Sendai Virus-Vectored Vaccines That Express Envelope Glycoproteins of Respiratory Viruses

Abstract

Human respiratory syncytial virus (HRSV), human metapneumovirus (HMPV), and human parainfluenza viruses (HPIVs) are leading causes of respiratory disease in young children, the elderly, and individuals of all ages with immunosuppression. Vaccination strategies against these pneumoviruses and paramyxoviruses are vast in number, yet no licensed vaccines are available. Here, we review development of Sendai virus (SeV), a versatile pediatric vaccine that can (a) serve as a Jennerian vaccine against HPIV1, (b) serve as a recombinant vaccine against HRSV, HPIV2, HPIV3, and HMPV, (c) accommodate foreign genes for viral glycoproteins in multiple intergenic positions, (d) induce durable, mucosal, B-cell, and T-cell immune responses without enhanced immunopathology, (e) protect cotton rats, African green monkeys, and chimpanzees from infection, and (f) be formulated into a vaccine cocktail. Clinical phase I safety trials of SeV have been completed in adults and 3-6-year-old children. Clinical testing of SeVRSV, an HRSV fusion (F) glycoprotein gene recombinant, has also been completed in adults. Positive results from these studies, and collaborative efforts with the National Institutes of Health and the Serum Institute of India assist advanced development of SeV-based vaccines. Prospects are now good for vaccine successes in infants and consequent protection against serious viral disease.

Keywords: HRSV; attachment protein; envelope glycoprotein; fusion glycoprotein; parainfluenza virus; paramyxovirus; pneumovirus; vaccine vector.

Figure 1

Phylogenetic tree of F proteins. Phylogenetic tree was created with CLC Main Workbench Version 20. Scale bar represents branch length as substitutions per site. CDV (canine distemper virus P12569.1), RPV (rinderpest virus P10864.1), Measles (measles virus AAF85680.1), Cedar (Cedar virus AFP87278.1), Nipah (Nipah virus AAM13405.1), Hendra (Hendra virus NP_047111.2), HMPV (ABM67072.1), PVM (pneumonia virus of mice AAS87365.1), BRSV (bovine respiratory syncytial virus P22167.1), HRSV (A P03420.1 and B AAR14266.1), NDV (Newcastle disease virus AAC28374.1), AMPV (avian metapneumovirus ABQ23891.1), HPIV4 (BAA08626.1), Mumps (mumps virus P11236.1), PIV5 (P04849.1), HPIV2 (NP_598404.1), AsaPV (Atlantic salmon paramyxovirus ABW38054.1), Fer-de-Lance (Fer-de-Lance virus AAN18264.1), Sendai (AAB06281.1), HPIV1 (P12605.1), HPIV3 (AAB21447.1), BPIV3 (AZB53083.1).

Figure 2

SeV replication cycle. (A) Genome structure of SeV. Polymerase complex genes nucleocapsid (N), phosphoprotein (P), and large polymerase (L) are color-coded red; matrix (M) is colored yellow; fusion (F) is colored blue; and hemagglutinin-neuraminidase (HN) is colored green. (B) Replication cycle of SeV. During step 1, HN binds sialic-acid-containing receptors, triggering irreversible conformational changes in the F protein that cause fusion of the viral envelope and host cell plasmid membrane. The genome and associated polymerase complexes are delivered into the cytoplasm, where they remain during replication. In step 2, the RNA-dependent RNA-polymerase transcribes viral genes serially starting from the 3′ end. In step 3, viral proteins are translated and processed. In steps 4 and 5, complementary genome is replicated and then serves as a template for replication of negative-sense genomes needed to produce progeny virions. In step 6, F and HN proteins traffic through the secretory pathway to the cell surface. The M protein associates with host cell proteins, viral ribonucleoproteins, envelope glycoprotein tails, and the plasmid membrane to help drive budding of progeny virions. HN receptor-destroying activity is needed for progeny virus release.

Figure 3

Intergenic junctions of SeV. Upstream of each gene is a gene start sequence that directs initiation of transcription and capping of the 5′ end of the transcript. Downstream of each gene is a gene end sequence that directs termination of transcription and synthesis of a 3′ poly-A tail. A trinucleotide sequence separates gene end from gene start between the genes. Differences in gene start sequences modulate the level of polymerase continuation of transcription versus termination.

Figure 4

Differential transcription of SeV genes. Transcription begins at the 3′ end of the genome with the N gene. At each gene junction, the polymerase may continue to transcribe downstream genes or terminate transcription. Transcription start sequences vary between genes with the sequence upstream of the F gene resulting in the highest level of termination. As a result, the relative expression levels of F, HN, and L genes are substantially less than those of the upstream genes. Insertion of a foreign gene adds another gene junction and usually alters the ratio of gene transcripts in addition to decreasing the abundance of downstream transcripts. Positioning of foreign genes near the 3′ end of the genome causes greater virus attenuation than positioning foreign genes nearer the 5′ end.

Figure 5

Genome structures of representative paramyxoviruses and pneumoviruses. (A) Genomes of human vaccines targeted by SeV-based vaccines. Due to high amino-acid similarity, unmodified SeV serves as a Jennerian vaccine against HPIV1. For the other human viruses, an envelope glycoprotein gene (yellow) is inserted into the SeV genome. (B) Sendai virus vaccine genome structures. The envelope glycoprotein gene (GENE, yellow) from a human virus may be inserted into any gene junction of the SeV genome. Most SeV-vectored vaccines studied in preclinical experiments contain a foreign gene inserted between the F and HN genes as shown. In the genomes, the 3′ leader is on the left and the 5′ trailer is on the right. Intergenic junctions (not shown) include a transcription stop, intergenic, and transcription start sequences. Genomes are drawn to scale with a scale bar at the bottom.

Figure 6

Schematic of gene delivery by SeV-vectored vaccines. (A) SeV vector genome structure. SeV genes are colored blue, and a foreign gene is colored yellow. Most SeV-vectored vaccines have been constructed with the inserted gene positioned between the F and HN genes, which allows for sufficient vaccine antigen expression while causing little to no attenuation of replication. (B) SeV virion structure. Vaccine is composed of SeV proteins (blue), SeV genome (blue), and a foreign gene (yellow). (C) List of genes inserted into SeV-vectored vaccines that have been tested in preclinical studies. (D) Schematic of vaccination. SeV-vectored vaccines are intranasally inoculated and infect respiratory epithelial cells. Replication occurs in the cytoplasm and produces both the foreign gene product (yellow) and SeV genes and genome (blue). New progeny virions are generated, but SeV replication is attenuated in species other than mice because its accessory genes are only able to counteract interferon responses in mice.