Publications by Author: Alexander A Merleev

OBJECTIVE: This study was undertaken to identify the mechanistic role of γδ T cells in the pathogenesis of experimental psoriatic arthritis (PsA).

METHODS: In this study, we performed interleukin-23 (IL-23) gene transfer in wild-type (WT) and T cell receptor δ-deficient (TCRδ-/- ) mice and conducted tissue phenotyping in the joint, skin, and nails to characterize the inflammatory infiltrate. We further performed detailed flow cytometry, immunofluorescence staining, RNA sequencing, T cell repertoire analysis, and in vitro T cell polarization assays to identify regulatory mechanisms of γδ T cells.

RESULTS: We demonstrated that γδ T cells support systemic granulopoiesis, which is critical for murine PsA-like pathology. Briefly, γδ T cell ablation inhibited the expression of neutrophil chemokines CXCL1 and CXCL2 and neutrophil CD11b+Ly6G+ accumulation in the aforementioned PsA-related tissues. Although significantly reduced expression of granulocyte-macrophage colony-stimulating factor (GM-CSF) and IL-17A was detected systemically in TCRδ-/- mice, no GM-CSF+/IL-17A+ γδ T cells were detected locally in the inflamed skin or bone marrow in WT mice. Our data showed that nonresident γδ T cells regulate the expansion of an CD11b+Ly6G+ neutrophil population and their recruitment to joint and skin tissues, where they develop hallmark pathologic features of human PsA.

CONCLUSION: Our findings do not support the notion that tissue-resident γδ T cells initiate the disease but demonstrate a novel role of γδ T cells in neutrophil regulation that can be exploited therapeutically in PsA patients.

ERAP1, ERAP2, and LNPEP are aminopeptidases implicated in autoimmune pathophysiology. In this study, we show that ERAP2 is upregulated and ERAP1 is downregulated in patients with psoriasis who are homozygous for autoimmune-linked variants of ERAP. We also demonstrate that aminopeptidase expression is not uniform in the skin. Specifically, the intracellular antigen-processing aminopeptidases ERAP1 and ERAP2 are strongly expressed in basal and early spinous layer keratinocytes, whereas granular layer keratinocytes expressed predominantly LNPEP, an aminopeptidase specialized in the processing of extracellular antigens for presentation to T cells. In psoriasis, basal keratinocytes also expressed the T-cell- and monocyte-attracting chemokine, CCL2, and the T-cell-supporting cytokine, IL-15. In contrast, TGF-β1 was the major cytokine expressed by healthy control basal keratinocytes. SFRP2-high dermal fibroblasts were also noted to have an ERAP2-high expression phenotype and elevated HLA-C. In psoriasis, the SFRP2-high fibroblast subpopulation also expressed elevated CXCL14. From these results, we postulate that (i) an increased ERAP2/ERAP1 ratio results in altered antigen processing, a potential mechanism by which ERAP risk alleles predispose individuals to autoimmunity; (ii) ERAP2-high expressing cells display a unique major histocompatibility complex-bound peptidome generated from intracellular antigens; and (iii) the granular layer peptidome is skewed toward extracellular antigens.

The epidermis is the outermost layer of skin. Here, we used targeted lipid profiling to characterize the biogeographic alterations of human epidermal lipids across 12 anatomically distinct body sites, and we used single-cell RNA-Seq to compare keratinocyte gene expression at acral and nonacral sites. We demonstrate that acral skin has low expression of EOS acyl-ceramides and the genes involved in their synthesis, as well as low expression of genes involved in filaggrin and keratin citrullination (PADI1 and PADI3) and corneodesmosome degradation, changes that are consistent with increased corneocyte retention. Several overarching principles governing epidermal lipid expression were also noted. For example, there was a strong negative correlation between the expression of 18-carbon and 22-carbon sphingoid base ceramides. Disease-specific alterations in epidermal lipid gene expression and their corresponding alterations to the epidermal lipidome were characterized. Lipid biomarkers with diagnostic utility for inflammatory and precancerous conditions were identified, and a 2-analyte diagnostic model of psoriasis was constructed using a step-forward algorithm. Finally, gene coexpression analysis revealed a strong connection between lipid and immune gene expression. This work highlights (a) mechanisms by which the epidermis is uniquely adapted for the specific environmental insults encountered at different body surfaces and (b) how inflammation-associated alterations in gene expression affect the epidermal lipidome.

IL-23-activation of IL-17 producing T cells is involved in many rheumatic diseases. Herein, we investigate the role of IL-23 in the activation of myeloid cell subsets that contribute to skin inflammation in mice and man. IL-23 gene transfer in WT, IL-23RGFP reporter mice and subsequent analysis with spectral cytometry show that IL-23 regulates early innate immune events by inducing the expansion of a myeloid MDL1+CD11b+Ly6G+ population that dictates epidermal hyperplasia, acanthosis, and parakeratosis; hallmark pathologic features of psoriasis. Genetic ablation of MDL-1, a major PU.1 transcriptional target during myeloid differentiation exclusively expressed in myeloid cells, completely prevents IL-23-pathology. Moreover, we show that IL-23-induced myeloid subsets are also capable of producing IL-17A and IL-23R+MDL1+ cells are present in the involved skin of psoriasis patients and gene expression correlations between IL-23 and MDL-1 have been validated in multiple patient cohorts. Collectively, our data demonstrate a novel role of IL-23 in MDL-1-myelopoiesis that is responsible for skin inflammation and related pathologies. Our data open a new avenue of investigations regarding the role of IL-23 in the activation of myeloid immunoreceptors and their role in autoimmunity.