B111-12 Microbial Signatures Are Associated With Malignant Effusions and Lung Cancer Related Somatic Mutations

Introduction The microbiome has emerged as a potential contributor to cancer biology with evidence suggesting that host-microbial interactions may influence cancer development via modulation of the tumour micro-environment and host immunity. In cancer, the pleura is a common site of disease spread, however little is known about the pleural microbiome. This study aimed to characterise the pleural microbiome in those with malignant pleural effusions (MPE) and explore associations between the pleural microbial composition and somatic mutations within the pleural space Methods All patients presenting to Beaumont Hospital, Dublin, Ireland with undifferentiated pleural effusions over a 14-month period were prospectively recruited. Skin, background and pleural samples were collected for each participant. 16S rRNA gene sequencing was performed and data was analysed at the amplicon sequence variant level using the Shannon index, Bray-Curtis dissimilarity and DESeq2 to assess alpha-diversity, beta-diversity and taxonomic enrichment, respectively. In a subset of 26 individuals with lung cancer-related effusions, targeted next-generation sequencing was performed on pleural supernatant cfDNA to assess association between pleural taxa and somatic mutations Results A total of 105 individuals were recruited: 51 with benign effusions, 31 with MPE and 23 with cytologically negative malignant effusions (CNMPE). No significant difference in alpha or beta-diversity was observed between the three groups. After excluding cases with pleural infection (n = 15), non-infectious benign effusions remained enriched with Alcinovorax relative to MPE and Sphingomnoas Yabuchieae, Crypotophyta and Halomonas when compared to CNMPE.In contrast, MPE samples were enriched with Bacillales relative to both CNMPE and benign effusions, while CNMPE demonstrated no significant taxonomic enrichment. (Figure 1A) Among lung cancer-related cases, individuals harbouring somatic mutations exhibited lower alpha-diversity and significantly different beta-diversity compared with mutation-negative individuals. (Figure 1B+1C) Enrichment analysis showed that mutation-positive samples were enriched with Lactobacillus. Analysis of specific mutations revealed that those with a KRAS mutation were enriched with Sphingomonadales and Bacillales. Those with TP53 were enriched with Hydrogenophaga whereas those with EGFR mutation showed no enrichment. Those with a PIK3CA mutation were enriched with a multitude of taxa including Akkermansia Muciniphilia, Bacillales, Enterococccus, and Ruminococcus Gnavus. (Figure 1D) Conclusion The pleural microbiome appears to differ across effusion subtypes and may be linked to specific somatic mutations in lung cancer. These findings suggest potential diagnostic and mechanistic roles for pleural microbiota in MPE. Larger studies integrating microbial-functional analyses as well as host response are warranted to validate these associations and clarify underlying biological interactions. This abstract is funded by: StAR MD programme, CHEST