Proton pump inhibitor attenutes acidic microenvironment to improve the therapeutic effects of MSLN-CAR-T cells on the brain metastasis of solid tumors.

The incidence of brain metastasis (BM) is gradually increasing, and the prognosis and therapeutic effect are poor. The emergence of immunotherapy has brought hope for the development of BM treatments. This study revealed that compared with primary cancers (PCs), BMs have a "colder" and more acidic tumor microenvironment (TME), resulting in reduced protein levels of mesothelin (MSLN), a promising target for chimeric antigen receptor-T (CAR-T) cell therapy for triple-negative breast cancer (TNBC) with BMs. These factors could significantly decrease the efficiency of MSLN-CAR-T cells in TNBC BMs. Pantoprazole (PPZ) administration at the most commonly used dose in the clinic notably increased the pondus hydrogenii (pH) of the TME, inhibited lysosomal activity, increased the membrane levels of the MSLN protein and improved the killing ability of MSLN-CAR-T cells both in vitro and in vivo. Similar results were obtained in non-small cell lung cancer (NSCLC) BMs. Hence, when administered in combination with CAR-T cells, PPZ, which increases the protein levels of target antigens, may constitute a new immunotherapeutic strategy for treating solid tumors with BMs.