Programmed downregulation of CCR6 is important for establishment of epidermal γδT cells by regulating their thymic egress and epidermal location

CCL27 is a crucial regulator of immune homeostasis of the skin and mucosal tissues

Abundant immune cells reside in barrier tissues. Understanding the regulation of these cells can yield insights on their roles in tissue homeostasis and inflammation. Here, we report that the chemokine CCL27 is critical for establishment of resident lymphocytes and immune homeostasis in barrier tissues. CCL27 expression is associated with normal skin and hair follicle development independent of commensal bacterial stimulation, indicative of a homeostatic role for the chemokine. Accordingly, in the skin of CCL27-knockout mice, there is a reduced presence and dysregulated localization of T cells that express CCR10, the cognate receptor to CCL27. Besides, CCL27-knockout mice have overreactive skin inflammatory responses in an imiquimod-induced model of psoriasis. Beyond the skin, CCL27-knockout mice have increased infiltration of CCR10⁺ T cells into lungs and reproductive tracts, the latter of which also exhibit spontaneous inflammation. Our findings demonstrate that CCL27 is critical for immune homeostasis across barrier tissues.

The molecular mechanisms supporting the homeostasis and activation of dendritic epidermal T cell and its role in promoting wound healing

The epidermis is the outermost layer of skin and the first barrier against invasion. Dendritic epidermal T cells (DETCs) are a subset of γδ T cells and an important component of the epidermal immune microenvironment. DETCs are involved in skin wound healing, malignancy and autoimmune diseases. DETCs secrete insulin-like growth factor-1 and keratinocyte growth factor for skin homeostasis and re-epithelization and release inflammatory factors to adjust the inflammatory microenvironment of wound healing. Therefore, an understanding of their development, activation and correlative signalling pathways is indispensable for the regulation of DETCs to accelerate wound healing. Our review focuses on the above-mentioned molecular mechanisms to provide a general research framework to regulate and control the function of DETCs.

Coordinated co-migration of CCR10+ antibody-producing B cells with helper T cells for colonic homeostatic regulation

In the intestine, IgA antibody-secreting B cells (IgA-ASCs) and helper T cells coordinate to maintain local homeostasis while their dysregulation could lead to development of intestinal inflammatory diseases. However, mechanisms underlying the coordinated localization and function of the B and T cells into the intestine, particularly the colon, are poorly understood. We herein report the first evidence that the gut-homing chemokine receptor CCR10⁺ IgA-ASCs form conjugates with helper T cells, preferentially regulatory T cells, at their differentiation sites of gut-associated lymphoid organs for their coordinated co-localization into the colon to promote local homeostasis. In CCR10-knockout mice, defective migration of IgA-ASCs also resulted in defective T-cell migration and homeostasis, and development of inflammatory symptoms in the colon. Antigen-specific interaction of CCR10⁺ IgA-ASCs and T cells is crucial for their homeostatic establishment in the colon. On the other hand, in IgA-knockout mice, preferential expansion of CCR10⁺ IgG1-ASCs with regulatory functions compensated for CCR10⁺ IgA-ASCs to help maintain colonic homeostasis. The preferential expansion of specific subclasses of CCR10⁺ IgG-ASCs with regulatory functions was also found in asymptomatic IgA-deficient patients. These findings suggest coordinated cell migration as a novel mechanism underlying localization and function of B and T cells in colonic homeostatic regulation.

Psoriasis-associated impairment of CCL27/CCR10-derived regulation leads to IL-17A/IL-22-producing skin T-cell overactivation

Psoriasis-associated suppression of the skin-specific chemokine/receptor CCL27/CCR10 axis leads to enhanced pathogenic IL-17A/IL-22-producing skin T cell activation and inflammation.

Thymus medulla fosters generation of natural Treg cells, invariant γδ T cells, and invariant NKT cells: what we learn from intrathymic migration

The organization of the thymus into distinct cortical and medullary regions enables it to control the step-wise migration and development of immature T-cell precursors. Such a process provides access to specialized cortical and medullary thymic epithelial cells at defined stages of maturation, ensuring the generation of self-tolerant and MHC-restricted conventional CD4⁺ and CD8⁺ αβ T cells. The migratory cues and stromal cell requirements that regulate the development of conventional αβ T cells have been well studied. However, the thymus also fosters the generation of several immunoregulatory T-cell populations that form key components of both innate and adaptive immune responses. These include Foxp3⁺ natural regulatory T cells, invariant γδ T cells, and CD1d-restricted invariant natural killer T cells (iNKT cells). While less is known about the intrathymic requirements of these nonconventional T cells, recent studies have highlighted the importance of the thymus medulla in their development. Here, we review recent findings on the mechanisms controlling the intrathymic migration of distinct T-cell subsets, and relate this to knowledge of the microenvironmental requirements of these cells.

CCR6 supports migration and differentiation of a subset of DN1 early thymocyte progenitors but is not required for thymic nTreg development

T-cell selection and development occurs as precursor cells journey through the thymus and interact with stromal cells residing in distinct microenvironments. Although the chemokines CCL19, CCL21, CCL25 and CXCL12 are known to have major roles in intrathymic migration of thymocytes and thymocyte precursors, the significance of other chemokines such as CCL20, which is also expressed in the thymus, is unknown. This is of particular interest given that the thymus is the location of development of the natural regulatory T-cell (nTreg) population and that the CCL20 receptor CCR6 has an important role in peripheral tolerance via control of Treg cell migration. However, whether the CCL20/CCR6 axis has a role in the formation or migration of nTregs in the thymus is unknown. In this study, we addressed this by analyzing expression of CCR6/CCL20 within the thymus and assessing their role in thymocyte development using Ccr6(-/-) mice. CCL20 is predominately expressed in the thymic medulla and CCR6 expression is restricted to nTregs and a subset of early thymocyte progenitor double-negative 1 (DN1) cells (CD4(-)CD8(-)CD25(-)CD44(+)CD117(+)). Ex vivo chemotaxis assays indicated that these two subsets were apparently the sole subsets of thymocytes responsive to CCL20. The data indicate that nTreg frequencies and localization are unperturbed by deletion of Ccr6. However, in Ccr6(-/-) thymi, reduced frequencies of DN2 and DN3 cells, the thymocyte progenitor subsets that follow the DN1 stage, were apparent. Together, these data indicate that CCR6 has a supplementary role in coordination of early thymocyte precursor migration events important for normal subsequent thymocyte precursor development, but is not required for normal nTreg development.

Chronic ethanol feeding induces subset loss and hyporesponsiveness in skin T cells

BACKGROUND

Chronic alcoholism is associated with increased incidence and severity of cutaneous infection. Skin-resident T cells orchestrate numerous immunological functions that are critically involved in both tissue homeostasis and cutaneous immunity. The impact of chronic ethanol (EtOH) exposure on skin T cells has not previously been examined; given their important role in maintaining the immune barrier function of the skin further study is warranted.

METHODS

Mice were administered EtOH in the drinking water for 12 to 16 weeks. Flow cytometry was used to evaluate impact of EtOH feeding on skin T cell numbers, rates of proliferation, and apoptosis as well as activation marker expression and cytokine production after ex vivo stimulation.

RESULTS

Chronic EtOH feeding caused a baseline reduction in dendritic epidermal T cell (DETC) numbers that corresponded with reduced expression of the activation marker JAML following phorbol 12-myristate 13-acetate (PMA)/ionomycin stimulation. Chronic EtOH feeding did not alter total numbers of dermal T cells, but specific subset loss was observed in Foxp3(+) regulatory T cells (Tregs) as well as CD3hi, Vγ3(+) and CD3int, Vγ3(-) dermal γδ T cells. EtOH-induced dysfunction in the latter population, which represents prototypical interleukin-17 (IL-17)-producing dermal γδT17s, was made evident by diminished IL-17 production following anti-CD3 stimulation. Additionally, the capacity of lymph node γδ T cells to produce IL-17 following anti-CD3 and PMA/ionomycin stimulation was impaired by chronic EtOH feeding.

CONCLUSIONS

Chronic EtOH feeding induced defects in both numbers and function of multiple skin T cell subsets. The decreased density and poor responsiveness of DETCs and γδT17 cells in particular would be expected to compromise immune effector mechanisms necessary to maintain a protective barrier and restrict pathogen invasion. These findings demonstrate the sensitivity of skin T cells to EtOH and provide new mechanisms to help explain the propensity of alcoholics to suffer skin infection.

Differential requirement for CCR4 in the maintenance but not establishment of the invariant Vγ5(+) dendritic epidermal T-cell pool

Thymocytes expressing the invariant Vγ5 γδT-cell receptor represent progenitors of dendritic epidermal T-cells (DETC) that play an important immune surveillance role in the skin. In contrast to the bulk of αβT-cell development, Vγ5(+) DETC progenitor development occurs exclusively in fetal thymus. Whilst αβT-cell development is known to require chemokine receptor mediated migration through distinct thymus regions, culminating in medullary entry and thymic egress, the importance and control of intrathymic migration for DETC progenitors is unclear. We recently revealed a link between Vγ5(+) DETC progenitor development and medullary thymic epithelial cells expressing Aire, a known regulator of thymic chemokine expression, demonstrating that normal Vγ5(+) DETC progenitor development requires regulated intramedullary positioning. Here we investigate the role of chemokines and their receptors during intrathymic Vγ5(+) DETC progenitor development and establishment of the DETC pool in the skin. We report that thymic medullary accumulation of Vγ5(+) DETC progenitors is a G-protein coupled receptor dependent process. However, this process occurs independently of Aire's influences on intrathymic chemokines, and in the absence of CCR4 and CCR7 expression by DETC progenitors. In contrast, analysis of epidermal γδT-cells at neonatal and adult stages in CCR4(-/-) mice reveals that reduced numbers of DETC in adult epidermis are not a consequence of diminished intrathymic embryonic development, nor deficiencies in initial epidermal seeding in the neonate. Collectively, our data reveal differences in the chemokine receptor requirements for intrathymic migration of αβ and invariant γδT-cells, and highlight a differential role for CCR4 in the maintenance, but not initial seeding, of DETC in the epidermis.

Ovine skin-recirculating γδ T cells express IFN-γ and IL-17 and exit tissue independently of CCR7

γδ T cells continuously survey extralymphoid tissues, providing key effector functions during infection and inflammation. Despite their importance, the function and the molecules that drive migration of skin-recirculating γδ T cells are poorly described. Here we found that γδ T cells traveling in the skin-draining afferent lymph of sheep are effectors that produce IFN-γ or IL-17 and express high levels of the skin- and inflammation-seeking molecule E-selectin ligand. Consistent with a role for chemokine receptor CCR7 in mediating T cell exit from extralymphoid tissues, conventional CD4 and CD8T cells in skin-draining lymph were enriched in their expression of CCR7 compared to their skin-residing counterparts. In contrast, co-isolated γδ T cells in skin or lymph lacked expression of CCR7, indicating that they use alternative receptors for egress. Skin-draining γδ T cells were unresponsive to many cutaneous and inflammatory chemokines, including ligands for CCR2, CCR4, CCR5, CCR8, CCR10, and CXCR3, but showed selective chemotaxis toward the cutaneously expressed CCR6 ligand CCL20. Moreover, IL-17(+) γδ T cells were the most CCL20-responsive subset of γδ T cells. The data suggest that γδ T cells survey the skin and sites of inflammation and infection, entering via CCR6 and E-selectin ligand and leaving independent of the CCR7-CCL21 axis.

Cross-talk between intraepithelial γδ T cells and epithelial cells

Intraepithelial γδ T cells play pivotal roles in homeostasis, tissue repair, inflammation, and protection from malignancy. In some tissues, γδ T cells are the only resident T cell population, whereas in others, they coexist with αβ T cells and other lymphocyte populations. γδ T cell function in the epithelium requires constant communication between cells in the form of cell-to-cell contacts and cell-to-matrix interactions. These interactions coordinate with the timely production of specific cytokines, chemokines, growth factors, and glycosaminoglycans, which have specialized effects on neighboring epithelial cells. Antigens that activate these T cells are not well-defined, and they do not express classic costimulatory or coreceptor molecules. As such, an understanding of the mechanisms used by epithelial γδ T cells to maintain homeostasis and facilitate wound repair has necessitated the identification of novel molecular interactions between γδ T cells and their neighboring epithelial cells.