In this episode, I will summarize the state of the art of Respiratory Syncytium Virus (RSV): it is one of the major viruses that almost “disappeared” during the winter of 2020-21 (see graph from ECDC below), as a consequence of “non-pharmacological interventions” to control COVID. Later that year, there was an unusually early start of the “RSV season” and this year, we see a high peak around the usual time. At present, pediatricians fear a ‘tripledemic’ of RSV, influenza and COVID-19, which worsen each other’s course. (Ep 302-1 and -2)
The diagnosis of RSV is mostly made in the infants and young children below 4 and in older adults over 60, because the infection is most symptomatic in those age groups (Fig 3), In reality, however, the infection is more evenly distributed across all ages, but it remains asymptomatic or with mild upper respiratory symptoms in older children and younger adults. The repeated nature of infection indicates that immunity is rather short-lived.
While RSV and its complications are mainly documented in high income countries, the largest burden and death toll is in LMIC
In this episode; I will remind schematically some of the major clinical characteristics in Par 1, the virology in Par 2, explore risk factors in Par 3, Immunology in Par 4. The main focus is on the recent advances in vaccination and monoclonal antibody therapy in Par 5.
Par 1 CLINICAL CHARACTERISTICS (Ep 302-3 till -6)
Clearly the most vulnerable people are infants (under 1 year). They may suffer from bronchiolitis, which is a key reason for hospitalization,
Par 2 VIROLOGY (Ep 302-7)
RSV is a negative stranded enveloped RNA virus, belonging to the Pneumoviridae subfamily of the Paramyxoviruses.
Phylogenetic tree of the pneumovirus & paramyxovirus subfamilies, built using fusion protein sequence comparison. CDV, canine distemper virus; HMPV, human metapneumovirus; HPIV3, human parainfluenza virus 3; HRSV, human respiratory syncytial virus; NDV, Newcastle disease virus; PIV5, parainfluenza virus 5.
There are 3 membrane/envelope viral proteins (Glycoprotein, Fusion protein and SH small hydrophobic protein), which have a role in attachment and fusion. The F protein is considered the most important target for neutralizing antibodies.
Schematic diagram of the RSV virion and its genome structure. (a) The general structure of the RSV virion and its encoded proteins. (b) The genome organization of RSV consists of 11 open reading frames (ORFs), including 2 ORFs adjacent to the 3′ leader region that encode nonstructural proteins related to evading the innate immune response, and ORFs that encode structural proteins: nucleoprotein (N), phosphoprotein (P), matrix protein (M), small hydrophobic (SH) protein, glycoprotein (G), fusion protein (F), M2 protein, and polymerase (L) protein.
Schematic of RSV Virion Structure and Genome. The RSV RNA genome consists of 10 genes of 15.2 kb length that encode 11 proteins. These proteins can be categorized into ribonucleocapsid proteins [phosphoprotein (P), nucleoprotein (N), RNA polymerase (L), M2-1, and M2-2], nonstructural proteins (NS1 and NS2), three surface proteins [small hydrophobic protein (SH) attachment glycoprotein (G), and fusion protein (F)], and matrix protein (M) which surrounds the envelope and nucleocapsid.
The Fusion protein exists in a pre- and a post-fusion form
Multiple molecules involved in viral entry
Attachment via G glycoprotein, followed by fusion by F protein in pre-fusion configuration
Potential receptors for G and F
Potential receptors for G glycoprotein = annexin II, HSPG/GAG and CX3CR1,
Potential receptors for F glycoprotein = TLR4, ICAM-1, EGFR, NCL and IGF1R
Schematic of the RSV life cycle. RSV first enters the host cell via macropinocytosis or cell-surface fusion by binding to the host cell receptor. The virus fuses with the cell membrane and releases the RNP complex to initiate replication and transcription in an inclusion body to produce the genomes, internal component proteins, and surface proteins required by the virus. Genomes are assembled to form new RNP complexes, and proteins are translated on the endoplasmic reticulum and then moved to the Golgi apparatus where they mature. Finally, the RNP complexes are transferred to the plasma membrane to germinate new filamentous RSV virions, completing the viral life cycle.
Par 3 BIOLOGICAL FACTORS for DISEASE SEVERITY
- Clinical risk factors
- Genetic diversity of RSV
There are 2 major subtypes (A and B), which co-circulate and each show polymorphism. As an illustration the global distribution and polymorphism 2015-2020 season (Ep 302-8 Langendijk)
Guzman (Ep 302-9) shows how these different genotypes within the subtypes A and B are associated with different disease severity.
- Host factors
Gene polymorphism ( Ep 302-10 Andrea Borchers 2013)
Microbiome and epigenetics (Ep 302-11 Fonseca)
Factors that predispose to the development of severe respiratory syncytial virus (RSV) disease. Factors that can impact the immune response during RSV infection are age; lung development is not complete in preterm infants, and the immune system development will continue until the first years of life, and it will be impacted by the microbiome composition of the mother and infant.
The infant microbiome would be shape since the prenatal stage by the mother microbiome, and it will continue modifying by postnatal factors such as environmental microbial exposure, mode of delivery, diet, and antibiotic use. The infant microbiome would have long-acting effects on RSV immune responses. All these components independently or together represent the most common elements that are involved in the development of RSV severe disease.
Epigenetic modification alters gene expression through multiple mechansims. (A) DNA methylation occurs at regions that are rich in cytosine and guanine (CpG islands), leading to the repression of gene transcription. (B) MicroRNAs are short, non-coding RNA molecules encoded in the genome that bind to mRNA, leading to mRNA degradation or translational suppression. These matches can either be fully complementary or may contain mismatched bases, allowing one miRNA to target several mRNA molecules. (C) Histone modifications include methylation and acetylation of lysine and arginine residues on histone tails, as well as phosphorylation of serine and threonine residues. Potential modifications of the tail of histone 3 are shown as an example. These modifications result in changes in chromatin structure that can be either repressive or permissive for transcription factor binding.
Par 4 IMMUNOLOGY
In Ep 302-12, Taylor explains why neonates are so susceptible to respiratory viruses, including RSV:
Baseline differences in the neonatal adaptive immune system
- Neonatal CD4+ T cells are skewed towards Th2 and Treg development. Less differentiation towards Th1 due to reduced IL-12, coupled with increased Th1 apoptosis due to IL-4 signaling.
- Follicular helper CD4. T (Tfh )cells, while stimulated by IL-4 to differentiate, have arrested development, with generation of short-lived pre-Tfh cells.
- IL-4 signaling s also limits IL-17 production and skews the humoral response towards IgE production.
- Immaturity of B cell response Both neonatal Tfh and B cells have poor migration to germinal centers, which structurally demonstrate poor organization. B cells also have increased production of IL-10 and spontaneous secretion of IgM.
- CD8+ T cells show increased proliferation, generation of reactive oxygen species (ROS) and antimicrobial peptides (AMPs), BUT reduced cytotoxicity and memory formation.
The immune response to respiratory viruses in neonates
- The neonatal response favors a type II response, with increased IL-33, IL-25 and TSLP released from the epithelium, increased type 2 innate lymphoid cells (ILC2), increased type 2 helper T cells (Th2), and differentiation of M2-like alveolar macrophages (= rather suppressive)
- CD8 T cells have reduced T cell receptor (TCR) avidity, while both CD8 T cells and natural killer (NK) cells show reduced effector functions.
- Germinal center (GC) reactions are diminished in neonates, with less T follicular helper (Tfh) cell differentiation and less IgG production from GC B cells
Ep 302-13: While each viral or bacterial infection in early life has been associated with development of asthma at age 7, Bonnelyckke shows that:
- In the unadjusted analysis, several viral as well as bacterial infections are associated development of asthma.
- However, after adjustment for the child’s total number of respiratory episodes there was no significant risk from any of the individual viruses or bacteria;
- Nevertheless, the association between the number of respiratory episodes and asthma remained significant after adjustment for any positive viral or bacterial aspirate (OR, 1.39 [95% CI, 1.21-1.59]
Par 5: PROPHYLAXIS BY MONOCLONAL ANTIBODIES AND VACCINES
The field of vaccination has been hampered by an incident in the sixties with a formaline-inactivated (FI) vaccine that caused exacerbation of a subsequent infection (Ep 302-14).
The emphasis is on the development of vaccines for pregnant women, to provide newborns with maternal Ab and vaccination of elderly at increased risk.
It has taken till very recently before two candidate vaccines from Pfizer and GSK received authorization in the US.
In the meantime, passive immunotherapy with the monoclonal Palivizumab was the standard for at-risk young infants. Today there a few new mAbs such as Nirsevimab and Clesrovimab which are being approved
Ep 302-14: Killikelly Sc Rep 2016: Vaccine development against RSV in the sixties started with a disaster: enhanced respiratory disease (ERD) syndrome in originally seronegative children who received the formalin-inactivated (FI)-RSV vaccine.
A three dose regimen (0, 1, 4 months) was used in subjects between 2 and 7 months of age,
- 16 were hospitalized of the 20 infected children in the FI-RSV-vaccinated group (N = 31)
- compared to only 1 hospitalized of 21 infected in the control groups (N = 40)
The immunological basis:
- FI-RSV induced high titers of binding antibody with weak neutralizing and fusion-inhibitory activity: in the context of large viral antigen load led to immune complex deposition and complement activation in airways upon subsequent RSV infection.
- Natural RSV infection after immunization with FI-RSV was associated with exaggerated peribronchiolar inflammation and infiltration of neutrophils and eosinophils into airways due to a Th2-biased immune responses and airway hypersensitivity characterized by up regulation of IL-4, IL-5, IL-13, and IgE.
- Clearly, immunological priming with the FI-RSV vaccine was responsible for aberrant responses to subsequent Infection.
- However, there was no enhanced RSV disease when individuals are first primed with live virus infection or vaccinated with attenuated replication-competent vaccines given intranasally or parenterally
The suspected culprit is the F protein in the post-fusion conformation, as this is induced by formaldehyde inactivation (FI)
Ep 302-15: Mazur Lancet Infect Dis 2023 provides a very nice state-of-the-art
Because of the history with FI-RSV and the deleterious Th2 bias, an RSV vaccine for RSV-naive recipients ideally elicits potent neutralising antibodies without a Th2 bias.
Although a definitive correlate of protection against RSV infection remains elusive, but cell-mediated immunity, mucosal IgA and neutralising antibodies have been associated with protection.
Overview of present candidates
- Live attenuated viruses (LAV)
- LAVs are considered safe after first exposure, because vaccine-enhanced disease has not been detected after LAV immunization
- Estimated efficacy (5 LAV candidates) = 67% (95% CI 24 to 85) against medically-attended RSV acute-respiratory illness and 88% (–9 to 99) against medically-attended RSV lower resp tract infections (LRTI).
- Responses were durable through 1 year after vaccination
Conclusion: LAVs provide an important needle-free tool for active intranasal immunisation of older infants who will not be sufficiently protected by a mAb or maternal vaccine
- Chimeric viruses: express RSV proteins in related attenuated viruses with favorable safety profiles:
Two candidates are in phase 1:
- Se/RSV: replication- deficient Sendai virus modified to express RSV F protein
- rBCG-N-hRSV: live-attenuated recombinant BCG vector expressing RSV N protein (nucleoprotein)
Pr-F: 8 different trials with F protein in pre-fusion conformation, because
- post-F is the form in the formalin inactivated -RSV
- a phase 3 trial with post-F failed.
But still: only restricted to pregnant women and older adults (avoiding young children, because of the history with FI-RSV.
Two of these candidates have successfully finished phase 3:
- Pfizer RSVpreF or PF-06928316 = a bivalent (subtype A + B) prefusion F vaccine.
- Ep 302-16: MATISSE trial Pfizer announced an efficacy of 81.8% against severe cases of RSV in the infants for the 90 days after birth by immunization of pregnant women
- Ep 302-17: RENOIR trial in older (> 60 yrs) subjects: efficacy of 85.7% against severe disease primary endpoint of lower respiratory tract illness (LRTI-RSV)
- GSK RSV PreF3 with ASO1 adjuvant: (Ep 302-18) similar results in older subjects with more details provided on vaccine efficacy
I could not find peer-reviewed or preprints on the Pfizer and GSK vaccines
Other RSV proteins in subunit:
- BARS13 uses RSV G protein as an antigen and cyclosporine A (CSA) immunosuppressant to induce regulatory T cells.
- DPX-RSV, uses the ectodomain of RSV-A-SHe protein with oil in water adjuvant
- VN-0200, uses VAGA-9001a as antigen and an MABH-9002b adjuvant (unclear which protein VAGA is)
- Particle-based vaccines:
- IVX-121/ DsCav1 = 20 copies of stabilized trimeric pre-F proteins
- V306-VLP = self-assembling lipopeptides containing a T-helper epitope and toll-like receptor ligand
These candidates are in early phase 1
- M-RNA: Moderna (mRNA-1345) encodes stabilised RSV pre-F with the same lipid nanoparticle formulation as for the SARS-CoV-2 vaccine.
Moderna intends to combine mRNA-1345 with mRNA-1653 (an mRNA vaccine against two other pediatric viruses, hMPV and parainfluenza virus type 3 intended for use in the pediatric population:
In phase 1
- Recombinant vectors
- MVA-BN-RSV = modified vaccinia Ankara virus, to express RSV surface antigens (F and G) and intracellular proteins (M2 and N): 79% reduction in symptomatic RSV in phase 2 with older subjects.
- Ad26.RSV.pre-F: showed 80% efficacy against RSV LRTI through the first RSV season in the older adult population. Interestingly: no interference when an RSV vaccine was co-administered with seasonal influenza vaccine in older adults in a phase 2 trial
Monoclonal antibodies = passive immunization in at risk infants
Ep 302-19: Antigenic sites of the respiratory syncytial virus fusion (F) protein and monoclonal antibodies (mAbs)
The conformation of the pre-F and post-F proteins and the specific antigenic sites are included in the left panel (pre-F) and right panel (post-F), respectively. Of the 5 major neutralizing sites (Ø, II, III, IV, and V) present on the pre-F surface, sites Ø and V are the most neutralization sensitive. The triangle illustrates the most to least neutralization sensitive epitopes.
Niservimab and Suptamuvab are pre-Fe specific mAbs that bind to antigenic sites Ø and V, respectively,
Palivizumab and MK-1654 (= Clesrovimab) bind to sites II and IV, respectively, in the pre-F and post-F
- Palivizimab = golden standard since 20 years
Ep 302-20: Meta-analysis of effectiveness of monthly injection to prevent RSV-related hospitalization in infants with congenital heart and lung disease or preterm infants between 29 and 35 weeks gestational age
- Nirsevimab has extended half-life (Fc engineering) → only 1 injection needed for an entire RSV season:
78 % efficacy against RSV-related hospitalization in phase 2 trial.
- Clesrovimab has also extended half-life and seems to have a similar efficacy.
CONCLUDING REMARKS and INDICATIONS
From Mazur (Ep 302-15) : RSV vaccine and monoclonal antibody agents by target population
Vaccine candidates and monoclonal antibodies are categorised into three different target populations: paediatric, maternal, and older adults (aged >60 years) and clinical phase of development (ie, phase 1, 2, or 3).
Different immunisation approaches are indicated by the key. Light grey text indicates development halted. IM=intramuscular. IN=intranasal. ID=intradermal. RSV=respiratory syncytial virus. PreF=prefusion protein. PostF=postfusion protein.
From Mejias Ep 302-19
All the best!
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