Hair follicle–derived cytokines control the tropism and retention of memory T cells in the skin and promote lymphoma. The skin harbors a variety of resident leukocyte subsets that must be tightly regulated to maintain immune homeostasis. Hair follicles are unique structures in the skin that contribute to skin dendritic cell homeostasis through chemokine production. We demonstrate that CD4+ and CD8+ skin-resident memory T cells (TRM cells), which are responsible for long-term skin immunity, reside predominantly within the hair follicle epithelium of the unperturbed epidermis. TRM cell tropism for the epidermis and follicles is herein termed epidermotropism. Hair follicle expression of IL-15 was required for CD8+ TRM cells, and IL-7 for CD8+ and CD4+ TRM cells, to exert epidermotropism. A lack of either cytokine in the skin led to impaired hapten-induced contact hypersensitivity responses. In a model of cutaneous T cell lymphoma, epidermotropic CD4+ TRM lymphoma cell localization depended on the presence of hair follicle–derived IL-7. These findings implicate hair follicle–derived cytokines as regulators of malignant and non-malignant TRM cell tissue residence, and they suggest that the cytokines may be targeted therapeutically in inflammatory skin diseases and lymphoma.

This document provides a comprehensive summary of evidence on the current situation of rare diseases (RDs) globally and regionally, including conditions, practices, policies, and regulations, as well as the challenges and barriers faced by RD patients, their families, and caregivers. The document builds on a review of academic literature and policies and a process of validation and feedback by a group of seven experts from across the globe. Panelists were selected based on their academic merit, expertise, and knowledge regarding the RD environment. The document is divided into five main sections: (1) methodology and objective; (2) background and context; (3) overview of the current situation and key challenges related to RDs covering six dimensions: burden of disease, patient journey, social impact, disease management, RD-related policies, and research and development; (4) recommendations; and (5) conclusions. The recommendations are derived from the discussion undertaken by the experts on the findings of this review and provide a set of actionable solutions to the challenges and barriers to improving access to RD diagnosis and treatment around the world. The recommendations can support critical decision-making, guiding efforts by a broad range of RDs stakeholders, including governments, international organizations, manufacturers, researchers, and patient advocacy groups.

Epidermal Langerhans cells (LCs) extend dendrites through tight junctions (TJs) to survey the skin surface, but their immunological contribution in vivo remains elusive. We show that LCs were essential for inducing IgG₁ responses to patch-immunized ovalbumin in mice that lacked skin dendritic cell subsets. The significance of LC-induced humoral responses was demonstrated in a mouse model of staphylococcal scalded skin syndrome (SSSS), a severe blistering disease in which the desmosomal protein Dsg1 (desmoglein1) is cleaved by Staphylococcus aureus–derived exfoliative toxin (ET). Importantly, ET did not penetrate TJs, and patch immunization did not alter epidermal integrity. Nevertheless, neutralizing anti-ET IgG₁ was induced after patch immunization and abolished upon LC depletion, indicating that antigen capture through TJs by LCs induced humoral immunity. Strikingly, the ET-patched mice were protected from developing SSSS after intraperitoneal ET challenge, whereas LC-depleted mice were susceptible to SSSS, demonstrating a vital role for LC-induced IgG₁ in systemic defense against circulating toxin in vivo. Therefore, LCs elicit humoral immunity to antigens that have not yet violated the epidermal barrier, providing preemptive immunity against potentially pathogenic skin microbes. Targeting this immunological process confers protection with minimal invasiveness and should have a marked impact on future strategies for development of percutaneous vaccines.

Japan has been facing challenges relating to specifically defined rare diseases, called Nan-Byo in Japanese (literally ‘difficult’+‘illness’), and has already taken measures for them since 1972. This governmental support has surely benefited Nan-Byo patients; however, those suffering from medically unidentified conditions do not fall into this scheme and thus still confront difficulty in obtaining an examination, a diagnosis, and a treatment. To identify such rare and often undiagnosed diseases, we must integrate systematic diagnosis by medical experts with phenotypic and genetic data matching. Thus, in collaboration with Nan-Byo researchers and the Japanese universal healthcare system, the Japan Agency for Medical Research and Development launched the Initiative on Rare and Undiagnosed Diseases (IRUD) in 2015. IRUD is an ambitious challenge to construct a comprehensive medical network and an internationally compatible data-sharing framework. Synergizing with existing next-generation sequencing capabilities and other infrastructure, the nationwide medical research consortium has successfully grown to accept more than 2000 undiagnosed registrants by December 2016. We also aim at expanding the concept of microattribution throughout the initiative; that is, proper credit as collaborators shall be given to local primary care physicians, nurses and paramedics, patients, their family members, and those supporting the affected individuals whenever appropriate. As it shares many challenges among similar global efforts, IRUD’s future successes and lessons learned will significantly contribute to ongoing international endeavors, involving players in basic research, applied research, and societal implementation.

Langerhans cells (LCs) are antigen-presenting cells in the epidermis whose roles in antigen-specific immune regulation remain incompletely understood. Desmoglein 3 (Dsg3) is a keratinocyte cell-cell adhesion molecule critical for epidermal integrity and an autoantigen in the autoimmune blistering disease pemphigus. Although antibody-mediated disease mechanisms in pemphigus are extensively characterized, the T cell aspect of this autoimmune disease still remains poorly understood. Herein, we utilized a mouse model of CD4⁺ T cell-mediated autoimmunity against Dsg3 to show that acquisition of Dsg3 and subsequent presentation to T cells by LCs depended on the C-type lectin langerin. The lack of LCs led to enhanced autoimmunity with impaired Dsg3-specific regulatory T cell expansion. LCs expressed the IL-2 receptor complex and the disruption of IL-2 signaling in LCs attenuated LC-mediated regulatory T cell expansion in vitro, demonstrating that direct IL-2 signaling shapes LC function. These data establish that LCs mediate peripheral tolerance against an epidermal autoantigen and point to langerin and IL-2 signaling pathways as attractive targets for achieving tolerogenic responses particularly in autoimmune blistering diseases such as pemphigus.