Modulation of Innate Epithelial Cell Responses by Oral Commensal Bacteria

The oral mucosa becomes continuously exposed to a wide array of bacterial taxa, genera, and species including commensal and pathogenic bacteria immediately after birth. However, the mechanisms through which a persistent recognition of oral commensal bacteria by oral epithelial cells (OECs) does not lead to an uncontrolled inflammatory response of the oral mucosa remain unknown. Evidence suggests that epithelial cells may discriminate between commensals and pathogenic bacteria through differential activation of responses, particularly cytokine/chemokine patterns, which ultimately influence both innate and adaptive arms of the immune response. Recent work in our laboratory has been identifying patterns of OECs responses following challenge with commensal and pathogenic oral bacteria. The results demonstrated a significantly more robust (quality and quantity) chemokine transcriptional activity in response to the oral commensal S. gordonii (Sg) compared to the outcomes following challenge with the oral pathogens F. nucleatum (Fn) and P. gingivalis. Subsequent protein analysis for a selected group of chemokines showed that there was a clear disconnect between the strong chemokine transcriptional activation induced by Sg and the reduced and limited protein levels found in OEC extracts and supernatants in comparison with the effect of Fn, which despite inducing lower mRNA chemokines levels (2 to 4-fold less) enhanced a robust protein chemokine production as previously shown. Therefore, we hypothesized that Sg and perhaps other oral commensal bacterial species have the ability to efficiently activate regulatory mechanisms for chemokine transcription and translation in order to minimize chronic pathological inflammatory responses driven by OECs in mucosal tissues. Consistently, preliminary analysis showed that Sg has the ability to significantly up-regulate (>2-fold) the expression of 115/2578 miRNAs in OECs, whereby miR-663a, miR-4516, miR492, and miR193a-5p have validated gene targets involved in TLR- and cytokine-induced chemokine transcription and translation. Importantly, variations or loss of these post-transcriptional regulatory mechanisms in OECs upon engagement of oral commensals could be related to pathologic inflammatory changes observed in disease. To test this hypothesis we propose the following two specific aims: (i) To determine the chemokine transcriptional and translational responses of OECs to oral commensal bacteria, and (ii) To determine the role of specific miRNAs in regulating the chemokine production induced by oral commensal bacteria in OECs. To address these knowledge gaps, we will use oral epithelial cell cultures to: (a) determine whether the disconnect between transcription and translation of chemokines in OECs is common feature of oral commensal bacterial species, and (b) determine the effect of oral commensals on the expression of specific miRNAs as regulators of these inflammatory molecules by OECs. The contribution of this investigation is expected to be the identification of the active regulation of inflammatory responses in OECs through miRNAs as a potential “tolerogenic mechanism” by which oral commensal bacteria maintain symbiosis with the host. This project’s significance will enable a better understanding of the cellular and molecular mechanisms involved in the “tipping point” between epithelial tolerogenic and pathologic responses to oral bacteria. The results are expected to contribute to a strong evidence-based foundation for future studies designed to target these mechanisms and identify new molecular target(s) associated with oral bacteria-regulated epithelial cytokine/chemokine production, and will ultimately provide new opportunities for the development of innovative approaches to prevent/treat periodontitis.