Shigella sp. is a Gram-negative bacterial pathogen that causes shigellosis in humans by infecting epithelial cells of the colon. Shigella primarily infects intestinal epithelial cells, expresses several proteins which provide a mechanism for delivering effectors inducing the bacterial uptake into host cells via phagocytosis. To accomplish the injection of the effectors, Shigella uses a type III secretion (TTS) system to induce the entry into epithelial cells and trigger apoptosis in infected macrophages.
Bacteria of Shigella sp., including Shigella dysenteriae (S. dysenteriae), Shigella flexneri (S. flexneri), Shigella boydii (S. boydii), and Shigella sonnei (S. sonnei), are responsible for shigellosis in humans, a disease characterized by the destruction of the colonic epithelium that is responsible for 1 million deaths per year in developing countries. Shigella dysenteriae has 15 serotypes, Shigella flexneri has 14 serotypes and subtypes, Shigella boydii has 20 serotypes, and Shigella sonnei has one serotype, but the prevalence of these strains is not evenly distributed.
Although it is possible to control and treat shigellosis outbreaks with antibiotics, the high cost of antibiotics and the constant emergence of antibiotic-resistant Shigella species, even against to the newest antibiotics, underscore a need for effective vaccines to help control Shigella and related enteroinvasive E. coli diseases in the developing regions of the world.
Natural Shigella infections confer immunity and provide protection against subsequent infections with homologous virulent Shigella. Epidemiologic and volunteer studies have revealed that protective immunity against Shigella is directed against the LPS or O-specific antigens, and thus is associated with serotypes of Shigella. Many studies have been conducted for Shigella vaccines including the use of live attenuated Shigella, dead Shigella whole bacteria, and Shigella lipopolyssacharides (LPSs) or O-polysaccharides conjugated to carriers such as proteosomes, tetanus toxoids, and ribosomes. Despite several years of extensive research, however, any effective and inexpensive vaccines against such Shigella species are not yet available.
When the attenuated Shigella strains are used as live oral vaccines, it has been demonstrated to induce protective efficacy. The results of clinical trials of genetically well characterized invasive Shigella vaccines are promising. It has been demonstrated orally administered CVD1208, SC602, WRSS1, and WRSd1 vaccines are safe and immunogenic in volunteer trials, and particularly that SC602 protects against shigellosis. Clinical trials using CVD1208 demonstrated that the symptoms of mild fever and diarrhea, which are observed when using some of the live Shigella vaccines, may be reduced by elimination of sen and set genes from the vaccine strains. Studies on Shigella diarrhea in six Asian countries indicated that a relative distribution of Shigella species isolated from patients varies for different countries and sites. Moreover, the Shigella flexneri serotypes are highly heterogeneous in a distribution thereof from site to site, and even from year to year. The heterogeneous distribution of Shigella species and serotypes suggest that multivalent or cross-protective Shigella vaccines will be required to prevent shigellosis all over the world. Vaccines that aim to confer a wide spectrum of coverage may need to include all important Shigella serotypes. To solve such a dilemma, a vaccine strategy based on the use of ‘pentavalent formulations’ including the attenuated Shigella sonnei and Shigella dysenteriae 1 strains along with Shigella flexneri 2a, 3a and 6 strains has been advocated. On the other hand, the use of complex structures consisting of Shigella-derived serotype-specific and cross-reactive antigens such as whole dead or live attenuated bacteria has, for example, been considered to be a promising approach to vaccinate against infections caused by the most common species and serotypes of Shigella (WO 2010/046778 A2).
Meanwhile, thick O-polysaccharides exist on a cell wall due to an action of a wzy enzyme (O-antigen polymerase) in the case of the Shigella species, and thus various protein antigens existing on a cellular membrane and common or specific to the species are buried in a cell wall to block exposure to immune cells. Therefore, such protein antigens have many limitations in use for immunity against Shigella. 
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