Macrophage dynamics and the role of TREM2 at the oral mucosal barrier

Macrophages have a critical role in mucosal barrier immunity, demonstrating dynamic phenotypes that promote and resolve inflammation. However, the regulation of this polarization is not fully understood. Triggering receptor expressed on myeloid cells 2 (TREM2) is an immunoregulatory receptor expressed on macrophages. The role of TREM2 in mucosal immunity has not been previously studied. Our objective was to investigate TREM2 on macrophages during periodontal disease (PD), a chronic inflammatory condition affecting the oral mucosa.

PD was induced using a ligature method in wild-type (WT) and TREM2^-/- mice. Macrophages were isolated from oral mucosa and analyzed via flow cytometry. PD severity was assessed via quantification of bone loss and local inflammatory cytokine expression. Lastly, oral mucosal macrophages from WT mice and humans were analyzed via single-cell RNA sequencing (scRNA-seq).

We demonstrated TREM2 expression on oral mucosal macrophages, and the proportion of TREM2+ macrophages significantly increased during resolution of PD and decreased during active PD. Analysis of TREM2^-/- mice revealed a more severe PD phenotype with significantly increased tissue destruction and increased Il1b, Tnfa, and Il6 expression in the oral mucosa compared to WT. Transcriptional characterization of oral mucosal macrophages from mice and humans using scRNA-seq demonstrated subpopulations of TREM2+ macrophages characterized by similar gene signatures and pathways consistent with immunoregulatory functions.

This study improves our understanding of macrophage polarization and introduces TREM2 as an important immunoregulator in the oral mucosa.