Date Published: September 9, 2016
Publisher: Public Library of Science
Author(s): Vicente Más, Laura Rodriguez, Eduardo Olmedillas, Olga Cano, Concepción Palomo, María C. Terrón, Daniel Luque, José A. Melero, Jason S. McLellan, Richard K Plemper.
Human metapneumovirus (hMPV) is a paramyxovirus that is a common cause of bronchiolitis and pneumonia in children less than five years of age. The hMPV fusion (F) glycoprotein is the primary target of neutralizing antibodies and is thus a critical vaccine antigen. To facilitate structure-based vaccine design, we stabilized the ectodomain of the hMPV F protein in the postfusion conformation and determined its structure to a resolution of 3.3 Å by X-ray crystallography. The structure resembles an elongated cone and is very similar to the postfusion F protein from the related human respiratory syncytial virus (hRSV). In contrast, significant differences were apparent with the postfusion F proteins from other paramyxoviruses, such as human parainfluenza type 3 (hPIV3) and Newcastle disease virus (NDV). The high similarity of hMPV and hRSV postfusion F in two antigenic sites targeted by neutralizing antibodies prompted us to test for antibody cross-reactivity. The widely used monoclonal antibody 101F, which binds to antigenic site IV of hRSV F, was found to cross-react with hMPV postfusion F and neutralize both hRSV and hMPV. Despite the cross-reactivity of 101F and the reported cross-reactivity of two other antibodies, 54G10 and MPE8, we found no detectable cross-reactivity in the polyclonal antibody responses raised in mice against the postfusion forms of either hMPV or hRSV F. The postfusion-stabilized hMPV F protein did, however, elicit high titers of hMPV-neutralizing activity, suggesting that it could serve as an effective subunit vaccine. Structural insights from these studies should be useful for designing novel immunogens able to induce wider cross-reactive antibody responses.
Human metapneumovirus (hMPV) was first isolated in 2001 from respiratory specimens collected from children with respiratory tract infections . Sequence analysis was used to classify hMPV in the Metapneumovirus genus of the Pneumovirinae subfamily of paramyxoviruses. This subfamily also includes the Pneumovirus genus in which human respiratory syncytial virus (hRSV) is the best known prototype. Like all members of the Paramyxovirus family, hMPV and hRSV are enveloped, single-stranded, negative-sense RNA viruses that share many characteristics of their respective life cycles with other paramyxoviruses . Sequence analysis of hMPV samples indicate that there are two main genetic lineages (A and B), each divided into at least two sub-lineages (A1, A2, B1 and B2) .
Data accumulated during the last 10–15 years have demonstrated extensive similarities between the clinical manifestations and epidemiology of hMPV and hRSV , and the two viruses share many steps of their respective life cycles. However, there are also important differences in gene order and number of genes encoded in their genomes  as well as in individual gene products. One of these differences resides in the protease maturation of their respective F glycoproteins. Whereas the hMPV F precursor is cleaved only once by trypsin-like proteases, the hRSV F precursor is cleaved at two distinct furin sites separated by 27 amino acids , which is unique among paramyxoviruses. It is still unresolved whether or not this difference has an impact on structural and functional properties of their respective F glycoproteins, but it has been shown that insertion of the two cleavage sites of hRSV F into Sendai virus F protein changed the hemagglutinin-neuraminidase (HN) dependence of Sendai F for activation  and altered Sendai virus thermostability .