Keiichi Yamanaka

The Vast Majority of CLA + T Cells Are Resident in Normal Skin

There are T cells within normal, noninflamed skin that most likely conduct immunosurveillance and are implicated in the development of psoriasis. We isolated T cells from normal human skin using both established and novel methods. Skin resident T cells expressed high levels of CLA, CCR4, and CCR6, and a subset expressed CCR8 and CXCR6. Skin T cells had a remarkably diverse TCR repertoire and were mostly Th1 memory effector cells with smaller subsets of central memory, Th2, and functional T regulatory cells. We isolated a surprising number of nonexpanded T cells from normal skin. To validate this finding, we counted T cells in sections of normal skin and determined that there are ∼1 × 106 T cells/cm2 normal skin and an estimated 2 × 1010 T cells in the entire skin surface, nearly twice the number of T cells in the circulation. Moreover, we estimate that 98% of CLA+ effector memory T cells are resident in normal skin under resting conditions. These findings demonstrate that there is a large pool of memory T cells in normal skin that can initiate and perpetuate immune reactions in the absence of T cell recruitment from the blood.

IL-18 induction of IgE: dependence on CD4+ T cells, IL-4 and STAT6.

Overproduction of immunoglobulin E (IgE) and T helper cell type 2 (TH2) cytokines, including interleukin 4 (IL-4), IL-5 and IL-13, can result in allergic disorders. Although it is known that IL-4 is critical to the polarization of naïve CD4+ T cells to a TH2 phenotype, both in vitro and in many in vivo systems, other factors that regulate in vivo IL-4 production and TH2 commitment are poorly understood. IL-18, an IL-1–like cytokine that requires cleavage with caspase-1 to become active, was found to increase IgE production in a CD4+ T cells−, IL-4– and STAT6–dependent fashion. IL-18 and T cell receptor–mediated stimulation could induce naïve CD4+ T cells to develop into IL-4–producing cells in vitro. Thus, caspase-1 and IL-18 may be critical in regulation of IgE production in vivo, providing a potential therapeutic target for allergic disorders.

The majority of human peripheral blood CD4+CD25highFoxp3+ regulatory T cells bear functional skin-homing receptors.

CD4+CD25+ T regulatory cells (Treg) are thought to be important in the peripheral tolerance. Recent evidence suggests that human peripheral blood CD4+CD25+ T cells are heterogeneous and contain both CD4+CD25high T cells with potent regulatory activity and many more CD4+CD25low/med nonregulatory T cells. In this study, we found that virtually all peripheral blood CD4+CD25highFoxp3+ Treg expressed high levels of the chemokine receptor CCR4. In addition, 80% of Treg expressed cutaneous lymphocyte Ag (CLA) and 73% expressed CCR6. These molecules were functional, as CLA+ Treg showed CD62E ligand activity and demonstrable chemotactic responses to the CCR4 ligands CCL22 and CCL17 and to the CCR6 ligand CCL20. The phenotype and chemotactic response of these Treg were significantly different from those of CD4+CD25med nonregulatory T cells. We further demonstrated that blood CLA+ Treg inhibited CD4+CD25− T cell proliferation induced by anti-CD3. Based on homing receptor profile, CLA+ Treg should enter normal skin. We next isolated CD4+CD25high T cells directly from normal human skin; these cells suppressed proliferation of skin CD4+CD25− T cells. Therefore, the majority of true circulating Treg express functional skin-homing receptors, and human Treg may regulate local immune responses in normal human skin.

IL-18 contributes to the spontaneous development of atopic dermatitis-like inflammatory skin lesion independently of IgE/stat6 under specific pathogen-free conditions

Atopic dermatitis (AD) is a pruritic inflammatory skin disease. Because IL-18 directly stimulates T cells and mast cells to release AD-associated molecules, Th2 cytokines, and histamine, we investigated the capacity of IL-18 to induce AD-like inflammatory skin disease by analyzing KIL-18Tg and KCASP1Tg, which skin-specifically overexpress IL-18 and caspase-1, respectively. They spontaneously developed relapsing dermatitis with mastocytosis and Th2 cytokine accumulation accompanied by systemic elevation of IgE and histamine. Stat6-deficient KCASP1Tg displayed undetectable levels of IgE but manifested the same degree of cutaneous changes, whereas IL-18-deficient KCASP1Tg evaded the dermatitis, suggesting that IL-18 causes the skin changes in the absence of IgE/stat6. KIL-18Tg and IL-1-deficient KCASP1Tg took longer to display the lesion than KCASP1Tg. Thus, AD-like inflammation is initiated by overrelease of IL-18 and accelerated by IL-1. Our present study might provide insight into understanding the pathogenesis of and establishing therapeutics for chronic inflammatory skin diseases including AD.

Skin-Specific Caspase-1-Transgenic Mice Show Cutaneous Apoptosis and Pre-Endotoxin Shock Condition with a High Serum Level of IL-18

To study the pathophysiological roles of overexpressed caspase-1 (CASP1), originally designated as IL-1β-converting enzyme, we generated transgenic mice in which human CASP1 is overexpressed in their keratinocytes. The transgenic mice spontaneously developed recalcitrant dermatitis and skin ulcers, characterized by the presence of massive keratinocyte apoptosis. The skin of the mice contained the active form of human CASP1 and expressed mRNA for caspase-activated DNase, an effector endonuclease responsible for DNA fragmentation. Their skin and sera showed elevated levels of mature IL-18 and IL-1β, but not of IFN-γ. The plasma from these animals induced IFN-γ production by IL-18-responsive NK cells. Administration of heat-killed Propionibacterium acnes, a potent in vivo type 1 cell inducer, caused IFN-γ-mediated lethal liver injury in the transgenic mice, which was completely inhibited by treatment with neutralizing anti-IL-18 Ab. These results indicated that in vivo overexpression of CASP1 caused spontaneous apoptotic tissue injury and rendered mice highly susceptible to exogenous type 1 cell-inducing condition in collaboration with endogenously accumulated proinflammatory cytokines.

Human skin cells support thymus-independent T cell development

Thymic tissue has previously been considered a requirement for the generation of a functional and diverse population of human T cells. We report that fibroblasts and keratinocytes from human skin arrayed on a synthetic 3-dimensional matrix support the development of functional human T cells from hematopoietic precursor cells in the absence of thymic tissue. Newly generated T cells contained T cell receptor excision circles, possessed a diverse T cell repertoire, and were functionally mature and tolerant to self MHC, indicating successful completion of positive and negative selection. Skin cell cultures expressed the AIRE, Foxn1, and Hoxa3 transcription factors and a panel of autoantigens. Skin and bone marrow biopsies can thus be used to generate de novo functional and diverse T cell populations for potential therapeutic use in immunosuppressed patients.

Human Mast Cell Chymase Cleaves Pro-IL-18 and Generates a Novel and Biologically Active IL-18 Fragment

Increased release of IL-18 in the skin causes atopic dermatitis (AD)-like skin lesions, suggesting a role of IL-18 in the pathogenesis of AD. Caspase-1 is a well-known activator of IL-18, but caspase-1 knockout mice still have biologically active IL-18. Normal human keratinocyte constitutively produces pro-IL-18, but it is unable to activate it, suggesting the existence of an alternative pathway for IL-18 in the skin. Dermal accumulation of mast cells is commonly observed in AD patients and in experimental mouse models of AD. Connective tissue mast cells contain high amounts of chymase and tryptase in their cytoplasmic granules. In the present study, we demonstrated that activation of IL-18 is a novel function of human mast cell chymase. Human mast cell chymase rapidly cleaves recombinant pro-IL-18 at 56-phenylalanine and produces a biologically active IL-18 fragment that is smaller than any other reported IL-18-derived species. The human mast cell chymase and the novel IL-18-derived active peptide may be novel therapeutic targets in AD- and IL-18-associated diseases

The Role of Cytokines/Chemokines in the Pathogenesis of Atopic Dermatitis

Atopic dermatitis (AD) is the most common and relapsing allergic disease of the skin. AD is characterized by a predominant expression of Th2-type cytokines associated with increased cellular infiltration in the skin, elevated circulating levels of IgE and eosinophilia. These findings are positively correlated with interleukin (IL)-4 and IL-13 expression in CD4+ T cells. In AD patients, Th2 cells, eosinophils, mast cells and dendritic cells are markedly increased in the skin lesions. However, Th1 cells are also involved in the development of AD lesions. In fact, Th1 cytokine mRNA expressions including γ-interferon and IL-12 are elevated in the chronic lesions as well as elevated Th2 cytokines in the acute AD lesions. The discovery of Th17 lineage and regulatory T (T(reg)) cells shifted the simple Th1/Th2 balance concept into a 4-way balance system. Th17/22 cells, Foxp3+ T(reg) and IL-10-producing T cells (Tr1) are involved in the mechanism of a local and systemic immunological milieu. In addition, super Th1 cells arranged from Th1 cells in high IL-18 milieu are also involved in the development of mouse AD lesions. Correction of Th2 cytokine predominance by Th1 inducers shows effectiveness in experimental models. However, fine-tuning of the delicate 4-way balance among Th1, Th2, Th17/22 and T(reg) cells is required for the control of AD. Efficacy of some biological agents in AD has been reported. However, further investigations are required to make possible the therapeutic application of biologicals, antigen-specific immunotherapy, non-antigen-specific immunotherapy, antagonists and biological response modifiers in the clinic. These novel approaches may constitute a potential curative therapy for AD.

Calcineurin Inhibitors Suppress Cytokine Production from Memory T Cells and Differentiation of Naïve T Cells into Cytokine-Producing Mature T Cells

T cells have been classified as belonging to the Th1 or Th2 subsets according to the production of defining cytokines such as IFN-γ and IL-4. The discovery of the Th17 lineage and regulatory T cells shifted the simple concept of the Th1/Th2 balance into a 4-way mechanistic pathway of local and systemic immunological activity. Clinically, the blockage of cytokine signals or non-specific suppression of cytokine predominance by immunosuppressants is the first-line treatment for inflammatory T cell-mediated disorders. Cyclosporine A (CsA) and Tacrolimus (Tac) are commonly used immunosuppressants for the treatment of autoimmune disease, psoriasis, and atopic disorders. Many studies have shown that these compounds suppress the activation of the calcium-dependent phosphatase calcineurin, thereby inhibiting T-cell activation. Although CsA and Tac are frequently utilized, their pharmacological mechanisms have not yet been fully elucidated.In the present study, we focused on the effects of CsA and Tac on cytokine secretion from purified human memory CD4(+)T cells and the differentiation of naïve T cells into cytokine-producing memory T cells. CsA or Tac significantly inhibited IFN-γ, IL-4, and IL-17 production from memory T cells. These compounds also inhibited T cell differentiation into the Th1, Th2, and Th17 subsets, even when used at a low concentration. This study provided critical information regarding the clinical efficacies of CsA and Tac as immunosuppressants.

Granzyme B is a novel interleukin-18 converting enzyme

BACKGROUND Granzyme B (GrB) is recognized to induce apoptosis; however, little is known about its possible role in other biological events. IL-18, a potent inflammatory cytokine, is produced as an inactive precursor (proIL-18). Several cells, including monocytes/macrophage lineage and non-hematopoietic cells such as keratinocytes, produce proIL-18. ProIL-18 requires appropriate processing to become active. Caspase-1 is the authentic IL-18 processing enzyme and is essential for IL-18 release from monocyte/macrophage lineage cells. However, caspase-1 is absent in non-hematopoietic cells, suggesting that there is another candidate to cleave proIL-18 except for caspase-1. OBJECTIVE GrB can invade and be active in cytoplasm of non-hematopoietic cells via perforin, therefore we investigated whether GrB converts proIL-18 into the biologically active form. METHODS Recombinant proIL-18 (rproIL-18) was produced and purified for protease reaction with GrB; this incubate was evaluated by immunoblotting. Biological activity of the proteolytic fragment cleaved by GrB was determined by IFN-gamma assay using KG-1 cells. IFN-gamma induction was also analyzed between extracts from GrB(+)/caspase-1(-) human CD8+ T cells and proIL-18 from normal human keratinocytes (NHK). RESULTS The proteolytic fragment that GrB cleaved proIL-18 had the same sequence and biological activity compared with mature IL-18 cleaved by caspase-1. Culture extracts from CD8+ T cells was able to cleave proIL-18 into authentic mature IL-18. IFN-gamma induction was also detected in NHK treated with CD8+ T cells. CONCLUSION GrB is a potent IL-18 converting enzyme and suggest that GrB secreted by CTLs and/or NK cells may initiate IL-18 release from target cells, leading to the development of inflammation.