All the time, it is a challenge for clinicians to make an accurate diagnosis about different levels of dental pulp inflammation, which is helpful to treatment planning. Since histological examinations show the extent of inflammation is not correlating with a clinical manifestation, old diagnosis approaches on account of symptoms, electrical or thermal pulp tests, should be improved[7, 18]. As new biological materials are developed, successful cases of vital pulp therapy are increasing, but district indication must be grasped.
During inflammation, cells in human dental pulp, including odontoblasts, macrophages, vascular endothelial cells, precursor cells and the other that can launch immune system, are secreting a lot of cytokines, chemokines and neuropeptides, of which have been reported to play critical roles in characteristics of inflammation[20, 21]. Sivakami et al found that the levels of IL-6 and cytokines interleukin-1β (IL-1β) clearly increased in saliva during pulpal and periapical inflammation. Compared with caries-exposed pulps, levels of IL-8 were significantly higher in irreversible pulpitis by detecting cytokine in pulpal blood. Some reported that several cytokines were expected as diagnostic markers of pulpal inflammation[12, 24]. However, up to now, to the best of our knowledge, no appropriate biomarkers with high degree of accuracy have been used in clinical examination to diagnose early pulpitis.
Researchers about microarray analysis are exploding due to the rapid development of transcriptomic studies, giving rise to more and more clarity of biological mechanisms of oral diseases. Through integration of several similar datasets, we can compose more biological significant expression profiles to identify key genes as biomarkers of pulpitis diagnosis and prognosis.
In the current study, expression data of pulpitis tissue from GEO database were integrated, then were used to identify underlying characteristics of DEGs as well as biomarker candidates for diagnosing dental pulp inflammation levels. 472 DEGs were screened and enrichment results from GO BP analysis manifested on inflammatory response, signal transduction and immune system, which demonstrate that the functions of DEGs are involved in processes of dental pulp inflammation. According to KEGG pathway analysis, some signaling pathways corelated with mechanisms of inflammation were enriched, containing cytokine-cytokine receptor interaction, osteoclast differentiation, chemokine signaling pathway, NF-κB signaling pathway and T cell receptor signaling pathway. Furthermore, GSEA results provided further functional annotations that complement system, IL6/JAK/STAT3 signaling pathway and inflammatory response pathways were possible mechanism of hub genes that induced inflammation.
As PPI network suggested, we also identified PTPRC, TLR8, CD86, IL6, CCL2, MMP9, CXCL8, SPI1, ICAM1 and LCP2 as hub genes of pulpitis. Some appropriate biomarkers are expected to become new methods for diagnosis and treatment in clinical practice.
Li et al reported DEGs and enrichment results by analyze data in GSE77459. Differently, we respectively extracted data from GSE77459 and GSE92681 and integrated the mRNA expression to remove batch effect and improve the quality of the data. Compared with them, common KEGG pathways were cytokine-cytokine receptor interaction and chemokine signaling pathway and common hub genes were IL6, CXCL8, PTPRC, CCL2 and ICAM1.
PTPRC encodes protein tyrosine phosphatase (PTP), known as signaling molecules that regulate various kind of cellular processes, playing a critical role in immune system. PTPRC can negatively regulate cytokine receptor signaling by suppressing JAK signaling pathway. Also, PTPRC expresses lowly or barely in normal dental pulp tissue, but highly in pulpitis tissue as indicated above. CD86 is the receptor that commonly participates in the processes of T-lymphocyte proliferation and IL-2 production, acting as a negative regulator for immune system. Miyuki Azuma et al studied the immune responses in mouse dental pulp, and founded that expression of CD86 were enhanced in dental pulp after cusp trimming, but disappeared within 2 h and migrated into regional lymph nodes at 24 h after acid treatment. As chemokines launch and boost the process of inflammation, an increased expression of CCL2 was observed in chronic periapical lesions, indicating the association between chemokines and dental pulp inflammation[31, 32] In addition, IL6, TLR8, CXCL8, MMP9, and ICAM1 are also associated with immunity and inflammation in dental pulp as reported[1, 33–37]