Studies on myeloid cell functions in periodontitis

Abstract

Periodontitis (PD) is a chronic inflammatory disease characterized by destruction of the tooth supporting tissues; gingiva, periodontal ligament, and alveolar bone. Myeloid cells, including monocytes and macrophages, play a crucial part in the inflammatory host response by clearing pathogens and producing a vast array of inflammatory mediators. However, during chronic inflammation, the actions of these cells can contribute to tissue degradation. By utilizing clinical samples and different cell culture systems, this thesis aimed to investigate the role of monocytes and macrophages, along with factors that regulate them in PD.

In study I, we found elevated gene and protein expression of MMP-12 in PD gingival tissue and the expression was mainly attributed to CD68+CD64+CD14+ monocyte-derived cells. These cells displayed low expression of the co- stimulatory molecule CD200R. Induced inflammation in oral tissue cultures reduced the CD200R and increased the MMP-12 expression in monocyte-derived cells. CSF-2 was found to potently induce MMP-12 and a CD200 ligand reduced MMP-12 production in monocyte-derived cells, suggesting that the CD200/CD200R pathway is a potential target to modulate harmful inflammatory processes in PD. In study II, the expression of S100A12 was found to be high in monocytes, decreased during macrophage differentiation, and unaltered upon macrophage polarization. Analysis of S100A12 in different monocyte subsets revealed that classical monocytes had the highest expression followed by the intermediate and non-classical subsets. Peripheral blood monocytes from PD patients were more frequently positive for S100A12 and showed higher secretion in vitro compared with controls. The importance of monocytes for S100A12 production was demonstrated in oral tissue cultures. The protein expression of S100A12 was increased in gingival tissue along with higher frequencies of S100A12+ monocyte-derived cells. S100A12 levels in saliva reflected the severity of PD, suggesting A100A12 as a potential biomarker for disease.

The expression of the macrophage growth factors CSF-1 and IL-34 in gingival tissue and gingival fibroblasts (GFs) was investigated in study III. Elevated CSF-1 protein expression was found in gingival tissue from PD patients while the levels of IL-34 were similar in PD and control tissue. CSF-1 and IL-34 were expressed and produced by GFs constitutively and proinflammatory stimuli induced their secretion. CSF-1 and IL-34 secretion did not differ in GFs isolated from PD patients compared with controls in unstimulated condition or in response to TNF-α or IL-1β stimuli. In study IV, we found elevated expression of CSF-1R in gingiva from PD patients. HLADR+CD64+ macrophages displayed similar frequencies in PD and controls with no difference regarding CSF-1R expression. In peripheral blood mononuclear cells, CSF-1R inhibition did not alter the monocyte subsets or the expression of the myeloid markers CD64, CD206 or CD163. However, CSF-1R blockade attenuated MMP production in PD gingival explants. Thus, CSF-1R may contribute to the inflammatory processes leading to tissue degradation in PD.

In summary, these studies investigated the involvement of monocytes and macrophages in PD as well as the expression of factors that regulate their functions. Furthermore, strategies to target tissue-destructive myeloid cell related processes were explored.