Differential utilization of nuclear factor-kappaB signaling pathways for gingival epithelial cell responses to oral commensal and pathogenic bacteria.

INTRODUCTION: Human beta-defensin-2 (hBD-2) is an antimicrobial peptide, induced by bacterial stimuli and inflammation, that plays a role in mucosal and skin innate immune defense. The nuclear factor-kappaB (NF-kappaB) transcription factor family is important in innate and adaptive immune responses to bacteria and proinflammatory cytokines. NF-kappaB operates via the traditional IKKbeta signaling, as well as an alternative pathway utilizing IKKalpha signaling, which is important in keratinocyte differentiation. Our previous studies showed that pathogenic, but not commensal, bacteria used NF-kappaB signaling in hBD-2 induction. The objective of this study was to understand which arm of the NF-kappaB pathway is involved in gingival epithelial cell responses to pathogenic bacteria, including hBD-2 induction.

METHODS: Cultured oral epithelial cells were transfected with synthetic small interfering RNAs (siRNAs) specific for various steps in each pathway, namely IKKbeta, TRAF6 and MyD88 in the canonical, and IKKalpha and TRAF3 in the alternative pathway, and subsequently stimulated with various oral bacteria.

RESULTS: The hBD-2 induction level was reduced to 21-61% in cells in which the alternative NF-kappaB pathway was blocked and subsequently stimulated with pathogenic bacteria, while cells in which the canonical pathway was blocked showed reduction to 78-99%. Cells stimulated with commensals showed little change in hBD-2 induction level regardless of the siRNA used. Microarray analysis showed that oral epithelia differentially regulated numerous innate immune markers in response to pathogens and commensals.

CONCLUSION: Our data suggest a role for the IKKalpha/TRAF3 pathway in NF-kappaB activation by pathogenic bacteria, while commensal bacteria do not utilize either NF-kappaB pathway, for hBD-2 induction.