Immunogenic Cell Death Induced by EEO in Breast Cancer Cells Promotes Dendritic Cell Activation in a CRT-Dependent Manner

Cancer immunotherapy faces significant challenges in treating “cold” tumors, which respond poorly to current strategies. Breast cancer is a major cause of cancer-related mortality in women and is a representative immunosuppressive cold tumor. The luminal A subtype of breast cancer exhibits an immune-cold phenotype, with MCF-7 cells used as a representative cell model. Here, we identified (3β,5α,8α)-5,8-epidioxyergost-6-en-3β-ol (EEO), isolated from Gymnopilus orientispectabilis, as an inducer of immunogenic cell death (ICD) hallmarks in MCF-7 cells, including cell surface exposure of calreticulin (CRT) and the extracellular release of damage-associated molecular patterns (DAMPs), such as ATP and High Mobility Group Box 1 (HMGB1). These ICD-associated events subsequently promoted CRT-dependent dendritic cell (DC) activation in a co-culture system with bone marrow-derived DCs and MCF-7 cells. Furthermore, surface-exposed CRT enhanced the phagocytosis of EEO-treated MCF-7 cells by DCs, and this phagocytic activity promoted DC maturation, as indicated by increased cell surface levels of the maturation markers major histocompatibility complex class II (MHC II) and cluster of differentiation 86 (CD86). Moreover, CRT knockdown in MCF-7 cells reduced surface CRT exposure following EEO treatment, thereby suppressing DC-mediated phagocytosis and subsequent DC maturation. Taken together, these findings suggest that EEO is a promising candidate for enhancing the antitumor efficacy of existing breast cancer immunotherapies through ICD-mediated DC maturation and converting immunologically cold tumors into hot tumors.