Abstract Lung cancer is the leading cause of cancer death. Surgery remains essential for cure. However, postoperative infections particularly pneumonia remain high and confer an increased risk for metastasis. Emerging evidence implicates Toll-like receptor (TLR) signalling in tumor progression after systemic infection. We hypothesize bacterial pneumonia increases lung cancer cell adhesion and metastasis and that these effects are mediated in part by TLR signalling. C57BL6 mice were intranasally inoculated with 10^5 CFU/mL of S. pneumonia and intrasplenically injected with H59 murine lung carcinoma 3 days later. Gross liver metastases were quantified at 2 weeks. Similarly, H59 cell adhesion to liver sinusoids was assessed by intravital microscopy (IVM) in wild type (WT) and TLR2 KO and TLR4 KO mice. WT, TLR2 KO, TLR4 KO, and MyD88 KO mice were intratracheally injected with lipoteichoic acid (LTA), lipopolysaccharide (LPS), or control (PBS). H59 cell adhesion was assessed 4 hours later with liver IVM. Using an in vitro pneumonia model, bronchoepithelial BEAS2B cells were stimulated with LPS, LTA, heat-inactivated E. coli or heat-inactivated S. pneumonia and supernatants collected. H59 cells were exposed to various supernatants and adhesion to extracellular matrix components was assessed. BEAS2B cells were also stimulated in the presence of anti-TLR2 antibodies or eritoran tetrasodium to inhibit TLR2 and TLR4 signalling respectively. Adhesion of supernatant treated cells to liver sinusoids was also assessed with IVM. Mice inoculated with S. pneumonia demonstrated a 2-3 fold increase in gross liver metastases compared to non-infected controls. Similarly WT and TLR4 KO mice infected with S. pneumonia demonstrated a 4-5 fold increase in H59 adhesion to liver sinusoids versus control. TLR2 KO mice infected with S. pneumonia demonstrated no significant increase in H59 cell adhesion to liver. WT mice intratracheally injected with LTA or LPS demonstrated a 3-4 fold increase in cancer cell adhesion to liver compared to control. The increased adhesion for LTA was negated in TLR2 KO and MyD88 KO mice. For LPS conditions this increase was absent in TLR4 KO and MyD88 KO mice. Incubation with TLR-activated BEAS2B supernatants increased H59 cell adhesion to collagen 1, 4 and fibronectin 2-3 , 3-6 and 4-6 fold respectively versus media control and adhesion to liver sinusoids in vivo increased 3-4 fold. These effects were abrogated by blocking TLR2 for LTA or S. pneumonia conditions and by blocking TLR4 for LPS or E. coli conditions. Bacterial pneumonia increases the metastatic potential of circulating lung cancer cells. These effects are facilitated in part by TLR2 or TLR4 activation and are mediated via interactions of the respiratory epithelium with the host systemically and lung cancer cells directly. TLR2 and TLR4 are potential therapeutic targets to decrease cancer recurrence in patients who suffer severe post-operative infections. Citation Format: Stephen D. Gowing, Simon C. Chow, Jonathan J. Cools-Lartigue, Crystal B. Chen, Betty Giannias, France Bourdeau, Simon Rousseau, Salman T. Qureshi, Lorenzo E. Ferri. Toll-like receptor activation in bacterial pneumonia increases lung cancer cell adhesion and metastasis formation. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 1963. doi:10.1158/1538-7445.AM2014-1963