Immunological consequences of apoptosis in a tumor system

Abstract

Cell lines genetically deficient in caspase-8 have been shown to be resistant to Fas-induced apoptosis, indicating that this pathway may be caspase-8-dependent. Some reports, however, have shown that Fas can induce cell death independent of caspase-8. In this study, we provide evidence for an alternative, caspase-8-independent, Fas death domain-mediated apoptotic pathway in the BCR-ABL positive leukemia cell line, 12B1-D1. Our data suggest that there is a novel, caspase-8-independent, Z-VAD-FMK inhibitable, apoptotic pathway in 12B1-D1 cells that targets mitochondria directly. In attempting to develop effective anti-cancer immunotherapies the relative ability of apoptotic cells to induce an immune response remains an important but controversial consideration. Apoptotic tumor cells can theoretically be a suitable antigen source for stimulation of anti-tumor responses. HSPs can act as danger signals to the immune system. We, therefore, hypothesize that the immunogenicity of apoptotic cell may be enhanced if endogenous HSP expression is induced, or an exogenous source of HSPs is present. To induce endogenous HSPs expression, the engineered 12B1-D1 cells are heat stressed before the induction of apoptosis by AP20187. These stressed apoptotic 12B1-D1 cells express HSP60 and HSP72 on their surface. Vaccination of mice with stressed apoptotic 12B1-D1 cells, but not non-stressed ones, elicits a potent cell-mediated anti-tumor immunity that significantly retards tumor progression. We have further demonstrated that, stressed apoptotic cells had higher abilities to upregulate the co-stimulatory molecules on the surface of DC, to stimulate DC to secrete proinflammatory cytokines, and to enhance their immunostimulatory functions. We further explored whether the immunogenicity of non-stressed apoptotic cell can be enhanced if an exogenous source of HSPs is present at the vaccination site. We used liver derived chaperone proteins co-injected with non-stressed apoptotic tumors to mice. Reproducibly this resulted in the generation of a durable and specific T-cell-mediated anti-tumor immunity. In summary, we have demonstrated that apoptotic tumor cells can be either immunogenic or non-immunogenic and DC may play a key role in determining the immunological consequences of apoptotic tumor cells. We have further demonstrated that normal tissue (liver) MCC may function as a danger signal for the immune system. These may provide new insights for combining immunotherapy with conventional therapies for treatment of cancers.