The neonatal Fc receptor (FcRn) for IgG has been shown to be responsible for IgG transport and to be involved in IgG catabolism. In this study, we show expression of FcRn in normal human epidermal keratinocytes. By RT-PCR, we demonstrate the FcRn alpha-chain mRNA obtained from cultured keratinocytes creating a 457 bp product as confirmed by sequence analysis. Northern blot analysis shows a 1.5 kb transcript. Real-time PCR reveals consistent expression of FcRn alpha-chain mRNA in human keratinocytes from different donors. Anti-FcRn alpha2-extracellular domain and anti-FcRn cytoplasmic tail antibody (Ab) directed against defined antigenic targets were generated and used for immunoblotting and immunoprecipitation revealing protein expression of the 46 kDa FcRn alpha-chain. By immunofluorescence microscopy, we find granular-vesicular staining for FcRn alpha-chain in keratinocytes. Fluorescence-activated cell sorting analysis gives predominantly an intracellular distribution of FcRn in keratinocytes. Biochemically, we demonstrate Fc-dependent binding of human IgG at acidic pH. In normal human epidermis, we find a cytoplasmic vesicular staining of predominantly basal and suprabasal keratinocytes. In summary, we demonstrate expression of a functional FcRn in normal human epidermal keratinocytes. These findings further emphasize the role of keratinocytes as immunomodulating cells in inflammatory and immunologic processes of the skin.

Emigration of tissue-resident memory T cells (TRMs) was recently introduced in mouse models and may drive systemic inflammation. Skin TRMs of patients undergoing allogeneic hematopoietic stem cell transplantation (HSCT) can coexist beside donor T cells, offering a unique human model system to study T cell migration. By genotyping, mathematical modeling, single-cell transcriptomics, and functional analysis of patient blood and skin T cells, we detected a small consistent population of circulating skin-derived T cells with a TRM phenotype (cTRMs) in the blood and unveil their skin origin and striking resemblance to skin TRMs. Blood from patients with active graft-versus-host disease (GVHD) contains elevated numbers of host cTRMs producing pro-inflammatory Th2/Th17 cytokines and mediating keratinocyte damage. Expression of gut-homing receptors and the occurrence of cTRMs in gastrointestinal GVHD lesions emphasize their potential to reseed and propagate inflammation in distant organs. Collectively, we describe a distinct circulating T cell population mirroring skin inflammation, which could serve as a biomarker or therapeutic target in GVHD.

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Retinoids are important modulators for cell growth and differentiation of normal skin. In plasma, retinol is transported coupled to plasma retinol-binding protein. In this study, we investigated gene and protein expression of RPE65, a putative receptor for plasma retinol-binding protein in human epidermal keratinocytes. We performed real-time PCR analysis to evaluate expression of RPE65 mRNA in proliferating and differentiating keratinocytes. Immunoblotting with anti-RPE65 antibody shows distinct reactivity to a 61-kDa protein. Indirect immunofluorescence on normal human epidermis reveals cell surface labeling of keratinocytes. Laser scan microscopy exhibits colocalization of plasma retinol-binding protein and RPE65 on cultured keratinocytes. Internalization experiments with [3H]retinoic acid-retinol-binding protein complex in the presence and absence of excess of retinol-binding protein indicates receptor-dependent uptake of retinoids. We further show isolation of RPE65 protein by affinity chromatography from lysates of keratinocytes using a retinol-binding protein-matrix gel column. In summary, we demonstrate mRNA and protein expression of RPE65 in epidermal keratinocytes. Colocalization of plasma retinol-binding protein with RPE65 and affinity binding suggest a direct interaction of RPE65 with plasma retinol-binding protein in cultured human keratinocytes that might be involved in retinoid uptake of keratinocytes.

The FDA-approved BRAF inhibitor vemurafenib achieves outstanding clinical response rates in patients with melanoma, but early resistance is common. Understanding the pathologic mechanisms of drug resistance and identification of effective therapeutic alternatives are key scientific challenges in the melanoma setting. Using proteomic techniques, including shotgun analysis and 2D-gel electrophoresis, we identified a comprehensive signature of the vemurafenib-resistant M24met in comparison with the vemurafenib-sensitive A375 melanoma cell line. The resistant cells were characterized by loss of differentiation, induction of transformation, enhanced expression of the lysosomal compartment, increased potential for metastasis, migration, adherence and Ca2(+) ion binding, enhanced expression of the MAPK pathway and extracellular matrix proteins, and epithelial-mesenchymal transformation. The main features were verified by shotgun analysis with QEXACTIVE orbitrap MS, electron microscopy, lysosomal staining, Western blotting, and adherence assay in a VM-1 melanoma cell line with acquired vemurafenib resistance. On the basis of the resistance profile, we were able to successfully predict that a novel resveratrol-derived COX-2 inhibitor, M8, would be active against the vemurafenib-resistant but not the vemurafenib-sensitive melanoma cells. Using high-throughput methods for cell line and drug characterization may thus offer a new way to identify key features of vemurafenib resistance, facilitating the design of effective rational therapeutic alternatives.