Abstract Background: Intratumoral microbiota can influence cancer progression through immune and metabolic interactions. Papillary thyroid cancer (PTC) frequently coexists with Hashimoto’s thyroiditis (HT), an autoimmune condition that remodels the tumor immune microenvironment. However, how microbe-derived molecules contribute to PTC metastasis, and how autoimmune cytokines modulate these effects, remains poorly understood. Methods: Tumor and matched adjacent tissues from 75 PTC patients were analyzed using 16S rRNA sequencing, and microbial functional pathways were inferred with PICRUSt2. The pro-metastatic activity of Pseudomonas-derived lipopolysaccharide (LPS) was evaluated using wound-healing and Transwell assays, with polymyxin B to confirm LPS specificity. NF-κB activation and EMT markers were analyzed by Western blotting. To model the autoimmune cytokine milieu of HT, PTC cells were treated with recombinant IFN-γ. For in vivo validation, LPS-stimulated PTC cells were injected intravenously into NCG mice to establish a spontaneous lung metastasis model. Results: Patients with lymph node metastasis (LNM) exhibited increased Pseudomonas abundance and enhanced predicted LPS biosynthetic activity. In contrast, HT+ tumors showed reduced Pseudomonas levels and diminished LPS pathway activity compared with HT- tumors. After adjusting for age and TNM stage, Pseudomonas abundance remained positively associated with LNM within the HT subgroup. Propensity score-matched mediation analysis further indicated that HT conferred a protective effect against LNM, whereas the pro-metastatic effect of Pseudomonas partially counteracted this protection. In vitro, Pseudomonas-derived LPS promoted PTC cell migration and invasion in a dose- and time-dependent manner through TLR4-NF-κB activation, accompanied by increased phosphorylated NF-κB and N-cadherin; these effects were substantially diminished by polymyxin B. Under an HT-like cytokine environment, IFN-γ markedly attenuated both LPS-induced NF-κB activation and invasive behavior. In vivo, mice receiving LPS-pretreated PTC cells developed significantly more pulmonary metastatic foci. Conclusions: Pseudomonas functions as a potential pro-metastatic bacterium in PTC with its LPS promoting PTC cell invasiveness through TLR4-NF-κB-EMT pathway. HT is associated with reduced Pseudomonas abundance, weakening this pro-metastatic signaling and contributing to the lower LNM risk observed in HT patients. These findings highlight the importance of microbial-immune interactions in PTC metastasis and suggest potential therapeutic targets for PTC prevention and treatment. Citation Format: Yue Che, Wei Kou, Haoran Feng, Qiwu Zhao, Zhuoran Liu, Jie Kuang, Weihua Qiu. Pseudomonas-derived LPS promotes metastatic behavior in papillary thyroid cancer metastasis is attenuated by Hashimoto’s thyroiditis abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 4894.
Che et al. (Fri,) studied this question.