A story of Shigella vaccine development in ICMR-NICED involving multidimensional approaches

Abstract

Enteric bacterial infection causes diarrhea throughout the world, especially in developing countries. Shigella is solely answerable to
almost 1.1 million deaths annually in the pediatric population. Vaccine development against diarrheal diseases is always an encouraged
concern. Our laboratory, dedicated to find a possible therapy against shigellosis, is working on a path of various potential methodologies
and immunogens. Over the years, we have concentrated and reported different immunogens with their advantages and drawbacks,
ultimately leading us to find the best possible vaccine candidate against bloody diarrhea.
The venture started with live attenuated vaccines that protect against multiple serotypes and subtypes of pathogens and found limited
host-serotype specific immune responses. It was observed that introducing a lipopolysaccharide biosynthesis gene pPR 1347 in Shigella
dysenteriae type 1, transformed it into an avirulent organism for candidate vaccine.
A mutant strain of Shigella flexneri 2a lacking the RNA-binding protein Hfq was made, leading to increased expression of the type III
secretion system via loss of regulation, resulting in attenuation of cell viability through repression of stress response sigma factors. Such
increased antigen production and simultaneous attenuation were expected to elicit protective immunity against homologous and a
limited number of heterologous serotypes subtypes.
Although we formulated the live attenuated vaccine through the introduction of a lipopolysaccharide gene and a mutant lacking
RNA binding protein Hfq, but due to lack of heterologous protective efficacy, these were not an ideal vaccine candidate to be made
available in the market although they showed a significant amount of immunogenicity. Moreover, live attenuated strains always have
a possibility to revert back to its virulent form.
Subsequently, monovalent and hexavalent heat-killed immunogens with single and six Shigella serotypes have shown significant
protective efficacy in mice, and rabbit models. Recently we have shown the homologous as well some extent of heterologus protective
efficacy of heat killed multi-serotype Shigella (HKMS) immunogens in a guinea pig colitis model.
A novel formulation for improved immunogen delivery system comprises substantially effective amounts of alginate chitosan
nanoparticles with OmpA protein of Shigella species. Alginate chitosan nano formulations of OmpA consists essentially of OmpA
protein as conserved active molecule, but efficacy study reveals partial protection efficacy against present circulating Shigella. Further
improving the delivery system, we have also formulated a subunit-based vaccine by nanoformulation of ipaC protein of Shigella. The
main drawback of OmpA and ipaC subunit based vaccines are they cannot provide a broad spectrum protection against 50 subtypes
and serotypes of Shigella, although they act as a conserved protein in Enterobacteriaceae family, indicating single epitope cannot be
the sole factor associated with the operational protective efficacy.
Eventually, our research moved a step ahead and found next-generation outer membrane vesicles (OMVs) based antigens from Shigella.
Disruption of tolA, one of the genes of the Tol–Pal system of Shigella membrane, has increased the OMVs release rate by approximately
80% higher. Recently we have reported only four serotype-subtype cross-protection among 50 subtypes of circulating Shigella in mice
models. Outer membrane vesicles based immunogen could be a potential cost-effective non-living, next-generation candidate vaccine
against shigellosis for humans.