Serological Correlates of Protection against Shigellosis in Humans

Abstract

Shigella infection causes severe diarrheal disease that results in protective immunity. Antibodies, particularly IgG, are believed to play an important role in host defenses against shigellosis. The mechanisms by which antibodies interact with Shigella and prevent infection remain to be understood. The goal of this project was to characterize immune responses against Shigella in humans and define the anti-microbial activity of Shigella-specific antibodies as potential immune correlates of protection (CoP). We established Shigella-specific functional antibody assays and determined serum bactericidal (SBA) and opsonophagocytic killing antibody (OPKA) titers as well as antigen-specific antibody levels pre- and post-challenge in serum from a Phase IIb study. Pre-challenge antibody titers, including SBA, OPKA, IpaB- and VirG(IscA)-specific IgG and IgG1, correlated with reduced severity of shigellosis while IpaC-, IpaD- and LPS-specific IgG levels did not. SBA, OPKA, IpaB-, and VirG-specific IgG antibodies increased after challenge, particularly in volunteers who experienced moderate disease. This is the first demonstration that functional and VirG-specific antibody levels represent CoP in a controlled human challenge study. In a small animal experiment, mice vaccinated intranasally with VirG developed VirG-specific IgG and experienced 60% protection against lethal Shigella pulmonary challenge. Next, we examined the presence of Shigella-specific SBA activity in (1) American volunteers who received oral live-attenuated Shigella vaccine candidates CVD 1204 and 1208S and (2) pregnant women living in an endemic region. We confirmed that live-oral vaccination mimics natural infection by inducing high rates of SBA responses after a single vaccination and that similar activity is present in women naturally exposed to the organism. SBA assays also successfully determined the immunogenicity of a prime-boost vaccination regimen that involved initial dosing of non-human primates with CVD 1208S followed by a boost with CVD 1208S expressing Escherichia coli CFA1 and CS3 antigens. Mechanistic studies to determine the antigenic specificity of SBA indicated that LPS and outer-membrane proteins may be involved, as depletions significantly reduced SBA activity of hyperimmune serum. In conclusion, we demonstrate potential CoP that may determine Shigella-specific immunity. This work provides support for further mechanistic analysis and future studies to determine if functional antibodies can predict protective immunity and vaccine efficacy.