Reverse vaccinology

Reverse vaccinology (RV) is the first application of genomic technologies in vaccine research, represented a major revolution in the process of discovering novel vaccines. By determining their entire antigenic repertoire, researchers could identify protective targets and design efficacious vaccines for pathogens where conventional approaches had failed. After genome sequencing in 1995, the chances of identifying the protein, or mixture of proteins, that could be used to develop an efficacious vaccine became higher. The RV offered two main advantages. First, it allowed identification of a much broader spectrum of candidates, including proteins that had not been identified before because they were masked by other, immunodominant targets. Second, it allowed the identification of potential vaccine targets in organisms that were difficult to cultivate in the laboratory.  The first RV protocol was developed to overcome the hurdles that had hampered the development of an efficacious vaccine against serogroup B N. meningitidis. N. meningitidis is coated by a polysaccharide capsule that, based on its chemical properties, is classified into five major serogroups: A, B, C, Y, and W135. Given its exposure on the surface of the cell and role in pathogenicity, the capsular polysaccharide constitutes the antigen of choice for the meningococcus, and is an excellent target for bactericidal antibodies elicited by conjugate vaccines against serogroups A, C, Y, and W135. However, in the case of serogroup B, the conjugate vaccine was not feasible because the capsular polysaccharide is an _(2, 8) polysialic acid, identical to the polysialic acid present in humanglycoproteins such as N-CAM. The capsule of MenB is thus a human self-antigen, and much effort has been directed toward the development of a protein-based vaccine specific for it. With the RV approach the entire genome of the virulent MC58 strain was sequenced, and from the genomic data, potential vaccine targets were selected. Bioinformatic analysis predicted that over 600 proteins were either exposed on the surface or secreted. Of these, 350 were cloned in Escherichia coli, successfully expressed in soluble form, purified, and used to immunize mice. The sera of immunized animals were then screened in a serum bactericidal assay that is known to correlate with protection. The process led to the identification of five previously unknown vaccine candidates that subsequently have completed clinical trials in a vaccine combination known as 4CMenB, and received a positive opinion from the EMA. After MenB the RV approach has been applied to a variety of other important pathogens, including Streptococcus pneumonia, Porphyromonas gingivalis, Chlamydia pneumoniae, Streptococcus agalactiae, E. coli, Leishmania major, and L. infantum.