AbstractIntroductionTriple‐negative breast cancer (TNBC) patients have the poorest clinical outcomes compared to other molecular subtypes of breast cancer. IL6/JAK/STAT3 signalling is upregulated in breast cancer; however, there is limited evidence for its role in TNBC. This study aimed to assess the expression of IL6/JAK/STAT3 in TNBC as a prognostic biomarker.MethodsTissue microarrays consisting of breast cancer specimens from a retrospective cohort (n= 850) were stained for IL6R, JAK1, JAK2 and STAT3 via immunohistochemistry. Staining intensity was assessed by weighted histoscore and analysed for association with survival/clinical characteristics. In a subset of patients (n= 14) bulk transcriptional profiling was performed using TempO‐Seq. Nanostring GeoMx® digital spatial profiling was utilised to establish the differential spatial gene expression in high STAT3 tumours.ResultsIn TNBC patients, high expression of stromal STAT3 was associated with reduced cancer‐specific survival (HR = 2.202, 95% CI: 1.148–4.224, log rankp= 0.018). TNBC patients with high stromal STAT3 had reduced CD4+T‐cell infiltrates within the tumour (p= 0.001) and higher tumour budding (p= 0.003). Gene set enrichment analysis (GSEA) of bulk RNA sequencing showed high stromal STAT3 tumours were characterised by enrichment of IFNγ, upregulation of KRAS signalling and inflammatory signalling Hallmark pathways. GeoMx™ spatial profiling showed high stromal STAT3 samples. Pan cytokeratin (panCK)‐negative regions were enriched for CD27 (p< 0.001), CD3 (p< 0.05) and CD8 (p< 0.001). In panCK‐positive regions, high stromal STAT3 regions had higher expression of VEGFA (p< 0.05).ConclusionHigh expression of IL6/JAK/STAT3 proteins was associated with poor prognosis and characterised by distinct underlying biology in TNBC.