Objectives: Periodontal disease is initiated from pathogenic bacteria-mediated uncoordinated inflammatory host response. However, whether commensals and pathogens follow different trajectories during their interactions with host cells is largely unknown. The present study aimed to examine the crosstalk of commensals and periodontopathogens in human oral keratinocytes (HOKs) and THP-1 monocytes.
Methods: The commensal oral bacteria including Streptococcus mutans (Sm), Streptococcus mitis (Smi) and Streptococcus sanguinis (Ss), and periodontopathogens including Aggregatibacter actinomycetemcomitans (Aa) and Porphyromonas gingivalis (Pg) were Selected. The interaction of bacteria with HOKs and THP-1 cells was analyzed in a co-culture model using a gradient of multiplicity of infection. The expression of IL-1b, IL-6, IL-8 and TNF-¦Á was assessed using ELISA and real-time qPCR. Cell surface receptors and signal transduction pathways were examined by western blots, and antibody-mediated blocking assays were undertaken for analysis of TLR2 and TLR4.
Results: The commensals upregulated the pro-inflammatory cytokines expression in both HOKs and THP-1 monocytes. In contrast, periodontopathogens induced higher levels of cytokines expression in HOKs, while in monocytes Pg down-regulated the TLR-mediated immuno-inflammatory response. THP-1 was more responsive to Smi and Ss in terms of IL-8 and TNF-¦Á production, and to Sm and Smi in terms of IL-1b than the controls and that treated with Pg (p<0.05). In HOKs, both Sm and Pg upregulated IL-6 and IL-8 expression as compared with the control (p<0.05). The cytokines expressions was through TLRs-mediated downstream pathways.
Conclusions : It seems that HOKs enable to detect the trajectories of oral commensal and pathogenic bacteria. However, once the periodontopathogens like Pg breach the epithelial barrier they may evade the host defense by paralysing the pro-inflammatory cytokine network of immune cells such as monocytes, thereby achieving proliferation in gingival tissues and contributing to periodontal disease.