Protective Efficacy and Immunogenicity of Vi‐Porin Conjugate against Salmonella typhi

Abstract

A conjugate vaccine against Salmonella typhi was prepared by covalently binding capsular polysaccharide (Vi) with porin, both isolated from S. typhi. First, Vi and porins were extracted. The Vi was purified from S. typhi Ty2. The purified Vi conformed to the requirements of the World Health Organization. Porins were purified from S. typhi 0901. The Vi was bound to the porins by a heterobifunctional cross‐linking reagent, N‐succinimidyl‐3‐(2‐pyridyl dithio)‐propionate (SPDP). After preparing the Vi‐porin conjugate, its protective ability and immunogenicity were studied in mice following systemic immunization. The results showed that the conjugate is 6.5‐fold more protective than Vi alone against S. typhi. The mice immunized with conjugate elicited higher anti‐Vi antibody (IgG) levels (P<0.01) than the mice immunized with Vi alone. Anti‐porin antibodies were also induced by the conjugate. To study the mucosal immune responses, secretory IgA (sIgA) in the intestinal fluid was measured. Conjugate‐immunized mice showed the induction of sIgA as compared to Vi alone. The results showed that when Vi is bound to porins, both isolated from same organism, the resultant conjugate induced both systemic and mucosal immune responses and provided better protection against S. typhi than Vi alone.

Protection in a mouse peritonitis model mediated by iron-regulated outer-membrane protein of Salmonella typhi coupled to its Vi antigen

“…In an earlier study, a conjugate of Vi bound to porin was shown to induce both systemic and mucosal immune responses, which were protective against challenge with S. typhi (Singh et al, 1999). A higher level of anti-Vi IgG on coupling of Vi to rEPA was noticed in 5-to 14-year-old children (Kossaczka et al, 1999).…”

“…In recent years, conjugates have been made by chemical coupling of Vi antigen with P. aeruginosa recombinant exoprotein A (rEPA) as the carrier (Kossaczka et al, 1999;Lin et al, 2001). Another approach has been to conjugate Vi to porins isolated from the same organism (Paniagua et al, 1992;Singh et al, 1999). In the present study we have conjugated the Vi antigen to IROMPs, as the latter have been shown to be protective in Gram-negative infections (Banerjee-Bhatnagar & Frasch, 1990;Chibber & Bajaj, 1995).…”

Salmonella porins induce a sustained, lifelong specific bactericidal antibody memory response

“…This notable immunogenicity of porins to sustain bactericidal antibody memory response could be explained by either of the following non‐exclusive mechanisms. First, because salmonella porins are good immunogens 20,21 , 27,33 , 36–42 and porins from other pathogens have been identified as TLR2 agonists 29–31 . Thus, during the initial immune response, it is possible that salmonella porins might be simultaneously recognized by B‐cell receptors and innate receptors, such as TLRs, thereby inducing the differentiation of naive B cells into effector antibody‐secreting plasma cells.…”

“…Because of the ability of porins to elicit a host immune response and thus protection of the host against infection, our group and others have studied porins from S. typhi as candidates for a vaccine against typhoid fever, 20,21 a life‐threatening systemic disease that remains an important health problem in some developing countries 22 . With up to 10 5 molecules per cell, porins are the most abundant proteins found in the outer membrane of Gram‐negative bacteria; porins assemble in trimers and form stable pores that allow the passive transport of nutrients 23 .…”

IFN-γ–Producing CD4+ T Cells Promote Generation of Protective Germinal Center–Derived IgM+ B Cell Memory against Salmonella Typhi

“…Salmonella and other Gram-negative bacteria express outermembrane proteins (Omps) that are highly immunogenic and elicit innate and adaptive immune responses in mice (9)(10)(11)(12)(13)(14). In humans, IgG and IgM Abs against pore-forming Omps (porins) can be found in individuals recovering from typhoid fever (15,16).…”