Cytokine gene therapy of mesothelioma. Immune and antitumor effects of transfected interleukin-12

Intracavitary Therapeutics for Pleural Malignancies

Pleural malignancies remain a serious therapeutic challenge, and are frequently refractory to standard treatment; however, they have the advantage of occurring in an enclosed cavity readily accessible for examination, biopsy, and serial sampling. Novel therapeutics can be administered via intracavitary delivery to maximize efficacy by targeting the site of involvement and potentially mitigating the adverse effects of systemic therapies. The easy accessibility of the pleural space lends itself well to repeated sampling and analysis to determine efficacy and toxicity of a given treatment paradigm. These factors support the rationale for delivery of novel therapeutics directly into the pleural space.

The "Trojan Horse" approach to tumor immunotherapy: targeting the tumor microenvironment

Most anticancer therapies including immunotherapies are given systemically; yet therapies given directly into tumors may be more effective, particularly those that overcome natural suppressive factors in the tumor microenvironment. The “Trojan Horse” approach of intratumoural delivery aims to promote immune-mediated destruction by inducing microenvironmental changes within the tumour at the same time as avoiding the systemic toxicity that is often associated with more “full frontal” treatments such as transfer of large numbers of laboratory-expanded tumor-specific cytotoxic T lymphocytes or large intravenous doses of cytokine. Numerous studies have demonstrated that intratumoural therapy has the capacity to minimizing local suppression, inducing sufficient “dangerous” tumor cell death to cross-prime strong immune responses, and rending tumor blood vessels amenable to immune cell traffic to induce effector cell changes in secondary lymphoid organs. However, the key to its success is the design of a sound rational approach based on evidence. There is compelling preclinical data for local immunotherapy approaches in tumor immunology. This review summarises how immune events within a tumour can be modified by local approaches, how this can affect systemic antitumor immunity such that distal sites are attacked, and what approaches have been proven most successful so far in animals and patients.

Immune responses and immunotherapeutic interventions in malignant pleural mesothelioma

Malignant pleural mesothelioma (MPM) is an aggressive, primary pleural malignancy with poor prognosis, hypothesized to originate from a chronic inflammatory state within the pleura. Similar to what has been observed in other solid tumors (melanoma, ovarian and colorectal cancer), clinical and pre-clinical MPM investigations have correlated anti-tumor immune responses with improved survival. As such, a better understanding of the complex MPM tumor microenvironment is imperative in strategizing successful immunotherapies. Herein, we review the immune responses vital to the development and progression of MPM, as well as assess the role of immunomodulatory therapies, highlighting recent pre-clinical and clinical immunotherapy investigations.

Inflammation precedes the development of human malignant mesotheliomas in a SCID mouse xenograft model

Asbestos fibers cause chronic inflammation that may be critical to the development of malignant mesothelioma (MM). Two human MM cell lines (Hmeso, PPM Mill) were used in a SCID mouse xenograft model to assess time-dependent patterns of inflammation and tumor formation. After intraperitoneal (IP) injection of MM cells, mice were euthanized at 7, 14, and 30 days, and peritoneal lavage fluid (PLF) was examined for immune cell profiles and human and mouse cytokines. Increases in human MM-derived IL-6, IL-8, bFGF, and VEGF were observed in mice at 7 days postinjection of either MM line, and a striking neutrophilia was observed at all time points. Free-floating tumor spheroids developed in mice at 14 days, and both spheroids and adherent MM tumor masses occurred in all mice at 30 days. Results suggest that inflammation and cytokine production precede and may be critical to the development of MMs.

CD40-activated B cells contribute to mesothelioma tumor regression

Targeting CD40, a member of the tumor necrosis factor superfamily, using agonist antibodies (Abs) produces dramatic antitumor effects. Indeed, high-dose intravenous anti-CD40 Ab 'licenses' dendritic cells (DCs) that instruct activated CD8(+) cytotoxic T cells to leave lymph nodes (LNs) and penetrate the mesothelioma tumor microenvironment. However, toxic side effects and the potential of an 'overwhelmed' immune response warrant an alternative approach. In this study, we show that injecting lower doses of anti-CD40 Ab directly into the tumor bed avoided toxic side effects and prolonged survival in 60% of mice, with most cured. Unexpectedly, DCs in tumors and LNs 'disappeared', CD8(+) tumor-specific T-cell numbers and function were not enhanced, and T cells did not infiltrate regressing tumors. CD4(+) or CD8(+) depletion only marginally hindered anti-CD40 Ab efficacy implying another effector mechanism. B-cell numbers significantly increased in tumors, draining LNs and spleens during intratumoral anti-CD40 Ab treatment. CD40 targeting had no effect on splenic B-1 cells, obliterated marginal zone B cells and promoted follicular (FO) B-cell activity. Adoptive transfer of tumor antigen-experienced, CD40-activated B cells, or their immunoglobulin products, which recognized autoantigens on mesothelioma cells, protected against tumor challenge. Finally, studies using B-cell knockout mice showed that successful treatment of established tumors required the presence of B cells. Thus, these data suggest that CD40-activated FO B cells can become an important component of an effective antitumor immune response.

Low-dose cyclophosphamide synergizes with dendritic cell-based immunotherapy in antitumor activity

Clinical immunotherapy trials like dendritic cell-based vaccinations are hampered by the tumor's offensive repertoire that suppresses the incoming effector cells. Regulatory T cells are instrumental in suppressing the function of cytotoxic T cells. We studied the effect of low-dose cyclophosphamide on the suppressive function of regulatory T cells and investigated if the success rate of dendritic cell immunotherapy could be improved. For this, mesothelioma tumor-bearing mice were treated with dendritic cell-based immunotherapy alone or in combination with low-dose of cyclophosphamide. Proportions of regulatory T cells and the cytotoxic T cell functions at different stages of disease were analyzed. We found that low-dose cyclophosphamide induced beneficial immunomodulatory effects by preventing the induction of Tregs, and as a consequence, cytotoxic T cell function was no longer affected. Addition of cyclophosphamide improved immunotherapy leading to an increased median and overall survival. Future studies are needed to address the usefulness of this combination treatment for mesothelioma patients.

Cytokine-armed vaccinia virus infects the mesothelioma tumor microenvironment to overcome immune tolerance and mediate tumor resolution

Intratumoral (i.t.) administration of cytokine genes expressed by viral vectors represents a rational approach that induces cytokine secretion at the site they are needed, and i.t. vaccinia virus (VV) has shown promise in mesothelioma patients. However, we and others have shown that the mesothelioma tumor microenvironment includes macrophages, dendritic cells (DCs), and T cells. Therefore, we investigated which of these cell types are infected after exposure to VV or Fowlpox virus (FPV)-cytokine gene constructs. In vitro studies showed that mesothelioma tumor cells were resistant to FPV infection yet highly permissive to infection by VV vectors resulting in significant cytokine production and impaired proliferation. Macrophages secreted low levels of cytokine suggestive of resistance to overt infection. DCs transiently secreted virally derived cytokines, but did not mature during VV infection. VV inhibition of T cell proliferation was rescued by the interleukin (IL)-2 and IL-12 VV constructs. In vivo studies of murine mesotheliomas showed that i.t. injection of the parent VV could not hinder tumor progression. In contrast, the VV-cytokine constructs induced profound tumor regression. These data suggest that i.t. VV-cytokine gene constructs retard tumor growth by infecting mesothelioma cells and targeting the immune system through tumor-infiltrating DC and T cells.

Evaluation of the antitumoral effect mediated by IL-12 and HSV-tk genes when delivered by a novel lipid-based system

In the present work, we used a novel albumin-associated lipoplex formulation, containing the cationic lipid 1-palmitoyl-2-oleoyl-sn-glycero-3-ethylphosphocholine (EPOPC) and cholesterol (Chol), to evaluate the antitumoral efficacy of two gene therapy strategies: immuno-gene therapy, mediated by IL-12 gene expression, and "suicide" gene therapy, mediated by HSV-tk gene expression followed by ganciclovir (GCV) treatment. Our data show that, in an animal model bearing a subcutaneous TSA (mouse mammary adenocarcinoma) tumor, intratumoral administration of the albumin-associated complexes containing the plasmid encoding IL-12 results in a strong antitumoral effect, as demonstrated by the smaller tumor size, the higher T-lymphocyte tumor infiltration and the more extensive tumor necrotic and hemorrhagic areas, as compared to that observed in animals treated with control complexes. On the other hand, the application of the "suicide" gene therapy strategy results in a significant antitumoral activity, which is similar to that achieved with the immuno-gene therapy strategy, although involving different antineoplastic mechanisms. For the tested model, albumin-associated complexes were shown to efficiently mediate intratumoral delivery of therapeutic genes, thus leading to a significant antitumoral effect. This finding is particularly relevant since TSA tumors are characterized for being poorly immunogenic, aggressive and exhibiting high proliferation capacity.

Gene Therapy for Malignant Pleural Mesothelioma

Gene therapy for mesothelioma is currently in its adolescence. The expansion of knowledge regarding molecular aspects of mesothelioma carcinogenesis has facilitated the development of promising gene therapy modalities that target specific oncoproteins and mutant tumor suppressor genes. Although implementation of any of these gene therapy approaches as part of standard medical care for patients who have mesothelioma remains years in the future, the field is finally progressing toward more definitive phase II/III efficacy studies. Unfortunately, the marginal benefits garnered from standard anticancer treatments in mesothelioma argue strongly for continued participation in clinical studies of various experimental approaches, particularly gene therapy. These trials serve multiple purposes: to establish safety, determine proper dosing, evaluate for efficacy, and, in an iterative fashion, guide future avenues of laboratory investigation.

Gene therapy of mesothelioma

Despite improved diagnostic skills and new chemotherapeutic regimens, malignant mesothelioma (MM) remains a pathological disease with survival expectations after diagnosis remaining < 1 year. As the incidence of this disease has yet to peak, there is a pressing need for new therapeutic approaches. One such approach is gene therapy, which inserts 'therapeutic' genes into (generally) tumour cells seeking to induce tumour regression via a number of different theoretical mechanisms. This approach may be particularly relevent for mesothelioma as it is localised to body cavities and is readily accessible for biopsy sampling or for gene delivery. Furthermore, as MM patients rarely die from distant metastases, treating the primary tumour site may result in significant symptomatic and survival benefit. Herein, the paper discusses past, present and future views on gene therapy in the treatment of MM.

Immunotherapy of Murine Malignant Mesothelioma Using Tumor Lysate–pulsed Dendritic Cells

RATIONALE

Exploiting the immunostimulatory capacities of dendritic cells holds great promise for cancer immunotherapy. Currently, dendritic cell-based immunotherapy is evaluated clinically in a number of malignancies, including melanoma and urogenital and lung cancer, showing variable but promising results.

OBJECTIVE

To evaluate if pulsed dendritic cells induce protective immunity against malignant mesothelioma in a mouse model.

METHODS

Malignant mesothelioma was induced in mice by intraperitoneal injection of the AB1 mesothelioma cell line, leading to death within 28 days. For immunotherapy, dendritic cells were pulsed overnight either with AB1 tumor cell line lysate, AB1-derived exosomes, or ex vivo AB1 tumor lysate, and injected either before (Days -14 and -7) at the day of (Day 0) or after (Days +1 and +8) tumor implantation.

MAIN RESULTS

Mice receiving tumor lysate-pulsed dendritic cells before tumor implantation demonstrated protective antitumor immunity with prolonged survival (> 3 months) and even resisted secondary tumor challenge. Tumor protection was associated with strong tumor-specific cytotoxic T-lymphocyte responses. Adoptive transfer of splenocytes or purified CD8+ T lymphocytes transferred tumor protection to unimmunized mice in vivo. When given after tumor implantation in a therapeutic setting, pulsed dendritic cells prevented mesothelioma outgrowth. With higher tumor load and delayed administration after tumor implantation, dendritic cells were no longer effective.

CONCLUSIONS

We demonstrate in this murine model that immunotherapy using pulsed dendritic cells may emerge as a powerful tool to control mesothelioma outgrowth. In the future, immunotherapy using dendritic cells could be used as adjuvant to control local recurrence after multimodality treatment for malignant mesothelioma.

Soluble type II transforming growth factor-beta receptor inhibits established murine malignant mesothelioma tumor growth by augmenting host antitumor immunity

PURPOSE

Transforming growth factor (TGF)-beta blockade has been proposed as an anticancer therapy; however, understanding which tumor patients might benefit most from such therapy is crucial. An ideal target of such inhibitory therapy might be malignant mesothelioma (MM), a highly lethal, treatment-resistant malignancy of mesothelial cells of the pleura and peritoneum that produces large amounts of TGF-beta. The purpose of this study was to explore the possible therapeutic utility of TGF-beta blockade on MM.

EXPERIMENTAL DESIGN

To evaluate this hypothesis, we tested the effects of a soluble TGF-beta type II receptor (sTGF-beta R) that specifically inhibits TGF-beta1 and TGF-beta 3 in three different murine MM tumor models, AB12 and AC29 (which produce large amounts of TGF-beta) and AB1 (which does not produce TGF-beta).

RESULTS

Tumor growth of both established AB12 and AC29 tumors was inhibited by sTGF-beta R. In contrast, AB1 tumors showed little response to sTGF-beta R. The mechanism of these antitumor effects was evaluated and determined to be primarily dependent on immune-mediated responses because (a) the antitumor effects were markedly diminished in severe combined immunodeficient mice or mice depleted of CD8(+) T cells and (b) CD8(+) T cells isolated from spleens of mice treated with sTGF-beta R showed strong antitumor cytolytic effects, whereas CD8(+) T cells isolated from spleens of tumor-bearing mice treated with of control IgG2a showed no antitumor cytolytic effects.

CONCLUSIONS

Our data suggest that TGF-beta blockade of established TGF-beta-secreting MM should be explored as a promising strategy to treat patients with MM and other tumors that produce TGF-beta.

Malignant pleural mesothelioma--an update

Exposure to asbestos is the most frequent, but not exclusive, cause of malignant mesothelioma. Clinical features include dyspnea, cough, nonspecific chest pain, weight loss and night sweats. Diagnosis may be complicated by histologic difficulties. Thoracoscopic techniques are proving beneficial, but no one method of imaging has proven superior, and disease staging is inconsistent. Conventional treatments such as chemotherapy, surgery, and radiotherapy have had variable impacts, although chemotherapy is useful in palliation and can improve both survival and quality of life. There is hope for new antimetabolite agents. The role of radical surgery is yet to be evaluated in a large trial. New radiotherapeutic techniques to improve local control are promising. Multimodality treatments appear to be the most successful for management of potentially resectable disease. It is likely that biological markers will improve accuracy in staging and prognosis. With new treatments based on better understanding of the biology of the disease, there is cautious optimism for the future for patients with malignant pleural mesothelioma.

Mesothelin-family proteins and diagnosis of mesothelioma

BACKGROUND

Mesothelioma is a highly aggressive tumour for which there are no reliable serum tumour markers. Identification of such a marker would be useful in diagnosis of mesothelioma and for monitoring responses to treatment and screening at-risk individuals.

METHODS

We assayed serum concentrations of soluble mesothelin-related proteins (SMR) using a double determinant (sandwich) ELISA in a blinded study of serum samples from 44 patients with histologically proven mesothelioma; 68 matched healthy controls, 40 of whom had been exposed to asbestos; and 160 patients with other inflammatory or malignant lung and pleural diseases.

FINDINGS

37 (84%) of 44 patients with mesothelioma had raised concentrations of SMR at a serum dilution of 1/80, compared with three (2%) of 160 patients with other cancers or other inflammatory lung or pleural diseases, and with none of 28 controls who had not been exposed to asbestos. SMR concentrations correlated with tumour size and increased during tumour progression. Seven of the 40 asbestos-exposed individuals had increased serum concentrations of SMR; three of those seven developed mesothelioma and one developed lung carcinoma within 1-5 years. None of the 33 asbestos-exposed participants whose serum samples had normal concentrations of SMR and who were followed up over 8 years developed mesothelioma.

INTERPRETATION

Determination of SMR in serum could be a useful marker for diagnosis of mesothelioma and to monitor disease progression. It might also prove helpful for screening asbestos-exposed individuals for early evidence of mesothelioma.

Regression of AK7 malignant mesothelioma established in immunocompetent mice following intratumoral gene transfer of interferon gamma

Malignant mesothelioma (MM) is a lethal tumor linked with a prior exposure to asbestos in which limited progress has been made so far using conventional therapies. MM is an example of a "nonimmunogenic" tumor characterized by a fibrous stroma and an absence of infiltrating T lymphocytes. High levels of transforming growth factor-beta (TGF-beta) produced by mesothelioma cells have been related to the immune tolerance towards the tumor. In order to evaluate the effect of local delivery of cytokines such as interferon gamma (IFN-gamma) by gene transfer, we characterized and used a murine model, AK7, which appeared very similar to human mesothelioma. AK7 cells expressed low levels of major histocompatibility class I and class II antigens and secreted high levels of latent TGF-beta. The TGF-beta pathway in AK7 cells is operative but inefficient because endogenous TGF-beta is predominantly inactive. Treatment of pre-established AK7 tumors by direct intratumoral injection of an adenovirus vector expressing murine IFN-gamma, Ad.mIFN-gamma, led to significant tumor regression. Peripheral tumor infiltration by CD4+ and CD8+ T lymphocytes in the treated tumors appeared to be because of the induction of an immune response. Tumor relapse was observed, which could be due to local TGF-beta secretion by remaining tumor cells.

Intrapleural infusion of activated macrophages and gamma-interferon in malignant pleural mesothelioma: a phase II study

STUDY OBJECTIVES

Intrapleural immunotherapy has shown some activity in patients with malignant mesothelioma. We conducted a multicentric pilot phase II study to evaluate the tolerance and the activity of intrapleurally infused autologous human activated macrophages (AM Phi) in patients with stage IA, IB, and IIA malignant pleural mesothelioma (MPM).

DESIGN

AM Phi derived from in vitro monocyte culture were infused into the pleura of patients every week for 8 consecutive weeks. Each infusion was followed 3 days later by an intrapleural injection of 9 millions units of gamma-interferon (gamma-IFN) in an attempt to prolong the in vivo activation of infused AM Phi. Response was assessed by CT scan and thoracoscopy when possible. If the patient's disease progressed after AM Phi treatment, an additional treatment was left to the choice of the investigator.

PATIENTS

Nineteen patients with histologically proven stage IA, IB, or IIA MPM were enrolled. Two patients were excluded before any AM Phi infusion because of complications impeding infusion. Seventeen patients were actually treated. After completion of the AM Phi cellular therapy, 10 patients were treated with chemotherapy as their diseases progressed.

RESULTS

The overall response rate of patients actually treated was 14%. When including the two patients enrolled but not treated, the overall response "in intention to treat" was 11%; two patients had a partial response, with a duration of response of 30 months and 3 months, respectively. One patient, who could not be evaluated by thoracoscopy because of pleural symphysis, is still alive without any clinical or radiologic sign of disease 69 months after treatment. No major adverse effects were observed during the infusion of either AM Phi or gamma-IFN, and there was no interruption of treatment because of toxicity. However, symphysis was observed in 7 of 14 patients who received the complete treatment. The median survival of patients actually treated, including those who received chemotherapy after AM Phi, was 29.2 months.

CONCLUSION

Combined infusion of AM Phi and gamma-IFN was well tolerated in patients with MPM; however, it had limited antitumor activity.

New approaches for mesothelioma: biologics, vaccines, gene therapy, and other novel agents

Although malignant mesothelioma is not a classically immunogenic cancer, there is abundant evidence for immune recognition. The relative ease of obtaining tumor tissue makes mesothelioma ideal for studying surrogate biomarkers such as lymphocytic infiltration or expression of transduced genes. There is evidence that malignant mesothelioma patients as well as asbestos-exposed persons without mesothelioma have impaired immune responsiveness. Substantial progress has been made in animal models using several biological and immunological techniques, but clinical application has been problematic. Systems studied have included lysis by interleukin-2 (IL-2)-activated lymphokine-activated killer (LAK) cells, tumor necrosis factor-alpha (TNF-alpha), a p16-expressing adenovirus vector, suicide gene therapy using the herpes simplex virus-tyrosine kinase (HSV-tk) followed by ganciclovir, and immunomodulatory gene therapy with IL-2, IL-4, interferon-gamma (IFN-gamma), IFN-alpha, TNF-alpha, granulocyte-macrophage colony-stimulating factor (GM-CSF), IL-6, and IL-1beta transfected into tumors. Vaccinia virus has been studied as a vector for cytokine gene transfer. Suicide gene therapy has been combined with a tumor vaccine. The University of Western Australia is initiating a pilot study of autologous vaccination in malignant mesothelioma. Novel agents under study include the angiogenesis inhibitors SU5416, bevacizumab, and thalidomide. ZD1839, an orally administered, highly selective inhibitor of the epidermal growth factor receptor (EGFR) tyrosine kinase, is being tested in a phase II trial. Since platelet-derived growth factor (PDGF) is thought to be an autocrine growth factor for mesothelioma STI-571 (Gleevec; Novartis, Basel, Switzerland), a highly selective inhibitor of the PDGF receptor tyrosine kinase, is being tested in a phase II trial. The development of more active cytotoxic combinations in this disease should facilitate further studies of chemoimmunotherapy. It seems likely that no single treatment modality will be effective by itself.

Endothelial cell-based systemic gene therapy of metastatic melanoma

Cancer metastasis accounts for a significant proportion of morbidity and mortality in patients. Effective means of treating disseminated disease remains elusive. The purpose of this study was to determine whether genetically modified endothelial cells (GMEC) can selectively target and deliver recombinant therapeutic molecules to sites of tumor metastases. Following the establishment of lung metastases of 4T1 mammary tumor in mice, intravenously (i.v.) administered, lacZ transgene-expressing endothelial cells (lacZ-GMEC) accumulated at the tumor sites. An average of 32% and 90% of the pulmonary metastases were X-gal stained following one and three tail vein injections of 10(5) lacZ-GMEC, respectively. The linear pattern of X-gal staining seen within the tumor sites and the histological appearance of the tumor vasculature were consistent with the incorporation of lacZ-GMEC into blood vessels. In C57Bl/6 mice harboring lung metastases of melanoma, the administration of three sequential i.v. injections of 10(5) endothelial cells expressing a human interleukin 2 transgene abrogated the tumor metastases and prolonged survival of the animals. These results demonstrate that i.v.-administered GMEC can selectively accumulate, survive, and stably express exogenous genes at multiple tumor sites. These findings support a role for i.v.-administered GMEC as a potential therapeutic strategy for the systemic treatment of cancer metastases.

The immune anti-tumor effects of GM-CSF and B7-1 gene transfection are enhanced by surgical debulking of tumor

Malignant mesothelioma (MM) is a solid tumor largely unresponsive to conventional therapies. Immunological gene therapy shows promise in murine models and human clinical trials; however, the role of surgery in combination with gene therapy has not been widely studied. The aim of this study was to determine if debulking surgery improved the effectiveness of gene therapy in a murine MM model. Mice were subcutaneously inoculated with the MM cell line, AC29, at two different sites, 4 days apart, to allow a surgical and distal site tumor to develop. Once tumors were established, the surgical site tumor was debulked and vaccination of syngeneic tumor transfectants encoding genes for IL-4, IL-2, GM-CSF, B7-1 or allogeneic MHC molecules commenced at a site away from both tumors, and tumor growth was measured. Neither debulking surgery nor gene therapy alone delayed tumor growth. However, there was a clear delay of tumor growth when debulking surgery was combined with vaccination of tumor transfectants expressing B7-1 or high levels of GM-CSF. Combinations of these two transfectants did not lead to a synergistic effect. This study demonstrates that debulking surgery can augment the immunostimulatory effects of immunological gene therapy and can delay tumor growth. This has implications for the future design of human gene therapy trials for solid tumors such as MM.

Localised spontaneous regression in mesothelioma -- possible immunological mechanism

Malignant mesothelioma (MM) is an aggressive tumor usually associated with asbestos exposure. Although it can remain stable for prolonged periods, it has not been described to spontaneously regress. MM tumors are thought to be immunogenic based both on animal studies and on the good responses in some humans treated with immunotherapy. Here we present a case of pleural MM in which a transient spontaneous regression was associated with tumor tissue infiltration with mononuclear cells and serological evidence of anti-MM reactivity. The patient's tumor eventually progressed and with this progression there was evidence of loss of serological reactivity to some, but not all, of her MM antigens. The patient survived for 20 months and, in contrast to her initial biopsy, no significant lymphoid infiltrate was detected in her MM tissue at post mortem examination.

IL-12 gene therapy of leukemia with hematopoietic progenitor cells without the toxicity of systemic IL-12 treatment

We have previously shown that the myeloid progenitor cell line 32Dc13 transduced with mIL-12 cDNAs (32DIL-12 cells) induces IFN-gamma and NK-cell-mediated cytotoxicity in vivo. Since systemic therapy with recombinant IL-12 protein has been shown to produce moderate to severe toxic side effects we examined whether IL-12 gene therapy with hematopoietic progenitor cells also induces systemic toxicities that are commonly associated with the administration of rIL-12 protein. Injection of large doses of IL-12 secreting 32DIL-12 cells (5 to 6 x 10(7) cells) significantly reduced mortality in mice injected with a lethal dose of 32Dp210 myeloid leukemia cells. More importantly, injection of similar doses of transduced cells failed to reduce body weight significantly or produce other visible signs of toxicity, i.e., fur ruffling or lethargy. There was no evidence of hematologic or hematopoietic toxicity resulting from the injection of transduced cells. In addition, microscopic examination of liver, kidney, lung, and intestine of mice injected with transduced cells revealed the absence of tissue necrosis or inflammatory response in any of these organs. Finally, 32DIL-12 cells were not found to interfere with the engraftment of syngeneic bone marrow transplant or the hematopoietic reconstitution of irradiated mice. These results demonstrate that IL-12 gene therapy with hematopoietic progenitor cells is nontoxic and provide a rationale for exploring the feasibility of treating minimal residual leukemia with IL-12 gene therapy.