Background/Purpose: The etiology and pathogenesis of systemic sclerosis (SSc) are poorly understood; however, an increasing body of evidence supports an early inflammatory phase that precedes fibrosis. Macrophages have been implicated in the inflammatory process that leads to the reorganization of the extracellular matrix (ECM) leading to the development of end-organ fibrosis. Macrophages also contribute to disease progression by production of reactive oxygen species, matrix-degrading enzymes, as well as by antigen presentation. However, macrophages are heterogeneous thus individual subsets may have differential functions in a tissue- and disease-specific context. Here we evaluate the transcriptional heterogeneity of macrophages present in the skin of SSc patients.

Methods: CD45⁺ immune cells were sorted from skin biopsies using multiparameter fluorescence-activated cell sorting (FACS) from early diffuse cutaneous (dc) SSc patients (630 cells from 5 patients). Cellular Indexing of Transcriptomes and Epitopes by Sequencing (CITE-Seq) was performed, and samples were processed using the 10X Genomics 3’ v3.0 pipeline and analyzed using the Seurat package with modifications when needed.

Results: Six individual clusters were resolved from immune cells isolated from SSc skin. Differentially expressed genes between clusters identified at least two distinct macrophage subsets that comprise one-third of all immune cells, including a monocyte-derived macrophage subset (LYV⁺ CD68⁺; 20% of all cells) and Langerhans cells (CD1a⁺ CD207⁺; 13% of all cells) that are associated with distinct pathways. Macrophage subsets identified by RNA expression were confirmed via antibody-derived tags and show heterogeneity in their gene and protein expression of CD11b, CD11c, and CCR2, among other key identifying features, that directly relate to their functional capabilities. These macrophage subsets also displayed differences in their expression of HLA-antigens, indicating distinct antigen presentation capabilities.

Conclusion: These data show for the first time that macrophages within fibrotic skin of patients with SSc come in many flavors corresponding to distinct functions. This highlights the importance of precisely identifying immune cells within an affected organ and their corresponding mechanisms contributing to the pathogenesis of SSc. Only then can we target specific immune cell populations that drive the inflammatory process, which may be more beneficial than the current approach of broadly targeting immune cells during disease progression. Future directions include interrogating how these macrophage subsets differ compared to those found in the skin of control patients as well as delving into potentially pathogenic lymphocytic populations.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/investigating-the-heterogeneity-of-skin-macrophages-in-systemic-scleroderma/