Lichen planus (LP) is a chronic inflammatory disease of the skin and mucous membranes, marked by T cell infiltration and keratinocyte apoptosis. However, its immune microenvironment remains poorly understood. Using single-cell RNA sequencing, spatial transcriptomics, and proteomics on samples from 28 patients and 18 healthy controls, we identify elevated interferon (IFN) and cytotoxic signatures in CXCL13+CD8+ T cells in both cutaneous and mucosal LP, but not in lichen planopilaris. T cells expressing TNF and IFNG are spatially linked to epithelial cells through ligand-receptor interactions, correlating with inflammation. We identify cDC2A cells as key contributors, proximal to CXCL13+CD8+ T cells, serving as a major source of IL-15. CXCL13+CD8+ T cells express TNFRSF9 (4-1BB), which enhances their cytotoxic responses in the skin. In summary, our data reveal a critical role for cDC2A in driving CXCL13+CD8+ T-epithelial cytotoxicity in cutaneous and mucosal LP through TNFRSF9.

IL-17 and TNF blockade effectively treats hidradenitis suppurativa and psoriasis, yet the relative importance of the different cell types that respond to IL-17 and TNF remains unresolved. Keratinocytes are viewed as the dominant effector cells, whereas fibroblasts have recently emerged as important contributors. In mice, topical imiquimod induces IL-17- and TNF-dependent skin inflammation and models psoriasis. Here, we demonstrate that intradermal injection of IL-17 and TNF elicits inflammation with features of hidradenitis suppurativa, including a transcriptional program that is distinct from that in psoriasis and imiquimod-induced inflammation. Single-cell transcriptomic network analysis identified fibroblasts as the dominant communication hub in hidradenitis suppurativa and in IL-17/TNF-injected mice. In contrast, both fibroblasts and keratinocytes showed strong network involvement in psoriasis and imiquimod-treated mice. Cell-type-specific deletion of IL-17RA revealed that imiquimod-induced inflammation depends equally on IL-17 signaling in fibroblasts and keratinocytes, whereas inflammation induced by intradermal IL-17/TNF requires only fibroblasts to recognize IL-17. Single-cell transcriptomic analysis of conditional knockout mice demonstrated that keratinocytes and fibroblasts activate divergent and disease-dependent transcriptional programs downstream of IL-17. These findings introduce a conceptual framework wherein IL-17 signaling is routed through distinct cellular and molecular pathways depending on disease context and establish complementary experimental systems for interrogating type 17 skin inflammation.

Cutaneous drug eruptions, including morbilliform drug eruption (MDE) and the less common symmetrical drug-related intertriginous and flexural exanthema (SDRIFE), are characterized by an erythematous rash following medication exposure, often to beta-lactam antibiotics. While MDE is thought to involve tissue-resident memory T cells, the immunopathogenesis of SDRIFE remains poorly understood. We sought to define the inflammatory patterns in SDRIFE compared to MDE, and to elucidate the spatial localization of immune cells within skin. To address these questions, we performed single-cell RNA sequencing, exploratory spatial transcriptomics, and immunohistochemistry to compare skin from patients with MDE and SDRIFE. MDE lesions featured a predominance of CD8+ cytotoxic T cells and Th1-type CD4+ T cells, high expression of CXCL9-11, and upregulation of interferon-responsive genes. In contrast, SDRIFE showed limited CD8+ infiltration, enrichment for CD4+ T cells with modest Th2 skewing and macrophages with an immunomodulatory profile. Exploratory spatial transcriptomics revealed substantial T cell and macrophage presence in the superficial dermis in both conditions. We conclude that MDE and SDRIFE display distinct immune landscapes: MDE is characterized by cytotoxic and interferon-mediated inflammation, while SDRIFE involves an immunomodulatory, non-classical response. These findings reveal inflammatory patterns in SDRIFE and highlight potential diagnostic and therapeutic targets.

Recent evidence implicates altered RNA editing and dysregulated type I IFN signaling in immune-mediated diseases, including psoriasis, although the underlying genetic mechanisms remain poorly defined. We investigated four unrelated multiplex families with early-onset plaque psoriasis, with or without psoriatic arthritis, segregating as a monogenic trait and characterized by a strong IFN signature in skin and blood. Whole-exome sequencing identified four rare heterozygous loss-of-function mutations in ADAR1 cosegregating with disease and elevated IFN-stimulated gene expression. Six additional rare variants were detected in an independent cohort of 125 psoriasis patients. Single-cell transcriptomics identified keratinocytes and melanocytes as major IFN sources. Functional studies showed that ADAR1 knockdown or expression of ADAR1G1119R and ADAR1P3A alleles pathogenic variants reduced adenosine-to-inosine RNA editing and increased IFN-stimulated genes and inflammatory cytokines, effects reversed by upadacitinib and deucravacitinib. These findings define a novel IFN-dependent psoriasis subtype caused by inborn defects of ADAR1-mediated RNA editing, with direct implications for precision medicine in psoriatic disease.

BACKGROUND:Preeclampsia affects approximately 1 in 10 pregnancies, leading to severe complications and long-term health risks for both mother and offspring. While the etiology remains unclear, preeclampsia has been linked to both autoimmunity and the timing of menarche.METHODS:Through human single-cell and spatial analyses, coupled with in vitro, in vivo, and ex vivo models, we demonstrate that VGLL3 (Vestigial-like family member 3), a transcription coregulator in the Hippo pathway, is upregulated in preeclamptic placentas.RESULTS:VGLL3 promotes immune activation, impairs trophoblast differentiation, and induces endothelial dysfunction, all of which contribute to pregnancy-related hypertension, fetal growth restriction, and offspring mortality. Our data reveal that VGLL3 acts upstream of preeclampsia-associated processes, including the production of sFLT1 (soluble fms-like tyrosine kinase 1), a key biomarker of the disease. Notably, targeting VGLL3, either by genetic deletion in mouse placentas or through therapeutic inhibition in human placentas, protects against preeclampsia and alleviates disease pathology.CONCLUSIONS:These findings position VGLL3 as a promising novel therapeutic target for preeclampsia.

Preeclampsia affects approximately 1 in 10 pregnancies, leading to severe complications and long-term health risks for both mother and offspring. While the etiology remains unclear, preeclampsia has been linked to both autoimmunity and the timing of menarche. Through human single-cell and spatial analyses, coupled with in vitro, in vivo, and ex vivo models, we demonstrate that VGLL3, a transcription co-regulator in the Hippo pathway, is upregulated in preeclamptic placentas. VGLL3 promotes immune activation, impairs trophoblast differentiation, and induces endothelial dysfunction, all of which contribute to pregnancy-related hypertension, fetal growth restriction, and offspring mortality. Our data reveal that VGLL3 acts upstream of preeclampsia-associated processes, including the production of sFLT1, a key biomarker of the disease. Notably, targeting VGLL3-either by genetic deletion in mouse placentas or through therapeutic inhibition in human placentas-protects against preeclampsia and alleviates disease pathology. These findings position VGLL3 as a promising novel therapeutic target for preeclampsia.

Generalized pustular psoriasis (GPP) is a severe subtype of psoriasis characterized by epidermal neutrophil infiltration, often presenting as acute, potentially life-threatening flares. However, the characterization of the immune micro-environment in GPP lesions remains largely unknown. Here, we use single-cell RNA profiling to interrogate the transcriptomes of 60,000 single cells from GPP lesional skin (n = 13) and healthy adult skin (n = 4), combined with spatial transcriptomics. We identify a neutrophil subset lacking CASP8 expression but exhibiting elevated levels of inflammatory pathway genes, including RIPK1, NFKB1, IL1B, CXCL1, and CXCL8 in GPP flares, illustrating neutrophil transition from pre-inflammatory to a pro-inflammatory state, and activation of a communication network between IL36G+ keratinocytes and neutrophils in GPP lesions, with TNFSF15 (TL1A) released from neutrophils exaggerating the inflammatory crosstalk. We further demonstrate that fibroblasts and capillary endothelial cells function as central communication hubs in GPP, through dynamic receptor-ligand interactions with several spatially proximate immune cells, including T cells, neutrophils, and macrophages. In this work, we provide an in-depth view of immune cell participation and highlight the role of neutrophil-keratinocyte crosstalk in GPP pathogenesis.

Introduction:Syphilis is a complex disease with variable clinical presentation where symptomatic and potentially infectious stages alternate with periods of latency, representing a fascinating model to study immune evasion and host immune responses. Methods:Immunohistochemistry (IHC), bulk, and single-cell RNA sequencing were performed on formalin-fixed paraffin-embedded skin biopsies collected from subjects with secondary syphilis. Additionally, PBMCs from healthy individuals and either primary or MyD88 knock-out keratinocytes were exposed to live Treponema pallidum cells to define initial skin responses to the bacteria. Results:Immunohistochemistry of secondary syphilis skin lesions showed a polymorphous immune infiltrate with colocalization of T cells, B cells and antigen-presenting cells. Single-cell analysis revealed distinct cellular contributions to the immune response, with prominent immune-stromal crosstalk accompanied by altered keratinocyte differentiation and decreased intraepidermal communication. Notably, prominent inflammatory signals were countered by concomitant regulatory responses, particularly in infiltrating myeloid cells. Exposure of PBMCs to live T. pallidum inhibited immune responses, while exposure to sonicated cells triggered CXCL1 and CXCL3 upregulation. Keratinocytes responded to both intact and sonicated T. pallidum with upregulation of type-I interferon responses that, however, were abolished in MYD88-deficient but not in STING-deficient keratinocytes. Discussion:Our data provide novel insights into the contribution of epidermal TLR sensing through MYD88 to the host response to syphilis infection, highlighting mechanisms by which T. pallidum evades immune responses in skin that may facilitate transmission of this pathogen through the skin.

BACKGROUND:Sweet syndrome is an inflammatory skin disease characterized by robust neutrophil infiltration into the dermis. The pathogenesis of Sweet syndrome and its distinguishing features compared to other neutrophilic dermatoses, such as pyoderma gangrenosum, remain poorly understood. OBJECTIVE:Our aim was to define the cellular and molecular landscape of the skin of patients with Sweet syndrome. METHODS:Single-nucleus and bulk transcriptomics were performed on archival clinical skin samples from patients with Sweet syndrome, patients with pyoderma gangrenosum, and healthy controls. For mechanistic validation, functional experiments were performed with primary human cells. Spatial transcriptomics with single-molecule resolution was used to map cell types to tissue location. RESULTS:A prominent interferon signature was identified in Sweet syndrome skin that was reduced in tissue samples from patients with pyoderma gangrenosum and healthy controls. This signature was observed in different subsets of cells, including fibroblasts that expressed interferon-induced genes. Functionally, this response was supported by analysis of cultured dermal fibroblasts that were observed to highly express neutrophil chemokines in response to activation by type I interferon. Furthermore, spatial transcriptomics revealed 2 positionally distinct interferon-activated fibroblast subsets: CXCL1-positive fibroblasts near neutrophil infiltrates and CXCL12-positive fibroblasts distal to these infiltrates. CONCLUSION:This study defines the cellular and molecular landscape of neutrophilic dermatoses and implicates dermal immune-acting fibroblasts in Sweet syndrome pathogenesis through type I interferon recognition and neutrophil recruitment.

Sweet’s syndrome is a poorly understood inflammatory skin disease characterized by neutrophil infiltration to the dermis. Single-nucleus and bulk transcriptomics of archived FFPE clinical samples revealed elevated plasmacytoid dendritic cells and a prominent interferon signature in Sweet’s syndrome skin that was reduced in tissue from other neutrophilic dermatoses (pyoderma gangrenosum and pustular psoriasis) and healthy controls. Interferon-stimulated genes were highly expressed in subsets of fibroblasts, keratinocytes, lymphocytes, and myeloid cells. Functionally, cultured primary human dermal fibroblasts highly expressed neutrophil chemokines in response to type I but not type II interferon. Subcellular resolution spatial transcriptomics of archived FFPE clinical samples from neutrophilic dermatoses and healthy skin was leveraged to locate these immune-acting fibroblasts. This approach identified two positionally distinct immune-acting fibroblast subsets in Sweet’s syndrome: a CXCL1+ subset proximal to the neutrophilic infiltrate in the upper dermis and a CXCL12+ subset in the lower dermis distal to neutrophils. Thus, this study defines the cellular landscape of neutrophilic dermatoses and identifies dermal immune-acting fibroblasts with a pathogenic role in Sweet’s syndrome through recognition of type I interferon and neutrophil chemoattraction. Supported by NSF GRFP2038238, NIH T32DK007202, NIH R01DK121760, NIH R01AR076082, NIHR01AI153185, U01AI152038, P50AR080594, and NIH R37AI052453. Immune Mechanisms of Human Disease (HUM)

Human skin is not a uniform organ but a mosaic of anatomically distinct niches, with each site finely tuned to unique environmental demands and immune pressures. Yet, the molecular determinants that define these regional identities and their relationship to site-specific vulnerability to inflammatory disease remain poorly understood. Here, we generate a high-resolution single-cell atlas of human skin, profiling 274,834 cells from 96 healthy samples across 7 anatomically distinct sites (acral, arm, axilla, back, face, leg and scalp). Our analysis reveals striking region-specific transcriptional and cellular networks, uncovering how local immune-stromal crosstalk governs tissue homeostasis and underpins anatomical susceptibility to distinct inflammatory diseases such as such as systemic lupus erythematosus (SLE), atopic dermatitis (AD), and psoriasis. These findings illuminate the tissue-intrinsic foundations of regional immune identity and provide a blueprint/resource for the development of precision therapies tailored to the distinct immunological microenvironments of specific anatomical skin sites.

BACKGROUND:Palmoplantar pustulosis (PPP) is an inflammatory disease characterized by relapsing eruptions of neutrophil-filled, sterile pustules on the palms and soles that can be clinically difficult to differentiate from non-pustular palmoplantar psoriasis (palmPP) and dyshidrotic palmoplantar eczema (DPE). OBJECTIVE:We sought to identify overlapping and unique PPP, palmPP, and DPE drivers to provide molecular insight into their pathogenesis. METHODS:We performed bulk RNA sequencing of lesional PPP (n = 33), palmPP (n = 5), and DPE (n = 28) samples, as well as 5 healthy nonacral and 10 healthy acral skin samples. RESULTS:Acral skin showed a unique immune environment, likely contributing to a unique niche for palmoplantar inflammatory diseases. Compared to healthy acral skin, PPP, palmPP, and DPE displayed a broad overlapping transcriptomic signature characterized by shared upregulation of proinflammatory cytokines (TNF, IL-36), chemokines, and T-cell-associated genes, along with unique disease features of each disease state, including enriched neutrophil processes in PPP and to a lesser extent in palmPP, and lipid antigen processing in DPE. Strikingly, unsupervised clustering and trajectory analyses demonstrated divergent inflammatory profiles within the 3 disease states. These identified putative key upstream immunologic switches, including eicosanoids, interferon responses, and neutrophil degranulation, contributing to disease heterogeneity. CONCLUSION:A molecular overlap exists between different inflammatory palmoplantar diseases that supersedes clinical and histologic assessment. This highlights the heterogeneity within each condition, suggesting limitations of current disease classification and the need to move toward a molecular classification of inflammatory acral diseases.

Systemic sclerosis (SSc) is a devastating autoimmune disease characterized by excessive production and accumulation of extracellular matrix, leading to fibrosis of skin and other internal organs. However, the main cellular participants in SSc skin fibrosis remain incompletely understood. Here using differentiation trajectories at a single cell level, we demonstrate a dual source of extracellular matrix deposition in SSc skin from both myofibroblasts and endothelial-to-mesenchymal-transitioning cells (EndoMT). We further define a central role of Hippo pathway effectors in differentiation and homeostasis of myofibroblast and EndoMT, respectively, and show that myofibroblasts and EndoMTs function as central communication hubs that drive key pro-fibrotic signaling pathways in SSc. Together, our data help characterize myofibroblast differentiation and EndoMT phenotypes in SSc skin, and hint that modulation of the Hippo pathway may contribute in reversing the pro-fibrotic phenotypes in myofibroblasts and EndoMTs.