Dendritic cells (DC) are recognized as sentinels, which capture antigens in tissue and migrate to the lymph node, where they initiate immune responses. However, when a vaccine strain of green fluorescent protein-expressing Salmonella abortusovis (SAO) was inoculated into sheep oral mucosa, it induced accumulation of myeloid non-DC in the subcapsular sinus and paracortex of the draining lymph node, and SAO was mainly found associated with these cells (granulocytes and macrophages) but rarely with DC. To analyze how bacteria reached lymph nodes, we used cervical pseudo-afferent lymph duct catheterization. We showed that Salmonella administered in the oral mucosa were traveling free in lymph or associated with cells, largely with lymph monocytes and granulocytes but less with DC. SAO also induced a strong influx of these phagocytic cells in afferent lymph. Migrating DC presented a semi-mature phenotype, and SAO administration did not alter their expression of major histocompatibility complex type 2 and coactivation molecules. Compared with blood counterparts, lymph monocytes expressed lower levels of CD40, and granulocytes expressed higher levels of CD80. The data suggest that immunity to bacteria may result from the complex interplay between a mixture of phagocytic cell types, which transport antigens and are massively recruited via lymph to decisional lymph nodes.

Dendritic cells (DC) have been reported to migrate in afferent lymph in the steady state. However, it is unknown whether DC traffic is modulated by the nature of the drained tissue. To analyze the influence of mucosal versus cutaneous microenvironments on the constitutive DC release, we exploited a novel technique of lymph cannulation in sheep, which allowed a comparison of afferent lymph DC migrating from the head mucosae [cervical DC (CerDC)] with DC migrating from skin [prescapular DC (PresDC)]. The migration rate was lower for CerDC than for PresDC. Compared with PresDC, CerDC contained a higher proportion of the CD26hi signal regulatory protein (SIRP)- DC subset. It is interesting that cytoplasmic apoptotic DNA as well as cytokeratin-positive inclusions were primarily detected among CD26hi SIRP- DC, an observation similar to that made in rats, which leads to the suggestion that this subset was involved in self-antigen presentation and tolerance induction. After the inoculation of cholera toxin (CT) onto the oro-nasal mucosae, migration of CD26hi SIRP- and CD26lo SIRP+ DC was accelerated in lymph, indicating that the effect of CT on DC mobilization is not subset-specific. Our results show that a mucosal environment influences DC output and the relative DC subset representation in lymph. This modulation of DC traffic to lymph nodes by mucosal surfaces is likely to affect the bias of the mucosal immune responses.