Dr. Wahied Khawar Balwan
The past two decades have seen the application of molecular genetics and its increased insights into immunology, microbiology and genomics applied to vaccinology. Reverse vaccinology is a milestone in silico method which employs computational techniques to select the best vaccine candidate eschewing rigorous wet lab techniques. Thus, this method has garnered attention in the modern time with a prime focus on pathogenic biology.
Vaccination is a medical practice of ancient origin that possibly started in Asia. Buddhist monks drank snake venom to confer immunity to snake bite. In China during 17th century smears from small pox lesions were used to transmit a mild infection and thereby protect against more serious disease. The practice was formally introduced into Western medicine in 1796 by Edward Jenner, who used infected materials isolated from cows to immunize against smallpox and introduced the terminology “Vaccine”. It was a century later, when it was discovered that microbes are the culprits for the cause of infections. It was Louis Pasteur who proposed the basic rules of vaccinology and started the rational development of vaccines. According to him, the tenet to be followed to produce a vaccine is “Isolate, inactivate and inject the microorganism” that causes the disease. Pasteur’s rules were followed for a century by various vaccine developers. Jonas Salk developed a vaccine containing a poliovirus that had been killed by formaldehyde treatment. Albert Sabin used poliovirus that had been attenuated by serial passage in vitro. Hilleman developed vaccines against measles, mumps, and rubella by attenuating the viruses causing the diseases.
Others, such as Ramon and Glenny, isolated essential components from bacterial or viral cultures, inactivated them, and paved the way for the development of vaccines against diphtheria and tetanus, Neisseria meningitidis, Streptococcus pneumoniae, Haemophilus influenzae, and so on. In the case of hepatitis B, it was found that the causative virus could not be cultured in vitro. As a result, the vaccine was initially developed by inactivating viral antigen present in the plasma of chronically infected people. The vaccines developed using Pasteur’s rules became powerful tools in the history of medicine and, in less than a century, led to the elimination of some of the most devastating infectious diseases globally. Most of the vaccines that could be developed by these traditional technologies had been developed by the end of the 20th century. Novel technologies were required to battle against the remaining pathogens. Milestones were achieved by introduction of new technologies such as recombinant DNA and chemical conjugation of proteins to polysaccharides, as well as advances in the use of novel adjuvants. In 1995, Craig Venter published the genome of the first free living organism. Thus, a new technology became available by which genomes of microorganisms could be accessed. This technological revolution allowed for the first time the capacity to move beyond the rules of Pasteur, using the computer to rationally design .vaccines starting with information present in the genome, without the need to grow the specific microorganisms. This new approach was denominated “reverse vaccinology”.
The first pathogen addressed by the reverse vaccinology approach was Meningococcus B (MenB), a pathogen that causes 50% of the meningococcal meningitis worldwide. This bacterium had been refractory to vaccine development because its capsular polysaccharide is identical to a human self-antigen, whereas the bacterial surface proteins are extremely variable. The basic idea behind reverse vaccinology is that an entire pathogenic genome can be screened for identifying suitable vaccine candidate by using bioinformatics approaches. Some of the traits that the genes are monitored for that may indicate antigenicity include genes that code for proteins with extracellular localization, signal peptides, and B-cell epitopes. Next, those genes are filtered for desirable attributes that would make good vaccine targets such as outer membrane proteins. Once the candidates are identified, they are produced synthetically and are screened in animal models of the infection. This allows the development of vaccines that were previously difficult or impossible to make and can lead to the discovery of unique antigens that may improve existing vaccines. The steps involved in development of vaccine using reverse vaccinology can be elucidated as follows:
- Computer analysis of the whole genome identifies the genes coding for predicted antigens and eliminates antigens with hornoloaies to human proteins.
- The identified antigens are screened for expression by the pathogen and for immunogenicity during infection.
- The selected antigens are then used to immunize animals and test whether immunization induces a protective response.
- Protective antigens are tested for their presence and conservation in a collection of strains representative of the species (molecular epidemiology).
- Selected antigens are manufactured in large scale for clinical trials and candidate vaccines are tested for safety and protective immunity in humans using established corrleates of protection or efficacy studies.
- Scientific, Clinical and technical information are then analysed and candidate vaccines are the tested for safety and approved by regulatory agencies Such as FDA and EMA.
- Policy making bodies such as ACIP and equivalent bodies from other nations makes the recommendations on how the vaccine should be used.
- The approved vaccine should be commercialized and used in large scale. At this point, phase-IV clinical studies confirm safety.
Unlike conventional methods, unraveling antigens and pathogens have become easier by employing the technique of reverse vaccinology. The only requirement for this technique is the availability of whole genome sequence of the organism from which the anti-genic sites can be predicted computationally. Thus, Reverse vaccinology can lead to swift identification of potential vaccine candidates even for highly pathogenic microbes.
‘“Any Error in this manuscript is silent testimony of the fact that it was a Human Effort”
Dr. Wahied Khawar Balwan
Senior Assistant Professor & Head
Department of Zoology
Govt. Degree College Kilhotran, Doda
E-mail: [email protected]