A Pan-Pneumovirus vaccine based on immunodominant epitopes of the fusion protein

Respiratory syncytial virus (RSV) and human metapneumovirus (hMPV) are two leading causes of severe respiratory infections in children, the elderly, and immunocompromised patients. The fusion (F) protein is the major target of neutralizing antibodies. Recent developments in stabilizing the pre-fusion conformation of the F proteins, and identifying immunodominant epitopes that elicit potent neutralizing antibodies have led to testing of numerous pre-fusion RSV F-based vaccines in clinical trials. We designed and tested the immunogenicity and protective efficacy of a chimeric fusion protein that contains immunodominant epitopes of RSV F and hMPV F (RHMS-1). RHMS-1 has several advantages over vaccination with pre-fusion RSV F or hMPV F, including a focus on recalling B cells to the most important protective epitopes and the ability to induce protection against two viruses with a single antigen. RHMS-1 was generated as a trimeric recombinant protein, and negative-stain EM analysis demonstrated the protein resembles the pre-fusion conformation. Probing of RHMS-1 antigenicity using a panel of RSV and hMPV F-specific monoclonal antibodies (mAbs) revealed the protein retains features of both viruses, including the pre-fusion site O epitope of RSV F. BALB/c mice immunized with RHMS-1 had serum binding and neutralizing antibodies to both viruses. RHMS-1 vaccinated mice challenged with RSV or hMPV had undetectable virus in lung homogenates for both viruses, in contrast to RSV F or hMPV F vaccinated mice, which had detectable virus for hMPV and RSV, respectively. Overall, this study demonstrates protection against two viruses with a single antigen and supports further testing of RHMS-1 in additional pre-clinical animal models. SignificanceRespiratory syncytial virus (RSV) and human metapneumovirus (hMPV) are two Pneumoviruses that cause substantial respiratory tract infections in young children, the elderly, and immunocompromised adults. Yet vaccines against either of them are still unavailable. Multiple promising vaccine designs based on the fusion proteins of RSV and hMPV have been tested, but none of them can induce cross-protective antibody responses despite their similar structures and closely related amino acid sequence identity. Here, we report a vaccine candidate, RHMS-1, which combines the immunodominant epitopes of the RSV and hMPV fusion proteins into a single antigen. RHMS-1 maintains the immunological features of both RSV F and hMPV F, which can be recognized by epitope-specific mAbs and human B cells pre-exposed to RSV or hMPV. Furthermore, this is the first immunogen that induced potent cross-neutralizing antibodies and protected mice from RSV and hMPV challenge. Our results suggest RHMS-1 is a promising Pan-Pneumovirus vaccine.