D Pic are serine-protease autotransporters involved in virulence and immune recognition [49]. The Sap protein continues to be uncharacterized but has higher sequence similarity to the gene encoding antigen 43, an autotransporter involved in E. coli autoaggregation [50]. A multivalent vaccine, named rMESF, containing a chimeric protein Tazarotenic acid Formula derived from the immunodominant epitopes from SigA, Pic, and Sap bound to GroEL of S. Typhi as an adjuvant was applied to I.n. immunize mice [48]. This vaccine elicited robust, rMESF-specific serum IgG and IgA and fecal IgA titers, and splenocytes from immunized mice elicited important levels of TNF, IL-17, and IFN-. Lastly, immunization with rMESF supplied 100 protection in mice against lethal I.n. challenge of S. flexneri [48]. Outer membrane proteins (OMPs) of Shigella species have been evaluated in multiple studies as subunit vaccines. OmpA is conserved and cross-reactive with numerous strains of S. flexneri and I.n. immunization in mice elicited OmpA-specific serum IgG and IgA [51]. In addition, it protected mice from lethal I.n. challenge with S. flexneri 2a [52]. The vaccine candidate EpiMix was produced by combining 5 synthetic epitopes derived from OmpA and OmpF Benzomalvin A custom synthesis conjugated to ovalbumin [53]. Intramuscular immunization of mice with EpiMix induced distinct serum IgG and fecal IgA and protected mice from building shigellosis following intraperitoneal (I.p.) challenge with S. flexneri 2b. On top of that, splenocytes taken from immunized mice developed significant levels of IFN- when stimulated with EpiMix, compared with non-immunized controls. Synthetic epitopes on the OMP OmpC have been also evaluated for immunogenicity [54]. Antibody responses for the synthetic linear or cyclic peptides of the most important OmpC epitope, conjugated with all the tetanus toxoid (TT) as adjuvant, have been compared, and there was much better recognition of OmpC from antibodies against the cyclic-TT peptides [54]. Even so, in vivo research must be performed to figure out if these synthetic OmpC peptides may well be protective. Generally, conjugate vaccines consist of capsular polysaccharides chemically conjugated to a protein carrier. Nonetheless, conjugate vaccines for Shigella make use of the LPS O-antigen simply because during natural infections, it elicits serotype-specific, short-lived protective antibodies [55,56]. One particular conjugation method that has been utilized in numerous studies is in vivo conjugation of Shigella O-antigen to Pseudomonas aeruginosa exotoxin A (EPA) utilizing an E. coli glycosylation system [57,58]. This involves functionally expressing the N-linked glycosylation technique from Campylobacter jejuni in E. coli as well as the carrier protein EPA. When the expression from the Shigella O-antigen is also incorporated, it can be enzymaticallyPathogens 2021, 10,7 ofconjugated, producing the O-antigen-EPA complicated that may be extracted and purified from the E. coli cells [57,58]. Several Shigella vaccine studies using this platform happen to be completed, like a phase III trial applying S. sonnei O-antigen-EPA [592]. This vaccine was discovered to be secure and immunogenic in both adults and children, when protection was only significant in kids older than 3 years of age. Clinical trials working with precisely the same carrier protein and conjugation technology have also been performed for S. flexneri and S. dysenteriae. The Flexyn2a (O-antigen from S. flexneri 2a) was evaluated for safety and immunogenicity within a phase I study, exactly where subjects received two intramuscular (I.m.) injections of Flexyn2a.