Investigation into the immune prompting mechanisms of calreticulin for targeted cancer cell death and immunotherapy

Abstract

Ovarian cancer is the leading cause of death from gynaecological malignancies. Late diagnosis following metastasis combined with frequent relapse after initial treatment, has led to a 5-year survival rate <50%. Thus, new and improved therapeutic strategies are urgently required. One possible novel therapy involves engaging the adaptive immune system in a process of immunogenic cell death (ICD). Apoptotic tumour cell death is usually immunologically silent because cells are cleared by phagocytes without the involvement of adaptive immunity. However, in ICD apoptotic cells have immunogenic properties and immune responses against cancer cell specific antigens or altered self-antigens can occur. Cells undergoing ICD also have surface expression of damage-associated molecular pattern molecules (DAMPs) which facilitate recognition and engulfment e.g. by dendritic cells, and subsequent activation of a cancer cell specific T lymphocyte response. A select group of cancer therapies induce ICD in addition to their known cytotoxic effects e.g. the anthracycline doxorubicin and radiotherapy. The endoplasmic reticulum (ER) chaperone protein calreticulin (CRT) is a key DAMP. CRT translocation from the ER to the cell surface during ICD induced by cytotoxic agents that induce ER stress promotes cancer cell immunogenicity. Thus, strategies to enhance ovarian cancer cell surface CRT may offer therapeutic options for patients. Therefore, the aims of this project were (a) to obtain a purified preparation of CRT (b) to determine the conditions that give rise to enhanced surface levels of CRT in ovarian cancer cells; specifically externalisation of endogenous CRT or cellular binding of exogenously added CRT and (c) to determine if ovarian cancer cells with enhanced surface CRT induce dendritic cell activation and subsequent T lymphocyte responses. Doxorubicin and the known ER stressor thapsigargin stimulated apoptosis in ovarian cancer cells. This was accompanied by enhanced binding of exogenous CRT, enhanced externalisation of endogenous CRT, increased cell expression of CRT and increased secretion of the protein. All of these actions were significantly inhibited by the ER stress inhibitor tauroursodeoxycholic acid, confirming the involvement of ER stress. Ovarian cancer cells with enhanced surface CRT induced maturation and activation of dendritic cells and subsequent activation i.e. proliferation, of cytotoxic T‑lymphocytes, hence stimulating an anticancer immune response.