Antibody responses induced at mucosal and nonmucosal sites demonstrate a significant level of autonomy. Here, we demonstrate a key role for mucosal interferon regulatory factor-4 (IRF4)-dependent CD103 þ CD11b þ (DP), classical dendritic cells (cDCs) in the induction of T-dependent immunoglobulin G (IgG) and immunoglobulin A (IgA) responses in the mesenteric lymph node (MLN) following systemic immunization with soluble flagellin (sFliC). In contrast, IRF8-dependent CD103 þ CD11b À (SP) are not required for these responses. The lack of this response correlated with a complete absence of sFliC-specific plasma cells in the MLN, small intestinal lamina propria, and surprisingly also the bone marrow (BM). Many sFliC-specific plasma cells accumulating in the BM of immunized wild-type mice expressed a 4 b 7 þ , suggesting a mucosal origin. Collectively, these results suggest that mucosal DP cDC contribute to the generation of the sFliC-specific plasma cell pool in the BM and thus serve as a bridge linking the mucosal and systemic immune system.

Systemic immunization with soluble flagellin (sFliC) from Salmonella Typhimurium induces mucosal responses, offering potential as an adjuvant platform for vaccines. Moreover, this engagement of mucosal immunity is necessary for optimal systemic immunity, demonstrating an interaction between these two semi-autonomous immune systems. Although TLR5 and CD103+CD11b+ cDC2 contribute to this process, the relationship between these is unclear in the early activation of CD4+ T cells and the development of antigen-specific B cell responses. In this work, we use TLR5-deficient mice and CD11c-cre.Irf4fl/fl mice (which have reduced numbers of cDC2, particularly intestinal CD103+CD11b+ cDCs), to address these points by studying the responses concurrently in the spleen and the mesenteric lymph nodes (MLN). We show that CD103+CD11b+ cDC2 respond rapidly and accumulate in the MLN after immunization with sFliC in a TLR5-dependent manner. Furthermore, we identify that whilst CD103+CD11b+ cDC2 are essential for the induction of primary T and B cell responses in the mucosa, they do not play such a central role for the induction of these responses in the spleen. Additionally, we show the involvement of CD103+CD11b+ cDC2 in the induction of Th2-associated responses. CD11c-cre.Irf4fl/fl mice showed a reduced primary FliC-specific Th2-associated IgG1 responses, but enhanced Th1-associated IgG2c responses. These data expand our current understanding of the mucosal immune responses promoted by sFliC and highlights the potential of this adjuvant for vaccine usage by taking advantage of the functionality of mucosal CD103+CD11b+ cDC2.

A novel Group B Streptococcus (GBS) vaccine, based upon the GBS alpha‐like surface proteins, is being developed by MinervaX for administration to pregnant women. The vaccine is intended to generate antibodies (IgG) capable of crossing the placenta, in order to passively immunize the baby and provide protection in utero and up to 3 months after birth. An initial vaccine candidate, GBS‐NN (based on the N‐terminal domains of Rib and AlphaC surface proteins) was replaced, due to insufficient cross‐reactivity with the two other N‐terminal proteins (Alp1 and Alp2/3), by a modified vaccine candidate designated GBS‐NN/NN2 that included all four AlpNs. Preclinical studies raised no safety concerns and the subsequent Phase I clinical trial demonstrated that the vaccine was well tolerated and strongly immunogenic. As the vaccine is intended for use during pregnancy for maternal immunization, an embryofetal study in rats and a fertility and embryofetal study in rabbits were performed, in both cases using GBS‐NN/NN2. Vaccination of female rats or rabbits did not adversely affect embryofetal development or survival in either species, or mating or fertility in rabbits. In both studies, the pregnant animals developed immune responses to GBS‐NN and GBS‐NN2 proteins and concentrations of antibodies to both fusion proteins were detected in the fetuses and in the amniotic fluid. Data generated during these reproductive studies indicated a suitable safety margin (approximately 40‐fold clinical dose) considered appropriate to support a subsequent human trial of GBS‐NN/NN2 administered in the second and third trimesters of pregnancy.