Elevated serum IgG to the Vi antigen has been shown to confer protection against typhoid fever in field trials with the Vi polysaccharide and with the Vi-rEPA conjugate vaccine [17] and [18]. The role of Vi-specific serum IgG to reduce bacterial load following challenge with a Vi-positive S. Typhimurium strain in a mouse model has also been demonstrated [4] and [19]. Interestingly, it has been recently published that unconjugated Vi polysaccharide is not only

a poor immunogen, but it suppresses the immune response to a subsequent boost with the conjugate vaccine [20]. On the contrary, no suppression has been observed MLN8237 when priming with conjugate and boosting with either conjugate or unconjugated Vi [20]. The local Vi-specific GDC0449 antibody response was analyzed in the intestinal tract. Significant levels of Vi-specific IgG were detected in intestinal washes from Vi-CRM197 immunized mice with a mean concentration of 9.3 μg/mg of total

IgG 10 days following the second immunization (P < 0.05 versus all groups), while no intestinal response was detected in mice immunized with Vi or CRM197 ( Fig. 2A). Significant correlation was observed between Vi-specific IgG detected in serum and intestinal washes in each mouse immunized with Vi-CRM197 (r = 0.87, P = 0.0002), suggesting that intestinal IgG may be derived from passive diffusion from blood. Notably, Vi-specific IgA was not observed in intestinal washes from any group ( Fig. 2B). These observations indicate the potential of parenteral vaccination to induce immunity in the intestinal tract, a feature generally associated with mucosal immunization routes.

Cellular proliferation was observed in splenocytes of Vi-CRM197 immunized mice, with a S.I. of 10 (P < 0.05 versus PBS group) while lower proliferation was observed in mice immunized with Vi antigen alone ( Fig. 3A). The CRM197 carrier component was necessary for the in vitro lymphoproliferative response, as confirmed by the absence of cell Adenosine proliferation after restimulation with unconjugated Vi (data not shown). Therefore, Vi-CRM197 is efficient in stimulating a T-cell response that in turn augments Vi-specific B cells to produce antibodies. Restimulated lymphocytes from mesenteric lymph nodes of mice immunized with Vi-CRM197 also showed a robust proliferation (S.I. 23, P < 0.001 versus PBS group, P < 0.01 versus Vi-immunized mice; Fig. 3B) and IFN-γ production (25 SFU/106 lymphocytes versus 0.2 SFU/106 lymphocytes in PBS control group, data not shown). These data suggest that antigen-specific T cells stimulated by parenteral immunization recirculate and migrate into lymph nodes draining the intestine. Recently, we have also demonstrated dissemination of antigen-specific activated T cells to distal non-draining lymph nodes, including mesenteric lymph nodes, following nasal immunization [21].