Pathogenicity and virulence of human respiratory syncytial virus: Multifunctional nonstructural proteins NS1 and NS2

Abstract

Human respiratory syncytial virus (hRSV) is a major cause of lower respiratory tract infections in young children, the elderly, and immunocompromised. Despite its discovery over 60 years ago and the global impact on human health, limited effective prophylactic or therapeutic options have been available for hRSV infections. This has largely been attributed to the legacy of vaccine failure in the 1960s using a formalin-inactivated RSV , which led to enhancement of disease post exposure to hRSV infection and hampered subsequent development of vaccine candidates. Recent FDA approval of vaccines for older adults and pregnant individuals are major advancements but leaves children between 6 months and 5 years of age unprotected. Part of this limitation is due to a lack of complete understanding of the factors that contribute to hRSV pathogenesis. The nonstructural proteins NS1 and NS2 are multifunctional virulence factors unique to hRSV and that play critical roles during hRSV infection, including antagonizing interferon (IFN) signaling to modulate host responses to hRSV infection. However, the molecular mechanisms by which the nonstructural proteins mediate their IFN inhibitory functions have not been completely defined. Current progress on the characterization of NS1 and NS2 during infection provides deeper insight into their roles. Furthermore, reverse genetics systems for hRSV provide a viable strategy to generate attenuated viruses by introduction of select mutations while maintaining immunogenicity required to elicit a long-term protective response. Here, we will review the current state of knowledge of the nonstructural proteins, their contributions to RSV pathogenesis, and their potential as targets for therapeutic development.

Keywords: Human respiratory syncytial virus; NS1; NS2; nonstructural proteins; viral IFN antagonists.

Figure 1.

hRSV genome and viral proteins. Domain organization of the hRSV RNA genome (A2 strain). The viral genome contains 10 genes encoding for 11 proteins. The M2 gene contains two ORFs encoding for M2–1 and M2–2 proteins. Shown above and below the genome are representative structures available for the different hRSV proteins (NS1, PDB 5VJ2; NS2, PDB 7LDK; N, PDB 2WJ8; P, PDB 6PZK; M, PDB 2QP; SH, PDB 2NB7 and 2NB8; G, PDB 6BLH; F, PDB 4JHW and 3RRR; M2–1, PDB 4C3D; and L, 6PZK). For SH, NMR coordinates are not available for the transmembrane helix and is represented as a rounded cylinder. NS1, nonstructural protein 1; NS2, nonstructural protein 2; N, nucleoprotein; P, phosphoprotein; M, matrix; SH, small hydrophobic; G, glycoprotein; F, fusion glycoprotein; M2–1, transcription processivity factor; M2–2, replication co-factor; L, large polymerase.

Figure 2.

hRSV replication cycle. Simplified schematic of the different stages of the viral replication cycle. 1, Several host factors are involved in attachment of hRSV to the host membrane that is cell type dependent. 2, Entry may involve endocytosis or micropinocytosis of the virion followed by fusion of viral and host cell membranes. 3, Replication and transcription occurs in inclusion bodies where the negative strand genome serves as template for generating the antigenome. The negative strand genome is also transcribed by a sequential start-stop mechanism that results in a gradient of mRNAs from the 3’ to the 5’ end. 4, After viral proteins are generated, some like NS1 and M partially localize to the nucleus and are later assembled along with the viral genome to produce new infectious viral particles.

Figure 3.

NS1 and NS2 antagonize multiple targets in the IFNα/β signaling pathway. Simplified representation of the IFNα/β induction and response signaling pathway. Cytosolic pattern recognition receptors, including RIG-I and MDA5, detect viral pathogen associated molecular patterns and stimulate the induction of IFNα/β. IFNα/β stimulates other downstream signaling pathways to activate the transcription of IFN-stimulated genes (ISG). hRSV NS1 and NS2 inhibit the function of different molecules at points indicated on the pathway. 5’OH, 5’-hydroxyl; 5’ppp, 5’-triphosphate; 5’m7Gppp, 5’-7-methylguanosine cap; PP1α/γ, protein phosphatase 1α/γ; RIG-I, retinoic inducible gene-I; MDA5, melanoma differentiation-associated protein 5; TRIM25, tripartite motif-containing protein 25; PACT, protein activator of PKR; LGP2, laboratory of genetics and physiology 2; MAVS, mitochondrial antiviral-signaling protein; NF?B, nuclear factor kappa-light-chain-enhancer of activated B cells; TRAF3, TNF receptor associated factor 3; TANK, TRAF family member-associated NF?B activator; I?B, inhibitor of NF?B; IKK, I?B kinase; TBK-1, TANK-binding kinase 1; IRF, interferon regulatory factor; IFN, interferon; IFNAR1/2, IFN alpha and beta receptor subunit 1/2; JAK1, Janus kinase1; Tyk2, non-receptor tyrosine kinase 2; STAT, signal transducer and activator or transcription; ISRE, IFN stimulated response elements; Ub, ubiquitin; P, phosphorylated.