Simon Milling

Intestinal CD103 − dendritic cells migrate in lymph and prime effector T cells

Intestinal dendritic cells (DCs) continuously migrate through lymphatics to mesenteric lymph nodes where they initiate immunity or tolerance. Recent research has focused on populations of intestinal DCs expressing CD103. Here we demonstrate, for the first time, the presence of two distinct CD103− DC subsets in intestinal lymph. Similar to CD103+ DCs, these intestine-derived CD103− DCs are responsive to Flt3 and they efficiently prime and confer a gut-homing phenotype to naive T cells. However, uniquely among intestinal DCs, CD103− CD11b+ CX3CR1int lymph DCs induce the differentiation of both interferon-γ and interleukin-17-producing effector T cells, even in the absence of overt stimulation. Priming by CD103− CD11b+ DCs represents a novel mechanism for the rapid generation of effector T-cell responses in the gut. Therefore, these cells may prove to be valuable targets for the treatment of intestinal inflammation or in the development of effective oral vaccines.

Intestinal macrophages and dendritic cells: what's the difference?

Mononuclear phagocytes (MPs) in the murine intestine, comprising dendritic cells (DCs) and macrophages (Mϕs), perform disparate yet complementary immunological functions. Functional analyses of these distinct MP subsets have been complicated by the substantial overlap in their surface phenotypes. Here, we review recent findings that have enabled more accurate definition of these MP subsets. We discuss these recent advances in the context of the current understanding of the functions of DCs and Mϕs in the maintenance of intestinal homeostasis, and how their functions may alter when homeostasis is disrupted.

Subsets of migrating intestinal dendritic cells

Dendritic cells (DCs) in the intestine are heterogeneous. Phenotypically different populations of conventional DCs have been identified in the intestinal lamina propria, Peyer’s patches, and in the draining mesenteric lymph nodes, to which these DCs constitutively migrate. Markers used to identify these populations include major histocompatibility complex class II, CD11c, CD8α, CD11b, and CD103. Extensive studies in rats, summarized here, which involved collection of migrating DCs by thoracic duct cannulation after mesenteric lymphadenectomy, have clearly demonstrated that the subsets of migrating intestinal lymph DCs have different functional properties. The subsets might play different roles in the induction of oral tolerance and in driving systemic immune responses after vaccination or intestinal stimulation with Toll‐like receptor ligands. The use of these surgical techniques allows investigation of the functions of purified subsets of migrating DCs. However, in the rat, these studies are limited by the range of available reagents and are difficult to compare with data from other species in this fast‐moving field. Recent refinements have enabled the collection of migrating intestinal DCs from mice; our initial results are described here. We believe that these studies will generate exciting data and have the potential to resolve important questions about the functions of migrating intestinal DC subsets.

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

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.

CCR7-dependent trafficking of RORγ + ILCs creates a unique microenvironment within mucosal draining lymph nodes

Presentation of peptide:MHCII by RORγ-expressing group 3 innate lymphoid cells (ILC3s), which are enriched within gut tissue, is required for control of CD4 T-cell responses to commensal bacteria. It is not known whether ILC populations migrate from their mucosal and peripheral sites to local draining secondary lymphoid tissues. Here we demonstrate that ILC3s reside within the interfollicular areas of mucosal draining lymph nodes, forming a distinct microenvironment not observed in peripheral lymph nodes. By photoconverting intestinal cells in Kaede mice we reveal constitutive trafficking of ILCs from the intestine to the draining mesenteric lymph nodes, which specifically for the LTi-like ILC3s was CCR7-dependent. Thus, ILC populations traffic to draining lymph nodes using different mechanisms.

Interleukin-22 binding protein (IL-22BP) is constitutively expressed by a subset of conventional dendritic cells and is strongly induced by retinoic acid

Interleukin-22 (IL-22) is mainly produced at barrier surfaces by T cells and innate lymphoid cells and is crucial to maintain epithelial integrity. However, dysregulated IL-22 action leads to deleterious inflammation and is involved in diseases such as psoriasis, intestinal inflammation, and cancer. IL-22 binding protein (IL-22BP) is a soluble inhibitory IL-22 receptor and may represent a crucial regulator of IL-22. We show both in rats and mice that, in the steady state, the main source of IL-22BP is constituted by a subset of conventional dendritic cells (DCs) in lymphoid and non-lymphoid tissues. In mouse intestine, IL-22BP was specifically expressed in lamina propria CD103+CD11b+ DC. In humans, IL-22BP was expressed in immature monocyte-derived DC and strongly induced by retinoic acid but dramatically reduced upon maturation. Our data suggest that a subset of immature DCs may actively participate in the regulation of IL-22 activity in the gut by producing high levels of IL-22BP.

D6 facilitates cellular migration, and fluid flow, to lymph nodes by suppressing lymphatic congestion

Lymphatic endothelial cells are important for efficient flow of antigen-bearing fluid and antigen-presenting cells (APCs) from peripheral sites to lymph nodes (LNs). APC movement to LNs is dependent on the constitutive chemokine receptor CCR7, although how conflicting inflammatory and constitutive chemokine cues are integrated at lymphatic surfaces during this process is not understood. Here we reveal a previously unrecognized aspect of the regulation of this process. The D6 chemokine-scavenging receptor, which is expressed on lymphatic endothelial cells (LECs), maintains lymphatic surfaces free of inflammatory CC-chemokines and minimizes interaction of inflammatory leukocytes with these surfaces. D6 does not alter the level of CCR7 ligands on LECs, thus ensuring selective presentation of homeostatic chemokines for interaction with CCR7(+) APCs. Accordingly, in D6-deficient mice, inflammatory CC-chemokine adherence to LECs results in inappropriate perilymphatic accumulation of inflammatory leukocytes at peripheral inflamed sites and draining LNs. This results in lymphatic congestion and impaired movement of APCs, and fluid, from inflamed sites to LNs. We propose that D6, by suppressing inflammatory chemokine binding to lymphatic surfaces, and thereby preventing inappropriate inflammatory leukocyte adherence, is a key regulator of lymphatic function and a novel, and indispensable, contributor to the integration of innate and adaptive immune responses.

Plasmacytoid Dendritic Cells Do Not Migrate in Intestinal or Hepatic Lymph

Plasmacytoid dendritic cells (pDCs) recognize pathogen-associated molecules, particularly viral, and represent an important mechanism in innate defense. They may however, also have roles in steady-state tolerogenic responses at mucosal sites. pDCs can be isolated from blood, mucosa, and lymph nodes (LNs). Although pDCs can express peripherally derived Ags in LNs and at mucosal sites, it is not clear whether pDCs actually migrate from the periphery in lymph or whether LN pDCs acquire Ags by other mechanisms. To determine whether pDCs migrate in lymph, intestine or liver-draining LNs were removed and thoracic duct leukocytes (TDLs) were collected. TDLs expressing MHC-II and CD45R, but not TCRαβ or CD45RA, were then analyzed. These enriched TDLs neither transcribe type I IFNs nor secrete inflammatory cytokines in response to viral stimuli in vitro or after a TLR7/8 stimulus in vivo. In addition, these TDLs do not express CD5, CD90, CD200, or Siglec-H, but do express Ig, and therefore represent B cells, despite their lack of CD45RA expression. Intestinal and hepatic lymph are hence devoid of bona fide pDCs under both steady-state conditions and after TLR7/8 stimulation. This shows that any role for pDCs in Ag-specific T cell activation or tolerance must differ from the roles of classical dendritic cells, because it cannot result from peripheral Ag capture, followed by migration of pDCs via lymph to the LN.

MyD88-dependent autoimmune disease in Lyn-deficient mice.

Recent evidence suggests that systemic autoimmune disease depends on signals from TLR ligands, but little is known about how TLR‐dependent pathways lead to the loss of self tolerance in vivo. To address this, we have examined the role of TLR signaling in Lyn‐deficient mice, which develop an autoimmune disease similar to SLE. We found that absence of the TLR signaling adaptor molecule MyD88 suppresses plasma cell differentiation of switched and unswitched B cells, and prevents the generation of antinuclear IgG antibodies and glomerulonephritis. In mixed chimeras the increased IgM and IgG antibody secretion in Lyn‐deficient mice is at least partially due to B cell‐independent effects of Lyn. We now show that MyD88 deficiency blocks the expansion and activation of DC in which Lyn is also normally expressed, and prevents the hypersecretion of proinflammatory cytokines IL‐6 and IL‐12 by Lyn‐deficient DC. These findings further highlight the important role of TLR‐dependent signals in both lymphocyte activation and autoimmune pathogenesis.

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

The intestinal innate immune system continually interacts with commensal bacteria, thus oral vaccines should induce extra/alternative activation of DC, potentially through TLR. To examine this we collected intestinal lymph DC (iL‐DC) under steady‐state conditions and after feeding resiquimod (R‐848), a synthetic TLR7/8 ligand, which we showed induces complete emptying of gut DC into lymph. iL‐DC are heterogeneous with subset‐specific functions. In this study we determined the kinetics of iL‐DC subset release, activation and cytokine secretion induced by R‐848. We show that L‐DC comprise three distinct subsets (CD172ahigh, CD172aint and CD172alow) present with similar frequencies in intestinal but not hepatic lymph. No iL‐DC express TLR7 mRNA, and only CD172a+ iL‐DC express TLR8. However, after oral R‐848 administration, output of all three subsets increases dramatically. CD172ahigh DC release precedes that of CD172alow DC, and the increased frequency of CD25high iL‐DC is restricted to the two CD172a+ subsets. After feeding R‐848 only CD172ahigh iL‐DC secrete IL‐6 and IL‐12p40. However, CD172aint and CD172ahigh DC secrete similar but markedly lower amounts when stimulated in vitro. These results highlight the importance of in vivo approaches to assess adjuvant effects on DC and give novel insights into the subset‐specific effects of an oral TLR ligand on intestinal DC.