Regression of tumors in mice vaccinated with professional antigen-presenting cells pulsed with tumor extracts

Vaccination with tumor extracts circumvents the need to identify specific tumor rejection antigens and extends the use of active immunotherapy to the vast majority of cancers, in which specific tumor antigens have not yet been identified. In this study we examined the efficacy of tumor vaccines comprised of unfractionated tumor material presented by professional antigen-presenting cells (APC): dendritic cells (DC) or macrophages (M phi). To enhance the relevance of these studies for human patients we used 2 poorly immunogenic murine tumor models and evaluated the effectiveness of the vaccination protocols in tumor-bearing animals. APC (in particular DC) pulsed with unfractionated extracts from these "poorly immunogenic" tumors were highly effective in eliciting tumor-specific cytotoxic T lymphocytes. A measurable CTL response could be detected after even a single immunization with tumor extract-pulsed DC. DC or M phi pulsed with tumor extract were also effective vaccines in tumor-bearing animals. In the murine bladder tumor (MBT-2) model a modest extension of survival and 40% cure rate was seen in the animal groups immunized with DC or M phi pulsed with MBT-2 tumor extract. DC or M phi pulsed with B16/F10.9 tumor extract were also remarkably effective in the B16 melanoma lung metastasis model, as shown by the observation that treatment with APC caused a significant reduction in lung metastases. Cumulatively, the CTL and immunotherapy data from the two murine tumor systems suggest that APC (in particular DC) pulsed with unfractionated cell extracts as a source of tumor antigen may be equally or more effective than genetically modified tumor vaccines.

Antigen-presenting cells pulsed with unfractionated tumor-derived peptides are potent tumor vaccines

Vaccination with peptides isolated from tumor cells circumvents the need for identifying specific tumor rejection antigens and extends the use of active immunotherapy to the majority of cancers where specific tumor antigens have not yet been identified. In this study, we examined the efficacy of tumor vaccines composed of unfractionated tumor peptides presented by antigen-presenting cells (APC) to induce cytotoxic T lymphocyte (CTL) responses and tumor immunity. RMA-S cells pulsed with peptides isolated from ovalbumin (OVA)-expressing tumor cells were highly effective at inducing primary, OVA-specific CTL responses in vitro and priming CTL responses in vivo. In addition, tumor peptide-pulsed RMA-S cells induced protective immunity in mice when challenged with OVA-expressing tumor cells. To enhance the clinical relevance of these studies, cells pulsed with tumor peptides were evaluated in the poorly immunogenic, B16/F10.9 melanoma post-surgical metastasis model. Treatment of tumor-bearing mice with peptide-pulsed RMA-S cells or with adherent splenocytes (enriched for professional APC) caused a significant reduction in lung metastases. The antimetastatic effect of peptide-pulsed splenocytes could be further enhanced by pretreating the cells with antisense oligonucleotides directed against the TAP-2 gene which was previously shown to increase the density of specific peptide/MHC class I complexes and thereby improve the APC function of the treated cells (Nair et el., J. Immunol. 1996. 156: 1772). This study suggests that APC loaded with unfractionated peptides derived from poorly immunogenic, highly metastatic tumor cells may represent a potent form of tumor vaccine.

Inhibition of established pancreatic cancers following specific active immunotherapy with interleukin-2 gene-transduced tumor cells

Pancreatic cancer has a poor prognosis even when complete resection can be accomplished. Recent studies have demonstrated that the immune system is capable of mounting effective tumor-specific immune responses even against "nonimmunogenic" tumors. The studies reported herein were conducted to determine if induction of tumor-specific immune responses of inhibiting in vivo pancreatic tumor growth could be achieved through active immunization with pancreatic tumor cells genetically engineered to secrete interleukin-2 (IL-2). A relevant poorly immunogenic subcutaneous model of murine ductal pancreatic cancer was first developed using an implantable tumor cell line Panc02 in C57BL/6 mice. Panc02 cells were then genetically engineered to secrete human IL-2 (Panc02/IL2). The ability of irradiated Panc02/IL2 cells to stimulate an immune response capable of rejecting a subsequent tumor challenge was first demonstrated. Ninety percent of animals vaccinated with irradiated parental Panc02 and subsequently challenged with parental Panc02 cells developed tumors by 48 days (mean tumor volume of 234 mm3) compared to only 40% (P < .05, chi square) of animals vaccinated with irradiated Panc02/IL2 and challenged with parental Panc02 (14 mm3, P < .004, tau test). The therapeutic benefit of active immunization in tumor-bearing animals was then examined. Mice with 3-day-old established subcutaneous tumors were administered a series of 4 weekly vaccinations with irradiated Panc02 or Panc02/IL2 cells. A significant reduction in tumor growth was present in those animals vaccinated with Panc02/IL2 (P < .005, tau test) versus Panc02 or saline. Animals whose established tumors regressed following vaccinations with IL-2-secreting Panc02 cells were found to have long-lasting immunity as demonstrated by rejection of a tumor challenge presented over 140 days following inoculation of the primary tumor. We conclude that an immune response capable of inhibiting established pancreatic tumors can be generated by vaccination with IL-2-secreting tumor cells. Furthermore, long-term immunological memory was established in mice that rejected the original established tumor. These studies provide preclinical evidence to support the use of cytokine gene-transduced tumor cell vaccinations in patients with pancreatic cancer.

Intranasal immunization with CTL epitope peptides from HIV-1 or ovalbumin and the mucosal adjuvant cholera toxin induces peptide-specific CTLs and protection against tumor development in vivo

To evaluate the ability of mucosal immunization protocols using peptide immunogens to induce CTL responses, BALB/c and C57BL/6 mice were immunized intranasally (i.n.) with peptides corresponding to a known CTL epitope in HIV-1 glycoprotein 120 or OVA, respectively, and the mucosal adjuvant cholera toxin (CT). Intranasal immunization of BALB/c mice with a 10- or 15-amino acid peptide corresponding to a CTL determinant in HIV-1 glycoprotein 120 and CT induced peptide-specific CTLs in spleen cells that persisted through 35 days after the last immunization. Intranasal immunization of C57BL/6 mice with the octameric OVA peptide and CT produced similar results with detectable peptide-specific CTL in both the cervical lymph node and spleen. To test whether CTL induced by i.n. immunization with OVA peptide and CT were functional in vivo, groups of C57BL/6 mice were injected with E.G7-OVA tumor cells that express the OVA protein and monitored for tumor growth. Animals immunized i.n. with OVA and CT were protected against tumor development as efficiently as animals immunized by the potent CTL induction protocol of i.v. injection with OVA-pulsed dendritic cells. Intranasal immunization with peptides corresponding to known CTL epitopes and CT provides a noninvasive route of immunization for the induction of CTL responses in vivo.

Intranasal immunization with CTL epitope peptides from HIV-1 or ovalbumin and the mucosal adjuvant cholera toxin induces peptide-specific CTLs and protection against tumor development in vivo

To evaluate the ability of mucosal immunization protocols using peptide immunogens to induce CTL responses, BALB/c and C57BL/6 mice were immunized intranasally (i.n.) with peptides corresponding to a known CTL epitope in HIV-1 glycoprotein 120 or OVA, respectively, and the mucosal adjuvant cholera toxin (CT). Intranasal immunization of BALB/c mice with a 10- or 15-amino acid peptide corresponding to a CTL determinant in HIV-1 glycoprotein 120 and CT induced peptide-specific CTLs in spleen cells that persisted through 35 days after the last immunization. Intranasal immunization of C57BL/6 mice with the octameric OVA peptide and CT produced similar results with detectable peptide-specific CTL in both the cervical lymph node and spleen. To test whether CTL induced by i.n. immunization with OVA peptide and CT were functional in vivo, groups of C57BL/6 mice were injected with E.G7-OVA tumor cells that express the OVA protein and monitored for tumor growth. Animals immunized i.n. with OVA and CT were protected against tumor development as efficiently as animals immunized by the potent CTL induction protocol of i.v. injection with OVA-pulsed dendritic cells. Intranasal immunization with peptides corresponding to known CTL epitopes and CT provides a noninvasive route of immunization for the induction of CTL responses in vivo.

Active immunization with tumor cells transduced by a novel AAV plasmid-based gene delivery system

Ex vivo genetically engineered cytokine-secreting tumor cell vaccines have been shown to prevent metastatic disease in animal models of lung and breast cancer. Because of the inefficiency of existing modes of gene delivery in transducing primary human tumor cells, it has been difficult to clinically apply this strategy. In this study, liposome-mediated delivery of an adeno-associated virus (AAV)-based plasmid containing the sequence for murine gamma-interferon (gamma-IFN) (pMP6A-mIFN-gamma) was used to generate cytokine-secreting murine tumor cell vaccines. High levels of gamma-IFN and elevated class I major histocompatibility complex expression after transfer of pMP6A-mIFN-gamma into the murine lung cancer cell line, D122, was demonstrated. The efficiency of gene transfer was determined by two different methods and was estimated to be 10-15%. Irradiated gamma-IFN D122 cells generated by this novel gene delivery system (D122/pMP6A-mIFN-gamma) and also by standard retroviral methods (DIF2) were administered as weekly vaccinations by intraperitoneal injection to animals bearing 7-day-old intrafootpad D122 tumors. Hindlimb amputation was performed when footpad diameters reached 7 mm, and lungs were harvested 28 days later. Animals vaccinated with gamma-IFN-secreting D122 cells produced by AAV-based plasmids delivery demonstrated a significant delay in footpad tumor growth when compared with controls and DIF2 cells. Fifty-seven percent of animals vaccinated with D122/pMP6A-mIFN-gamma were free of pulmonary metastases 28 days after amputation, significantly improved from the 0, 7, and 15% observed in animals vaccinated with irradiated parental D122 cells, irradiated D122 cells lipofected with an empty-cassette vector (pMP6A), or DIF2 cells, respectively. These results and the ability to transfer genes with this delivery system to a broad range of tumor types support its use in the generation of cytokine-secreting tumor cell vaccinations for use in clinical trials.

Transient protection of human T-cells from human immunodeficiency virus type 1 infection by transduction with adeno-associated viral vectors which express RNA decoys

RNA decoys are oligonucleotides corresponding to the TAR and RRE sequences of HIV which inhibit the HIV-encoded regulatory proteins Tat and Rev, respectively. Adeno-associated viral vectors encoding RNA decoys stably transduced into the human T-cell line CEM-SS expressed transactivating region (TAR) and Rev-responsive element (RRE) RNA decoys from tRNA polIII promoters at high levels, without any apparent deleterious effects on cell growth or expression of CD4. DNA blot analysis indicated that RNA decoy-encoding vectors were not rearranged and were integrated into the genomic DNA of selected cell lines. Vector DNA with the appropriate TAR and RRE sequences was isolated from transduced cell lines after prolonged growth in culture, further confirming that the vector DNA was present in a stable form through multiple cell cycles. Cell lines expressing TAR and RRE decoys transiently inhibited HIV gene expression and replication by 70-99% as determined by measurement of intracellular and extracellular HIV p24 production. Adeno-associated vectors encoding RNA decoys may be useful for gene therapy of HIV infection.

High-efficiency retroviral vector mediated gene transfer into human peripheral blood CD4+ T lymphocytes

Genetic modification of peripheral blood T lymphocytes has been proposed as a therapeutic strategy for treating congenital disorders, cancer and viral diseases. Central to all T lymphocyte-based gene therapy strategies is the ability to efficiently and stably deliver genes into primary T lymphocytes. In this study, we sought to increase the gene transfer efficiency in CD4+ peripheral blood T lymphocytes using procedures which could be utilized in clinical applications. In order to quantity the gene transfer efficiency in primary CD4+ T cells, a high-titer retroviral vector which efficiently expresses a truncated version of the human low-affinity nerve growth factor receptor (delta LNGFR) was constructed. Transduced cells were then accurately enumerated with immunofluorescence staining and fluorescence activated cell sorting (FACS) analysis and rapidly isolated at high purity for further analysis. Using this system, a supernatant-based gene transfer procedure was developed which routinely yields gene transfer efficiencies of 25-40% into a wide repertoire of both freshly obtained and cryopreserved peripheral blood CD4+ T lymphocytes.

Dendritic cells pulsed with RNA are potent antigen-presenting cells in vitro and in vivo

Immunization with defined tumor antigens is currently limited to a small number of cancers where candidates for tumor rejection antigens have been identified. In this study we investigated whether pulsing dendritic cells (DC) with tumor-derived RNA is an effective way to induce CTL and tumor immunity. DC pulsed with in vitro synthesized chicken ovalbumin (OVA) RNA were more effective than OVA peptide-pulsed DC in stimulating primary, OVA-specific CTL responses in vitro. DC pulsed with unfractionated RNA (total or polyA+) from OVA-expressing tumor cells were as effective as DC pulsed with OVA peptide at stimulating CTL responses. Induction of OVA-specific CTL was abrogated when polyA+ RNA from OVA-expressing cells was treated with an OVA-specific antisense oligodeoxynucleotide and RNase H, showing that sensitization of DC was indeed mediated by OVA RNA. Mice vaccinated with DC pulsed with RNA from OVA-expressing tumor cells were protected against a challenge with OVA-expressing tumor cells. In the poorly immunogenic, highly metastatic, B16/F10.9 tumor model a dramatic reduction in lung metastases was observed in mice vaccinated with DC pulsed with tumor-derived RNA (total or polyA+, but not polyA- RNA). The finding that RNA transcribed in vitro from cDNA cloned in a bacterial plasmid was highly effective in sensitizing DC shows that amplification of the antigenic content from a small number of tumor cells is feasible, thus expanding the potential use of RNA-pulsed DC-based vaccines for patients bearing very small, possibly microscopic, tumors.

Induction of antitumor immunity using bone marrow-generated dendritic cells

We have previously shown that bone marrow-generated dendritic cells (DC) pulsed with a class I-restricted peptide are potent inducers of CD8+ CTL. In the present study we have investigated whether bone marrow-generated DC are capable of inducing antitumor immunity. We show that a single immunization with DC pulsed with OVA peptide was highly effective in eliciting a protective immune response against a challenge with tumor cells expressing the OVA gene (E.G7-OVA), more so than immunization with irradiated E.G7-OVA cells, OVA peptide-pulsed RMA-S cells, or free OVA peptide mixed with adjuvant. The addition of free OVA protein to day 4 or day 7 bone marrow cultures, but not to day 9 mature DC, was also effective in eliciting CTL and engendering antitumor immunity, but was less effective than peptide-pulsed DC. Induction of CTL and antitumor immunity by bone marrow-generated DC pulsed with the class I-restricted OVA peptide correlated with the expression of syngeneic MHC class II molecules on the DC. This and the fact that induction of tumor immunity was dependent on CD4+ T cells suggest that in vivo priming of CTL and induction of antitumor immunity by bone marrow-generated DC also require the presentation of MHC class II-restricted epitopes and activation of CD4+ T cells. This observation has potentially important implications to the use of peptide-pulsed DC in clinical immunotherapy.

Induction of antitumor immunity using bone marrow-generated dendritic cells

We have previously shown that bone marrow-generated dendritic cells (DC) pulsed with a class I-restricted peptide are potent inducers of CD8+ CTL. In the present study we have investigated whether bone marrow-generated DC are capable of inducing antitumor immunity. We show that a single immunization with DC pulsed with OVA peptide was highly effective in eliciting a protective immune response against a challenge with tumor cells expressing the OVA gene (E.G7-OVA), more so than immunization with irradiated E.G7-OVA cells, OVA peptide-pulsed RMA-S cells, or free OVA peptide mixed with adjuvant. The addition of free OVA protein to day 4 or day 7 bone marrow cultures, but not to day 9 mature DC, was also effective in eliciting CTL and engendering antitumor immunity, but was less effective than peptide-pulsed DC. Induction of CTL and antitumor immunity by bone marrow-generated DC pulsed with the class I-restricted OVA peptide correlated with the expression of syngeneic MHC class II molecules on the DC. This and the fact that induction of tumor immunity was dependent on CD4+ T cells suggest that in vivo priming of CTL and induction of antitumor immunity by bone marrow-generated DC also require the presentation of MHC class II-restricted epitopes and activation of CD4+ T cells. This observation has potentially important implications to the use of peptide-pulsed DC in clinical immunotherapy.

Cells treated with TAP-2 antisense oligonucleotides are potent antigen-presenting cells in vitro and in vivo

Treatment of RMA and EL4 cells or freshly isolated splenocytes with antisense (AS) oligonucleotides directed against the TAP-2 gene recreates the phenotype seen in cells that are genetically deficient in TAP function. Cells incubated with AS oligonucleotides exhibit reduced MHC class I expression on the cell surface, which can be increased by incubating the oligonucleotide-treated cells at 28 degrees C or by adding MHC haplotype-matched peptides to the culture medium. RMA cells or splenocytes treated with AS oligonucleotides and incubated with peptide were highly effective in generating primary CTL responses in vitro. The bulk of the AS oligonucleotide-responsive and CTL-inducing cells resided in the adherent fraction of splenocytes. Moreover, TAP-2 AS oligonucleotide-treated adherent splenocytes pulsed with OVA peptide elicited potent OVA-specific CTL responses in vivo and provided effective protection from challenge with tumor cells expressing the corresponding Ag. AS oligonucleotide technology provides a simple approach to develop broadly applicable methods for generating potent APC to study TAP function in normal cells and to identify other gene products involved in MHC class I presentation.

Cells treated with TAP-2 antisense oligonucleotides are potent antigen-presenting cells in vitro and in vivo

Treatment of RMA and EL4 cells or freshly isolated splenocytes with antisense (AS) oligonucleotides directed against the TAP-2 gene recreates the phenotype seen in cells that are genetically deficient in TAP function. Cells incubated with AS oligonucleotides exhibit reduced MHC class I expression on the cell surface, which can be increased by incubating the oligonucleotide-treated cells at 28 degrees C or by adding MHC haplotype-matched peptides to the culture medium. RMA cells or splenocytes treated with AS oligonucleotides and incubated with peptide were highly effective in generating primary CTL responses in vitro. The bulk of the AS oligonucleotide-responsive and CTL-inducing cells resided in the adherent fraction of splenocytes. Moreover, TAP-2 AS oligonucleotide-treated adherent splenocytes pulsed with OVA peptide elicited potent OVA-specific CTL responses in vivo and provided effective protection from challenge with tumor cells expressing the corresponding Ag. AS oligonucleotide technology provides a simple approach to develop broadly applicable methods for generating potent APC to study TAP function in normal cells and to identify other gene products involved in MHC class I presentation.

Immunotherapy of cancer with genetically modified tumor vaccines

Distant metastasis is the major cause for therapeutic failures in Clinical oncology. Active immunotherapy in the adjuvant setting of patients with low tumor volume would contribute to further reduction of the remaining tumor and establish long-lasting immunity that could protect the patient from recurrence of disease. Studies employing rodent tumor models with little or no intrinsic immunogenicity have shown that genetically modified tumor cell preparations consisting of irradiated tumor cells transduced with and expressing cytokines, such as interleukin-2 (IL-2), IL-6, interferon-gamma (IFNgamma), or granulocyte-macrophage colony stimulating factor (GM-CSF), or co-stimulatory molecules, such as B7-I, were capable of inducing the regression of preexisting tumors and cure animals from their disease. Moreover, in some instances, the cured animals have retained immunological memory, as indicated by the fact that animals have resisted a second challenge with the parental tumor cells. Induction of potent immune responses in tumor-bearing animals against nonimmunogenic or weakly immunogenic tumors supports the view that active immunization of cancer patients deserves consideration despite lack of demonstrable immunogenicity in many human tumors.

Inhibition of HIV-1 in CEM cells by a potent TAR decoy

TAR decoys are short RNA oligonucleotides, corresponding to the HIV TAR sequence, which inhibit HIV expression and replication by blocking the binding of the HIV regulatory protein Tat to the authentic TAR region. In previous studies, TAR decoys expressed from a tRNA polIII promoter were moderately effective at inhibiting HIV in isolated human T cell lines and less effective at inhibiting HIV in peripheral blood CD4+ T cells. In this study, a series of modifications was introduced into the tRNA expression cassette in order to improve their effectiveness. These modifications included the addition of sequences which are predicted to have stem-loop secondary structures and addition of a wild-type tRNA processing site. TAR decoy RNA expressed in CEM cells from modified tRNA-based expression cassettes yielded five- to 20-fold more TAR transcripts than unmodified tRNA-based expression cassettes. HIV replication, as measured by a flow cytometric method to quantify intracellular viral p24 expression, was significantly reduced in polyclonal populations of CEM cells expressing a modified tRNA-TAR transcript that contains a wild-type tRNA processing site and stem-loops 5' and 3' to the TAR sequence. Similar modifications to the tRNA expression cassette also increased the intracellular concentration of a random test oligonucleotide, indicating that this improved expression system may also be useful for antisense and ribozyme based gene inhibition strategies.

Bone marrow-generated dendritic cells pulsed with a class I-restricted peptide are potent inducers of cytotoxic T lymphocytes

It has previously been shown that bone marrow-generated dendritic cells (DC) are potent stimulators in allogeneic mixed leukocyte reactions and are capable of activating naive CD4+ T cells in situ in an antigen-specific manner. In this study we have investigated whether bone marrow-generated DC are capable of inducing antigen-specific CD8+ T cell responses in vivo. Initial attempts to induce specific cytotoxic T lymphocyte (CTL) responses in mice injected with bone marrow-generated DC pulsed with ovalbumin (OVA) peptide were frustrated by the presence of high levels of nonspecific lytic activity, which obscured, though not completely, the presence of Ag-specific CTL. Using conditions that effectively differentiate between antigen-specific and nonspecific lytic activity, we have shown that bone marrow-generated DC pulsed with OVA peptide are potent inducers of OVA-specific CTL responses in vivo, compared with splenocytes or RMA-S cells pulsed with OVA peptide, or compared with immunization with free OVA peptide mixed with adjuvant. Antibody-mediated depletion experiments have shown that the cytotoxic effector cells consist primarily of CD8+ cells, and that induction of CTL in vivo is dependent on CD4+ as well as on CD8+ T cells. These results provide the basis for exploring the role of bone marrow-generated DC in major histocompatibility class I-restricted immune responses, and they provide the rationale for using bone marrow-generated DC in CTL-mediated immunotherapy of cancer and infectious diseases.

Efficient gene transfer with adeno-associated virus-based plasmids complexed to cationic liposomes for gene therapy of human prostate cancer

We have shown previously that treatment of rats bearing the Dunning R3327 MatLyLu prostatic tumor with human interleukin 2 (IL-2) gene-modified tumor cell preparations induces potent antitumor immunity in the animal. To test the clinical feasibility of using genetically modified tumor vaccines for the treatment of prostate cancer, we have explored the use of a simplified gene delivery system based on liposomes to introduce and express the IL-2 gene in the Dunning rat R3327 MatLyLu prostatic tumor cell line (MatLyLu) and in short-term cultures of primary human prostatic tumor cells. Liposome-DNA complexes containing the adeno-associated virus inverted terminal repeats exhibited 3-10-fold higher levels of gene transfer and IL-2 expression than did liposome complexes with non-adeno-associated virus containing plasmids. Single transfections resulted in IL-2 expression for an extended period of time that exceeded severalfold the amount of IL-2 secreted from retrovirally transduced MatLyLu cells. X-irradiation of cells (4000 rads) prior to transfection did not affect cytokine secretion, indicating that liposome-mediated gene transfer does not depend on cell proliferation. High levels of gene transfer and IL-2 expression were also achieved in short-term cultures of primary human prostatic tumor cells established from tumor specimens obtained following radical prostatectomy of cancer patients. Depending on the type of liposome used, IL-2 levels secreted from the human prostatic tumor cells were comparable to or exceeded the levels of IL-2 secreted from retrovirally transduced MatLyLu cells, which induced antitumor immunity in the rat model. The ability to culture and expand ex vivo human prostatic tumor cells, and the use of a simple and highly efficient gene transfer method to generate genetically modified tumor vaccines, set the stage for clinical exploration of gene-based immunotherapy of prostate cancer.

RG-2 glioma growth attenuation and severe brain edema caused by local production of interleukin-2 and interferon-gamma

Two aspects of cytokine therapy of intracerebral tumors are considered in this study: modulation of tumor growth in vivo and central nervous system toxicity. Coimplantation of RG-2 glioma cells and retroviral vector producer cell lines was performed to provide a local source of interleukin-2 (IL-2) or IFN-gamma within the tumor and coinitiate an antitumor immune response. We demonstrated that local intratumoral production of IL-2 and IFN-gamma generates a cell-mediated antitumor response in vivo. This response was manifest as a diffuse infiltration of monocytes/macrophages, CD4+ and CD8+ T cells, and activation of microglial OX42+ cells in intracerebral RG2 tumors. The cell-mediated antitumor immune response resulted in the early suppression of intracranial and subcutaneous tumor growth, but the effect was not sustained and there were no tumor regressions. The absence of increased survival of animals with intracranial tumors is explained in part by the severe central nervous system toxicity caused by local production of IL-2 and IFN-gamma. Central nervous system toxicity induced blood-brain barrier disruption, vasogenic brain edema, and dislocation of the brain midline structures, as observed by dynamic magnetic resonance imaging and direct measurements of tissue water content. The clinical application of IL-2 and IFN-gamma gene transfer therapy for intracerebral tumors must consider the potential for severe vasogenic brain edema associated with intracerebral production of these cytokines.

Gene therapy approaches in urologic oncology

The tools and concepts of cytokine gene-based immunotherapy are being applied to the development of potentially effective new adjuvant treatment modalities for urologic malignancies. In preclinical models for the most prevalent urologic cancers, such as renal cell, bladder, and prostate carcinoma, it was shown that cytokine secreting, growth inactivated, tumor cell preparations (1) are capable of inducing a T-cell response against even nonimmunogenic tumors, (2) have considerable therapeutic benefit in tumor bearing animals, and (3) establish effective immunologic memory in cured animals. These studies have advanced further our understanding of the efficacy and therapeutic use of cytokine secreting tumor cells and form the rationale for translating these preclinical results into a clinical setting. It is realistic to speculate that in the foreseeable future alternative or complementary approaches to cytokine gene-based immunotherapy will be developed that would augment immune responses in cancer patients. Genetically modified dendritic cells transduced with genes encoding isolated tumor rejection antigens or costimulatory signals, such as B7, may be even more potent immune stimulators to induce systemic immune responses. Although animal studies have shown considerable promise and investigational clinical trials are underway, additional research and further development still is required to realize the full benefit of this approach, and some forms of cancer eventually may respond to this form of cancer immunotherapy.

Considerations for the use of cytokine-secreting tumor cell preparations for cancer treatment

Limited efficacy of chemotherapy in most solid tumors has revived interest in immunotherapeutic approaches for cancer. One novel form of immunotherapy is the use of cancer vaccines consisting of tumor cells genetically engineered to secrete cytokines. The rationale for this immunization strategy is based on the existence of tumor-specific antigens, on the importance of the cellular arm of the immune system in mediating an effective antitumor response, and on the role of cytokines in regulating the cellular immune response. Such tumor vaccines showed considerable promise in various animal models and induced potent antitumor immunity in the host, which led to regression of established tumors and, moreover, produced immunological memory protecting animals from a subsequent tumor challenge at a distant site. Translated to the human patient, this implies that genetically modified tumor vaccines may be able to eradicate or reduce existing tumor deposits to subclinical levels as well as provide long-term protection from regrowth of tumor cells. This report will review and discuss the concept and rationale for the use of cytokine-secreting tumor vaccines for the treatment of human malignancies.

Inhibition of human immunodeficiency virus type 1 in human T cells by a potent Rev response element decoy consisting of the 13-nucleotide minimal Rev-binding domain

Intracellular immunization is an anti-viral gene therapy strategy based on the introduction of DNA templates into cells to stably express genetic elements which inhibit viral gene expression and replication. We have recently developed an intracellular immunization strategy for human immunodeficiency virus (HIV) infection that uses RNA decoys. RNA decoys are short RNA oligonucleotides corresponding to the HIV trans activation response element (TAR) or Rev response element (RRE) sequences, which function by inhibiting the binding of the HIV regulatory proteins Tat and Rev to the authentic HIV RNA TAR and RRE regions, respectively. In this report we describe the characterization of potent RRE decoys containing the minimal 13-nucleotide primary Rev binding domain of the RRE. Using an improved tRNA cassette to express high levels of RRE transcripts in CEM cells, we found that this new generation of minimal RRE decoys were more potent inhibitors of HIV in isolated cell lines than previously described TAR or RRE decoys. CEM cells expressing RRE decoys exhibited diminished Rev function in cotransfection assays, confirming the specificity of inhibition of HIV by RRE decoys and indicating that the 13-nucleotide minimal Rev binding domain defined by using in vitro binding studies also binds Rev in vivo. Significant differences in the degree of HIV inhibition between individual CEM cell lines transduced with RRE decoy vectors which were not due to sequence alterations in the tRNA-RRE DNA template, differences in RRE decoy expression level, or endogenous variations in the resistance of CEM clonal cell lines to HIV were observed. In order to evaluate the efficacy of RRE decoys in a more realistic fashion than by comparison of individual clonal cell lines, polyclonal populations of transduced CEM cells were infected with HIV. By using a novel flow cytometric method for quantitating intracellular p24 expression, one version of the RRE decoys tested in this study was found to be capable of durably protecting polyclonal populations of CEM cells from HIV.

Immunotherapy of cancer using cytokine gene-modified tumor vaccines

Distant metastasis is the major cause for therapeutic failures in clinical oncology. Active immunotherapy of patients with low tumor burden would not only contribute to further reduction of the remaining tumor burden to subclinical levels, but it would also establish a constant state of immunity, i.e. immunological memory, that could protect the patient from recurrence of disease. Studies employing rodent tumor models with little or no intrinsic immunogenicity have shown that genitically modified tumor cell preparations consisting of irradiated tumor cells transduced with and expressing cytokines such as IL-2, IL-6, IFN-gamma or GM-CSF were capable of inducing the regression of a preexisting tumor burden and cure animals from their disease. Moreover, in some instances the cured animals have retained immunological memory, as indicated by the fact that such animals have resisted a second challenge with the parental tumor cells. Induction of potent immune responses in tumor bearing animals against non-immunogenic tumors supports the view that active immunization of cancer patients deserves consideration despite lack of demonstrable immunogenicity in many human tumors.

Immunotherapy of bladder cancer with cytokine gene-modified tumor vaccines

This study explored the use of cytokine gene-modified tumor cells as cellular vaccines for the treatment of bladder cancer. The mouse MBT-2 tumor is an excellent model for human bladder cancer. This carcinogen-induced tumor of bladder origin resembles human bladder cancer in its etiology and histology and responds to treatment in a manner similar to that of its human counterpart. In a previous study we have shown that interleukin 2 (IL-2)-secreting, irradiated, MBT-2 cell preparations were capable of curing animals from orthotopically established tumors and engendered protective immunological memory in the cured animals. In this study we have compared the effectiveness of several cytokines and found that while IL-1 alpha, IL-1 beta, and gamma-interferon were only weakly effective in the therapeutic vaccination protocol, granulocyte-macrophage colony-stimulating factor was almost as effective as but not superior to IL-2, as reported previously for another tumor model system. Induction of cytotoxic T-lymphocyte correlated only poorly with the therapeutic benefit of the cytokine gene-modified tumor cell preparations, questioning its prognostic value for the development of improved genetically modified tumor vaccines.

An experimental model simulating local recurrence and pelvic lymph node metastasis following orthotopic induction of prostate cancer

In order to develop an animal model that more closely simulates the organ environment and metastatic routes of human prostatic cancer, R3327-MatLyLu tumors were induced by orthotopic implantation in the ventral prostatic lobe of Copenhagen rats. This procedure reproducibly resulted in metastatic spread of the intraprostatic tumor to the pelvic and retroperitoneal lymph nodes, and invariably to the lungs. Further, a tumor recurrence model was established using an approach that combined orthotopic tumor implantation and subsequent surgical resection of the primary tumor. When prostatectomy was carried out 4 days or more after induction, tumors recurred locally in all animals. The surgical procedures described may provide an animal model to test the in vivo response to experimental adjuvant treatment protocols for advanced prostate cancer. Immunological studies are now in progress using cytokine gene-modified prostatic tumor cells as cellular antitumor vaccines in orthotopically established R3327-MatLyLu tumors.