A reassessment of the role of B7-1 expression in tumor rejection.

Introduction of the B7-1 gene into murine tumor cells can result in rejection of the B7-1 transductants and, in some cases, systemic immunity to subsequent challenge with the nontransduced tumor cells. These effects have been largely attributed to the function of B7-1 as a costimulator in directly activating tumor specific, major histocompatibility class I-restricted CD8+ T cells. We examined the role of B7-1 expression in the direct rejection as well as in the induction of systemic immunity to a nonimmunogenic murine tumor. B-16 melanoma cells with high levels of B7-1 expression did not grow in C57BL/6 recipient mice, while wild-type B-16 cells and cells with low B7-1 expression grew progressively within 21 d. In mixing experiments with B7-1hi and wild-type B-16 cells, tumors grew out in vivo even when a minority of cells were B7-1-. Furthermore, the occasional tumors that grew out after injection of 100% B-16 B7-1hi cells showed markedly decreased B7-1 expression. In vivo antibody depletions showed that NK1.1 and CD8+ T cells, but not CD4+ T cells, were essential for the in vivo rejection of tumors. Animals that rejected B-16 B7-1hi tumors did not develop enhanced systemic immunity against challenge with wild-type B-16 cells. These results suggest that a major role of B7-1 expression by tumors is to mediate direct recognition and killing by natural killer cells. With an intrinsically nonimmunogenic tumor, this direct killing does not lead to enhanced systemic immunity.

Murine tumor antigens: is it worth the search?

Standardized techniques that allow the direct identification of tumor antigens are now available. Several murine antigens recognized by T cells have already been identified. So far, the majority of these antigens derive from cellular proteins similar to those that give rise to human tumor antigens. While many of the known human tumor antigens are widely shared, most of the murine tumor antigens appear to be unique to the individual tumor from which they were isolated. Nonetheless, common features between murine and human tumor antigens are emerging, suggesting that these murine antigens will provide essential tools in the evaluation of antigen-based vaccines for the future treatment of cancer.

HER-2/neu is a tumor rejection target in tolerized HER-2/neu transgenic mice.

HER-2/neu (neu-N) transgenic mice, which express the nontransforming rat proto-oncogene, develop spontaneous focal mammary adenocarcinomas beginning at 5-6 months of age. The development and histology of these tumors bears a striking resemblance to what is seen in patients with breast cancer. We have characterized the immunological responses to HER-2/neu (neu) in this animal model. neu-positive tumor lines, which were derived from spontaneous tumors that formed in neu-N animals, are highly immunogenic in parental, FVB/N mice. In contrast, a 100-fold lower tumor challenge is sufficient for growth in 100% of transgenic animals. Despite significant tolerance to the transgene, neu-specific immune responses similar to those observed in breast cancer patients can be demonstrated in neu-N mice prior to vaccination. Both cellular and humoral neu-specific responses in transgenic mice can be boosted with neu-specific vaccination, although to a significantly lesser degree than what is observed in FVB/N mice, indicating that the T cells involved are less responsive than in the nontoleragenic parental strain. Using irradiated whole-cell and recombinant vaccinia virus vaccinations we are able to protect neu-N mice from a neu-expressing tumor challenge. T-cell depletion experiments demonstrated that the observed protection is T cell dependent. The vaccine-dependent neu-specific immune response is also sufficient to delay the onset of spontaneous tumor formation in these mice. These data suggest that, despite tolerance to neu in this transgenic model, it is possible to immunize neu-specific T cells to achieve neu-specific tumor rejection in vivo. These transgenic mice provide a spontaneous tumor model for identifying vaccine approaches potent enough to overcome mechanisms of immune tolerance that are likely to exist in patients with cancer.

High efficiency gene transfer into primary human tumor explants without cell selection.

Preclinical studies with murine tumor models have demonstrated that autologous tumor cell vaccines engineered to secrete certain cytokines in a paracrine fashion elicit systemic immune responses capable of eliminating small amounts of established tumor. These results have engendered much interest in developing this strategy for gene therapy of human cancer. The major limitation to creating genetically modified autologous human tumor vaccines is efficient gene transfer into primary tumor explants, since the majority of human tumors fail to proliferate in long-term culture. Using the retroviral vector MFG in conjunction with short-term culture techniques, we have achieved, in the absence of selection, a mean transduction efficiency of 60% in primary renal, ovarian, and pancreatic tumor explants, and we have developed an autologous granulocyte-macrophage colony-stimulating factor secreting tumor vaccine for clinical trials.

Controlled release, biodegradable cytokine depots: a new approach in cancer vaccine design.

Experimental studies using murine tumor models have demonstrated that potent systemic immunity can be generated using tumor vaccines engineered by gene transfer to secrete certain cytokines. The underlying physiological principle behind these strategies involves the sustained release of high doses of cytokine at the site of the tumor. In some cases, this paracrine approach appears to enhance tumor antigen presentation and avoids systemic cytokine toxicity. The widespread clinical use of autologous cytokine gene transduced tumor vaccines may be limited by the technical difficulty and labor intensity of individualized gene transfer. We have therefore explored an alternate approach to generating sustained release of cytokines local to the tumor cells. High doses of granulocyte-macrophage colony-stimulating factor encapsulated in cell-sized gelatin-chondroitin sulfate microspheres were mixed with irradiated tumor cells prior to s.c. injection. This vaccination scheme resulted in systemic anti-tumor immune responses comparable to granulocyte-macrophage colony-stimulating factor gene transduced tumor vaccines.

Consistent chromosome abnormalities in adenocarcinoma of the pancreas.

Little is known about the somatic genetic changes which characterize pancreatic adenocarcinoma. The identification of acquired genomic alterations would further our understanding of the biology of this neoplasm. We have studied 62 primary pancreatic adenocarcinomas obtained from surgical resections using classical cytogenetics and fluorescent in situ hybridization methods. Clonally abnormal karyotypes were observed in 44 neoplasms. Karyotypes were generally complex (greater than three abnormalities) and included both numerical and structural chromosome abnormalities. Many tumors contained at least one marker chromosome. The most frequent whole chromosomal gains were chromosomes 20 (eight tumors) and 7 (seven tumors). Losses were much more frequent: chromosome 18 was lost in 22 tumors followed in frequency by chromosomes 13 (16 tumors), 12 (13 tumors), 17 (13 tumors), and 6 (12 tumors). Structural abnormalities were frequent. Two hundred nine chromosome breakpoints were identified. Excluding Robertsonian translocations, the chromosomal arms most frequently involved were 1p (12); 6q (11); 7q and 17p (9 each); and 1q, 3p, 11p, and 19q (8 each). Portions of the long arm of chromosome 6 appeared to be lost in nine tumors. To determine whether the apparent losses of portions of 6q are real, four tumors with 6q deletions were hybridized with a biotin-labeled microdissection probe from 6q24-ter. Loss of one copy of this region was verified in three of four tumors. In addition, double minute chromosomes were identified in eight cases. To our knowledge, these represent the first primary specimens of pancreatic adenocarcinoma with cytogenetic evidence of gene amplification.

The collaboration of both humoral and cellular HER-2/neu-targeted immune responses is required for the complete eradication of HER-2/neu-expressing tumors.

HER-2/neu (neu) transgenic mice (neu-N mice), which express the nontransforming rat proto-oncogene, demonstrate immunological tolerance to neu that is similar to what is encountered in patients with neu-expressing breast cancer. We have shown previously that a significant increase in neu-specific T cells, but no induction of neu-specific antibody, is seen after neu-specific vaccination in neu-N mice. In contrast, a significant induction of both neu-specific T-cell and antibody responses is found in nontoleragenic FVB/N mice after vaccination. These mice are fully protected from a s.c. challenge with NT cells, a mammary tumor cell line derived from a spontaneous tumor that arose in a neu-N mouse, whereas neu-N mice are not. In this study, we demonstrate that CD4+ T cell-depleted FVB/N mice show no induction of neu-specific IgG after vaccination and are unable to reject an NT challenge (0 of 10 mice were tumor free). Conversely, the depletion of natural killer cells has no effect on vaccine-mediated tumor rejection (100% of mice were tumor free). In CD8+ T cell-depleted animals, where vaccine-induced neu-specific IgG titers were normal, NT growth was delayed, but only 10% of mice remained tumor free, demonstrating that neu-specific IgG alone is insufficient for protection from NT challenge. To directly assess the necessity for the combination of neu-specific cellular and humoral immune responses, severe combined immunodeficient mice were given an adoptive transfer of CTLs plus IgG derived from FVB/N mice. Animals that were given CTLs that recognized an irrelevant antigen plus neu-specific IgG developed tumors at a rate similar to CD8+ T cell-depleted FVB/N mice. Animals receiving an adoptive transfer of neu-specific CTLs plus control IgG derived from naive FVB/N mice were only partially protected from NT challenge (50% of animals were tumor free). However, only animals receiving the combination of neu-specific CTLs and neu-specific IgG were fully protected from NT challenge (100% of animals were tumor free). These studies specifically define the immunological requirements for the eradication of neu-expressing tumors in this model system, demonstrating that both cellular and humoral neu-specific responses are necessary for protection from an NT challenge. These data suggest that vaccines optimized to induce maximal T- and B-cell immunity to neu, and possibly to similar putative tumor-rejection antigens, may lead to more potent in vivo antitumor immunity.

Cyclophosphamide, doxorubicin, and paclitaxel enhance the antitumor immune response of granulocyte/macrophage-colony stimulating factor-secreting whole-cell vaccines in HER-2/neu tolerized mice.

Tumor-specific immune tolerance limits the effectiveness of cancer vaccines. In addition, tumor vaccines alone have a limited potential for the treatment of measurable tumor burdens. This highlights the importance of identifying more potent cancer vaccine strategies for clinical testing. We tested immune-modulating doses of chemotherapy in combination with a granulocyte/macrophage-colony stimulating factor (GM-CSF)-secreting, HER-2/neu (neu)-expressing whole-cell vaccine as a means to treat existing mammary tumors in antigen-specific tolerized neu transgenic mice. Earlier studies have shown that neu transgenic mice exhibit immune tolerance to the neu-expressing tumors similar to what is observed in patients with cancer. We found that cyclophosphamide, paclitaxel, and doxorubicin, when given in a defined sequence with a GM-CSF-secreting, neu-expressing whole-cell vaccine, enhanced the vaccine's potential to delay tumor growth in neu transgenic mice. In addition, we showed that these drugs mediate their effects by enhancing the efficacy of the vaccine rather than via a direct cytolytic effect on cancer cells. Furthermore, paclitaxel and cyclophosphamide appear to amplify the T helper 1 neu-specific T-cell response. These findings suggest that the combined treatment with immune-modulating doses of chemotherapy and the GM-CSF-secreting neu vaccine can overcome immune tolerance and induce an antigen-specific antitumor immune response. These data provide the immunological rationale for testing immune-modulating doses of chemotherapy in combination with tumor vaccines in patients with cancer.

Discovery of new markers of cancer through serial analysis of gene expression: prostate stem cell antigen is overexpressed in pancreatic adenocarcinoma.

Serial analysis of gene expression (SAGE) can be used to quantify gene expression in human tissues. Comparison of gene expression levels in neoplastic tissues with those seen in nonneoplastic tissues can, in turn, identify novel tumor markers. Such markers are urgently needed for highly lethal cancers like pancreatic adenocarcinoma, which typically presents at an incurable, advanced stage. The results of SAGE analyses of a large number of neoplastic and nonneoplastic tissues are now available online, facilitating the rapid identification of novel tumor markers. We searched an online SAGE database to identify genes preferentially expressed in pancreatic cancers as compared with normal tissues. SAGE libraries derived from pancreatic adenocarcinomas were compared with SAGE libraries derived from nonneoplastic tissues. Three promising tags were identified. Two of these tags corresponded to genes (lipocalin and trefoil factor 2) previously shown to be overexpressed in pancreatic carcinoma, whereas the third tag corresponded to prostate stem cell antigen (PSCA), a recently discovered gene thought to be largely restricted to prostatic basal cells and prostatic adenocarcinomas. PSCA was expressed in four of the six pancreatic cancer SAGE libraries, but not in the libraries derived from normal pancreatic ductal cells. We confirmed the overexpression of the PSCA mRNA transcript in 14 of 19 pancreatic cancer cell lines by reverse transcription-PCR, and using immunohistochemistry, we demonstrated PSCA protein overexpression in 36 of 60 (60%) primary pancreatic adenocarcinomas. In 59 of 60 cases, the adjacent nonneoplastic pancreas did not label for PSCA. PSCA is a novel tumor marker for pancreatic carcinoma that has potential diagnostic and therapeutic implications. These results establish the validity of analyses of SAGE databases to identify novel tumor markers.

Bioactivity of autologous irradiated renal cell carcinoma vaccines generated by ex vivo granulocyte-macrophage colony-stimulating factor gene transfer.

Granulocyte-macrophage colony-stimulating factor (GM-CSF) gene-transduced, irradiated tumor vaccines induce potent, T-cell-mediated antitumor immune responses in preclinical models. We report the initial results of a Phase I trial evaluating this strategy for safety and the induction of immune responses in patients with metastatic renal cell carcinoma (RCC). Patients were treated in a randomized, double-blind dose-escalation study with equivalent doses of autologous, irradiated RCC vaccine cells with or without ex vivo human GM-CSF gene transfer. The replication-defective retroviral vector MFG was used for GM-CSF gene transfer. No dose-limiting toxicities were encountered in 16 fully evaluable patients. GM-CSF gene-transduced vaccines were equivalent in toxicity to nontransduced vaccines up to the feasible limits of autologous tumor vaccine yield. No evidence of autoimmune disease was observed. Biopsies of intradermal sites of injection with GM-CSF gene-transduced vaccines contained distinctive macrophage, dendritic cell, eosinophil, neutrophil, and T-cell infiltrates similar to those observed in preclinical models of efficacy. Histological analysis of delayed-type hypersensitivity responses in patients vaccinated with GM-CSF-transduced vaccines demonstrated an intense eosinophil infiltrate that was not observed in patients who received nontransduced vaccines. An objective partial response was observed in a patient treated with GM-CSF gene-transduced vaccine who displayed the largest delayed-type hypersensitivity conversion. No replication-competent retrovirus was detected in vaccinated patients. This Phase I study demonstrated the feasibility, safety, and bioactivity of an autologous GM-CSF gene-transduced tumor vaccine for RCC patients.

Breast cancer vaccines: maximizing cancer treatment by tapping into host immunity.

Optimizing standard treatment modalities for breast cancer has improved the outlook for women afflicted with it, but the fact that 40% still ultimately die from the disease highlights the need for new therapies. Remarkable advances in molecular immunology and biotechnology have created a unique opportunity for developing active vaccination strategies that engage the patient's own immune system in the fight against breast cancer. Early clinical trials have established the safety and bioactivity of some breast cancer vaccine approaches, with a hint of clinical response. They have also highlighted the importance of elucidating the pharmacodynamic interactions between established therapies for breast cancer, such as tamoxifen, aromatase inhibitors, chemotherapy, the HER-2/neu-specific monoclonal antibody trastuzumab (Herceptin), and breast cancer vaccines. Preclinical studies have simultaneously defined the importance of developing targeted approaches for circumventing established immune tolerance to breast cancer during the vaccination process. The first strategies targeting the negative influence of CD4(+)CD25(+)T regulatory cells and the CTLA-4 signaling pathway are just entering clinical testing in combination with tumor vaccines. Developing the most potent approach for activating antitumor immunity while maintaining the efficacy of standard approaches to breast cancer management will ensure that active immunotherapy is successfully integrated into the standard of care.

Cancer vaccines.

It has been more than 100 years since the first reported attempts to activate a patient's immune system to eradicate developing cancers. Although a few of the subsequent vaccine studies demonstrated clinically significant treatment effects, active immunotherapy has not yet become an established cancer treatment modality. Two recent advances have allowed the design of more specific cancer vaccine approaches: improved molecular biology techniques and a greater understanding of the mechanisms involved in the activation of T cells. These advances have resulted in improved systemic antitumor immune responses in animal models. Because most tumor antigens recognized by T cells are still not known, the tumor cell itself is the best source of immunizing antigens. For this reason, most vaccine approaches currently being tested in the clinics use whole cancer cells that have been genetically modified to express genes that are now known to be critical mediators of immune system activation. In the future, the molecular definition of tumor-specific antigens that are recognized by activated T cells will allow the development of targeted antigen-specific vaccines for the treatment of patients with cancer.

Novel allogeneic granulocyte-macrophage colony-stimulating factor-secreting tumor vaccine for pancreatic cancer: a phase I trial of safety and immune activation.

PURPOSE: Allogeneic granulocyte-macrophage colony-stimulating factor (GM-CSF)-secreting tumor vaccines can cure established tumors in the mouse, but their efficacy against human tumors is uncertain. We have developed a novel GM-CSF-secreting pancreatic tumor vaccine. To determine its safety and ability to induce antitumor immune responses, we conducted a phase I trial in patients with surgically resected adenocarcinoma of the pancreas. PATIENTS AND METHODS: Fourteen patients with stage 1, 2, or 3 pancreatic adenocarcinoma were enrolled. Eight weeks after pancreaticoduodenectomy, three patients received 1 x 10(7) vaccine cells, three patients received 5 x 10(7) vaccine cells, three patients received 10 x 10(7) vaccine cells, and five patients received 50 x 10(7) vaccine cells. Twelve of 14 patients then went on to receive a 6-month course of adjuvant radiation and chemotherapy. One month after completing adjuvant treatment, six patients still in remission received up to three additional monthly vaccinations with the same vaccine dose that they had received originally. RESULTS: No dose-limiting toxicities were encountered. Vaccination induced increased delayed-type hypersensitivity (DTH) responses to autologous tumor cells in three patients who had received >or= 10 x 10(7) vaccine cells. These three patients also seemed to have had an increased disease-free survival time, remaining disease-free at least 25 months after diagnosis. CONCLUSION: Allogeneic GM-CSF-secreting tumor vaccines are safe in patients with pancreatic adenocarcinoma. This vaccine approach seems to induce dose-dependent systemic antitumor immunity as measured by increased postvaccination DTH responses against autologous tumors. Further clinical evaluation of this approach in patients with pancreatic cancer is warranted.

Enhanced tumor protection by granulocyte-macrophage colony-stimulating factor expression at the site of an allogeneic vaccine.

Murine tumor models have demonstrated that whole tumor cell vaccines engineered to secrete certain cytokines in a paracrine fashion elicit systemic immune responses capable of eliminating small amounts of established tumor. In particular, autologous tumors that express the cytokine GM-CSF induce potent systemic immune responses against poorly immunogenic murine tumors. However, phase I clinical trials have demonstrated the technical difficulty of routinely expanding primary autologous human tumor cells to the numbers required for vaccination, making the generalization of autologous vaccines impractical. Dissection of the mechanism by which antitumor immunity is generated has demonstrated that GM-CSF recruits professional antigen-presenting cells that act as intermediates in presenting tumor antigen to and activating effector T cells. Furthermore, the identification of commonly recognized murine and human tumor antigens indicates that many are shared rather than unique. These findings would suggest that allogeneic as well as autologous tumor cells can be used as the vaccinating cells for activating antitumor immunity. A major concern in the application of allogeneic vaccines relates to the potential interference of allogeneic MHC expression at the vaccine site with priming of tumor-specific T cell responses. Here we describe a series of experiments that directly examines the effects of allogeneic MHC molecules on the immune-priming capabilities of a whole cell tumor vaccine engineered to secrete GM-CSF. The results demonstrate that the expression of an allogeneic MHC molecule by a vaccine cell can actually enhance the induction of systemic antitumor immunity. In addition, allogeneic MHC expression has no inhibitory effect on the ability of GM-CSF-transduced vaccines to induce systemic antitumor immunity. These findings support the design of clinical trials for testing this more feasible and generalizable allogeneic whole tumor cell vaccine approach.

Herpes simplex-1 virus thymidine kinase gene is unable to completely eliminate live, nonimmunogenic tumor cell vaccines.

Recent experiments with genetically engineered tumors have generated renewed interest in active cellular immunotherapy as a cancer treatment modality. In order to consider the use of live tumor cells for immunotherapy in human cancer patients, it will be important to ensure that these cells do not themselves produce morbidity in the event the immune system fails to eliminate them. Toward this end, we have examined a strategy for eliminating genetically manipulated nonimmunogenic tumors in vivo. When B16F10 melanoma cells were transfected with the Herpes simplex virus 1 thymidine kinase (HSV-TK) gene, cells were rendered susceptible to killing by the nucleoside analogs acyclovir (ACV) and ganciclovir (GCV). B16-HSV-TK+ tumors established in C57BL6 mice were successfully "suicided" in vivo when GCV was administered by continuous infusion. However, late recurrences were observed even after 1 month of continuous GCV treatment. In vivo growth kinetics suggested that the recurrences resulted from a tiny number (< 20) of cells that had survived the GCV treatment. Interestingly, recurrent tumors were as sensitive to GCV as the parental B16-HSV-TK+ line. While these results demonstrate potential feasibility of the suicide gene strategy for active immunotherapy with live tumor cells, they also illustrate that approaches dependent on the intracellular generation of cell cycle-dependent toxins may fail to eliminate small numbers of cells that temporarily exit cell cycle or that are pharmacologically sequestered.

Intensified adjuvant combined modality therapy for resected periampullary adenocarcinoma: acceptable toxicity and suggestion of improved 1-year disease-free survival.

PURPOSE: (1) To determine the toxicity of an intensified postoperative adjuvant regimen for periampullary adenocarcinoma (pancreatic and nonpancreatic) utilizing concurrent 5-fluorouracil (5-FU), leucovorin (LV), dipyridamole (DPM), and mitomycin-C (MMC) combined with split-course locoregional external beam radiotherapy (EBRT) to 50 Gy. This was followed by 4 cycles of the same chemotherapy as adjuvant therapy. (2) To determine preliminary estimates of the overall and disease-free survival associated with the use of this regimen. (3) To compare the toxicities and early survival results of patients treated with the current regimen to those of patients who completed our prior trial of concurrent chemoradiation infusion with 5-FU/LV chemotherapy and regional nodal and prophylactic hepatic irradiation. METHODS: Postpancreaticoduodenectomy, patients received every 4 weeks bolus administration of 5-FU, (400 mg/m(2)), and LV, (20 mg/m(2), Days l-3), DPM (75 mg p.o., 4 times per day, Days 0-3, and every 8 weeks), MMC, (10 mg/m(2); maximum of 20 mg, Day l during EBRT). This was followed by 4 months of the same chemotherapy, beginning 1 month following the completion of EBRT. EBRT consisted of split-course 5000 cGy/20 fractions with a 2-week planned rest after the first 10 fractions (2500 cGy). RESULTS: From 4/96 to 6/99, 45 patients were enrolled and treated. Their experience constitutes the basis of this analysis. There were 29 patients with pancreatic cancer and 16 with nonpancreatic periampullary cancer. Seventeen patients had tumors of 3 cm or more, and 39 patients had at least 1 histologically involved lymph node. Thirteen patients had a histologically positive margin of resection. The mean time to start of treatment was 63 days following surgery. During chemoradiation therapy there were no Grade 3 or worse nonhematologic toxicities and 47% Grade 3 or Grade 4 hematologic toxicities of short duration. Following chemoradiation, during chemotherapy treatment only, there was one Grade 3 hepatic and one Grade 3 pulmonary toxicity which was nondebilitating (2% each case) and 42% Grade 3 or 4 hematologic toxicity. There were 2 episodes of neutropenic fever requiring admission and no treatment-related mortalities. One patient developed a mild case of HUS, which responded to standard management. One patient developed persistent shortness of breath (nondebilitating), and another patient had occasional dyspnea on exertion, both occurring after all therapy. The majority of patients complained of increased fatigue (Grade 1-2), greatest during the combined therapy and improving post all treatment. As of 6/23/99, 20 of 45 patients have relapsed, 13 in the liver. Twelve patients have died. Median follow-up for surviving patients is 14.3 months. Disease-free survival at 12 months following surgery is 66% (as compared to 25% in our prior study), and the median disease-free survival is 17 months (as compared to 8. 3 months in our prior study). Median survival has not yet been reached, but will be greater than 17 months. CONCLUSION: With a 14.3-month median follow-up, acute toxicity has been acceptable and manageable. Observed relapses were seen 9-13 months following surgical resection. Early survival analysis suggests a trend toward increased median disease-free survival (8.3 vs. 17 months), especially for patients with nonpancreatic periampullary adenocarcinoma.

Humoral and cellular immune responses: independent forces or collaborators in the fight against cancer?

Recent advances in our understanding of immune function with regard to the generation of a potent antitumor response have resulted in a renewed interest in cancer vaccines and point to a role for immunotherapy in the treatment of cancer. Currently, the majority of vaccine strategies for the treatment of solid malignancies focus on the generation of cytotoxic T lymphocytes (CTL) that destroy tumor cells that express a given target protein, or antigen. However, antibody therapies have already been successful against some cancers. Current humoral immunotherapy typically involves the passive infusion of monoclonal antibodies, which usually target a specific tumor-encoded antigen. However, vaccines can be engineered to induce humoral immunity. By focusing on the cellular arm of the immune response at the expense of humoral immunity (or the converse), we may have inadvertently limited the potential efficacy of our anticancer vaccines. This article seeks to explore the notion that a vaccine designed to optimally activate both arms of the immune system may well generate an antitumor immune response greater than the sum of the two individual effector mechanisms alone.

Chemotherapy: friend or foe to cancer vaccines?

Cancer vaccines are on the threshold of taking their place alongside the more traditional cancer treatment modalities of surgery, radiation therapy and chemotherapy. The toxicology and immunopharmacology of therapeutic cancer vaccines, particularly those that secrete granulocyte macrophage colony stimulating factor (GM-CSF), are currently under active clinical investigation. Interestingly, drugs traditionally used for tumor cytoreduction can have both positive and negative effects on host immunity. Exploration of the potential pharmacodynamic interactions of antineoplastic drugs with GM-CSF-secreting vaccines has revealed that low doses of some chemotherapeutics can augment the antitumor immunity induced by GM-CSF-secreting vaccines. These interactions will require thorough preclinical evaluation to maximize the clinical impact of this type of therapeutic cancer vaccine.

Immunotherapy of cancer.

The goal of cancer treatment is to develop modalities that specifically target tumor cells, thereby avoiding unnecessary side effects to normal tissue. Vaccine strategies that result in the activation of the immune system specifically against proteins expressed by a cancer have the potential to be effective treatment for this purpose. An early vaccine approach that was developed by our group involves the insertion of the granulocyte-macrophage colony stimulating factor (GM-CSF) gene into cancer cells that are then used to immunize patients. These genetically modified tumor cells produce the immune activating protein GM-CSF in the local environment of the tumor cells, specifically activating the patient's T cells to eradicate cancer at metastatic sites. We have performed many studies that demonstrate that this vaccine can cure mice of cancer. We recently demonstrated that this approach can activate an immune response in patients with renal cell carcinoma. We are currently testing a similar approach in patients with pancreatic cancer. Until recently, whole tumor cells were used to produce the vaccine because the proteins expressed by the tumor cells that can be recognized by the immune system were unknown. However, recent advances have allowed the identification of many of the proteins expressed by some cancers. In addition, significant attention has been focused on the mechanisms by which antitumor immunity can be modulated. These active areas of research will undoubtably lead to the development of more specific and more potent vaccine strategies in the near future. The first part of this paper focuses on data from two recent clinical trials that evaluated the whole tumor cell approach. The second part of this paper discusses some of the more exciting antigen-specific vaccine approaches that are under development for the treatment of cancer.

Potential role of tumor vaccines in GI malignancies.

Although surgery remains the only curative option for patients with gastrointestinal (GI) malignancies, the use of adjuvant chemotherapy and/or localized radiation is considered standard therapy for patients who present with locoregional disease. Even with adjuvant therapy, however, the 5-year survival rate for such patients ranges from 2% to 50%, depending on the specific cancer type and stage. As a result, more effective interventions are necessary for all but the earliest stages of GI malignancies. Colon cancer represents the paradigm for the management of GI malignancies, not only because it is, by far, the most common cancer in this group, but also because the biological progression to disease is well characterized. Immunotherapy is an alternative approach for treating GI malignancies that can either: (1) activate tumor-specific T-cells; or (2) use monoclonal antibodies derived from tumor-specific antigens. Monoclonal antibodies act by a mechanism that is distinct from that of chemotherapy and, thus, represent a non-cross-resistant treatment with an entirely different spectrum of toxicities. Thanks to an improved understanding of tumor immunology, as well as the events needed to generate an optimal immune response, the possibility of designing an effective colon cancer vaccine approach that induces both humoral and cellular responses has become even more realistic.