared with its wt RSV 2B parent virus. Despite this attenuation, chimpanzees immunized with RSV 2B33F were completely resistant to respiratory tract disease and virus replication upon challenge with wt virus. The ts phenotype of the RSV 2B33F mutant exhibited some alteration during replication in vivo in three of four chimpanzees tested. Virus present in nasopharyngeal swab or tracheal lavage secretions of these three chimpanzees was biologically cloned by plaque passage in Vero cells at permissive temperature. The plaque progeny retained the ts phenotype, but uniformly produced plaques at 39 and 40 degrees C to a level intermediate between that of the 2B33F input virus and the 2B wt parent virus, indicating that partial loss of the level of temperature sensitivity occurred following replication in vivo. The implications of these findings for RSV vaccine development are discussed.

Immunological properties of plaque purified strains of live attenuated respiratory syncytial virus (RSV) for human vaccine

Respiratory syncytial virus (RSV) causes severe lower respiratory tract disease in infants, young children, and the elderly. Efforts to develop satisfactory live or inactivated vaccines have not yet been proven successful. Our research focuses on the development of four purified live attenuated RSV sub-type A human vaccine clones. Temperature sensitive (ts) and attenuated purified clones of either cold-adapted (ca) RSV or high-passage (hp) RSV were administered intra-nasally (i.n.) to BALB/c mice and tested for immunogenicity. All four clones produced significant anti-RSV F IgG2a and IgG1 titres in the sera of mice, RSV-specific neutralizing titres higher than those produced by their wild-type progenitor viruses, cytotoxic T-lymphocyte (CTL) activity, and total protection against wild-type (wt) viral challenge. These purified vaccine candidates await testing in humans to determine which contain the required balance between immunogenicity and attenuation.

Respiratory syncytial virus (RSV) disease and prospects for its control

Respiratory syncytial virus (RSV) is a major virus pathogen of infants and young children, an important cause of disease in adults and is responsible for a significant amount of excess morbidity and mortality in the elderly. It also can be devastating in immunosuppressed populations. Vaccines are being developed, but none are currently licensed. Moreover, even if one or more are approved, they may not be suitable for some populations vulnerable to RSV (e.g. very young infants and the immunosuppressed). Ribavirin and immunoglobulin preparations with high titers of RSV-specific neutralizing antibodies are currently approved for use to treat and prevent RSV infection. However, neither of these is cost-effective or simple to administer. New agents are needed to reduce the impact of RSV. This review is concerned with the means currently available for controlling RSV, the search for new agents effective against this virus, and future prospects for preventing and treating RSV infections.

Respiratory syncytial virus vaccines

Respiratory syncytial virus (RSV) is the most important cause of viral lower respiratory tract illness (LRI) in infants and children worldwide and causes significant LRI in the elderly and in immunocompromised patients. The goal of RSV vaccination is to prevent serious RSV-associated LRI. There are several obstacles to the development of successful RSV vaccines, including the need to immunize very young infants, who may respond inadequately to vaccination; the existence of two antigenically distinct RSV groups, A and B; and the history of disease enhancement following administration of a formalin-inactivated vaccine. It is likely that more than one type of vaccine will be needed to prevent RSV LRI in the various populations at risk. Although vector delivery systems, synthetic peptide, and immune-stimulating complex vaccines have been evaluated in animal models, only the purified F protein (PFP) subunit vaccines and live attenuated vaccines have been evaluated in recent clinical trials. PFP-2 appears to be a promising vaccine for the elderly and for RSV-seropositive children with underlying pulmonary disease, whereas live cold-passaged (cp), temperature-sensitive (ts) RSV vaccines (denoted cpts vaccines) would most probably be useful in young infants. The availability of cDNA technology should allow further refinement of existing live attenuated cpts candidate vaccines to produce engineered vaccines that are satisfactorily attenuated, immunogenic, and phenotypically stable.

Biochemical characterizations of two temperature-sensitive and attenuated strains of respiratory syncytial virus subgroup B

Cold-adapted, temperature-sensitive (ts), attenuated strains of respiratory syncytial virus have been developed from a B subgroup clinical isolate for potential use as vaccine candidates. The replication of two B subgroup ts mutant viruses (2B33F and 2B20L) at the permissive and nonpermissive temperatures have been compared with that of the parental 2B virus to establish differences that may account for their ts and/or attenuated phenotypes. We have shown that the ts restriction at 39 degrees C in the replication of the two mutant viruses in tissue culture occurs at a step after virus adsorption but before or including initiation of virus-specific mRNA transcription. At the permissive temperature of 32 degrees C a 12- to 24-h delay in the accumulation of mRNA for both mutant viruses in comparison to that of the parental 2B virus was exhibited. This effect was mirrored by equivalent delays in viral protein synthesis and production of infectious virus. By 36 h postinfection both mutants had produced levels of viral mRNA, protein, and infectious virus that were similar to those of the parent virus at 32 degrees C. ts+ revertant viruses derived from both mutants have also reverted in their viral mRNA, protein, and infectious virus production kinetics at 32 degrees C to rates more like those exhibited by the parental 2B virus. This suggests a positive correlation between the ts step in the replication of the mutant viruses and the initial delay in mRNA production that occurs at the permissive temperature.

Strain-specific reverse transcriptase PCR assay: means to distinguish candidate vaccine from wild-type strains of respiratory syncytial virus

Candidate live-virus vaccines for respiratory syncytial virus are being developed and are beginning to be evaluated in clinical trials. To distinguish candidate vaccine strains from wild-type strains isolated during these trials, we developed PCR assays specific to two sets of candidate vaccine strains. The two sets were a group A strain (3A), its three attenuated, temperature-sensitive variant strains, a group B strain (2B), and its four attenuated, temperature-sensitive variant strains. The PCR assays were evaluated by testing 18 group A wild-type strains, the 3A strains, 9 group B wild-type strains, and the 2B strains. PCR specific to group A wild-type strains amplified only group A wild-type strains, and 3A-specific PCR amplified only 3A strains. PCR specific to group B wild-type strains amplified all group A and group B strains but gave a 688-bp product for group B wild-type strains, a 279-bp product for 2B strains, a 547-bp product for all group A strains, and an additional 688-bp product for some group A strains, including 3A strains. These types of PCR assays can, in conjunction with other methods, be used to efficiently distinguish candidate vaccine strains from other respiratory syncytial virus strains.