Entry route of Salmonella typhimurium directs the type of induced immune response.

Secretory immunoglobulin A (SIgA) represents a first line of defense against mucosal pathogens by limiting their entrance. By using different strains of Salmonella typhimurium that target the two mechanisms of bacterial entry (microfold cell [M cell]- or dendritic cell-mediated), we demonstrated here that the distribution of bacteria after oral infection directed the type of induced immune response. M cell-penetrating invasive, but not noninvasive, S. typhimurium was found in large numbers in Peyer's patches (PPs), leading to the activation of immune cells and the release of fecal IgA. In contrast, both strains of bacteria were equally capable of reaching the mesenteric lymph node and the spleen and inducing IgG responses. These data suggest that PPs are absolutely required for the initiation of an IgA response to Salmonella, whereas they are dispensable for a systemic response. This compartmentalization could allow the fast generation of both mucosal and systemic acquired immunity to pathogens.