Dendritic Cell Responses to a Polymicrobial Challenge with Oral Bacteria

Periodontitis is a polymicrobial disease. The polymicrobial challenge by subgingival biofilms results in a chronic inflammatory response in host tissues. Dendritic cells (DCs) are critical for antigen processing and presentation and link innate and adaptive immunity. Objective: This study used an in vitro model of immature DCs (iDC) to examine bacterial activation of the HIV promoter and cytokine production, particularly related to a polymicrobial challenge. Methods: THP-1 cells transfected with the HIV-1 LTR promoter driving expression of luciferase was treated with GM-CSF/IL-4 to differentiate into an iDC phenotype. Oral bacteria were used to stimulate these cells (eg. P. gingivalis, F. nucleatum, T. denticola, S. mutans, C. rectus, P. intermedia). Results: iDCs reacting to the microbial challenge indicated that an optimal dose of the bacteria was noted at 1x107 for Pg, Fn, Sm, and Cr with increased challenge levels showing decreased activity. In contrast, Pi demonstrated a broader range of stimulation. iDCs produced a high level of IL-6, IL-12, and TNFĄ after challenge with various oral bacteria, compared with basal levels. Interestingly, both Sm and Cr appeared to be the most broadly active in triggering the production of these cytokines by the iDCs. Challenge of the iDCs with a mixture of these bacteria resulted in different outcomes related to the species and relative amounts of each in the polymicrobial challenge. Thus, we observed minimal interactions, additive or synergistic effects, and negative interactions evaluating both HIV promoter activation and cytokine production by the iDCs. Conclusions: Differences in the magnitude of responses to oral bacteria by iDCs suggested different receptors and/or intracellular circuits are engaged for activation. This supports the hypothesis that bacteria related to periodontal infections can trigger latently HIV-infected iDCs, and that a polymicrobial challenge may elicit a different pattern of responses versus the individual species. Supported by P20 RR020145.