Dendritic cells are early cellular targets of Listeria monocytogenes afterintestinal delivery and are involved in bacterial spread in the host

We studied the sequence of cellular events leading to the dissemination of Listeria monocytogenes from the gut to draining mesenteric lymph nodes (MLN s) by confocal microscopy of immunostained tissue sections from a rat ligat ed ileal loop system. OX-62- positive cells beneath the epithelial lining o f Peyer's patches (PPs) were the first Listeria targets identified after in testinal inoculation. These cells had other features typical of dendritic c ells (DCs): they were large, pleiomorphic and major histocompatibility comp lex class IIhi. Listeria were detected by microscopy in draining MLNs as ea rly as 6 h after inoculation. Some 80-90% of bacteria were located in the d eep paracortical regions, and 100% of the bacteria were present in OX-62-po sitive cells. Most infected cells contained more than five bacteria each, s uggesting that they had arrived already loaded with bacteria. At later stag es, the bacteria in these areas were mostly present in EDI-positive mononuc lear phagocytes. These cells were also infected by an actA mutant defective in cell-to-cell spreading. This suggests that Listeria are transported by DCs from PPs to the deep paracortical regions of draining MLNs and are then transmitted to other cell populations by mechanisms independent of ActA. A nother pathway of dissemination to MLNs was identified, probably involving free Listeria and leading to the infection of ED3-positive mononuclear phag ocytes in the subcapsular sinus and adjacent paracortical areas. This study provides evidence that DCs are major cellular targets of L. monocytogenes in PPs and that DCs may be involved in the early dissemination of this path ogen. DCs were not sites of active bacterial replication, making these cell s ideal vectors of infection.