Column Honing The Host's Death Mechanisms To Cure Cancer Dr. Anil Shanker
 For a number of years there has been great enthusiasm for the idea of triggering the immune system to eradicate tumors. However, in contrast to bacteria or viruses, tumor cells closely resemble the normal tissue from which they were originally derived. Therefore, in order for an immune response to tumor cells to occur, normal self-tolerance must be overcome and tumor antigens must be identified.
Over the last decade, a lot of progress has been made in tumor immunology, particularly in the identification of numerous tumor antigens and encouragingly, in some tumor models the T lymphocyte-mediated antigen-specific response efficiently eradicated tumor. Functional effector T cells, but not anti-bodies, have been shown to develop against self-tumor antigens in some melanoma patients pancreatic cancer patients, Epstein-Barr virus-associated malignancies and some breast cancer patients. Since fully activated T cells have the ability to directly lyse tumor cells in vitro, it has been assumed that this cell-mediated cytotoxicity is crucial to their antitumor effector function.
Cell-Mediated Lysis of Tumors
T lymphocytes, along with natural killer (NK) cells, can directly lyse various tumor cells in vitro following appropriate recognition of distinct structures on the tumor cell surface. Currently, two major molecular pathways are known to be used by these cytolytic lymphocytes for lysing the tumor cells.
One pathway involves the directed release of cytotoxic granules (granule exocytosis) found in both activated T cells and NK cells. These granules contain the pore-forming protein ‘perforin’ as well as various other constituents including a unique family of serine proteases known as ‘granzymes’. Perforin promotes the access and trafficking of the granzymes into target cells, causing target cell death by caspase dependent or independent mechanisms.
The second mechanism is direct apoptosis (programmed cell death) of tumor cells by cytotoxic T cells and NK cells. This pathway also known as 'death receptor pathway' engages appropriate cell surface receptors on tumor cells with various proteins of the tumor necrosis factor (TNF) superfamily such as TNF- alpha, FasL or TNF-related apoptosis-inducing ligand (TRAIL or Apo2L) called ‘death ligands’ expressed on the effector cells. Intracellular signaling following receptor engagement activates pro-apoptotic proteins and a cascade of caspases resulting in the death of the target cells. Interestingly, in contrast to granzymes and perforin, expression of these death ligands is not restricted to activated T cells and NK cells. They are also produced by variety of other cells of the innate immune system such as dendritic cells and macrophages.
The intracellular signaling required to engage these two lytic pathways differ substantially. Once appropriate recognition and triggering of granule release from the cytotoxic cell has occurred, it seems unlikely that the tumor target cell can influence its own demise. By contrast, many cancer cells are resistant to the apoptotic effects following engagement of death receptors with their ligands. This is presumably because the target cell itself must participate in the transmission of the apoptotic signal. Indeed, the resistance of tumor cells to apoptotic signaling is now thought to be one of the prerequisites of cancer development. This loss of apoptotic signaling potential can occur at multiple points along the apoptotic signaling pathway. In the response to death ligands, a loss of the appropri-ate receptor or any of the intracellular components of the apoptotic signaling can prevent cellular apoptosis. Nonetheless, it has been known for many years that agents such as cycloheximide or actinomycin D can sensitize some tumor cells to apoptosis triggered by death ligands. Thus, it may be that the apoptotic signaling pathway in response to death ligands still exists in many tumors, but this suicide pathway is blocked at some particular stage resulting in tumor cell survival.
Tumor Destruction in Vivo
The recent availability of a variety of gene-targeted or mutant mice has helped demonstrate that tumor destruction by the activated T cells (in mouse tumor models eg: B16 melanoma, methylcholanthrene-induced sarcomas, renal carcinoma Renca etc) could occur in the total absence of perforin. This suggests that granule-mediated cytotoxic effects may not be as crucial for tumor rejection in vivo as it was originally believed. On the other hand, death ligands produced by T cells or NK cells may be major mediators of tumor destruction in vivo. But tumors differ in their response to various death ligands and prior knowledge of their apoptotic signaling could be helpful in determining the success of different immunotherapeutic approaches. Since activated T cells are major source of FasL, their direct cytotoxic effects could be the major mediator for the destruction of tumor cells uniquely sensitive to this death ligand. However in solid tumors, the mass is often infiltrated with other cells of the innate immune system such as macrophages and dendritic cells which locally can produce TNF-alpha or TRAIL and might be crucial for the destruction of tumors particularly sensitive to these death ligands. In these cases, the major antitumor role of the activated T cells may be to cause proinflammatory cytokine milieu, which in turn might increase the levels of death ligands produced by other cells in the tumor stroma.
Augmenting Immune-Mediated Tumor Eradication
The antitumor effects of death ligands may be more amenable to pharmacological modifications at the level of the tumor itself. Since tumor cells undergo a high protein turnover that relies on ubiquitin-proteasome pathway, recently there has been a lot of interest in blocking proteasome function as an approach to sensitize tumor cells to death ligand-mediated apoptosis. Pioneering work from the laboratory of Dr. Thomas J Sayers at NCI-Frederick and others have shown that bortezomib, a reversible and specific inhibitor of proteasome can sensitize a number of mouse and human tumor cells to TRAIL-mediated apoptosis in vitro. It was also demonstrated that bortezomib treatment of tumor cells resulted in an enhanced activation of caspase 8 following TRAIL binding suggesting a possibility that the major effects of bortezomib in sensitizing tumor cells to TRAIL may occur at a proximal level of the apoptotic signaling cascade.
Interestingly, bortezomib could be combined with allogeneic bone marrow transfer allowing reduction of graft-versus-host disease while maintaining beneficial graft-versus-tumor effects. Recent studies indicate that the combination of bortezomib with an agonist antibody to TRAIL death receptor 5 can provide therapeutic benefit in mice bearing renal cell carcinoma. It seems likely that bortezomib could also be effectively combined with agents that induce TRAIL production in vivo, particularly if this induction could be localized in the vicinity of the tumor or its metastases. Human Genome Sciences have recently commenced with a clinical trial to investigate therapeutic benefit of bortezomib together with agonist antibodies to TRAIL death receptors in patients with multiple myeloma.
Future Directions
The molecular basis underlying bortezomib sensitization of tumor cells to death ligand-mediated apoptosis remains unclear. Bortezomib could be affecting multiple components of the apoptotic signaling pathway. Identification of the molecular mechanism underlying bortezomib sensitization of tumor cells to TRAIL or other death ligands could be important in two respects: (1) It may allow for molecular profiling of tumor cells or tissue samples in advance, thus allowing for selection of tumors based on the characteristics associated with the “sensitive” phenotype and (2) more information on the critical components of the apoptotic pathway could result in the development of more specific novel agents to augment death ligand-mediated apoptosis of tumors.
In conclusion, it is hoped that these strategies to sensitize tumor cells to the apoptotic effects of death ligands could in the future be coupled to the recent advances in adoptive T cell transfer protocols, in order to improve the beneficial effects of immunotherapy in cancer patients.
Dr. Anil Shanker is a talented scientist working in area of Tumor Immunology at National Cancer Institute (NCI), Frederick, NIH, USA. |