Large cohorts of recent clinical studies have firmly established that increased levels of tumor-infiltrating lymphocytes (TILs) in TNBC and HER2+ subtypes predicted better clinical outcome compared to the luminal subtype. These observations led to the hypothesis that women with TNBC or HER2+ subtypes may respond to a checkpoint blockade. However, early results from these trials using check point inhibitors alone or in combination with chemotherapy have shown very little promise in breast cancer patients, despite the remarkable long-lasting responses in other hard to treat malignancies such as non-small cell lung and melanoma. Although the outcome falls short of the expectation, it has suggested that the combinations of check point blockade with therapeutics that target immunosuppression may potentiate its efficacy. TNFα exhibits paradoxical roles; it may fuel tumor cell growth, invasion and metastasis in some tumor types, while in others it induces cytotoxic cell death. We recently demonstrated that TNFα distinctly induces A20 in TNBC subtype and protects these cells from TNFα-induced cytotoxic cell death by upregulating HSP70 protein and maintaining EMT/CSC phenotype. In contrast, luminal MCF7 or ZR75-1 cells display approximately 70% apoptosis when treated with TNFα. Overexpression of A20 in luminal cells not only protected them from TNFα-induced cytotoxicity by upregulating HSP70 and EMT/CSC phenotype, but also exhibited aggressive metastatic properties in mouse xenograft models. Furthermore, we show that A20/HSP70 pathway attracts tumor-infiltrating lymphocytes (TILs) while inducing the accumulation of immunosuppressive MDSCs in syngeneic mouse models. Interestingly, pulmonary DTCs as well as the immune infiltrates from 4T1 tumor-bearing mice exhibited significantly higher HSP70 expression. Therefore, we proposed that targeting HSP70 may potentiate the efficacy of immunotherapy in preclinical models of breast cancer. As previously reported, murine 4T1 tumors do respond to check point inhibitors. We reasoned that this may be an appropriate model to test the efficacy of HSP70 inhibitor, JG-231. Expectedly, there was no difference in tumor growth and metastasis between control and anti-PDL1 treated animals, however, combination of anti-PDL1 antibody ed with JG-231 and chemotherapy (cyclophosphamide-CTX) significantly reduced primary tumor growth (>10 fold) and eliminated metastasis. Collectively, our pilot experiments provide a strong rationale for testing our hypothesis and may lead to a rapid translation into the clinical utility. Citation Format: Hasan Korkaya, Fulya Koksalar Alkan, Z. Ece Arslan, Esra Altintas, Raziye Piranlioglu, Eunmi Lee, Max S. Wicha. Dual function of HSP70 in cytoprotection of tumor cells and generation of immunosuppressive tumor microenvironment [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 1900.