Regulatory T cells (Tregs), which are characterized by expression of the transcription factor Foxp3, are a dynamic and heterogeneous population of cells that control immune responses and prevent autoimmunity. We recently identified a subset of Tregs in murine skin with properties typical of memory cells and defined this population as memory Tregs (mTregs). Due to the importance of these cells in regulating tissue inflammation in mice, we analyzed this cell population in humans and found that almost all Tregs in normal skin had an activated memory phenotype. Compared with mTregs in peripheral blood, cutaneous mTregs had unique cell surface marker expression and cytokine production. In normal human skin, mTregs preferentially localized to hair follicles and were more abundant in skin with high hair density. Sequence comparison of TCRs from conventional memory T helper cells and mTregs isolated from skin revealed little homology between the two cell populations, suggesting that they recognize different antigens. Under steady-state conditions, mTregs were nonmigratory and relatively unresponsive; however, in inflamed skin from psoriasis patients, mTregs expanded, were highly proliferative, and produced low levels of IL-17. Taken together, these results identify a subset of Tregs that stably resides in human skin and suggest that these cells are qualitatively defective in inflammatory skin disease.

Thymic Foxp3-expressing regulatory T cells are activated by peripheral self antigen to increase their suppressive function, and a fraction of these cells survive as memory Tregs (mTregs). Memory Tregs persist in non-lymphoid tissue after cessation of antigen expression and have enhanced capacity to suppress tissue-specific autoimmunity. Here, we show that murine mTregs express specific effector memory T cell markers and localize preferentially to hair follicles in skin. Memory Tregs express high levels of both IL-2Rα and IL-7Rα. Using a genetic deletion approach, we show that IL-2 is required to generate mTregs from naive CD4+ T cell precursors in vivo. However, IL-2 is not required to maintain these cells in the skin and skin-draining lymph nodes. Conversely, IL-7 is essential for maintaining mTregs in skin in the steady state. These results elucidate the fundamental biology of mTregs and show that IL-7 plays an important role in their survival in skin.

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Immune homeostasis in peripheral tissues is achieved by maintaining a balance between pathogenic effector T cells (Teff) and protective Foxp3+ regulatory T cells (Treg). Using a mouse model of an inducible tissue-antigen we demonstrate that antigen (Ag) persistence is a major determinant of the relative frequencies of Teff and Treg cells. Encounter of transferred naïve CD4+ T cells with transiently expressed tissue-Ag leads to generation of cytokine-producing Teff cells and peripheral Treg cells. Persistent expression of Ag, a mimic of self Ag, leads to functional inactivation and loss of the Teff cells with preservation of Treg in the target tissue. The inactivation of Teff cells by persistent Ag is associated with reduced ERK phosphorylation (pERK), whereas Treg cells show less reduction in pERK and are relatively resistant to ERK inhibition. Our studies reveal a crucial role for Ag in maintaining appropriate ratios of Ag-specific Teff to Treg cells in tissues.

Migratory dendritic cell (DC) subsets deliver tissue Ags to draining lymph nodes (DLNs) to either initiate or inhibit T cell-mediated immune responses. The signals mediating DC migration in response to tissue self-antigen are largely unknown. Using a mouse model of inducible skin-specific self-antigen expression, we demonstrate that CD103 dermal DCs (DDCs) rapidly migrate from skin to skin DLN (SDLNs) within the first 48 h after Ag expression. This window of time was characterized by the preferential activation of tissue-resident Ag-specific effector T cells (Teffs), with no concurrent activation of Ag-specific Teffs in SDLNs. Using genetic deletion and adoptive transfer approaches, we show that activation of skin-resident Teffs is required to drive CD103 DDC migration in response to tissue self-antigen and this Batf3-dependent DC population is necessary to mount a fulminant autoimmune response in skin. Conversely, activation of Ag-specific Teffs in SDLNs played no role in DDC migration. Our studies reveal a crucial role for skin-resident T cell-derived signals, originating at the site of self-antigen expression, to drive DDC migration during the elicitation phase of an autoimmune response.

Adult-onset Niemann–Pick disease type C (NPC) is an infrequent presentation of a rare neurovisceral lysosomal lipid storage disorder caused by autosomal recessive mutations in NPC1 (∼95%) or NPC2 (∼5%). Our patient was diagnosed at age 33 when he presented with a 10-yr history of difficulties in judgment, concentration, speech, and coordination. A history of transient neonatal jaundice and splenomegaly with bone marrow biopsy suggesting a lipid storage disorder pointed to NPC; biochemical (“variant” level cholesterol esterification) and ultrastructural studies in adulthood confirmed the diagnosis. Genetic testing revealed two different missense mutations in the NPC1 gene—V950M and N1156S. Symptoms progressed over >20 yr to severe ataxia and spasticity, dementia, and dysphagia with aspiration leading to death. Brain autopsy revealed mild atrophy of the cerebrum and cerebellum. Microscopic examination showed diffuse gray matter deposition of balloon neurons, mild white matter loss, extensive cerebellar Purkinje cell loss with numerous “empty baskets,” and neurofibrillary tangles predominantly in the hippocampal formation and transentorhinal cortex. We performed whole-genome sequencing to examine whether the patient harbored variants outside of the NPC1 locus that could have contributed to his late-onset phenotype. We focused analysis on genetic modifiers in pathways related to lipid metabolism, longevity, and neurodegenerative disease. We identified no rare coding variants in any of the pathways examined nor was the patient enriched for genome-wide association study (GWAS) single-nucleotide polymorphisms (SNPs) associated with longevity or altered lipid metabolism. In light of these findings, this case provides support for the V950M variant being sufficient for adult-onset NPC disease.

Mutations in ADAR, the gene that encodes the ADAR1 RNA deaminase, cause numerous human diseases, including Aicardi-Gouti&egraveres Syndrome (AGS). ADAR1 is an essential negative regulator of the RNA sensor MDA5, and loss of ADAR1 function triggers inappropriate activation of MDA5 by self-RNAs. However, the mechanisms of MDA5-dependent disease pathogenesis in vivo remain unknown. Here, we introduce a knockin mouse that models the most common ADAR AGS mutation in humans. These Adar-mutant mice develop lethal disease that requires MDA5, the RIG-I-like receptor LGP2, type I interferons, and the eIF2α kinase PKR. We show that a small molecule inhibitor of the integrated stress response (ISR) that acts downstream of eIF2α phosphorylation prevents immunopathology and rescues the mice from mortality. These findings place PKR and the ISR as central components of immunopathology in vivo and identify new therapeutic targets for treatment of human diseases associated with the ADAR1-MDA5 axis.