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. 2013 Sep;1(3):113–123. doi: 10.1177/2051013613500428

An update on vaccines against Shigella

Shai Ashkenazi 1,✉, Dani Cohen 2
PMCID: PMC3967666  PMID: 24757519

Abstract

Despite intensive research efforts for more than 60 years, utilizing diverse vaccine strategies, a safe and efficacious vaccine against shigellosis is not available yet. We are currently witnessing innovative approaches based on elucidation of the virulence mechanisms of Shigella, understanding the immune response to the pathogen and progress in molecular technology for developing Shigella vaccines. It is hoped that these will lead to a licensed effective Shigella vaccine to protect humans against the significant worldwide morbidity and mortality caused by this microorganism.

Keywords: Shigellosis, diarrhea, gastroenteritis, prevention, vaccine

Background

Epidemiology

Shigella spp. cause acute gastrointestinal infections by invasion of the mucosa, toxin production and induction of local inflammation [Page et al. 1999; Sansonetti et al. 2001; Ingersoll et al. 2002], sometimes with extraintestinal manifestations [Ashkenazi et al. 1990; Baiulescu et al. 2002; Anatoliotaki et al. 2003]. These infections are of major public health relevance, especially in developing countries, where they cause significant pediatric morbidity and mortality [Bennish et al. 1990; Goren et al. 1992]. It is estimated that approximately 1.1 million deaths result from the 164.7 million annual cases worldwide, with about 70% of episodes and 60% of deaths involving children younger than 5 years [Kotloff et al. 1999]. In developed countries the infection is associated with considerable morbidity; the incidence of Shigella infections in the United States is 4–8 per 100,000, with 10,000–15,000 cases reported annually [Gupta et al. 2004; Centers for Disease Control and Prevention, 2011; Shiferaw et al. 2012]. The age group with the highest risk is children between 1 and 4 years old (particularly those in the second and third years of life), followed by those between 5 and 9 years [Ashkenazi, 2004]. Shigella spp. are also important etiologic agents of diarrhea in travelers and among soldiers deployed to endemic regions [DuPont 2009; Putman et al. 2006; Cohen et al. 2001]. In the United States, seasonality of shigellosis has changed from a peak in summer to a higher incidence in the late summer and autumn. In developing countries with tropical climates, Shigella infection is common throughout the year, but higher isolation rates are found during the summer and rainy seasons.

About 50 serotypes of Shigella have been identified, belonging to four serogroups (or species): group A (S. dysenteriae), group B (S. flexneri), group C (S. boydii), and group D (S. sonnei). The relative prevalence of the Shigella serotypes varies over time and by geography. Currently, S. sonnei consists of about 80% of Shigella isolates in developed countries, with an increasing relative prevalence in the last decades. By contrast, in developing countries, S. flexneri is the most common cause of bacillary dysentery [von Seidlein et al. 2006], while epidemics caused by S. dysenteriae serotype 1 that have occasionally occurred in the past have not been reported recently.

Clinical presentation

The typical incubation period is 12–48 h, but it may last for up to a week. In mild infections, especially in adults, the only complaints may be watery or loose stools for few days with minimal constitutional symptoms of fever. In contrast, children in particular have acute onset of high fever, malaise, abdominal cramps, and watery diarrhea that is followed by appearance of nausea, vomiting, and passage of frequent, mucous, bloody stools associated with severe abdominal pain and tenesmus. Other children have bloody mucous diarrhea from the onset of illness [Ashkenazi, 2004]. Physical findings include increased body temperature, usually 38.5°C or higher, general toxicity, mild dehydration (<30 ml/kg/day of diarrheal fluid is usually lost in the dysenteric phase), lower abdominal tenderness, and increased bowel sounds. Rectal examination elicits severe pain. In most cases, symptoms resolve without antibiotic therapy in about a week, but therapy shortens significantly the clinical illness. Neonates and children with underlying immune deficiency (including human immunodeficiency virus infection) or malnutrition are at increased risk of bacteremia and other complications of shigellosis [Struelens et al. 1985; Martin et al. 1983; Viner et al. 2001; Greenberg et al. 2003].

Immunity to shigellosis

Although mucosal secretory immunoglobulin (Ig) A antibodies and serum IgG antibodies develop against the virulence invasion plasmid antigens (ipa) ABCD after natural infection [Hayani et al. 1991; Levine et al. 2007], it has been shown that acquired natural immunity is serotype [lipopolysaccharide (LPS)] specific [Ferreccio et al. 1991; Cohen et al. 1991]. While there is no established correlate of immunity, case–control and prospective studies documented a strong association between pre-existing Shigella LPS IgG antibodies and protection against serotype-specific infection [Cohen et al. 1988, 1991]. The level of serum Shigella IgG antibodies induced by S. sonnei conjugate vaccine correlated with the extent of protection conferred by the vaccine among young adults and children [Cohen et al. 1997; Passwell et al. 2010]. Recent analysis of data generated by challenge studies showed an inverse correlation between the magnitude of prechallenge IgG antibodies to LPS and IpaB, as well as IgA IpaB B memory cells (BM) and postchallenge IgA LPS BM with disease severity, suggesting a role for antigen-specific BM in protection [Wahid et al. 2013]. It is unclear how occasional naïve children can be infected with Shigella and yet remain well [Guerrero et al. 1994]. Cellular immunity and cytokine production, which also develop during acute illness, may be an explanation. Tumor necrosis factors α and β, interleukin (IL)-1α, IL-1β, IL-1ra, IL-4, IL-8, IL-6, and transforming growth factor β are induced while interferon γ is suppressed [Raqib et al. 1997].

Treatment

Rapid correction of fluid and electrolyte deficits, early reinstitution of feeding (<12 h after initiation of treatment), and continued replacement of ongoing losses are key elements to therapy. Oral rehydration is the treatment of choice, although patients in shock or coma, as well as those with severe vomiting or ileus, should be treated with intravenous replacement of fluids and electrolytes.

Appropriate antibiotic therapy of shigellosis shortens the duration of fever, diarrhea and fecal excretion of the pathogen (thereby reducing infectivity) and apparently also reduces the risk of complications [Varsano et al. 1991; Eidlitz-Marcus et al. 1993; Ashkenazi et al. 1993; Martin et al., 2000; Basualdo and Arbo, 2003]. Empiric antibiotic treatment is generally recommended for people with colitis or dysentery until results of culture and clinical response are known [Ashkenazi, 2004; Christopher et al. 2010]. The major problem, however, is the worldwide increasing antibiotic resistance of Shigella spp. [Chuang et al. 2006; Centers for Disease Control and Prevention, 2013]. Antimotility agents should be avoided in shigellosis as well as in other infectious causes of colitis [Ashkenazi, 2004].

Need for a vaccine

Clean running water and appropriate sanitation are crucial for preventing infectious diarrhea, including shigellosis, in developing countries. For toddlers and older children, good hygiene practices are helpful but are difficult to implement [Khan, 1982]. It has been shown that prolonged breastfeeding is an important strategy to reduce shigellosis in young children [Clemens et al. 1986; Mata et al. 1969]. Specific protective antibodies present in the milk of women living in endemic areas decrease the severity of infection in infants [Hayani et al. 1991].

Because of the very low infectious dose, control of shigellosis only by hygiene measures is limited. As shown for other infectious disease, prevention by active vaccination is optimal, and can present an important measure in controlling the worldwide morbidity and mortality of shigellosis. Since Shigella spp. naturally infect mainly humans, an efficacious vaccine can potentially lead to a significant global reduction, or even to eradication, of shigellosis.

Approaches to development of Shigella vaccines

Diverse vaccine strategies (Table 1) have been utilized over several decades in an attempt to develop a safe and efficacious Shigella vaccine [Levine et al. 2007; Camacho et al. 2013; Barry et al. 2013]. Although a licensed vaccine is not available yet, these attempts have helped in better understanding the immune response to Shigella, and together with recent innovative strategies led to promising vaccines [Barry et al. 2013].

Table 1.

Shigella vaccine development strategies (past and present).

Live attenuated vaccines
Vaccine strain Attenuation/mutation Phase of development Reference
Attenuated mutants obtained by serial passages in vitro
SmD in Yugoslavia Serial passages in vitro Phase III Mel et al. [1971, 1974]
Istrati T32 in Romania Serial passages in vitro Phase III and IV Meitert et al. [1984]
Hybrid live vector Shigella vaccine
Escherichia coli K12–Shigella flexneri 2a Escherichia coli K12 vector of Shigella invasiveness plasmid; aroD Phase IIb Cohen et al. [1994];
Attenuated mutants obtained by recombinant DNA technology
S. flexneri 2a CVD 1208S guaBA, sen, set Phase II Kotloff et al. [2007]
Shigella sonnei WRSs1 virG Phase II Orr et al. [2005]
S. sonnei WRSs2, WRSs3 virG, senA, senB, msbB2 Preclinical Kotloff et al. [2002]; Barnoy et al. [2011]; Bedford et al. [2011]
S. flexneri 2a SC602 virG, iuc Phase IIb Sansonetti and Arondel [1991]; Coster et al. [1999]; Rahman et al. [2011]
S. flexneri 2a WRSf2G11, 12, 15 virG, senA, senB, msbB2 Preclinical Ranallo et al. [2012]
Shigella dysenteriae 1 WRSd1 virG, stxAB Phase I McKenzie et al. [2006]
Inactivated vaccines Vaccine candidate Phase of development Reference
Whole cell killed vaccines S. flexneri 2a, 3a, S. sonnei Preclinical McKenzie et al. [2006]
Subunit vaccines
Chemical glycoconjugates S. flexneri 2a, S. sonnei, PS- rec. exoprotein A Preclinical and phases I, II and III Cohen et al. [1997];
Ashkenazi et al. [1999]; Passwell et al. [2010]
Synthetic glycoconjugates S. flexneri 2a synt O ag-tetanus toxoid Preclinical Pozsgay et al. [2007]; Phalipon et al. [2009]
Bioglycoconjugates S. dysenteriae 1 LPS exoprotein A Phase I Fernandez et al. [2009]
Proteosomes LPS S. flexneri 2a, S. sonnei Phase II Fries et al. [2001]
Ribosomes LPS S. sonnei, S. flexneri Preclinical Levenson et al. [1995]; Shim et al. [2007]
Purified Ipa proteins Shigella spp. Preclinical Martinez-Becerra et al. [2013]
GMMA protein vesicles S. flexneri 2a, S. sonnei Preclinical Scorza et al. [2012]
Combined LPS and common proteins S. flexneri 2a Phase I Oaks and Turbyfill [2006]; Riddle et al. [2011]
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Ipa, invasion plasmid antigen; LPS, lipopolysaccharide; SmD, streptomycin dependent; GMMA, generalized modules of membrane antigens.

Observational and challenge studies have shown that natural Shigella infection confers protection for a limited duration against the homologous serotype [Ferreccio et al. 1991; Cohen et al. 1991]. Therefore, the main attempts to develop Shigella vaccines were targeted towards inducing a good immune response against Shigella O polysaccharide, which determines the Shigella serogroup and serotype. The diversity of the worldwide Shigella serotype isolates and their variable relative importance in developing versus developed countries led to the conclusion that a multivalent Shigella vaccine will have to be developed to address the needs of the different potential target populations for an efficacious Shigella vaccine. These include young children in developing countries, children living in developed countries but under conditions of particular crowding, travelers from industrialized countries to highly endemic countries, military personnel, men who have sex with men, and so on. It is expected that a vaccine which will include S. dysenteriae type 1, S. sonnei, S. flexneri 2a, S. flexneri 3, and S. flexneri 6 will cover more than 75% of the global Shigella-associated episodes of diarrhea [Levine et al. 2007]. This is based on the assumption (from the analysis of Shigella O antigens and cross protection studies) that inclusion of S. flexneri 2a, 3a, and 6 in the vaccine will provide cross protection against the other 11 S. flexneri serotypes because of shared group antigens [Noriega et al. 1999; Levine et al. 2007].

There are two basic approaches to attain such a wide range in protection against shigellosis: either by combining efficacious serotype-targeted vaccines in a multivalent vaccine or by using a cross-reactive antigen which will confer extended cross protection against Shigella strains. To reach this ultimate goal, it is first necessary that monovalent Shigella candidate vaccines will demonstrate protection in clinical trials. Regrettably, only two prototype Shigella vaccines, one which uses attenuated strains as live oral vaccines [Mel et al. 1974; Meitert et al. 1984] and another which uses parenteral conjugates of Shigella O polysaccharide covalently linked to a carrier protein [Cohen et al. 1997; Passwell et al. 2010], have conferred significant protection in controlled field trials.

The Shigella vaccine development strategies of the last 50 years and the current ones include the two main distinct categories of live-attenuated vaccine strains and inactivated Shigella vaccine candidates (subunit and whole cell).

Live-attenuated Shigella strains

Attenuated mutants obtained by serial passages in vitro

In the early 1960s, in Yugoslavia, David Mel serially passed different Shigella serotypes on streptomycin-containing media until they became streptomycin resistant and streptomycin dependent (SmD), losing in parallel the capability of mucosal invasiveness [Sereny, 1957]. These vaccine strains were well tolerated; in controlled field trials, Mel and colleagues demonstrated the efficacy of the SmD vaccines, showed that multiple strains could be mixed together in combination vaccines and reported that protection was serotype specific [Mel et al. 1971, 1974]. Protection persisted for a year following primary immunization of children, but administration of a single booster extended the protection for an additional year [Mel et al. 1971, 1974].

A similar live-attenuated Shigella vaccine (S. flexneri 2a strain T32) was developed in Romania by repeated subculturing. Large doses of 5 × 1010–2 × 1011 colony-forming units (CFUs) were shown to be well tolerated and significantly protective in large field studies in Romania and China [Meitert et al. 1984]. The field trials also suggested that T32 conferred significant (albeit lower level) protection against shigellosis due to S. sonnei, S. flexneri 1b and S. boydii 1–6. Later, it was shown that T32 harbored a large deletion in the invasiveness plasmid, resulting in the loss of three loci, ipaADCB, invA, and virG, which diminished the ability of this strain to invade epithelial cells [Venkatesen et al. 1991].

These studies provided proof of concept for future modern multivalent vaccines that aim to confer broad protection. Unfortunately, they had drawbacks that prevented licensure and further large-scale use, such as the multiple doses needed for primary vaccination, need for annual boosters, lack of clear knowledge on the exact segment of the bacterial genome which had been modified by the serial passages in vitro and the occasional back mutations, reverting the streptomycin-dependent strain to streptomycin-independent strains in the case of the SmD vaccine strain [Levine, 1975; Levine et al. 2007].

Attenuated mutants obtained by recombinant DNA technology

Advances in recombinant DNA technology and more recently whole genome sequencing of shigellae enabled the development of live-attenuated oral Shigella candidates with defined deletion mutations, knocking out virulence genes on the invasiveness plasmid that encode for intracellular spread and multiplication (icsA or virG), altering key metabolic pathways, such as aro and guaAB that introduce severe auxotrophy, impairing synthesis of nucleic acids, impairing the capacity to compete for ferric iron, via the production of siderophores (i.e. aerobactin). The knowledge on the exact changes in the bacterial genome associated with different levels of attenuation together with the various advantages of manufacturing and delivery of oral live-attenuated vaccines strongly supported investment of research efforts and funding in the development of this vaccination strategy. This has been the leading approach in Shigella vaccine development at the Walter Reed Army Institute of Research (WRAIR), University of Maryland Center for Vaccine Development (CVD), Institut Pasteur and Karolinska Institute.

Two major obstacles emerged and significantly slowed down the process of development of these promising candidates. The first was the narrow window between immunogenicity and safety of these candidates. The development process of the hybrid Escherichia coli K12–S. flexneri 2a (EcSf2a1 and 2), S. flexneri 2a SC602 vaccine strain, S. dysenteriae 1 (SC599 and WRSd1), the series of S. sonnei WRSs1, WRSs2, and WRSs3, and the series of CVD 1204, CVD 1207, CVD 1208, and CVD 1208S exemplified this delicate balance along the way.

The invasiveness plasmid of S. flexneri 5 and the genes that allow expression of S. flexneri 2a type and group-specific O antigens were introduced into E. coli K12, producing a hybrid strain EcSf2a-1 which was further attenuated by an aroD mutation, becoming the vaccine candidate EcSf2a-2. At high dosage levels, this strain, with the ability to penetrate epithelial cells, was immunogenic but also caused some adverse reactions [Cohen et al. 1994]. The vaccine conferred only 36% protection against illness (fever, diarrhea, or dysentery) upon experimental challenge.

Deletions in the aerobactin-encoding system (iuc iut) and icsA in S. flexneri 2a led to the development of a vaccine candidate SC602 [Sansonetti and Arondel, 1991] that has undergone phase I and II clinical trials, with encouraging results in Western volunteers [Coster et al. 1999]. SC602 was reactogenic at 2 × 106CFUs, but relatively safe at 104 with seroconversion (IgG or IgA) and significant antibody-secreting cell (ASC) response in 60–70% of the volunteers, and protection when vaccinees were challenged with a wild type pathogenic S. flexneri strain of a similar serotype. Subsequent studies demonstrated the absence of accidental transmission of the live vaccine strain. A delta icsA S. sonnei and a delta icsA and delta sxtA and sxtB (encoding Shiga toxin) S. dysenteriae type 1vaccine candidate constructed by scientists at WRAIR showed similar results with regard to tolerance and immunogenicity balance [Orr et al. 2006].

A second generation of more attenuated S. sonnei mutants, WRSs2 and WRSs3, were constructed at WRAIR [Barnoy et al. 2011; Bedford et al. 2011]. Besides the loss of VirG (IcsA), WRSs2, and WRSs3, the mutants also lacked plasmid-encoded enterotoxins ShET2-1 and ShET2-2. WRSs3 further lacks MsbB2 that reduces the endotoxicity of the lipid A portion of the bacterial LPS. Studies in cell cultures and in gnotobiotic piglets demonstrate that WRSs2 and WRSs3 have the potential to cause less diarrhea due to loss of ShET2-1 and ShET2-2, as well as alleviate febrile symptoms by loss of MsbB2 [Barnoy et al. 2011; Bedford et al. 2011]. A combination of these gene deletions with the addition of the knockout of the chromosomal set locus encoding for the ShET1 were applied for S. flexneri 2a candidate vaccine strains WRSf2G11, WRSf2G12, and WRSf2G15 [Ranallo et al. 2012].

A series of strains were constructed at the CVD. CVD 1203 strain, incorporating aroA and virG deletions, was too reactogenic at doses of 108 and 109 CFUs when it showed good immunogenicity and strain 1207, which harbored deletion mutations in guaBA, virG, set, and sen, was well tolerated at 108 and 109 CFUs but poorly immunogenic [Kotloff et al. 2000]. Recently, S. flexneri 2a strain CVD 1208 with a guaAB mutation that has been combined with a sen and a set mutation leaving the virG gene unknocked showed excellent tolerance, thereby allowing administration of vaccine doses up to 109 CFUs without side effects. CVD 1208 induced a geometric mean IgA ASC of 62 per 100 peripheral blood mononuclear cells, with 71% of subjects exhibiting fourfold rises in serum IgA or IgG, and 86% exhibiting fourfold rises in fecal IgA at 109 CFUs [Kotloff et al. 2004]. Similar good safety and immunogenicity results were generated when the same vaccine strain (CVD 1208) was reconstructed using animal-free media to conform to regulatory guidelines, and designated CVD1208S [Kotloff et al. 2007].

The studies on the new generations of live-attenuated vaccine strains delivered orally to volunteers from industrialized countries are encouraging in their achievement of a broader and safer interval between immunogenicity and safety following immunization with oral live-attenuated Shigella vaccine strains.

A second and emerging concern related to orally delivered live-attenuated Shigella vaccine strains is associated with their performance in terms of immunogenicity and efficacy in children in developing countries. A series of clinical trials with the live-attenuated S. flexneri 2a SC602 strain, which was well tolerated and performed excellently in North American volunteers at a dose of 104 CFUs in terms of replication, immunogenicity, and protection, demonstrated very poor excretion and very low immunogenicity in Bangladeshi adults and children [Rahman et al. 2011]. We must be cautious with generalization of these results to other live-attenuated Shigella strains administered orally, especially those with different attenuating gene deletions (e.g. S. flexneri 2a strain CVD 1208). Nevertheless, we have to keep in mind that S. flexneri 2a SC602 was the strain which showed very good local and systemic immunogenicity and some residual reactogenicity in North American volunteers while the other current Shigella live candidate vaccine strains are more attenuated albeit sometimes with other attenuating gene deletions. The immunogenicity data related to S. flexneri 2a SC602 vaccine strain in Bangladesh corroborate findings on weaker immunogenicity and efficacy in children in developing countries compared with developed countries documented with other oral enteric vaccines, including the two recently licensed live-attenuated rotavirus vaccines [Levine, 2010]. Different hypotheses have been raised to explain the ‘intestinal barrier’ of volunteers in developing countries against this S. flexneri 2a SC602 live vaccine strain [Rahman et al. 2011] and other oral enteric vaccines [Levine, 2010], and solutions proposed to overcome this obstacle. There is no doubt that a significant research effort will be invested to resolve this complex issue.

Inactivated Shigella vaccine candidates

Whole-cell vaccines

In the early attempts to develop a vaccine, inactivated, whole-cell preparations of Shigella were developed and administered parenterally [Levine et al. 2007]. Although serum antibodies were elicited, unfortunately no significant protection was observed either in challenge studies among volunteers or in field trials. McKenzie et al. [2006] examined an orally delivered, formalin-inactivated, whole-cell Shigella vaccine candidate; there were no significant adverse events, but the immune response to several Shigella antigens was only moderate. Because of these limitations, the whole-cell inactivated approach to develop an efficacious Shigella vaccine was not continued.

Lipopolysaccharide-based vaccines

LPS-based vaccines were developed in concert with case–control and prospective studies demonstrating an association between serum LPS IgG antibodies and serotype-specific protection against shigellosis [Cohen et al. 1991; Passwell et al. 1995]. Since the lipid A moiety of the LPS is associated with significant adverse events, the O-specific polysaccharides (O-PS) were used as vaccine candidates; however, as the immune response to sugars is limited, particularly in infants and young children, methods to increase the immunogenicity were needed.

Shigella conjugate vaccines

Investigators at the National Institutes of Health developed parenterally administered conjugate Shigella vaccines by covalently binding the serotype-specific polysaccharides of Shigella to a carrier protein, thus obtaining T-cell-dependent antigens, immune memory, and a better immune response. Conjugates against S. sonnei, S. flexneri 2a, and S. dysenteriae 1 were examined first in mice, and then in adults, children aged 4–7 years, and infants and children aged 1–4 years [Cohen et al. 1997; Ashkenazi et al. 1999; Passwell et al. 2003]. They were found as very safe, with minimal systemic adverse events (mainly fever in 2–5%, which was usually of low level and self limited) and local adverse events (mainly mild local pain and swelling) [Ashkenazi et al. 1999; Passwell et al. 2003]. They were highly immunogenic in terms of inducing homologous serum LPS antibodies, although the immune response in younger children was lower than that obtained in older children and in adults [Ashkenazi et al. 1999; Passwell et al. 2003].

The efficacy of these conjugates was examined in two field studies. A randomized, double-blind, double-dummy study in Israeli soldiers demonstrated that a single dose of S. sonnei conjugate conferred 74% protection against S. sonnei gastroenteritis [Cohen et al. 1997]. A randomized, double-blind efficacy study was done in 1–4-year-old children at 15 sites throughout Israel [Passwell et al. 2010]. The 2799 children enrolled received two doses of either S. sonnei or S. flexneri 2a conjugate vaccines, each serving as a control for the other, and followed for 2 years. The study demonstrated that the S. sonnei conjugate had a protective efficacy of 71% in 3–4-year-old children, but not in younger ones [Passwell et al. 2010]. The reduced efficacy of the Shigella conjugate vaccines in young children is probably related to the lower antibody levels against S. sonnei LPS that have been elicited in this age group [Passwell et al. 2003]. It is well known from other conjugate vaccines, such as Haemophilus Influenzae type b and Streptococcus pneumoniae vaccines, that a higher number of doses is needed to protect infants and young children. The number of S. flexneri 2a cases cultured during the study was too low to enable conclusions regarding its efficacy [Passwell et al. 2010].

Attempts are currently being made to prepare a more immunogenic conjugate Shigella vaccine. One approach is based on synthetic oligosaccharides and another one involves core fragments of S. sonnei O-PS, both further bound to carrier proteins. Such conjugates have been developed by investigators at the National Institutes of Health in the USA [Pozsgay et al. 2007] and at the Pasteur Institute in France [Phalipon et al. 2009]. The use of the synthetic technology allows flexibility and control in the production of vaccine antigens which mimic the protective antigenic configuration carried by Shigella O antigen, and showed promising preliminary results in animal studies [Pozsgay et al. 2007; Phalipon et al. 2009]. A synthetic pentadecasaccharide, representing three biological repeating units of the O-PS of S. flexneri 2a recognized by sera of naturally infected subjects, elicited a protective serum anti-LPS 2a response in mice [Phalipon et al. 2009]. Further studies with these new conjugates, especially testing in human field trials, are needed to prove their efficacy.

Other subunit Shigella vaccines

S. flexneri 2a and S. sonnei LPS were hydrophobically complexed with proteosomes, meningococcal outer membrane proteins, for another approach to increase the immunogenicity of LPS-based vaccines [Fries et al. 2001; Kweon 2008]. These vaccines were first administered orally, and then intranasally, with a good immune response. Relatively purified Shigella ribosomal subunit vaccines, composed of Shigella O-PS and ribosomes, were initially developed by Levenson and his group in the former USSR and later on in the USA (WRAIR) [Levenson et al. 1995]. New preparations of a Shigella ribosomal vaccine candidate were prepared in South Korea. They were safe, immunogenic, and demonstrated protection against Shigella pneumonia in a murine model following intranasal administration [Shim et al. 2007].

With the idea of developing a multivalent subunit Shigella vaccine, candidate vaccines based on the type three secretion system, which is utilized by all shigellae, were developed. IpaB- and IpaD-based Shigella vaccines, with adjuvants, given to mice intranasally [Martinez-Becerra et al. 2012], parenterally [Martinez-Becerra et al. 2013], or orally [Heine et al. 2013], were promising, shown as immunogenic and protective against lethal pulmonary infection with Shigella. Oaks and and Turbyfill prepared a macromolecular complex composed of S. flexneri 2a LPS, ipaB, ipaC, and ipaD [Oaks and Turbyfill 2006]. When given intranasally, this experimental vaccine induced serum and mucosal antibodies and also antibody-secreting cells [Riddle et al. 2011]. In a non-LPS-based approach, outer membrane vesicles released from shigellae and encapsulated into nanoparticles were immunogenic and protected mice against experimental Shigella infection [Camacho et al. 2013]. Genetically derived outer membrane particles composed of predicted Shigella outer membrane and periplasmic proteins without LPS, obtained by the novel protein vesicle technology, represent another promising subunit vaccine candidate with expected wide coverage [Scorza et al. 2012].

Conclusion

We currently witness diverse innovative approaches based on progress in molecular technology, elucidation of the virulence mechanisms of Shigella, and lessons learnt from previous vaccine studies for developing Shigella vaccines. It is hoped that these will lead to a licensed safe and efficacious Shigella vaccine to protect humans against this pathogen and the related morbidity and mortality.

Footnotes

Funding: The study was supported in part by the European Commission (grant agreement number 261472 STOPENTERICS).

Conflict of interest statement: The authors declare no conflicts of interest in preparing this article.

Contributor Information

Shai Ashkenazi, Department of Pediatrics A, Schneider Children’s Medical Center, 14 Kaplan Street, Petach Tikva 49202, Israel.

Dani Cohen, School of Public Health, Sackler Faculty of Medicine, Tel Aviv University, Israel.

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