Galectin-3 as a new negative checkpoint of the immune response is the key target for effective immunotherapy against prostate cancer

COMPAGNO, DANIEL

Simposio; XXXIV jornadas multidisciplinarias del instituto de oncología Angel. H. Roffo; 2019

Based on animal model results and the analysis of clinical data from metastatic and castration resistant prostate cancer (mCRPC) patients, we were able to demonstrate the advantages and the mechanisms by which Docetaxel (DTX)-based chemotherapy induces the correct pre-conditioning of the tumor microenvironment to allow high cytotoxic T cell activity to avoid prostate tumor growth and recurrence. First, analysis of the gene expression of human PCa cells showed that treatment with DTX decreases the expression of Galectin 3 (Gal-3) in either chemo-sensitive or -resistant prostate cancer (PCa) cell lines. In addition, using a murine PCa model (derived from TRAMP mice), treatment of TRAMP-C1 (TC1) cells or tumors with a low dose of DTX lowered the expression of Gal-3 but not Gal-1 (among others) in this cell line and tumors as well (reversible silencing of about 98% (in vitro) and 55% (in vivo)). More importantly, Gal-3 downregulation was confirmed in clinical samples of mCRPC patients treated by taxane-based chemotherapy. To evaluate whether this DTX-dependent Gal-3 downregulation in PCa tumors could be one of the principal molecular factors responsible for chemotherapy enhancement of immunotherapy, we decide to inoculate mice with an autologous BM-DC vaccine loaded with lysates from TC1 cells pretreated by DTX in vitro or expressing an anti-Gal-3 shRNA. This protocol completely protects the animals to the growth of Gal-3 deficient-PCa tumors. We then decided to go further in the understanding of the mechanisms that promote this effective anti-PCa immune response, and analyzed the proliferation ability of lymphocytes in the presence or absence of TC1 tumor cells, expressing or not Gal-3. We thus showed that Gal-3-expressing tumor promotes the expansion of CD8+/CD122+, so called CD8 T regulator cells, known as inhibitors of CD8+ T effector cells proliferation. The results were then encouraging as we demonstrated that Gal-3 down-regulation in tumors (through the use of TC1 shGal-3 cells or TC1-treated with low doses of DTX) was sufficient to restore the capability of CD8+ T cells to be activated and to proliferate as well as in the absence of tumor cells. Interestingly, we showed by ex vivo cytotoxicity assays that these activated and proliferating CD8+ T lymphocytes after vaccination of mice with TC1-lysate loaded BM-DC are capable of killing Gal-3 downregulated TC1 cells and protect prostate cancer recurrence and metastasis development in mice after primary tumor-resection. In conclusion, our results strongly suggest that Gal-3 is one of the principal negative checkpoint in prostate cancer that limits the success of immunotherapy. We also show that DTX-based chemotherapy would promote a decrease of Gal-3 expression by the tumors to create a permissive tumor microenvironment for successful immunotherapy. Finally, we propose a rational protocol combining no toxic/low doses of Docetaxel with simplified immunotherapy to completely control tumor recurrence for all PCa patients and chemoresistant as well.