ABSTRACT
Shigella spp. are a major global cause of diarrhea and dysentery. Children 2-5 years of age in low- and middle-
income countries are the most affected. Mortality is second only to rotavirus among diarrheal diseases in young
children, and repeated bouts of disease cause lifelong disability. In industrialized nations, Shigella outbreaks
have occurred in day care centers and medical institutions. Shigella spp. rapidly acquire genetic elements that
confer antimicrobial resistance. A safe, effective, and affordable vaccine could make a major public health
impact, yet none is currently approved. Candidates based on the Shigella-O-antigen are in clinical development.
However, that approach is impractical and costly, requiring multiple vaccines to prevent disease caused by
Shigella strains with different O antigens. This proposal seeks to develop a safe, practical, and effective broad-
spectrum Shigella vaccine based on highly conserved Shigella Type III secretion system (TTSS) proteins,
invasion plasmid antigen (Ipa) B, IpaH, and the virulence antigen VirG (IcsA). Our laboratory was the first to
report the high immunogenic and broad protective capacity of Shigella IpaB in mice, and the association of IpaB-
and VirG- serum IgG (IgG1) levels with clinical protection against shigellosis in humans. Preliminary data in this
application demonstrate that VirG also elicits potent and protective immunity. In addition, we have found that
adults living in endemic regions who acquire natural immunity to Shigella have high levels of IpaB-, VirG-, and
IpaH- serum antibodies. Maternal IgG against these proteins are more efficiently transferred to infants through
the placenta as compared to IgG against the O-polysaccharide. High levels of antibodies are also present in
breast milk. This robust maternally-derived immunity is consistent with the low incidence of infection at <6 months
of life. Together, our findings provide a strong premise for the success of a broad-spectrum subunit (IpaB, IpaH,
and/or VirG) Shigella vaccine. Given parenterally, this vaccine is expected to be well-tolerated and to elicit robust
immunity in young children, the main target group.
In Aim 1, we will purify and characterize Shigella IpaB, IpaH, and VirG using an innovative cell-free protein
synthesis system. This technological breakthrough allows the production of high-quality vaccine antigens at large
yield and can be easily scaled-up for manufacturing. In Aim 2, we will evaluate the capacity of IpaB, IpaH, and
VirG to elicit protective immunity and to confer broad-spectrum protection in mice and guinea pigs. The proteins
will be administered parenterally, alone or combined, with alum as adjuvant ¿ a strategy that can be readily
translated to humans. In Aim 3, we will investigate the protective role of IpaB, IpaH, and VirG antibodies through
passive transfer experiments in mouse and guinea pig infection models and specific steps of infection targeted
by each antibody using in vitro cell culture. We have unique expertise, tools, and novel technology to produce a
simple and efficacious new subunit vaccine to prevent multidrug-resistant Shigella. If successful, this concept
could be easily tested in Phase I studies in humans.