© 2020, The Author(s), under exclusive licence to Cancer Research UK. Background: Low pH suppresses the proliferation and cytotoxic activity of CD8+ cytotoxic and natural killer lymphocytes. The hypoxia-regulated transmembrane protein, carbonic anhydrase CA9, converts carbon dioxide produced by the Krebs cycle to bicarbonate and protons that acidify the extracellular milieu. We examined whether CA9 is also involved in intratumoural immunosuppression pathways. Methods: A series of 98 tissue samples of primary non-small-cell lung carcinomas (NSCLC) from patients treated with surgery were analysed for the expression of CA9 and programmed-death ligand PD-L1 by cancer cells, and of FOXP3 by tumour-infiltrating lymphocytes (TILs). Results: There was no direct association of CA9 with PD-L1 expression or the density of TILs in the tumour stroma, but CA9 was directly related to the extent of FOXP3+ TIL density (p = 0.008). Double-stratification survival analysis showed that patients with high CA9 expression and low TIL score had significantly poorer survival compared with all other groups (p < 0.04). In a multivariate analysis stage (p < 0.0001, HR 1.95, 95% CI: 1.3–2.7), TIL score (p = 0.05, HR 0.55, 95% CI: 0.2–1.0) was an independent prognostic variable of death events. CA9 expression by cancer cells is associated significantly with FOXP3+ regulatory T-cell abundance in the tumour stroma of NSCLC. Conclusion: The study provides a basis for testing CA9 as a marker of resistance to immune-checkpoint inhibitors and as a therapeutic target to enhance the efficacy of immunotherapy.
British Journal of Cancer