Mononuclear phagocytes contribute to intestinal invasion and dissemination of Yersinia enterocolitica

Enteropathogenic Yersinia enterocolitica (Ye) enters the host via contaminated food. After colonisation of the small intestine Ye invades the Peyer's patches (PPs) via M cells and disseminates to the mesenteric lymph nodes (MLNs), spleen and liver. Whether Ye uses other invasion routes and which pathogenicity factors are required remains elusive. Oral infection of lymphotoxin-β-receptor deficient mice lacking PPs and MLNs with Ye revealed similar bacterial load in the spleen 1 h post infection as wild-type mice, demonstrating a PP-independent dissemination route for Ye. Immunohistological analysis of the small intestine revealed Ye in close contact with mononuclear phagocytes (MPs), specifically CX3CR1+ monocyte-derived cells (MCs) as well as CD103+ dendritic cells (DCs). This finding was confirmed by flow cytometry and imaging flow cytometry analysis of lamina propria (LP) leukocytes showing CD103+ DCs and MCs with intracellular Ye. Uptake of Ye by LP CD103+ DCs and MCs was dependent on the pathogenicity factor invasin, whereas the adhesin YadA was dispensable as demonstrated by Ye deletion mutants. Furthermore, Ye were found exclusively associated with CD103+ DCs in the MLNs from wild-type mice, but not from CCR7−/− mice, demonstrating a CCR7 dependent transport of Ye by CD103+ DCs from LP to the MLNs. In contrast, dissemination of Ye to the spleen was dependent on MCs as significantly less Ye could be recovered from the spleen of CX3CR1GFP/GFP mice compared to wild-type mice. Altogether, MCs and CD103+ DCs contribute to immediate invasion and dissemination of Ye. This together with data from other bacteria suggests MPs as general pathogenic entry site in the intestine.