AbstractLymphocyte trafficking in the gastrointestinal tract is primarily mediated by interactions with the mucosal addressin cell adhesion molecule 1 and its lymphocyte ligand, α4β7, and partly by L-selectin (L-Sel) interactions with peripheral node addressin coexpressed on some mucosal addressin cell adhesion molecule 1. We inquired whether intestinal responses in mice lacking L-Sel would be enhanced. L-Sel-deficient (L-Sel−/−) mice were orally immunized with either Salmonella vaccine vector or Salmonella vector-expressing colonization factor Ag I (CFA/I) from enterotoxigenic Escherichia coli. In L-Sel−/− mice, mucosal IgA anti-CFA/I fimbrial responses were greatly reduced, and systemic IgG2a anti-CFA/I fimbrial responses were 26-fold greater compared with C57BL/6 (L-Sel+/+) mice. L-Sel−/− Peyer’s patch (PP) CD4+ Th cells revealed IFN-γ-dominated responses and an unprecedented absence of IL-4, whereas the expected mixed Th cell phenotype developed in L-Sel+/+ mice. PP CD4+ Th cell anti-Salmonella responses were nearly nonexistent in L-Sel−/− mice immunized with either Salmonella vaccine. Splenic CD4+ Th cell anti-Salmonella responses were reduced but did show cytokine production in Ag restimulation assays. Increased colonization of PP and spleen was noted only with the Salmonella vector in L-Sel−/− mice, resulting in increased splenomegaly, suggesting that the Salmonella-CFA/I vaccine was not as infectious or that the presence of the fimbriae improved clearance, possibly because of reduced neutrophil recruitment. However, sufficient anti-Salmonella immunity was induced, because Salmonella vector-immunized L-Sel−/− mice showed complete protection against wild-type Salmonella challenge, unlike L-Sel+/+ mice. This evidence shows that L-Sel is important for development of mucosal immunity, and absence of L-Sel is protective against salmonellosis.