Elizabeth B. Sobolik

Paramyxoviruses including measles, Nipah, and parainfluenza viruses are public health threats with pandemic potential. Human parainfluenza virus type 3 (HPIV3) is a leading cause of illness in pediatric, older, and immunocompromised populations. There are no approved vaccines or therapeutics for HPIV3. Neutralizing monoclonal antibodies (mAbs) that target viral fusion are a potential strategy for mitigating paramyxovirus infection, however their utility may be curtailed by viral evolution that leads to resistance. Paramyxoviruses enter cells by fusing with the cell membrane in a process mediated by a complex consisting of a receptor binding protein (HN) and a fusion protein (F). Existing atomic resolution structures fail to reveal physiologically relevant interactions during viral entry. We present cryo-ET structures of pre-fusion HN-F complexes in situ on surfaces of virions that evolved resistance to an anti-HPIV3 F neutralizing mAb. Single mutations in F abolish mAb binding and neutralization. In these complexes, the HN protein that normally restrains F triggering has shifted to uncap the F apex. These complexes are more readily triggered to fuse. These structures shed light on the adaptability of the pre-fusion HN-F complex and mechanisms of paramyxoviral resistance to mAbs, and help define potential barriers to resistance for the design of mAbs.

Measles virus (MeV) presents a public health threat that is escalating as vaccine coverage in the general population declines and as populations of immunocompromised individuals, who cannot be vaccinated, increase. There are no approved therapeutics for MeV. Neutralizing antibodies targeting viral fusion are one potential therapeutic approach but have not yet been structurally characterized or advanced to clinical use. We present cryo–electron microscopy (cryo-EM) structures of prefusion F alone [2.1-angstrom (Å) resolution], F complexed with a fusion-inhibitory peptide (2.3-Å resolution), F complexed with the neutralizing and protective monoclonal antibody (mAb) 77 (2.6-Å resolution), and an additional structure of postfusion F (2.7-Å resolution). In vitro assays and examination of additional EM classes show that mAb 77 binds prefusion F, arrests F in an intermediate state, and prevents transition to the postfusion conformation. These structures shed light on antibody-mediated neutralization that involves arrest of fusion proteins in an intermediate state.

Measles cases have surged pre-COVID-19 and the pandemic has aggravated the problem. Most measles-associated morbidity and mortality arises from destruction of pre-existing immune memory by measles virus (MeV), a paramyxovirus of the morbillivirus genus. Therapeutic measles vaccination lacks efficacy, but little is known about preserving immune memory through antivirals and the effect of respiratory disease history on measles severity. We use a canine distemper virus (CDV)-ferret model as surrogate for measles and employ an orally efficacious paramyxovirus polymerase inhibitor to address these questions. A receptor tropism-intact recombinant CDV with low lethality reveals an 8-day advantage of antiviral treatment versus therapeutic vaccination in maintaining immune memory. Infection of female ferrets with influenza A virus (IAV) A/CA/07/2009 (H1N1) or respiratory syncytial virus (RSV) four weeks pre-CDV causes fatal hemorrhagic pneumonia with lung onslaught by commensal bacteria. RNAseq identifies CDV-induced overexpression of trefoil factor (TFF) peptides in the respiratory tract, which is absent in animals pre-infected with IAV. Severe outcomes of consecutive IAV/CDV infections are mitigated by oral antivirals even when initiated late. These findings validate the morbillivirus immune amnesia hypothesis, define measles treatment paradigms, and identify priming of the TFF axis through prior respiratory infections as risk factor for exacerbated morbillivirus disease.

Mitigation measures against the COVID-19 pandemic affected the RSV seasonality and led to an unexpectedly high number of RSV cases in Washington State since October 2022. Here we describe the RSV genomic characteristics and evolutionary relationship of 2022 outbreak compared to the previous RSV outbreaks in the region and globally.