Production and immunogenicity of chimeric virus-like particles containing the spike glycoprotein of infectious bronchitis virus

Lv, Lishan; Li, Xiaoming; Liu, Genmei; Li, Ran; Liu, Qiliang; Shen, Huifang; Wang, Wei; Xue, Chunyi; Cao, Yongchang

Vaccination is the most effective method of preventing virus infection and controlling the spread of IBV. Both inactivated and live attenuated virus vaccines have been used to protect chickens against IBV [4, 9] . Inactivated virus vaccines are produced from IBV-infected allantoic fluids, but do not illicit high levels of S1-specific antibody responses [33] . Live attenuated viruses offer long lasting and protective immunity, but pose the risk of returning to virulent form and co-infection with Escherichia coli or other bacteria [10, 34] . Accordingly, it is necessary to develop a new IBV vaccine. Spike (S) protein, one of the four structural proteins of IBV, is a heavily glycosylated spike glycoprotein expressed on virion surfaces [13] . This protein be cleaved into two subunits, N-terminal S1 and C-terminal S2. The S1 subunit, which is the bulbous head of the S protein, is responsible for attachment of the virus to cells [7] . Analysis of S-specific monoclonal antibodies has shown that many of the amino acids of virus neutralization (VN) epitopes are located within the first and third quarters of the linear S1 polypeptide [11, 12, 19] . Immune responses induced by the S1 subunit have been studied using S1 protein prepared from purified virus and derived from baculovirus-based expression systems [16, 23, 33] . Virus-like particles (VLPs) are multi-protein structures that mimic the organization and conformation of authentic native viruses without viral genomes. VLPs are generated by assembling structural viral proteins and lipids into particles [18, 30] . VLPs have been widely investigated for use in the development of safe and effective vaccines because the viral antigens on the surfaces of VLPs can induce humoral and cellular responses [24, 26, 27] . Two VLP-based vaccines have already been licensed for use in humans against hepatitis B virus and HPV, and more VLP-based vaccines are being evaluated in preclinical and clinical trials. In addition, chimeric VLPs have been generated by substituting part or all of the extracellular domain of a surface antigen of a VLP derived from one virus with one from another virus, and these VLPs have been shown to induce immune responses against the surface antigen from the other virus [35] . VLPs based on IBV structural proteins have been reported and employed for investigations of proteinprotein interactions and assembly of virons [2, 11, 17, 22, 31] . Influenza virus is a major threat to human health that causes significant morbidity and mortality worldwide and is therefore always at the forefront of vaccine research. Influenza VLPs have been generated by co-infecting insect cells with recombinant baculoviruses expressing structural influenza proteins of matrix 1 (M1)/hemagglutinin (HA), HA/neuraminidase (NA)/M1, or HA/NA/M1/matrix 2 (M2). [14, 15, 21, 28, 33] . Influenza VLPs have been found to induce protective immunity in preclinical and clinical studies [20] . In light of the above findings, this study was conducted to investigate whether influenza VLPs could serve as a platform for the expression of IBV S1 protein, and whether VLPs containing S1 protein could serve as a candidate IBV vaccine.