Regulation of Intestinal Dendritic Cell Migration and Activation by Plasmacytoid Dendritic Cells, TNF-α and Type 1 IFNs after Feeding a TLR7/8 Ligand

Abstract

Dendritic cells (DCs) migrating via lymph are the primary influence regulating naive T cell differentiation, be it active immunity or tolerance. How DCs achieve this regulation in vivo is poorly understood. Intestinal DCs are in direct contact with harmless or pathogenic luminal contents, but may also be influenced by signals from epithelial cells, macrophages, or other resident or immigrant cells. To understand the role of TLR7 and TLR8 in regulating intestinal DC function, we fed a TLR7/8 ligand (resiquimod (R-848)) to rats and mice and examined DC in pseudoafferent lymph (rat) and mesenteric lymph nodes (MLNs). Oral R-848 induced a 20- to 30-fold increase in DC output from the intestine within 10 h due to a virtually total release of lamina propria DCs. This resulted in an accumulation of DCs in the MLNs that in mice was completely TNF-α dependent. Surprisingly, intestinal lymph DCs (iL-DCs) released by R-848 did not up-regulate CD86, but did up-regulate CD25. In contrast, MLN-DCs from R-848-stimulated rats and mice expressed high levels of CD86. This DC activation in MLNs was dependent on type 1 IFNs. The major source of these rapidly released cytokines is plasmacytoid DCs (pDCs) and not classical DCs, because depletion of pDCs significantly reduces the R-848-stimulated increase in serum cytokine levels as well as the accumulation and activation of DCs in MLNs. These experiments show that TLR-mediated regulation of iL-DC functions in vivo is complex and does not depend only on direct iL-DC stimulation, but can be regulated by pDCs.

A distinct subset of intestinal dendritic cells responds selectively to oral TLR7/8 stimulation

“…We have shown that feeding R-848 leads to a marked increase in output of all three DC subsets into lymph, accompanied by complete emptying of DC from the LP. However, by immuno-histochemistry and FACS analysis, there is no decrease in PP DC numbers ( [17] and unpublished observation). That DC do not migrate from the PP to the MLN under SS conditions has recently been suggested by studies in minipigs [11].…”

“…Oral administration of R-848 dramatically and rapidly stimulates the entry of L-DC into intestinal [17] and hepatic lymph. Output of CD172a high DC increases before that of the other subsets, but the release of CD172a low DC in both intestinal and hepatic lymph also increases after feeding of R-848; indeed, the CD172a low subset constitutes the majority of iL-DC at the peak of increased output (8-10 h post-feeding).…”

“…It is possible that the rapid increase in output of CD172a + DC represents direct stimulation by TLR8 on these cells, whereas the delayed release of CD172a low DC that do not express TLR8 represents an indirect effect. The indirect effect could be mediated by TNFa, as it is in mice where the bulk of this cytokine is secreted by pDC [17].…”

“…For the intestine this is due to complete emptying of DC from the LP [9]. We have recently observed that feeding rats with R-848, a TLR7/8 ligand, also results in a dramatic but transient increase in the numbers of DC in intestinal and liver lymph ( [17] and unpublished observations). To investigate the effects of oral R-848 on the kinetics of DC subset release, cannulated lymphadenectomised rats were fed R-848 and lymph collected over 2-h intervals.…”

“…In a recent study, we demonstrated that feeding the adjuvant R-848 to rats and mice results in complete emptying of DC from the lamina propria (LP) of the small intestine into afferent lymph [17]. We now examined the expression of TLR7/8 by iL-DC subsets and their in vivo response to oral R-848.…”