Throughout life, millions of cells die each day in an orderly process (apoptosis) and are replaced by new cells to maintain homeostasis. From an immunologic perspective, apoptosis does not elicit an inflammatory response and is generally thought to be immunologically silent — this is important as otherwise autoimmunity could ensue. Cancer cells overall have a lower propensity to undergo apoptosis. It is now recognized that a dominant mode of cell death caused by chemotherapy is apoptosis. Since chemotherapy also suppresses the immune response, and apoptosis is thought to be immunologically silent, it is generally assumed that chemotherapy-induced cancer cell death does not trigger an immune response.
Recent work by Casares et al. challenges this somewhat simplistic view. In this paper, the authors show that even in the absence of any adjuvant, tumor cells dying in response to doxorubicin (DX) could elicit an effective antitumor immune response that suppressed the growth of inoculated tumor cells and even led to regression of established tumors in immunocompetent mice. In contrast, tumor cells killed by treatment with mitomycin C (MC) were immunologically silent and did not elicit an antitumor immune response. This surprising and important difference would not be expected a priori as both agents induce apoptosis in cancer cell lines. Upon detailed inspection, the authors showed that while both DX and MC induced caspase activation and chromatin condensation, only MC led to detectable DNA fragmentation. To test the immunogenicity of these cells, chemotherapy-treated cancer cells (and necrotic cells induced by freeze/thaw [F/T] as controls) were injected into one flank while live, untreated cells were injected into the opposite flank. Only DX-treated, but not MC- or F/T-treated, cells reduced tumor formation by live, untreated cells by ~ 45 percent on day 30 and ~ 30 percent on day 120. Furthermore, protected animals in the DX-treated group were immune to a second challenge with the same live, untreated cancer cells, but not to an unrelated cancer cell line, and splenic T cells from protected animals could transfer protection to naïve animals. Vaccination with DX-treated apoptotic cells was ~ 80 percent protective after one prophylactic injection prior to challenge with live, untreated cells — this represents a higher degree of protection than MC- or F/T-treated, or γ irradiated cells. Other anthracyclines including daunorubicin and idarubicin conferred similar effects. Taken together, these results indicate that anthracyclines induce a form of apoptosis which is immunogenic and can lead to protective antitumor immune responses. While the mechanisms involved remain unclear, the role of caspase in the immunogenicity of apoptotic cells induced by DX was shown using a broad spectrum caspase inhibitor Z-VAD-fmk. When combined with DX (together referred to as DXZ), DXZ-treated cells exhibited delayed death and were significantly less immunogenic than DX-treated cells. Depletion of dendritic cells or CD8 T cells abolished the immune response against DX-treated tumor cells in vivo. These results suggest that full caspase activation is a necessary step in the immunogenicity of DX-induced apoptosis, and that the response is mediated by dendritic and CD8 T cells.
In Brief
Together, these results show that not all apoptosis is the same. Importantly, at least one form — that induced by DX and other anthracyclines — may be immunogenic and could lead to protective antitumor immune responses. This may have significant implications in cancer therapy. DX (and other anthracylines) is a common agent in the treatment of acute leukemia. It may be that long-term remission, perhaps cure, in some patients may arise through a synergistic effect of, initially, cytoreduction by chemotherapy, followed by an effective antileukemia immune response that eliminates residual tumor cells, perhaps including cancer stem cells and escape variants. If true, it could be worthwhile to revise anthracycline-containing chemotherapy regimens to minimize their immunosuppressive activity, and potentially couple these with active immunotherapeutic (vaccination) approaches. On the flip side, these data also raise caution that normal cells induced into apoptosis via a DX-like mechanism may lead to autoimmunity. While autoimmunity after anthracycline treatment is rare, other drugs or processes may induce death of normal tissues via an analogous mechanism. Hence, a better understanding into the mechanism underlying the immunogenicity of DX-induced apoptosis could lead to better cancer treatment strategies as well as, possibly, ways to prevent certain autoimmune processes.
Competing Interests
Dr. Lee indicated no relevant conflicts of interest.
