Tailoring subunit vaccine immunity with adjuvant combinations and delivery routes using the Middle East respiratory coronavirus (MERS-CoV) receptor-binding domain as an antigen.

Jiaming Lan,Guangwen Lu,Wen Wang,Yao Deng,George F Gao,Zhuozhuang Lu,Xiaojuan Guo,Hong Chen,Wenjie Tan,Yuxian He

doi:10.1371/journal.pone.0112602

Abstract

The development of an effective vaccine is critical for prevention of a Middle East respiratory syndrome coronavirus (MERS-CoV) pandemic. Some studies have indicated the receptor-binding domain (RBD) protein of MERS-CoV spike (S) is a good candidate antigen for a MERS-CoV subunit vaccine. However, highly purified proteins are typically not inherently immunogenic. We hypothesised that humoral and cell-mediated immunity would be improved with a modification of the vaccination regimen. Therefore, the immunogenicity of a novel MERS-CoV RBD-based subunit vaccine was tested in mice using different adjuvant formulations and delivery routes. Different vaccination regimens were compared in BALB/c mice immunized 3 times intramuscularly (i.m.) with a vaccine containing 10 µg of recombinant MERS-CoV RBD in combination with either aluminium hydroxide (alum) alone, alum and polyriboinosinic acid (poly I:C) or alum and cysteine-phosphate-guanine (CpG) oligodeoxynucleotides (ODN). The immune responses of mice vaccinated with RBD, incomplete Freund’s adjuvant (IFA) and CpG ODN by a subcutaneous (s.c.) route were also investigated. We evaluated the induction of RBD-specific humoral immunity (total IgG and neutralizing antibodies) and cellular immunity (ELISpot assay for IFN-γ spot-forming cells and splenocyte cytokine production). Our findings indicated that the combination of alum and CpG ODN optimized the development of RBD-specific humoral and cellular immunity following subunit vaccination. Interestingly, robust RBD-specific antibody and T-cell responses were induced in mice immunized with the rRBD protein in combination with IFA and CpG ODN, but low level of neutralizing antibodies were elicited. Our data suggest that murine immunity following subunit vaccination can be tailored using adjuvant combinations and delivery routes. The vaccination regimen used in this study is promising and could improve the protection offered by the MERS-CoV subunit vaccine by eliciting effective humoral and cellular immune responses.

Highlights

In 2012 a novel human coronavirus, Middle East respiratory syndrome coronavirus (MERS-CoV), caused outbreaks of a SARS-like illness in the Middle East, and is considered a threat to global public health [1,2]
The results indicated that receptor-binding domain (rRBD) protein combined with any adjuvant, including alum, incomplete Freund’s adjuvant (IFA), CpG or poly(I:C), could induce a receptor-binding domain (RBD)-specific IgG antibody response in the majority of mice after the second immunisation
No neutralizing antibodies were detected in the sera of mice immunised with the individual adjuvants without rRBD antigen. These results suggest that the MERS-CoV S rRBD protein can induce potent anti-MERS-CoV neutralizing antibody responses when combined with certain adjuvants

Summary

Introduction

In 2012 a novel human coronavirus, Middle East respiratory syndrome coronavirus (MERS-CoV), caused outbreaks of a SARS-like illness in the Middle East, and is considered a threat to global public health [1,2]. Studies show that camels are a likely primary source of the MERS-CoV that is infecting humans [4,5,6]. Highly purified proteins are typically not inherently immunogenic, as they usually lack the means to directly stimulate the innate immune system [11]. They call for efficient delivery systems and potent immunostimulants, jointly denoted as adjuvant(s) to evoke the desired antigen-specific immune response phenotype enabling successful vaccination [12]

Methods

Results

Conclusion