Necrobiosis is a histologic term used to describe abnormal deposits of "degenerating" collagen within the skin. It can be found as an incidental finding in various granulomatous conditions, but is a hallmark of necrobiosis lipoidica (NL) and necrobiotic xanthogranuloma (NXG). There is limited prior research on necrobiosis. Here, we employed single-cell analysis of lesional and nonlesional skin to study the pathophysiology of necrobiosis. Our findings demonstrate that necrobiotic lesional skin is characterized by SPP1hi macrophages expressing MARCO
NKG7-expressing effector CD8+ T cells coexpressing CCL5, IFNG, GZMs, and PRF1
CCL5hi fibroblasts coexpressing CXCL9, diverse collagens (e.g., COL4A4, COL11A1, COL8A1), and TIMP1
and IGHM-expressing plasma cells. Integrative analysis of signaling ligands and receptor expression identified strong cell-cell communication between NKG7+ T cells, CCL5hi fibroblasts, and SPP1-expressing macrophages. In contrast, these cell populations were not dominant features of systemic sclerosis, another collagen deposition disease. Furthermore, although SPP1-expressing macrophages were detectable in sarcoidosis, IFNG-expressing T cells were a more defining feature of sarcoidosis compared with NL and NXG. From these findings, we speculate that necrobiosis results from the deposition of diverse collagens and ECM proteins through a process driven by CCL5-expressing fibroblasts and SPP1-expressing macrophages.