[Deciphering the fate of granulation tissue in the human inflammatory alveolar microenvironment using single-cell RNA...

Objective: To investigate the cellular composition and heterogeneity of granulation tissue in human alveolar sockets and construct single-cell transcriptomic maps to elucidate the potential outcomes of natural resolution in the inflammatory microenvironment. Methods: Granulation tissues from 12 alveolar sockets undergoing tooth extraction due to periodontitis and scheduled for delayed site preservation or autologous tooth transplantation were collected in the Department of Oral and Maxillofacial Surgery, School of Stomatology,The Fourth Military Medical University from September 2022 to August 2023. This study used HE staining and single-cell RNA sequencing (scRNA-seq) to observe the cellular composition and morphological changes of different types of granulation tissues. This approach enabled us to identify cellular sequence variations in the inflammatory dental alveolar granulation tissue within specific microenvironments, construct single-cell atlases for different types of human dental alveolar granulation tissues, and explore the spatiotemporal patterns of cell type distribution and key gene changes during the resolution process of inflammatory granulation tissue. Results: HE staining revealed extensive infiltration of inflammatory cells in the dental alveolar inflammatory granulation tissue. After allowing the inflammatory granulation tissue to naturally resolve for three weeks, its histological morphology was essentially consistent with that of reparative granulation tissue. ScRNA-seq captured a total of 20 448 cells from granulation tissues, and the gene expression quantification analysis revealed total gene counts of 33 835 for inflammatory granulation tissue, 36 058 for naturally resolved granulation tissue, and 34 719 for reparative granulation tissue. At the single-cell level, granulation tissue was annotated into ten cell subgroups, including vascular endothelial cells, mesenchymal stem cells, and fibroblasts. Differences were observed in the proportion of cell compositions between inflammatory and naturally resolved granulation tissues. Pseudo-temporal analysis illustrated two main outcomes in tissue resolution and healing, involving immune responses and angiogenesis. Among these, genes associated with inflammation regulation and immune response, such as Igbp5, Zfp36, and Hspa1b, as well as genes involved in extracellular matrix secretion and the formation of vessels and fibers such as Timp3, Postn, and Rgs5, showed significant differences in expression between the two types of granulation tissues. Conclusions: Inflammatory granulation tissue exhibits heterogeneity in cell composition, gene expression, and biological functions compared to naturally resolved granulation tissue. When the inflammatory granulation tissue in the alveolar socket is left undisturbed to undergo natural resolution, it displays a cellular composition similar to that of reparative granulation tissue at both the histological and single-cell levels. Moreover, it modulates the inflammatory response and the healing process through immune reactions and tissue remodeling.