An Overview of Subunit Vaccine (Part One)

Posted by beauty33 on November 26th, 2019

A vaccine is a pathogenic protein, polypeptide, polysaccharide or nucleic acid in the form of a single component or a complex particle containing active ingredient that produces specific immune responses by activating attenuated pathogens or carriers into the body. Subunit vaccine is made up of components that have the major protective immunogens of pathogenic bacteria, also known as component vaccine. Since macromolecular antigens carry multiple specific antigenic determinants, only a small number of antigenic sites contribute to a protective immune response. Vaccine can be prepared by extracting proteins having an immunological function from pathogens or using recombinant technique to express specific protein structure of bacterium and virus and to screen for immunologically active fragments. These vaccines have only a few major single-component, effective, and non-pathogenic surface proteins, thereby reducing vaccine side effects and vaccine-related diseases. The shortcoming of the subunit vaccine is that it is less immunogenic and needs to be combined with adjuvant to produce good immune effects.

Characteristics of subunit vaccine:

1. Compared with whole-virus vaccine, the subunit vaccine is safer and has better stability.

2.  Immunization of subunit vaccine is persistent. Long-term immunity can be obtained by one-time vaccination and there is no need to repeatedly strengthen the immunization.
3.  Subunit vaccine can be produced on a large scale by fermentation of escherichia coli, baculovirus and pichia pastoris.

4. Subunit vaccines not only induce humoral immune responses, but also cause cytotoxic T lymphocyte activation to induce cellular immunity.

Here are several classic subunit vaccines:

1 Vaccines against avian influenza virus

Avian influenza (AI) is an infectious disease characterized by mild respiratory symptoms or acute sepsis caused by various types of avian influenza A viruses. The avian influenza virus is a retrovirus. Like other retroviruses, avian influenza viruses are characterized by multiple variability and weak cross-protection between different serotypes of avian influenza viruses. The most effective means for controlling avian influenza is still vaccination. Traditional avian influenza vaccines mainly include chicken embryo inactivated vaccines and avian influenza attenuated vaccines. Chicken embryo inactivated vaccine is currently the main vaccine used to prevent and control avian influenza and plays a key role in the treatment of avian influenza.

The avian influenza virus subunit vaccine is prepared by prokaryotic or eukaryotic expression of a gene encoding a main immunogen of avian influenza virus, and the extracted protein is added with an appropriate adjuvant. Since the vaccine is not infectious, there is no safety hazard during use and it can stimulate the body to produce a good immune response.

The avian influenza virus genome consists of 8 negative-stranded RNAs encoding a total of 10 proteins, of which hemagglutinin (HA) and neuraminidase (NA) glycoproteins are viral virulence-related proteins, and antibodies against these two proteins can inhibit the infection of avian influenza viruses and prevent the onset of the disease. HA protein is the most abundant glycoprotein on the surface of the virus. It is responsible for the adsorption of sialic acid receptors at the terminal end of host cell receptors, mediating the fusion of viruses and cells, and is the main protective antigen. The NA protein is primarily involved in the release of the virus from infected cells. According to the different antigenicity of HA and NA of avian influenza virus, it can be divided into 15 H subtypes and 9 N subtypes. Avian influenza virus not only has a large number of serotypes, but also has strong variability. Low-pathogenic or non-virulent strains may be mutated into highly pathogenic strains in a short period of time, making it difficult to provide diagnosis and control of avian influenza. The highly pathogenic avian influenza (HPAI) caused by H5 and H7 subtype avian influenza viruses has caused serious economic losses to the poultry industry worldwide.

Although the inactivated vaccine with traditional oil emulsion can induce sufficient immune protection in chickens, the rate of antibody production after immunization is slow, especially the anti-NP or M1 protein antibodies produced after vaccine immunization cannot distinguish antibodies produced after vaccine immunization from antibodies produced by wild virus infection. And weak vaccine could lead to the risk of genetic recombination of RNA produced by vaccine strain and wild strains. The recombinant poxvirus vaccine cannot be used for poultry vaccine immunization of chickens. Subunit vaccine can overcome the shortcoming of the recombinant poxvirus vaccine, and therefore a powerful weapon against the outbreak of HPAI.

Traditional subunit vaccines are vaccines prepared by chemical separation of protective antigens from virions. The influenza virus nucleic acid is removed by protease treatment and its hemagglutinin (HA)/neuraminidase (NA) subunit is extracted to make a vaccine with an adjuvant. This subunit vaccine is part of the virion, but it does not contain nucleic acids, has good safety, and can stimulate the body to produce sufficient immunity. However, the subunit vaccine is low in immunogenicity and short in duration and needs to be used in combination with an adjuvant or coupled to a suitable carrier. However, the traditional method for preparing subunit vaccines is complicated and costly. The development of recombinant DNA and molecular cloning techniques, the HA gene can be ligated into a vector plasmid and introduced into an expression system for amplification and expression. The yeast or mammalian expression system is used to express the corresponding avian influenza virus surface proteins, such as H5 and H7, and the expression product is purified and used as a vaccine to immunize the organism. The antigenic proteins produced by genetic engineering expression are not only immunogenic, but also have high yield and low cost. Since neutralizing antibodies are mainly directed against HA production on the surface of the virus, the subunit vaccines currently being studied are mostly directed against HA. The influenza virus HA gene is cloned by recombinant DNA technology and expressed in eukaryotic cells, and the protein product can be used as a subunit vaccine after effective purification. In the vaccine, HA is in an unsplit configuration and has a highly antigenic epitope that elicits a strong immune response. The baculovirus expression system is used to produce H5 and H7 recombinant HA adjuvant vaccines, and the recombinant adjuvant vaccination group is inoculated with lethal avian influenza. As a result, all the immunized chickens did not develop disease, and the unimmunized chickens died after vaccination against lethal avian influenza. It has been demonstrated that no adjuvant or low-dose recombinant HA adjuvant vaccine is only partially protective.

2 Vaccines against Newcastle virus

Newcastle virus belongs to the paramyxovirus family. The mature virion has a diameter of 100-250 nm, its vesicle vesicles are usually round, and the vesicles have a fiber length of about 8 nm. The core and nucleocapsid structures of the Newcastle virus are approximately 17 nm in diameter and have HA and NA on the surface of the capsule. Newcastle virus can be inactivated at 55 °C for 45 min or under direct sunlight for 30 min. The infection ability of newcastle virus can be maintained for several weeks at 4 °C, for several months at 20 °C, for several years at 70 °C. The disinfection effect of sodium carbonate and sodium hydroxide on newcastle virus is unstable. Detergents can quickly inactivate Newcastle virus. In general, NDV is quite resistant to physical and chemical factors.