A single-dose intranasal immunization with a novel bat influenza A virus-vectored MERS vaccine provides effective protection against lethal MERS-CoV challenge

Abstract

The threat to global health security posed by Middle East respiratory syndrome coronavirus (MERS-CoV) and emerging MERS-like coronaviruses highlights the need to develop safe and efficient vaccines. Viral vector vaccines have been shown to be effective and are widely used to prevent various viral diseases because they mimic natural infection and induce a more comprehensive immune response. Herein, we developed a novel bat influenza A virus-based vaccine vector by replacing the open reading frame of either bat influenza hemagglutinin or neuraminidase with that of the hemagglutinin-esterase-fusion gene from influenza D virus, which can infect multiple species, including humans and camels. We then generated a temperature-sensitive, cold-adapted, and attenuated MERS vaccine candidate expressing the clade A MERS-CoV spike S1, referred to as Len_S1, using the developed bat influenza vector and demonstrated its safety and immunogenicity. A single-dose intranasal immunization with Len_S1 protected human dipeptidyl-peptidase-4 (hDPP4) transgenic mice from a lethal MERS-CoV challenge. Notably, a two-dose immunization with Len_S1 completely blocked viral replication and lung damage in challenged mice. Further studies revealed that intranasal immunization with Len_S1 in mice elicited mucosal, humoral, and cellular immune responses. Moreover, sera collected from Len_S1-immunized mice were able to cross-neutralize multiple clades of MERS-CoVs. Collectively, these results indicate that Len_S1 is a safe and effective MERS vaccine that induces a comprehensive immune response and provides cross-protection against diverse clades of MERS-CoVs.IMPORTANCEMiddle East respiratory syndrome coronavirus (MERS-CoV) is an important zoonotic virus with pandemic potential that continues to evolve within dromedary camels. However, no licensed vaccine is currently available. Viral vector-based vaccines represent a promising platform, with demonstrated efficacy in preventing viral diseases. In this study, we developed a bat influenza virus-vectored MERS vaccine, Len_S1, that is safe and immunogenic. Intranasal immunization of human dipeptidyl-peptidase-4 (hDPP4)-transgenic mice with Len_S1 induced humoral, mucosal, and cellular immune responses and provided effective protection against a lethal MERS-CoV challenge. Importantly, sera collected from immunized mice cross-neutralized three distinct clades of MERS-CoVs. Our results indicate that Len_S1 is a promising vaccine candidate with the potential to prevent MERS-CoV infection and mitigate the risk of future epidemics and pandemics.

Keywords: MERS-CoV; bat influenza vectored MERS vaccine; cross-protection; safety and immunogenicity.

Fig 1

Construction and characterization of a bat influenza virus-based vector and potential MERS vaccine candidates expressing the spike S1 subunit. (A) Schematic diagrams of the construction of the bat influenza A virus-based vector and MERS vaccine candidates expressing the MERS-CoV spike S1 subunit. ORFs of influenza D virus HEF and MERS-CoV spike S1 were flanked with either Bat09 HA or NA packaging signals, as indicated. Recombinant viruses containing seven or eight segments were rescued and designated as vector, N10ps_S1, and H17ps_S1. (B) Madin-Darby canine kidney (MDCK) cells were infected with the indicated viruses at a multiplicity of infection (MOI) of 0.01. Supernatants were harvested at 24, 36, 48, 60, and 72 hours post-infection (hpi), and viral growth curves were determined. (C) MDCK cells were infected with the indicated viruses at an MOI of 0.1. At 24 hpi, cells were fixed and stained to detect MERS-CoV S1 expression. Nuclei were stained with DAPI. Scale bar: 150 µm. (D) MDCK cells were infected with the indicated virus at an MOI of 0.1, and cell lysates were collected at 24 hpi. Expression of MERS-CoV S1 and Bat09 NP was detected by western blotting. (E) MERS-CoV S1 expression in N10ps_S1- and H17ps_S1-infected cells was relatively quantified by normalizing to glyceraldehyde-3-phosphate dehydrogenase (GAPDH). The data represent results from three independent experiments.

Fig 2

Attenuation of bat influenza-vectored MERS vaccine candidates. (A, B) Schematic diagrams of two approaches used to attenuate bat influenza-vectored MERS vaccine candidates. (A) The NS1 protein was truncated by introducing three consecutive stop codons at the 128th codon and deleting nucleotides 412-474. (B) Amino acids responsible for the ts, ca, and att phenotypes in AA and Len viruses were introduced into the corresponding genes of Bat09. (C) MDCK cells were infected with vector, H17ps_S1, or NS128_S1 at an MOI of 0.01 at 37°C. Supernatants were collected at 24, 36, 48, 60, and 72 hpi to compare viral growth dynamics. (D–F) MDCK cells were infected with H17ps_S1, AA_S1, or Len_S1 at an MOI of 0.01 and cultured at 33°C, 37°C, or 39°C. Supernatants were harvested at the indicated time points and titrated on MDCK cells at 37°C to determine the growth kinetics of parental H17ps_S1 (D), AA_S1 (E), and Len_S1 (F). (G) MDCK cells were infected with H17ps_S1, AA_S1, or Len_S1 at an MOI of 0.1 and cultured at 33°C, 37°C, or 39°C. At 24 hpi, cells were fixed and stained for S1 expression using an MERS-CoV S1 antibody. Nuclei were stained with DAPI. Scale bar: 150 µm. (H) MDCK cells were infected with 10-fold dilutions (10⁻¹ to 10⁻⁶) of the vector or the indicated viruses. At 3 days post-infection (dpi), cells were fixed with cold methanol and stained with crystal violet. (I) Plaque sizes formed by each virus were quantified using ImageJ software.

Fig 3

Len_S1 demonstrates safety and immunogenicity in mice. (A) Schematic overview of the experimental design to evaluate the safety and immunogenicity of MERS vaccine candidates in C57BL/6J mice. (B) Weight change over 14 days in mice intranasally mock-immunized with DMEM or immunized with 10⁶ TCID₅₀ of the vector, H17ps_S1, NS128_S1, or Len_S1. (C, D) Virus replication in organ tissues of immunized mice. Three mice per group were necropsied at 3 and 5 dpi, and the indicated tissues were collected, homogenized, and analyzed for viral titers on MDCK cells. The dashed line indicates the limit of detection (10^1.5 TCID₅₀/mL). (E) Detection of neutralizing antibodies in sera from mice immunized with the indicated viruses. Serum samples were collected at 2, 3, and 4 weeks post-immunization, serially diluted, and incubated with rVSV-eGFP-S to determine neutralizing antibody titers using Vero E6 cells.

Fig 4

A single-dose intranasal immunization with Len_S1 elicits a comprehensive immune response in mice. (A) Experimental design to evaluate immune responses and efficacy of Len_S1 in hDPP4-transgenic C57BL/6J mice. (B) Body weight changes of mice immunized with the vector or Len_S1, monitored over 14 days post-immunization. (C) Neutralizing antibody titers induced by one or two doses of Len_S1. Sera were collected at 4 wpi, serially diluted, and incubated with rVSV-eGFP-S to determine neutralizing titers. (D) Serum IgG levels in Len_S1-immunized mice. Sera collected at 4 wpi were analyzed for MERS-CoV spike-specific IgG titers by ELISA. The dashed line indicates the detection limit (10-fold dilution). (E) Nasal IgA levels in Len_S1-immunized mice. Nasal turbinates were collected from three euthanized mice at 4 wpi, and nasal washes were assessed for spike-specific IgA titers by ELISA. (F) T cell responses in Len_S1-immunized mice. Spleens were collected from three mice per group at 4 wpi. Splenocytes (5 × 10⁵ cells) were stimulated with 5 µg/mL purified MERS-CoV spike trimers, and spike-specific responses were measured by ELISPOT assay. (G) Number of spot-forming units per well was quantified using ImmunoSpot software.

Fig 5

A single-dose intranasal immunization with Len_S1 provides effective protection against lethal MERS-CoV challenge. Groups of hDPP4-transgenic C57BL/6J mice were intranasally challenged with 10-fold LD₅₀ of the mouse-adapted MERS-CoV Clone 6.1.2 at 4 weeks post-immunization with the vector or Len_S1. Clinical signs and body weight were monitored daily. Three mice per group were necropsied for the collection of nasal turbinates, trachea, and lungs for viral titration and histopathological analysis. (A) Body weight of mice in each group over 14 days post-challenge. (B) Survival percentage of mice in each group over 14 days post-challenge. (C) Viral titers in the nasal turbinates, trachea, and lungs of three mice per group. Titers were determined using Vero 81 cells. Dashed line indicates the limit of detection (10^1.5 TCID₅₀/mL). (D) Detection of MERS-CoV N antigen in mouse lung tissues. Lungs were fixed, and N antigen was visualized by immunohistochemistry. Arrows indicate N antigen-positive alveolar and bronchiolar cells. Scale bar: 50 µm.