Mechanisms involved in the recognition of apoptotic cells by B lymphocytes

Abstract

As many tumours are poorly immunogenic, one approach for cancer therapy aims to improve natural anti-tumour immunity. Recently it has been shown that anthracycline-induced apoptosis in tumour cells results in the relocation of the ER resident chaperone calreticulin to the cell surface. Interestingly, this relocation directly stimulates T cell-mediated anti-tumour immunity following tumour cell phagocytosis by dendritic cells. In addition, evidence from medullary carcinoma of the breast patients shows a correlation between tumour-infiltrating B cells and a more favourable prognosis. Surprisingly, many of these infiltrating B cells recognise proteins relocated from their normal intracellular distribution to the tumour cell surface, also as a result of apoptosis. As B cells have recently been shown to be an important population of antigen presenting cells capable of extracting membrane antigens from target cells, these findings raise the possibility that tumour infiltrating B cells may be capable of presenting tumour antigens promoting the generation of anti-tumour immunity. In particularly, this thesis envisages that following apoptosis induction in tumour cells, antigen specific B cells will be able to acquire relocated tumour antigens, therefore directly promoting antigen-specific T cells. To provide a proof of principle I have characterized the processes involved in the extraction of membrane-tethered antigens by B cells from target cells using flow cytometry and a novel proliferation bio-assay. Unexpectedly, surface antigen acquisition by B cells has been demonstrated to be rapid, proceeding in times as short as 1 min. Furthermore, I have also characterised the processes involved in the relocation of antigens to the cell surface of dying target cells from various intracellular locations using anthracyclines and UVC as immunogenic apoptotic agents. This has allowed me to demonstrate that antigens from several intracellular compartments following apoptosis induction are acquired by antigen-specific B cells. Importantly, this antigen acquisition was sufficient to induce CD4+ T cell activation as confirmed by proliferation assay. These in vitro findings demonstrate that B cells are capable of activating CD4+ T cells following interaction with apoptotic target cells. Therefore, further investigations concerning the role of B cells in natural anti-tumour immunity may improve current anti-cancer therapies.