Acute immune responses to microbial insults in the oral cavity often progress to chronic inflammatory diseases such as periodontitis and apical periodontitis. Chronic oral inflammation causes destruction of the periodontium, potentially leading to loss of the dentition. Previous investigations have demonstrated that the composition of oral immune cells, rather than the overall extent of cellular infiltration, determines the pathological development of chronic inflammation. The role of T lymphocyte populations, including Th1, Th2, Th17, and Treg cells, has been extensively described. Studies now propose pathogenic Th17 cells as a distinct subset, uniquely classifiable from traditional Th17 populations. In situ differentiation of pathogenic Th17 cells has been verified as a source of destructive inflammation, which critically drives pathogenesis in chronic inflammatory diseases such as diabetes, rheumatoid arthritis, and inflammatory bowel disease. Pathogenic Th17 cells resemble a Th1 penotype and produce not only interleukin 17 (IL-17) but also γ-interferon (IFN-γ) and granulocyte-macrophage colony-stimulating factor (GM-CSF). The proinflammatory cytokine-specific mechanisms known to induce IL-17 expression in Th17 cells are well characterized; however, differentiation mechanisms that lead to pathogenic Th17 cells are less understood. Recently, Ca2+ signaling through Ca2+ release-activated Ca2+ channels (CRAC) in T cells has been uncovered as a major signaling axis involved in the regulation of T-cell-mediated chronic inflammation. In particular, pathogenic Th17 cell–mediated immunological diseases appear to be effectively targeted via such Ca2+ signaling pathways. Pathogenic plasticity of Th17 cells has been extensively illustrated in autoimmune and chronic inflammatory diseases. Although their specific causal relationship to oral infection-induced chronic inflammatory diseases is not fully established, pathogenic Th17 cells may be involved in the underlining mechanism. This review highlights the current understanding of T-cell phenotype regulation, calcium signaling pathways in this event, and the potential role of pathogenic Th17 cells in chronic inflammatory disorders of the oral cavity.

S.S. conceptualized the study, generated KO cells, and performed imaging experiments. B.W. performed all the molecular biology and flow cytometry experiments, and A.R. performed bioinformatics analysis of the RNA sequencing data with help from S.H. G.G. and V.A. generated the severe acute respiratory syndrome coronavirus 2 virus and performed all infection experiments. S.S. wrote the manuscript with input from all authors.The RNA sequencing data have been submitted to the National Center for Biotechnology Information Gene Expression Omnibus (https://www.ncbi.nlm.nih.gov/geo/query/acc. cgi?acc=GSE173707) under accession number GSE173707.