338PA new insight into tumour immune-evasion: Crosstalk between cancer stem cells and T regulatory cells

Abstract Background Cancer development is aggravated by a rare population of cells, termed cancer stem cells (CSCs), that uniquely initiates and sustains the disease. Although CSCs can orchestrate the immune systems in favour of tumor, the detailed mechanisms underlying their immunomodulatory effects remain elusive. Recent reports indicate the contribution of exosomes, secreted from various cells, as mediators of cell-to-cell communication and also as a mediator of immune modulation in the tumor microenvironment. Methods Breast CSCs (bCSCs) were purified from breast cancer cell lines and human breast tumor tissues following which CSC-derived exosomes (CDEs) were isolated. Peripheral blood mononuclear cells were isolated from the blood of healthy donors and breast cancer patients, co-cultured with CDEs, and T regulatory (Treg) cell generation was evaluated. Results We found that bCSCs are able to generate CD4+CD25+FOXP3+ Treg cells having suppressive activity. Our search for the underlying mechanism revealed for the first time that bCSC-secreted exosomes contain FOXP3 protein that uniquely initiates acquisition of Treg characteristics in the infiltrating T lymphocytes. Exosomes from FOXP3-ablated bCSCs failed to augment Treg cell generation indicating the requirement of bCSC-gifted FOXP3 in Treg generation. Interestingly, perturbation of transcription or translation in T naive cells significantly decreased CDE-augmented FOXP3 level signifying that CDE-FOXP3 utilizes the transcription and translational machineries of T naive cells to induce Treg cell generation. In fact, exosome-shuttled FOXP3 binds to the CNS2 region of the Foxp3 promoter in T lymphocytes, thus resulting in a sustained expression of FOXP3 and further leading to induction of other genes of Foxp3 interactome, thereby phenotypically converting the T cells to Treg cells. Conclusions Collectively our data demonstrates that bCSC-shed exosomal FOXP3 plays an important role in procreation of Treg cells within the tumor microenvironment thus leading to tumor-induced immune suppression. Breaching the deadly-liaison between them might be a promising strategy for breast cancer therapy. Legal entity responsible for the study Prof. Tanya Das, Professor, Division of Molecular Medicine, Bose Institute. Funding Department of Science and Technology, Govt. of Inda Council of Scientific & Industrial Research (CSIR), Govt. of India. Disclosure All authors have declared no conflicts of interest.