COVID-19 vaccine development: What lessons can we learn from TB?

Chemical and Biological Characterization of Mycobacterium Tuberculosis-Specific ESAT6-Like Proteins and their Potentials in the Prevention of Tuberculosis and Asthma

Early Secreted Antigenic Target 6 kDa (ESAT6) is a potent immunogenic protein secreted by the bacteria causing tuberculosis, i.e., Mycobacterium tuberculosis. Another highly immunogenic culture filtrate protein whose gene is linked to ESAT6/ESXA is known as CFP10/ESXB. Because of their high immunogenicity and specificity to M. tuberculosis, these proteins have been proposed as a vaccine to prevent tuberculosis and diagnose the active/latent disease. However, the same proteins cannot be used for prevention and diagnosis because immunized but healthy people will also show a positive response and be falsely reported as diseased. Therefore, in this review article, the search was made to identify if any other ESAT6-like proteins exist in the M. tuberculosis genome. The search identified 21 additional ESAT-like proteins, i.e., ESXC to ESXW. Immunological characterization has shown that some of them (especially ESXV) were able to induce immune responses in vitro with cells obtained from tuberculosis patients and healthy donors. When the protein ESXV was tested in different recombinant forms (expressed in Escherichia coli, mycobacterial vectors, and DNA plasmids) and injected in mice, immune responses were induced to multiple epitopes of the protein. Furthermore, immunization of mice with ESXV protected them from infection with M. tuberculosis. The same protein was also able to protect mice against the induction of asthma. These results suggest that ESXV has the potential to protect against two major diseases of the world, i.e., tuberculosis and asthma, and hence may be used as a common vaccine for both diseases.

The findings of glucosyltransferase enzymes derived from oral streptococci

Glucosyltransferase enzymes (Gtfs) distribute among some streptococcal species in oral cavity and are known as key enzymes contributing to the development of oral biofilm such as dental plaque. In 18 streptococcal species, 45 glucosyltransferase genes (gtf) are detected from genome database. Gtfs catalyze the synthesis of the glucans, which are polymers of glucose, from sucrose and they are main component of oral biofilm. Especially, the Gtfs from Streptococcus mutans are recognized as one of dental caries pathogens since they contribute to the formation of dental plaque and the establishment of S. mutans in the tooth surface. Therefore, Gtfs has been studied particularly by many researchers in the dentistry field to develop the anti- caries vaccine. However, it is not still accomplished. In these days, the phylogenetic and crystal structure analyses of Gtfs were performed and the study of Gtfs will enter new situation from the technique in the past old viewpoint. The findings from those analyses will affect the development of the anti-caries vaccine very much after this.　In this review, we summarize the findings of oral streptococcal Gtfs and consider the perspectives of the dental caries prevention which targeted Gtf.

Adjuvants and Antigen-Delivery Systems for Subunit Vaccines against Tuberculosis

The only licensed vaccine against tuberculosis is BCG. However, BCG has failed to provide consistent protection against tuberculosis, especially pulmonary disease in adults. Furthermore, the use of BCG is contraindicated in immunocompromised subjects. The research towards the development of new vaccines against TB includes the use of Mycobacterium tuberculosis antigens as subunit vaccines. Such vaccines may be used either alone or in the prime-boost model in BCG-vaccinated people. However, the antigens for subunit vaccines require adjuvants and/or delivery systems to induce appropriate and protective immune responses against tuberculosis and other diseases. Articles published in this Special Issue have studied the pathogenesis of BCG in children and the use of BCG and recombinant BCG as potential vaccines against asthma. Furthermore, the use of different adjuvants and delivery systems in inducing the protective immune responses after immunization with subunit vaccines has been described.

Immunological Characterization of Proteins Expressed by Genes Located in Mycobacterium tuberculosis-Specific Genomic Regions Encoding the ESAT6-like Proteins

The 6 kDa early secreted antigen target (ESAT6) is a low molecular weight and highly immunogenic protein of Mycobacterium tuberculosis with relevance in the diagnosis of tuberculosis and subunit vaccine development. The gene encoding the ESAT6 protein is located in the M. tuberculosis-specific genomic region known as the region of difference (RD)1. There are 11 M. tuberculosis-specific RDs absent in all of the vaccine strains of BCG, and three of them (RD1, RD7, and RD9) encode immunodominant proteins. Each of these RDs has genes for a pair of ESAT6-like proteins. The immunological characterizations of all the possible proteins encoded by genes in RD1, RD7 and RD9 have shown that, besides ESAT-6 like proteins, several other proteins are major antigens useful for the development of subunit vaccines to substitute or supplement BCG. Furthermore, some of these proteins may replace the purified protein derivative of M. tuberculosis in the specific diagnosis of tuberculosis by using interferon-gamma release assays and/or tuberculin-type skin tests. At least three subunit vaccine candidates containing ESAT6-like proteins as antigen components of multimeric proteins have shown efficacy in phase 1 and phase II clinical trials in humans.