Exploring mechanisms that govern immune homeostasis in skin

我们身体最大的器官，皮肤，是抵御环境压力的重要屏障，如热变化，阳光，过敏原，毒素和微生物。这个复杂的器官由许多细胞类型的阵列组成，包括上皮细胞、成纤维细胞、神经元和脉管系统，它们一致地起作用以确保其功能。免疫系统的细胞已经成为皮肤生物学许多方面的协调者，共同加强抵御外部威胁的屏障。免疫细胞的组成、功能和激活状态不断受到皮肤结构、微生物和炎症刺激的调节。反过来，皮肤驻留和招募的免疫细胞与组织实质进行积极对话，以调节肠道细菌，限制微生物入侵并直接炎症和修复。以前的研究主要集中在表皮中独特的抗原呈递细胞朗格汉斯细胞（LC）的功能和生物学上，自发现以来，其体内功能一直不清楚。我们证明了LC具有独特的机制，以接近已经突破皮肤最外层屏障（角质层）但仍在表皮紧密连接之外的外源抗原（Kubo等人，JEM 2009）。这使我们假设LC调查在某些情况下也可能是致病性的肠道细菌。为了解决这个问题，我们将金黄色葡萄球菌衍生的毒素接种到完整的皮肤上。虽然这不会引起皮肤炎症，但我们发现LC通过完整的紧密连接屏障获得毒素，并诱导产生中和抗体，保护小鼠免受实验性葡萄球菌烫伤皮肤综合征中毒素的全身性攻击（Ouchi et al，JEM 2011）。我们进一步证明，LC摄取表皮自身抗原以诱导抑制自身免疫性皮肤病的调节性T细胞的扩增（Kitashima et al，eBiomedicine，2018）。总的来说，这些研究确立了LC在宿主防御和自身免疫中的关键作用。导致当前组织免疫串扰主题的一个重要成就是阐明了LC网络如何由毛囊维持，这是哺乳动物的定义特征之一。我们发现毛囊是免疫活性结构，其在感测到机械应力时产生趋化因子以吸引LC前体并充当其再增殖到表皮中的通道（Nagao等人，Nat Immunol 2012）。这一发现代表了组织特异性信号如何与免疫细胞通信以维持免疫稳态的新概念。我们最近通过证明毛囊产生细胞因子使记忆T细胞在表皮中持续存在来扩展上述发现。重要的是，当驻留记忆T细胞经历恶性转化为淋巴瘤时，它们仍然依赖于毛囊来源的细胞因子（阿达奇等人，Nat Med 2015）。我们还研究了宿主-微生物在疾病环境中的相互作用，通过产生异位性皮炎（AD）的ADAM 17 cKO小鼠模型，该模型自发地发展与以S.金黄色葡萄球菌，这是一种重现人类AD的特征。S. AD皮肤上的金黄色葡萄球菌定殖是湿疹性炎症的原因还是仅仅是慢性炎症的结果一直存在争议。我们在我们的AD小鼠模型中确定S.金黄色葡萄球菌是湿疹形成的关键组分，为长期存在的临床问题提供了答案（小林等人，Immunity 2015）。使用来自ADAM 17 cKO小鼠的皮肤的批量RNA-seq分析，我们证明了这些小鼠中的转录组重现了人类AD的转录组（Woodring et al，J Invest Dermatol，2018）。我们进一步证明了表皮中的先天性淋巴样细胞（ILC）依赖于毛囊衍生的细胞因子和趋化因子在皮脂腺附近的持久性和定位。在那里，ILC调节皮脂腺的功能，以调节驻留在皮肤表面上的微生物的平衡（小林等人，细胞，2019）。在NIAMS的皮肤白细胞生物学部分，我们进一步探索了稳态和炎症过程中组织免疫和宿主微生物相互作用的基本机制。这些过程涉及先天性和适应性免疫细胞，我们假设改变上皮或基质室之间的串扰是参与皮肤病的病理生理。我们采用临床相关的模型，包括AD的小鼠模型，以扩大我们对组织免疫串扰的基础知识，并建立一个基础，以促进更好地了解皮肤免疫和炎症性皮肤病。

Our body's largest organ, the skin, is a vital barrier against environmental pressures such as thermal changes, sunlight, allergens, toxins, and microbes. This complex organ is composed of an array of many cell types including epithelia, fibroblasts, neurons, and vasculature that act in unison to ensure its function. Cells of the immune system have emerged as orchestrators of many facets of skin biology that collectively serve to reinforce the barrier against external threats. The composition, function, and activation status of immune cells is constantly tuned by skin structures, microbial and inflammatory stimuli. In turn, skin-resident and recruited immune cells engage in an active dialogue with the tissue parenchyma to regulate commensal bacteria, limit microbial invasion and direct inflammation and repair. Previous research program primarily focused on the functions and biology of Langerhans cells (LC), antigen presenting cells unique in the epidermis, who's in vivo functions had remained unclear since their discovery. We demonstrated that LCs were equipped with a unique mechanism to gain access to foreign antigens that had breached the skin's outermost barrier, the stratum corneum, but that were still outside of epidermal tight junctions (Kubo et al, JEM 2009). This led us to hypothesize that LCs survey for commensal bacteria which may also be pathogenic under certain circumstances. To address this, we inoculated Staphylococcus aureus-derived toxin onto intact skin. While this caused no skin inflammation, we found that LCs acquired the toxin through intact tight junction barriers and induced the production of neutralizing antibodies that protected mice from systemic challenge of the toxin in experimental Staphylococcal scalded skin syndrome (Ouchi et al, JEM 2011). We further demonstrated that LCs took up an epidermal autoantigen to induce the expansion of regulatory T cells that suppressed autoimmune skin disease (Kitashima et al, eBiomedicine, 2018). These studies, in aggregate, established crucial roles of LCs during host defense and autoimmunity. An important accomplishment that led to the current theme of tissue-immune crosstalk was elucidating how the LC network is maintained by hair follicles, one of the defining features of mammals. We discovered that hair follicles are immunologically active structures that, upon sensing mechanical stress, produced chemokines to attract LC precursors and served as a gateway for their repopulation into epidermis (Nagao et al, Nat Immunol 2012). This finding represented a novel concept of how tissue-specific signals communicate with immune cells to maintain immunological homeostasis. We have recently expanded on the above findings by demonstrating that hair follicles produced cytokines that enabled the persistence of memory T cells in the epidermis. Importantly when resident memory T cells underwent malignant transformation to lymphoma, they remained dependent on hair follicle-derived cytokines (Adachi et al, Nat Med 2015). We also studied host-microbe interactions in a disease setting by generating an ADAM17 cKO mouse model of atopic dermatitis (AD) that spontaneously developed eczematous skin inflammation associated with dysbiosis (imbalance of the bacterial flora) that was predominated by S. aureus, a feature that recapitulates human AD. Whether S. aureus colonization on AD skin contributed to eczematous inflammation or was merely a result of chronic inflammation had been debated. We determined in our AD mouse model that S. aureus was a crucial component of eczema formation, providing an answer to a long-standing clinical question (Kobayashi et al, Immunity 2015). Using bulk RNA-seq analysis of skin from ADAM17 cKO mice we demonstrated that the transcriptome in these mice recapitulates that of human AD (Woodring et al, J Invest Dermatol, 2018). We further demonstrated that innate lymphoid cells (ILC) in the epidermis rely on hair follicle-derived cytokines and chemokines for persistence and localization near the sebaceous glands. There, the ILCs regulate the functions of sebaceous glands to tune the equilibrium of microbes that reside on skin surface (Kobayashi et al, Cell, 2019). In the Cutaneous Leukocyte Biology Section at NIAMS, we further explore fundamental mechanisms that underlie tissue-immune and host-microbe interactions during homeostasis and inflammation. These processes involve innate and adaptive immune cells, and we hypothesize that altered crosstalk between epithelial or stromal compartments is involved in the pathophysiology of skin diseases. We employ clinically relevant models, including mouse models for AD, to expand our fundamental knowledge of tissue-immune crosstalk and to build a foundation that will promote better understanding of skin immunity and inflammatory skin diseases.