Immunotherapy III: Combinatorial molecular immunotherapy--a synthesis and suggestions

Specific immunotherapy of cancer in elderly patients

The concept of immunotherapy of cancer is more than a century old, but only recently have molecularly defined therapeutic approaches been developed. In this review, we focus on the most promising approach, active therapeutic vaccination. The identification of tumour antigens can now be accelerated by methods allowing the amplification of gene products selectively or preferentially transcribed in the tumour. However, determining the potential immunogenicity of such gene products remains a demanding task, since major histocompatibility complex (MHC) restriction of T cells implies that for any newly defined antigen, immunogenicity will have to be defined for any individual MHC haplotype. Tumour-derived peptides eluted from MHC molecules of tumour tissue are also a promising source of antigen. Tumour antigens are mostly of weak immunogenicity, because the vast majority are tumour-associated differentiation antigens already 'seen' by the patient's immune system. Effective therapeutic vaccination will thus require adjuvant support, possibly by new approaches to immunomodulation such as bispecific antibodies or antibody-cytokine fusion proteins. Tumour-specific antigens, which could be a more potent target for immunotherapy, mostly arise by point mutations and have the disadvantage of being not only tumour-specific, but also individual-specific. Therapeutic vaccination will probably focus on defined antigens offered as protein, peptide or nucleic acid. Irrespective of the form in which the antigen is applied, emphasis will be given to the activation of dendritic cells as professional antigen presenters. Dendritic cells may be loaded in vitro with antigen, or, alternatively, initiation of an immune response may be approached in vivo by vaccination with RNA or DNA, given as such or packed into attenuated bacteria. The importance of activation of T helper cells has only recently been taken into account in cancer vaccination. Activation of cytotoxic T cells is facilitated by the provision of T helper cell-derived cytokines. T helper cell-dependent recruitment of elements of non-adaptive defence, such as leucocytes, natural killer cells and monocytes, is of particular importance when the tumour has lost MHC class I expression. Barriers to successful therapeutic vaccination include: (i) the escape mechanisms developed by tumour cells in response to immune attack; (ii) tolerance or anergy of the evoked immune response; (iii) the theoretical possibility of provoking an autoimmune reaction by vaccination against tumour-associated antigens; and (iv) the advanced age of many patients, implying reduced responsiveness of the senescent immune system.

Unexpected induction of unresponsiveness by vaccination with transformed Salmonella typhimurium

Rats vaccinated with attenuated Salmonella typhimurium transformed with a vector containing the v2 exon of CD44 (SL-v2) were not protected and developed thymic metastases at a high rate. This was surprising because there was evidence for concomitant induction of a CD44v2-specific helper and cytotoxic T-cell response. The inefficacy of vaccination was partly caused by tumor escape and tumor-induced immunosuppression. More important were the facts that (i) BSpl2v2 cells migrated from the intraperitoneal implantation site to the thymus and (ii) after vaccination with transformed attenuated Salmonella typhimurium, a small number of dendritic cells, which had transcribed the cDNA insert, were detected in the thymus. In the thymic environment, these v2 presenting dendritic cells, as well as the BSp12v2 tumor cells, supported tolerance induction. Thus, vaccination with tumor-associated differentiation antigens, which in many instances have induced antitumor response, may deteriorate survival time and rate if vaccination is accompanied by presentation of the antigen during intrathymic T-cell selection.

Cancer gene therapy: hard lessons and new courses

Gene therapy for the treatment of cancer was initiated with high levels of optimism and enthusiasm. Recently, this perception has had to be tempered by the realisation that efficiency and accuracy of gene delivery remain the most significant barriers to its success. So far, there has been a disappointing inability to reach target cells with sufficient efficacy to generate high enough levels of direct killing and this has necessitated the invocation of bystander effects in order for any potential strategy to be convincing. At least in the foreseeable future, clinical advance will come from co-operation with other more established disciplines - such as chemotherapy, radiotherapy and immunotherapy. This is inevitable - and necessary - in order to prove that gene therapy can have efficacy as part of a combinatorial therapy, before hoping to move clinical mountains alone. In addition, there will have to be a thorough understanding of the clinical situations in which gene therapy will be used in order both to understand its own limitations, and to exploit its full potential. This will enable it to find the appropriate clinical niche in which its abilities will be optimally useful. Finally, anyone wishing to practise clinical cancer gene therapy will rapidly have to learn the ways of the free market and be able to juggle commercial necessities with ideological purity. Gene Therapy (2000) 7, 2-8.

Disabled infectious single-cycle herpes simplex virus as an oncolytic vector for immunotherapy of colorectal cancer

New modalities of treatment for colorectal cancer are required to support and improve those currently available. One such approach is immunotherapy by transfer of immunostimulatory genes to tumor cells. Here, we report the use of a herpes simplex virus (HSV) vector that is capable of a single round of infection (disabled infectious single-cycle [DISC]-HSV) as a gene transfer vehicle for colorectal cancer. This vector has potential advantages over other vectors for cancer immunotherapy in that it lyses infected tumor cells. Infection with DISC-HSV inhibited tumor cell growth both in vitro and in vivo. In addition, DISC-HSV-mediated cell killing occurs by both apoptotic and necrotic mechanisms. A range of colorectal tumor cell lines could be rapidly transduced with DISC-HSV/lacZ (14-90% in 4 hr). Both tumor prevention and tumor therapy protocols showed clear antitumor effects with DISC-HSV/mGM-CSF. In the prophylactic approach, an infected/irradiated whole cell vaccine protected up to 80% of mice from rechallenge. In addition, intratumoral injection of established tumors with DISC-HSV/GM-CSF caused rejection in 40% of mice and generated some protection from subsequent rechallenge. In both cases, however, it is clear that a dominant therapeutic effect of the DISC-HSV vector derives from its oncolytic properties, irrespective of the transduced gene. As a prelude to taking these studies forward to human clinical trials, we demonstrate that tumor cells could be successfully grown from freshly obtained human colorectal cancer resections (within 1 week of surgery), were transduced with DISC-HSV/hGM-CSF, and secreted the cytokine. This study provides the preclinical basis for trials of immunotherapy of colorectal cancer using DISC-HSV.

Cancer therapy: new concepts on active immunization

There is increasing evidence that tumors express putative target molecules for a therapeutic immune reaction. Yet, tumor cells lack the prerequisites for appropriate antigen presentation and--hence--the immune system does not respond. This difficulty can probably be circumvented when tumor antigens are processed by conventional antigen presenting cells. Thus, the identification of immunogenic tumor-associated antigens may allow new modes of vaccination with the hope of adding a fourth and hopefully powerful weapon to surgery, radiation and chemotherapy in the fight against cancer.

Heat Shock Protein 70 Induced During Tumor Cell Killing Induces Th1 Cytokines and Targets Immature Dendritic Cell Precursors to Enhance Antigen Uptake

Previously, we reported that killing tumor cells in vivo with the HSV thymidine kinase/ganciclovir system generates potent antitumor immunity, determined in part by the mechanism by which the cells die and by the levels of inducible heat shock protein (hsp) expression induced during the process of cell death. Here, we show that induction of hsp70 expression induces an infiltrate of T cells, macrophages, and predominantly dendritic cells (DCs) into the tumors as well as an intratumoral profile of Th1 cytokine expression (IFN-γ, TNF-α, and IL-12) and enhances immunogenicity via a T cell-mediated mechanism. In addition, the protection conferred by hsp70 is both tumor and cell specific. We also demonstrate that hsp70 targets immature APC to make them significantly more able to capture Ags. This is likely to optimize cross-priming of the infiltrating APC with tumor Ags, which are simultaneously being released by the dying cells. In addition, using an Myc epitope-tagged hsp70 expression vector, we present evidence that hsp70 released from dying tumor cells is taken up directly into DCs and may, therefore, be involved in direct chaperoning of Ags into DCs. Taken together, our data suggest that hsp70 induction serves to signal the immune system of the presence of an immunologically relevant (dangerous) situation against which an immune reaction should be raised.

Antigen loss variants of a murine renal cell carcinoma: implications for tumor vaccination

Vaccination with tumour cells genetically modified to support induction of an immune response either by production of cytokines or expression of co-stimulatory molecules provides a promising therapeutic approach. We have evaluated the efficiency of tumour vaccination using RENCA cells, a renal cell carcinoma of the BALB/c strain, which were stably transfected with MHC class II, B7.1 or both. Tumour growth after vaccination with MHC class II and/or B7.1 transfected RENCA cells was extremely variable, with protection close to 100% after vaccination with some clones and no effect of vaccination with others. To unravel the underlying mechanism, untransfected RENCA cells were cloned, and individual clones were tested for immunogenicity; that cloned RENCA cells varied considerably in immunogenicity. Whereas all clones displayed comparable growth rates in nude mice, some grew very slowly in immunocompetent syngenetic hosts. Vaccination with rapidly growing clones was ineffective and, importantly, this feature remained unaltered by vaccination with MHC class II and/or B7.1 transfected clones. Instead, 8 of 10 mice rejected the parental line after immunisation with a pool of MHC class II and B7.1 transfected clones. Finally, by cloning RENCA cells, we obtained one highly immunogenic clone (P2). Vaccination with this clone led to an individual-specific response, which indicates that during the cloning procedure a new strongly immunogenic entity must have arisen. Taken together, our results indicate that vaccination with MHC II and/or B7.1 transfected tumour cells induces an efficient immune response, but only if the tumour is weakly immunogenic. Since tumours may be composed of clones displaying different antigenicities, it is mandatory to use bulk cell populations for transfection and vaccination.

Gene therapy for lung cancer

Gene therapy has received considerable attention and some speculation as to its value. Although few patients have been treated, the preliminary results of the phase I lung cancer gene therapy clinical trials are very promising. Clinically relevant basic research in the molecular pathogenesis and immunology of lung cancer is progressing. As improved vector technologies are developed, new opportunities will be available to initiate lung cancer gene therapy trials that are based on a more detailed understanding of lung cancer biology. In conclusion, although important biologic and technical questions remain unanswered, recent research suggests that gene therapy will have a profound impact on lung cancer treatment.

Tumor immunogenicity is determined by the mechanism of cell death via induction of heat shock protein expression

In situ killing of tumor cells using suicide gene transfer to generate death by a non-apoptotic pathway was associated with high immunogenicity and induction of heat shock protein (hsp) expression. In contrast, a syngeneic colorectal tumor line, CMT93, killed predominantly by apoptosis, showed low levels of hsp expression and less immunogenicity. When apoptosis was inhibited in CMT93 cells by overexpression of bcl-2, hsp was also induced. Furthermore, when cDNA encoding hsp70 was stably transfected into B16 and CMT93 cells, its expression significantly enhanced the immunogenicity of both tumors. Increased levels of hsp, induced by non-apoptotic cell killing, may provide an immunostimulatory signal in vivo which helps break tolerance to tumor antigens. These findings have important implications for the development of novel anti-cancer therapies aimed at promoting patients' immune responses to their own tumors.

Strategies for achieving multiple layers of selectivity in gene therapy

Here we review the progress towards the development of targeted vectors for direct in vivo delivery in gene therapy. Currently, there are many separate approaches. These include: simple physical/anatomical localization of administration of the vector at the site where gene transfer is required; exploitation of natural tropisms of plasmid, viral and cellular vectors; and the use of molecular engineering to change the specificity of proteins and nucleic acids so that they specifically recognize target ligands expressed on/in the target cells. Unfortunately, each of these approaches is usually imperfect by itself. However, combinations of these strategies might produce vectors in which several layers of imperfect targeting give an overall level of specificity that can justify systemic delivery of vectors to treat human disease.

A family of bicistronic vectors to enhance both local and systemic antitumor effects of HSVtk or cytokine expression in a murine melanoma model

The herpes simplex virus-thymidine kinase/ganciclovir (HSVtk/GCV) system produces both direct and immune-mediated tumor cell killing. Here, we compare the efficacy of HSVtk/GCV with cytokines, alone and in combination, on the tumorigenicity and immunogenicity of B16 cells. With respect to single gene modifications, only HSVtk/GCV, or high-level interleukin-2 (IL-2) secretion, completely prevented tumor growth, whereas granulocyte-macrophage colony-stimulating factor (GM-CSF) generated the best levels of long-term systemic protection. To augment both local killing and immune activation, we constructed bicistronic constructs that express HSVtk and a cytokine within the same cell. Co-expression of HSVtk with IL-2 or GM-CSF enhanced the local antitumor activity of any gene alone. In a tumor-prevention model, HSVtk killing, in an environment preprimed with GM-CSF, generated the best long-term immune protection. However, in a short-term therapy model, continued IL-2 expression was most effective against 3-day established tumors. This probably reflects differences in the activities of IL-2 and GM-CSF in generating short-term, nonspecific immune stimulation compared to long-term immunological memory, respectively. As a prelude to in vivo delivery experiments, we also demonstrated that these bicistronic cassettes can be packaged normally into retroviral (5 x 10(5) virus/ml from pooled populations) and adenoviral vectors (5 x 10(9) virus/ml) and function as predicted within virally infected cells. This family of bicistronic vectors can be used to stimulate synergy between suicide and cytokine genes, overcomes the problems of delivering two genes on separate vectors, and should allow easier preparation of vectors for the delivery of multiple genes to patients' tumor cells.