Decoding the role of macrophages in periodontitis and type 2 diabetes using single‐cell RNA‐sequencing

Macrophages are resident myeloid cells in the gingival tissue which control homeostasis and play a pivotal role in orchestrating the immune response in periodontitis. Cell heterogeneity and functional phenotypes of macrophage subpopulations in periodontitis remain elusive. Here, we isolated gingival tissue from periodontitis-affected and healthy sites of patients with and without type 2 diabetes mellitus (T2DM). We then used single-cell RNA-sequencing (scRNA-seq) to define the heterogeneity of tissue-resident macrophages in gingival tissue in health vs. periodontitis. scRNA-seq demonstrated an unforeseen gene expression heterogeneity among macrophages in periodontitis and showed transcriptional and signaling heterogeneity of identified subsets in an independent cohort of patients with periodontitis and T2DM. Our bioinformatic inferences indicated divergent expression profiles in macrophages driven by transcriptional regulators CIITA, RELA, RFX5, and RUNX2. Macrophages in periodontitis expressed both pro-inflammatory and anti-inflammatory markers and their polarization was not mutually exclusive. The majority of macrophages in periodontitis expressed the monocyte lineage marker CD14, indicating their bone marrow lineage. We also found high expression and activation of RELA, a subunit of the NF-κB transcription factor complex, in gingival macrophages of periodontitis patients with T2DM. Our data suggested that heterogeneity and hyperinflammatory activation of macrophages may be relevant to the pathogenesis and outcomes of periodontitis, and may be further augmented in patients with T2DM.