Porins, purified from Salmonella @phi strain 0901 provided 90% protection to BALB/c mice against a lethal dose (300 X LD50) of S. typhi Ty2 when given intraperitoneally. To measure the porin-induced cellular immune responses, macrophages and lymphocytes were isolated from spleen and lamina propria (LP) of porin immunised-challenge mice and of infected and control mice; T-cell phenotypes, lymphocyte proliferation and cytokine production were studied. The secretory IgA (sIgA) antibody level in the intestinal fluid was also measured to study mucosal immune response. After immunisation, the splenic lymphocytes exhibited a significant increase in total T-cell count and CD4+/CD8+ ratio, while the LP lymphocytes (LPL) exhibited an increase in CD4+/CD8+ ratio only. They also exhibited a significant increase in porin-specific proliferative response and cytokine levels (IL-1, IL-2, IFN-y and IL-4). After immunisation, slgA antibody was also found to be increased. These results suggest that porins given intraperitoneally induce cellular and humoral immune responses both at systemic and mucosal levels.

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.

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