Exogenous stress proteins enhance the immunogenicity of apoptotic tumor cells and stimulate antitumor immunity.

We have previously reported that apoptotic tumor cells can be either immunogenic or nonimmunogenic in vivo, depending on whether or not these cells are heat stressed before induction of apoptosis. Stressed apoptotic cells express heat shock proteins on their plasma membranes and dendritic cells are capable of distinguishing them from nonstressed apoptotic cells. Here we provide evidence that when purified heat shock protein 70 or chaperone-rich cell lysate (CRCL) from syngeneic normal tissue is used as an adjuvant with nonimmunogenic apoptotic tumor cells in vaccination, potent antitumor immunity can be generated. This antitumor immunity is mediated by T cells because antitumor effects are not observed in either severe combined immunodeficiency or T cell-depleted mice. We further demonstrate that vaccination of mice with apoptotic tumor cells mixed with liver-derived CRCL as adjuvant were capable of enhancing the production of T(H)1 cytokines, inducing specific cytotoxic T lymphocytes and eliciting long-lasting antitumor immunity. Stress proteins from autologous normal tissue components therefore can serve as danger signals to enhance the immunogenicity of apoptotic tumor cells and stimulate tumor-specific immunity

Stressed apoptotic tumor cells stimulate dendritic cells and induce specific cytotoxic T cells.

We have previously reported that stressed apoptotic tumor cells are more immunogenic in vivo than nonstressed ones. Using confocal microscopy we have confirmed our previous observation that heat-stressed apoptotic 12B1-D1 leukemia cells (BCR-ABL(+)) express HSP60 and HSP72 on their surface. To explore how the immune system distinguishes stressed from nonstressed apoptotic tumor cells, we analyzed the responses of dendritic cells to these 2 types of apoptotic cells. We found that nonstressed and heat-stressed apoptotic 12B1-D1 cells were taken up by dendritic cells in a comparable fashion. However, when stressed apoptotic 12B1-D1 cells were coincubated with immature dendritic cells for 24 hours, this resulted in greater up-regulation of costimulatory molecules (CD40, CD80, and CD86) on the surface of dendritic cells. Moreover, stressed apoptotic 12B1-D1 cells were more effective in stimulating dendritic cells to secrete interleukin-12 (IL-12) and in enhancing their immunostimulatory functions in mixed leukocyte reactions. Furthermore, we demonstrated that immunization of mice with stressed apoptotic 12B1-D1 cells induced the secretion of T helper-1 (T(H)1) profile of cytokines by spleen cells. Splenocytes from mice immunized with stressed apoptotic cells, but not nonstressed ones, were capable of lysing 12B1-D1 and the parental 12B1 line, but not a B-cell leukemia line, A20. Our data indicate that stressed apoptotic tumor cells are capable of providing the necessary danger signals, likely through increased surface expression of heat shock proteins (HSPs), resulting in activation/maturation of dendritic cells and, ultimately, the generation of potent antitumor T-cell responses.