ABSTRACT: Oral squamous cell carcinoma (OSCC) claims the lives of thousands in the U.S. and hundreds of thousands worldwide annually. We have reported that an altered expression profile of hBD-3, an epithelial cell derived antimicrobial peptide (AMP) and hBD-2, another epithelial cell AMP, is an early event in OSCC, and that the ratio of hBD-3 and -2 in the lesion, when compared to the contralateral site, could be exploited in distinguishing OSCC from other lesions in the oral cavity. We refer to this ratio as the beta defensin index (BDI). Our ongoing clinical study of 81 subjects with oral lesions demonstrated high sensitivity (94%) and specificity (97%) of BDI in distinguishing cancerous from benign oral lesions (P<0.0001). There has been a surge in microbiome studies over the last several years in attempts to discover signature microbes with various cancers and their links to cancer progression, including OSCC. A common theme arising from these studies is that the genus Fusobacterium, and specifically the oral commensal species Fusobacterium nucleatum and F. periodonticum are enriched in OSCC when compared to contralateral normal sites in the same patient. F. nucleatum also has demonstrated tumorigenic properties and has been shown to induce tumor progression in an oral cancer murine model. We and others have shown that, while hBD-3 is a potent AMP with a broad spectrum of activity, there is differential resistance of various oral microorganisms to hBD-3. Most recently, we discovered that F. nucleatum demonstrates strain specific resistance to hBD-3 and have identified a unique strategy that these organisms use to protect themselves from electrostatic disruption of their outer membranes by hBDs. We now hypothesize that overexpression of hBD-3 in the context of OSCC promotes selective persistence of key pathobionts, such as hBD-3-resistant Fusobacterium. To address this hypothesis, our interdisciplinary team proposes the following two aims: Aim 1. Correlate BDI scores with the outcome of a 16S rRNA gene-based microbiome study in OSCC vs. contralateral sites and vs. non- OSCC lesions. Aim 2. Identify cultivable fusobacterial isolates resistant to hBD-3 in lesions vs. contralateral sites. Ours is the first study to pin-point a specific OSCC-derived agent; i.e., hBD-3, that can contribute to compositional shifting of the microbiota in cancerous vs. non-cancerous oral lesions. We surmise that high BDI scores will dictate the persistence of signature hBD3-resistant fusobacteria in OSCC lesions when compared to contralateral normal sites and non-OSCC lesions. Results obtained from this proposal will lead to future studies to (1) determine the utility of bacterial signatures as cancer biomarkers, (2) identify genomic determinants of hBD resistance and (3) decipher the organisms? pathogenic properties in promoting cancer progression.