Oral commensal/pathogenic bacteria-host cells crosstalk : immuno-inflammatory response, microenvironmental regulation and signaling mechanism

Abstract

Periodontal disease is a common infectious/inflammatory disease, and remains a major global oral disease burden. It is initiated by the shift of commensal plaque biofilms to pathogenic ones which results in aberrant host response and periodontal destruction. Currently, it remains unclear whether periodontal commensals and pathogens interact differentially with host cells. The present study investigated the profiles of immuno-inflammatory response, microenvironmental regulation and the underlying mechanisms in the crosstalk of human oral keratinocytes (HOKs) and THP-1 monocytes with selected periodontal commensals or pathogens.

Firstly, a standardized bacteria-HOKs interactive model was established, with dynamic bacteria-HOKs interactions in oral keratinocyte media without antibiotics at a mulplicity of infection of 1 for 2 h in an aerobic condition for optimal viability of both bacteria and HOKs (Chapter III). This protocol with defined cultural parameters was used to further investigate the immuno-inflammatory response to oral commensals and pathogens in HOKs. The expression of pro-inflammatory cytokines (IL-6 and IL-8) in HOKs was significantly upregulated by the periodontal commensal Streptococcus mutans (p<0.05) and periodontopathogen Porphyromonas gingivalis (p<0.01) as compared to the control. All the species tested could upregulate the expression of IL-6 and IL-8 from 15 min to 2 h (Chapter IV). Streptococcus mitis, Streptococcus sanguinis and P. gingivalis induced higher levels of human β-defensin (hBD)-2 in HOKs than the control (p<0.01). The P. gingivalis-HOKs interactive group exhibited a significantly higher level of hBD-2 as compared with that in the S. mitis-HOKs group at 30 or 60 min, through toll-like receptor (TLR) 2 and TLR4 pathways (Chapter V). With reference to HOKs, the expression profiles of pro-inflammatory cytokines in the bacteria-THP-1 groups varied considerably according to bacterial characteristics. S. mitis and S. saguinis significantly induced higher levels of IL-8 and IL-1βin THP-1 than the control through TLR2 and TLR4, while P. gingivalis failed to show such effects (Chapter VI). Interestingly, the interactions of HOKs with P. gingivalis after 2 and 18 h elicited less chemotactic attraction to THP-1 as compared with the commensal S. sanguinis, suggesting a possible underlying mechanism of P. gingivalis to evade host response (Chapter VII). Lastly, the hypothesis that P. gingivalis prior grown at 37℃, 39℃ and 41℃ differentially interact with THP-1 was examined. P. gingivalis prior grown at 37℃ significantly upregulated the expression of IL-8 and TNF-α proteins and mRNAs in THP-1, with reference to that prior grown at 41℃, in line with the expression profiles of NF-κB signaling molecules (Chapter VIII).

Taken together, this pioneering study demonstrates that periodontal commensals and pathogens may differentially modulate immuno-inflammatory response in HOKs and THP-1 monocytes through TLR2 and TLR4. Notably, the microenvironmental factors such as the increased temperature at periodontal niches in uncontrolled inflammation may significantly affect bacteria-host crosstalk and the resultant immuno-inflammatory response. Therefore, early and effective control of periodontal inflammation can contribute to the prevention and management of periodontal disease for optimal oral and general health.