Immunotherapy Enhancement by Targeting Extracellular Tumor pH in Triple-Negative Breast Cancer Mouse Model

SIMPLE SUMMARY: Despite remarkable progress in immunotherapy, most advanced cancer patients still show intrinsic or naturally acquired resistance to immune checkpoint blockade strategies leading to treatment failure. Breast cancer produces lactate and protons in tumor cells, which are secreted to maintain intracellular homeostasis, causing extracellular pH (pH(e)) of the tumor microenvironment to become acidic. Immune cells are unfortunately unable to mount effective responses under these harsh conditions—thereby causing immunotherapy treatments to fail. Therefore, reducing tumor acidity can be effective in cancer immunotherapy. The role of neutralizing tumor acidosis in improving treatment response in triple negative breast cancer (TNBC) environment is not known and is thus the focus of our study. In this study, oral administration of either sodium bicarbonate or sodium bicarbonate plus anti-PD-L1 combination enhanced responses to anti-tumor immunity by tumor growth inhibition and improving survival time in TNBC. Our key finding is a pH-neutralizer for improving cancer immunotherapy. The combination of these therapies can provide exceptional impact by providing powerful personalized medicine paradigm. ABSTRACT: Triple-negative breast cancer (TNBC), as one of the most aggressive forms of breast cancer, is characterized by a poor prognosis and a very low rate of disease-free and overall survival. In recent years, immunotherapeutic approaches targeting T cell checkpoint molecules, such as cytotoxic lymphocyte antigen-4 (CTLA-4), programmed death1 (PD-1) or its ligand, programmed death ligand 1 (PD-L1), have shown great potential and have been used to treat various cancers as single therapies or in combination with other modalities. However, despite this remarkable progress, patients with TNBC have shown a low response rate to this approach, commonly developing resistance to immune checkpoint blockade, leading to treatment failure. Extracellular acidosis within the tumor microenvironment (also known as the Warburg effect) is one of the factors preventing immune cells from mounting effective responses and contributing to immunotherapy treatment failure. Therefore, reducing tumor acidity is important for increasing cancer immunotherapy effectiveness and this has yet to be realized in the TNBC environment. In this study, the oral administration of sodium bicarbonate (NaHCO(3)) enhanced the antitumor effect of anti-PD-L1 antibody treatment, as demonstrated by generated antitumor immunity, tumor growth inhibition and enhanced survival in 4T1-Luc breast cancer model. Here, we show that NaHCO(3) increased extracellular pH (pH(e)) in tumor tissues in vivo, an effect that was accompanied by an increase in T cell infiltration, T cell activation and IFN-γ, IL2 and IL12p40 mRNA expression in tumor tissues, as well as an increase in T cell activation in tumor-draining lymph nodes. Interestingly, these changes were further enhanced in response to combined NaHCO(3) + anti-PD-L1 therapy. In addition, the acidic extracellular conditions caused a significant increase in PD-L1 expression in vitro. Taken together, these results indicate that alkalizing therapy holds potential as a new tumor microenvironment immunomodulator and we hypothesize that NaHCO(3) can enhance the antitumor effects of anti-PD-L1 breast cancer therapy. The combination of these treatments may have an exceptional impact on future TNBC immunotherapeutic approaches by providing a powerful personalized medicine paradigm. Therefore, our findings have a great translational potential for improving outcomes in TNBC patients.