Husein Hadeiba

Deletion of a coordinate regulator of type 2 cytokine expression in mice

Mechanisms that underlie the patterning of cytokine expression in T helper (TH) cell subsets remain incompletely defined. An evolutionarily conserved ∼400-bp noncoding sequence in the intergenic region between the genes Il4 and Il13, designated conserved noncoding sequence 1 (CNS-1), was deleted in mice. The capacity to develop TH2 cells was compromised in vitro and in vivo in the absence of CNS-1. Despite the profound effect in T cells, mast cells from CNS-1−/− mice maintained their capacity to produce interleukin 4. A T cell–specific element critical for the optimal expression of type 2 cytokines may represent the evolution of a regulatory sequence exploited by adaptive immunity.

Plasmacytoid dendritic cells transport peripheral antigens to the thymus to promote central tolerance.

Central tolerance can be mediated by peripheral dendritic cells (DCs) that transport innocuous antigens (Ags) to the thymus for presentation to developing T cells, but the responsible DC subsets remained poorly defined. Immature plasmacytoid DCs (pDCs) express CCR9, a chemokine receptor involved in migration of T cell precursors to the thymus. We show here that CCR9 mediated efficient thymic entry of endogenous or i.v. transfused pDCs. pDCs activated by Toll-like receptor (TLR) ligands downregulated CCR9 and lost their ability to home to the thymus. Moreover, endogenous pDCs took up subcutaneously injected fluorescent Ag and, in the absence of TLR signals, transported Ag to the thymus in a CCR9-dependent fashion. Injected, Ag-loaded pDCs effectively deleted Ag-specific thymocytes, and this thymic clonal deletion required CCR9-mediated homing and was prevented by infectious signals. Thus, peripheral pDCs can contribute to immune tolerance through CCR9-dependent transport of peripheral Ags and subsequent deletion of Ag-reactive thymocytes.

Comparative transcriptional and functional profiling defines conserved programs of intestinal DC differentiation in humans and mice

Dendritic cells (DCs) that orchestrate mucosal immunity have been studied in mice. Here we characterized human gut DC populations and defined their relationship to previously studied human and mouse DCs. CD103+Sirpα− DCs were related to human blood CD141+ DCs and to mouse intestinal CD103+CD11b− DCs and expressed markers of cross-presenting DCs. CD103+Sirpα+ DCs aligned with human blood CD1c+ DCs and mouse intestinal CD103+CD11b+ DCs and supported the induction of regulatory T cells. Both CD103+ DC subsets induced the TH17 subset of helper T cells, while CD103−Sirpα+ DCs induced the TH1 subset of helper T cells. Comparative analysis of transcriptomes revealed conserved transcriptional programs among CD103+ DC subsets and identified a selective role for the transcriptional repressors Bcl-6 and Blimp-1 in the specification of CD103+CD11b− DCs and intestinal CD103+CD11b+ DCs, respectively. Our results highlight evolutionarily conserved and divergent programming of intestinal DCs.Dendritic cells (DCs) that orchestrate mucosal immunity have been studied in mice. Here we characterize human gut DC populations, and define their relationship to previously studied human and mouse DCs. CD103+Sirpα− DCs were related to human blood CD141+ and to mouse intestinal CD103+CD11b− DCs and expressed markers of cross-presenting DCs. CD103+Sirpα+ DCs aligned with human blood CD1c+ DCs and mouse intestinal CD103+CD11b+ DCs and supported regulatory T cell induction. Both CD103+ DC subsets induced TH17 cells, while CD103−Sirpα+ DCs induced TH1 cells. Comparative transcriptomics revealed conserved transcriptional programs among CD103+ DC subsets and uncovered a selective role for Bcl-6 and Blimp-1 in CD103+Sirpα− and intestinal CD103+CD11b+ DC specification, respectively. These results highlight evolutionarily conserved and divergent programming of intestinal DCs.

Lung CD25 CD4 Regulatory T Cells Suppress Type 2 Immune Responses But Not Bronchial Hyperreactivity

To study the effects of chronic Ag deposition in the airway mucosa on CD4+ T cell priming and subsequent airway disease, transgenic mice were generated that expressed OVA under the control of the surfactant protein C promoter. CD4 T cells from these mice were tolerant to OVA but this was overcome among spleen CD4 T cells by crossing to OVA-specific DO11.10 TCR-transgenic mice. Lungs from the double-transgenic mice developed lymphocytic infiltrates and modest mucus cell hyperplasia. Infiltrating cells were unaffected by the absence of either Rag-1 or Stat6, although the latter deficiency led to the disappearance of mucus. In the lung of double-transgenic mice, a large number of Ag-specific CD4 T cells expressed CD25 and functioned as regulatory T cells. The CD25+ CD4 T cells suppressed proliferation of CD25− CD4 T cells in vitro and inhibited type 2 immune responses induced by aerosolized Ags in vivo. Despite their ability to suppress allergic type 2 immunity in the airways, however, CD25+ CD4 regulatory T cells had no effect on the development of bronchial hyperreactivity.

Retinoic acid regulates the development of a gut-homing precursor for intestinal dendritic cells.

The vitamin A metabolite retinoic acid (RA) regulates intestinal immune responses through immunomodulatory actions on intestinal dendritic cells (DCs) and lymphocytes. Here, we show that RA also controls the generation of gut-tropic migratory DC precursors, referred to as pre-mucosal DCs (pre-μDCs). Pre-μDCs express the gut trafficking receptor α4β7 and home preferentially to the intestines. They develop in the bone marrow (BM), can differentiate into CCR9+ plasmacytoid DCs as well as conventional DCs (cDCs), but preferentially give rise to CD103+ intestinal cDCs. Generation of pre-μDCs in vivo in the BM or in vitro is regulated by RA and RA receptor α (RARα) signaling. The frequency of pre-μDCs is reduced in vitamin A-deficient animals and in animals treated with RAR inhibitors. The results define a novel vitamin A-dependent, RA-regulated developmental sequence for DCs and identify a targeted precursor for CD103+ cDCs in the gut.

The chemoattractant chemerin suppresses melanoma by recruiting natural killer cell antitumor defenses

Chemerin recruits NK cells to suppress melanoma growth.

Role and species-specific expression of colon T cell homing receptor GPR15 in colitis

Lymphocyte recruitment maintains intestinal immune homeostasis but also contributes to inflammation. The orphan chemoattractant receptor GPR15 mediates regulatory T cell homing and immunosuppression in the mouse colon. We show that GPR15 is also expressed by mouse TH17 and TH1 effector cells and is required for colitis in a model that depends on the trafficking of these cells to the colon. In humans GPR15 is expressed by effector cells, including pathogenic TH2 cells in ulcerative colitis, but is expressed poorly or not at all by colon regulatory T (Treg) cells. The TH2 transcriptional activator GATA-3 and the Treg-associated transcriptional repressor FOXP3 robustly bind human, but not mouse, GPR15 enhancer sequences, correlating with receptor expression. Our results highlight species differences in GPR15 regulation and suggest it as a potential therapeutic target for colitis.

Leukocyte Trafficking to the Small Intestine and Colon

Leukocyte trafficking to the small and large intestines is tightly controlled to maintain intestinal immune homeostasis, mediate immune responses, and regulate inflammation. A wide array of chemoattractants, chemoattractant receptors, and adhesion molecules expressed by leukocytes, mucosal endothelium, epithelium, and stromal cells controls leukocyte recruitment and microenvironmental localization in intestine and in the gut-associated lymphoid tissues (GALTs). Naive lymphocytes traffic to the gut-draining mesenteric lymph nodes where they undergo antigen-induced activation and priming; these processes determine their memory/effector phenotypes and imprint them with the capacity to migrate via the lymph and blood to the intestines. Mechanisms of T-cell recruitment to GALT and of T cells and plasmablasts to the small intestine are well described. Recent advances include the discovery of an unexpected role for lectin CD22 as a B-cell homing receptor GALT, and identification of the orphan G-protein-coupled receptor 15 (GPR15) as a T-cell chemoattractant/trafficking receptor for the colon. GPR15 decorates distinct subsets of T cells in mice and humans, a difference in species that could affect translation of the results of mouse colitis models to humans. Clinical studies with antibodies to integrin α4β7 and its vascular ligand mucosal vascular addressin cell adhesion molecule 1 are proving the value of lymphocyte trafficking mechanisms as therapeutic targets for inflammatory bowel diseases. In contrast to lymphocytes, cells of the innate immune system express adhesion and chemoattractant receptors that allow them to migrate directly to effector tissue sites during inflammation. We review the mechanisms for innate and adaptive leukocyte localization to the intestinal tract and GALT, and discuss their relevance to human intestinal homeostasis and inflammation.

Baseline airway hyperreactivity in A/J mice is not mediated by cells of the adaptive immune system

Human asthma is characterized by increased airway hyperreactivity to a variety of bronchoconstricting agents. Aberrant type 2 immune responses in the lung have been associated with airway hyperreactivity in both human asthma and in murine models of allergic airways disease. Despite their intrinsically elevated basal airway reactivity to smooth muscle constricting agents, A/J mice demonstrated no inherent inflammatory cell infiltration nor elevation of type 2 cytokines in the lung. Crossed bone marrow reconstitution experiments between A/J and MHC congenic B10.A mice revealed enhanced airway reactivity only in A/J recipients, irrespective of whether they had been reconstituted with A/J or B10.A hemopoietic cells. Further, A/J-derived bone marrow cells did not affect the reactivity of B10.A recipients. Although mice on RAG-deficient and IL-4-deficient backgrounds demonstrate substantial abrogation of allergen-induced airway hyperreactivity, these gene deletions had no impact on the elevated baseline reactivity when backcrossed onto A/J mice. Thus, in these mice, basal airway hyperreactivity is maintained independently of type 2 immunity induced by allergens.

Thymus‐homing dendritic cells in central tolerance

Central tolerance is critical in establishing a peripheral T‐cell repertoire purged of functional autoreactive T cells. One of the major requirements for effective central tolerance is the presentation of self and other innocuous antigens (Ags), including food, gut flora, or airway allergens, to developing T cells in the thymus. This seemingly challenging task can be mediated in some cases by ectopic expression of tissue‐specific Ags by thymic epithelial cells or by entry of systemic blood‐borne Ags into the thymus. More recently, thymic homing peripheral dendritic cells (DCs) have been proposed as cellular transporters of peripheral tissue‐specific Ags or foreign innocuous Ags. The aim of this viewpoint is to discuss the three principal thymic DC populations and their trafficking properties in the context of central tolerance. We will first discuss the importance of peripheral DC trafficking to the thymus and then compare and contrast the three DC subsets. We will describe how they were characterized, describe their trafficking to and their microenvironmental positioning in the thymus, and discuss the functional consequence of thymic trafficking and localization on thymic selection events.