Clinical trials with oncolytic reovirus: moving beyond phase I into combinations with standard therapeutics

It is time for those working on oncolytic viruses to take stock of the status of the field. We now have at our disposal an array of potential therapeutic agents, and are beginning to conduct early-phase clinical trials in patients with relapsed/metastatic cancers. By drawing on lessons learned during the development of other biological therapies, such as monoclonal antibodies and targeted small molecule inhibitors, we are now in a position to chart the course of the next wave of trials that will go beyond the phase I studies of safety and feasibility. In this article we review our approach to the development of oncolytic viruses as cancer therapeutics. In doing so, we emphasise the fact that this process is modular and involves multiple iterative steps between the laboratory and the clinic. Ultimately, at least in the medium term, the future of oncolytic virotherapy lies in combination regimens with standard anti-cancer agents such as radiation and chemotherapy.

Phase I/II trial of oncolytic reovirus (Reolysin) in combination with carboplatin/paclitaxel in patients (pts) with advanced solid cancers

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Background: Reolysin, a wild type reovirus (Dearing strain), replicates preferentially in Ras-activated cancer cells. Preclinical data have demonstrated synergistic tumor kill when reolysin is combined with standard chemotherapies including platinum agents and taxanes, justifying the clinical evaluation of this drug combination. Methods: Pts were initially treated in an open-label, dose-escalating, phase I trial and received iv reolysin, d1–5, iv carboplatin (AUC5), d1, and paclitaxel (175mg/m2), d1, qw3. Reolysin was administered at a starting dose of 3x109 TCID50 and then increased to 1x1010 and 3x1010 TCID50 in cohorts of 3 pts. Primary endpoints for the dose escalation trial were to determine the maximum tolerated dose, dose limiting toxicity (DLT) and to recommend a dose for phase II studies. Secondary endpoints were to evaluate pharmacokinetics, immune response and anti-tumour activity. The primary endpoint for the phase II expansion cohort in head and neck (H&N) pts is to characterize response rate. Results: 17 heavily pre-treated pts (11 M, median age 55 yrs) with advanced cancer: H&N (10), melanoma (4), peritoneal/endometrial cancer (2), and sarcoma (1) have received 82 cycles of treatment to date; 4 pts are still on study. There were no DLTs in the dose escalation. Toxicities were mainly grade 1 and 2 and included: nausea, fatigue, vomiting, myalgia, fever, neutropenia, lymphopenia, thrombocytopenia and hypotension. This combination resulted in a blunting of antiviral immune response as compared to monotherapy virus. Response rates in 15 evaluable patients were partial response (PR) (4 pts), stable disease (SD) (6 pts) and progressive disease (5 pts). Of note, all PRs and 4/5 SDs were in H&N disease. Conclusions: The combination of reolysin and carboplatin/paclitaxel was well tolerated and resulted in disease control in the majority of pts. Significant responses in refractory H&N pts recommended this combination for phase II evaluation. Enrollment is ongoing and randomized studies are planned.

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Results of a phase II study to evaluate the biological effects of intratumoral (ITu) reolysin in combination with low dose radiotherapy (RT) in patients (Pts) with advanced cancers

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Background: Reolysin, a wild type reovirus serotype 3 Dearing strain, replicates preferentially in Ras-activated cancer cells. In vitro and in vivo data have shown that combining reolysin and radiation (RT) significantly increases RT-induced cytotoxicity. A completed phase I trial of ITu reolysin and RT demonstrated that the combination was well tolerated and resulted in local and systemic responses. Methods: This open-label, single-arm, multicenter Phase 2 study combined ITu reolysin with low-dose fractionated RT. 20 Gy was given in 5 consecutive daily 4 Gy fractions combined with 2 ITu injections of reolysin (1x1010 TCID50) on days 2 & 4. The primary endpoint was objective tumor response rate in treated lesions. Secondary endpoints were to evaluate: viral replication, immune response and safety. Pts with ECOG performance status ≤2, with refractory advanced or metastatic cancers were eligible. Results: 16 heavily pre-treated pts (9 male, median age 66 yrs, ECOG 0:4pts; 1:12pts) with advanced cancer: melanoma (5), colorectal (4), gastric (1), ovarian (1), pancreas (1), lung (1), cholangiocarcinoma (1), sinus (1), and thyroid (1) were enrolled since Dec 2006. Most pts had received prior chemotherapy (13 pts) or RT (5 pts). No related serious adverse effects were observed during the study. Toxicities related to treatment were Grade 1 or 2: chills, pyrexia, headache, lethargy, anorexia, vomiting, shivering, nausea, and mild injection site pain. Of 14 pts evaluable for response, 13 pts had stable disease or better in the treated target lesion. Of these, partial responses were observed in 4 pts (lung, melanoma x 2, gastric) and minor responses were observed in 2 pts (thyroid, ovarian). Antibody responses to reolysin were delayed compared to previous results with intravenous administration. Conclusions: The combination of ITu reolysin and low dose RT was well tolerated and resulted in marked responses or stabilization in the treated target lesions for most of the pts evaluated to date. Further study in the radical setting is warranted.

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Reovirus: viral therapy for cancer 'as nature intended'

Oncolytic viruses are tumour selective and able to lyse cancer cells after infection. Reovirus is an example of a wild-type oncolytic virus and is currently being investigated as a potential novel therapy for cancer. This overview gives a brief description of what is known about reovirus biology and summarises the preclinical data related to its oncolytic ability. The completed and ongoing clinical trials involving reovirus, both as a single agent and in combination with chemotherapy and radiotherapy, will be reviewed and their results discussed. Many of these clinical studies are being conducted by centres in the UK.

Efficacy and Tolerability of Nisoldipine Coat-Core vs Diltiazem Retard in Combination with a Beta-Blocker in Patients with Stable Exertional Angina Pectoris

A randomised, double-blind, placebo-controlled, parallel-group trial with forced titration study to investigate possible equivalence of efficacy and tolerability between nisoldipine coat-core (CC) 40mg once daily, and diltiazem retard 120mg twice daily, was carried out in 176 patients with stable angina pectoris who were already receiving beta-blocker therapy. A total of 164 patients were included in the tolerability analysis and 135 patients were evaluable for efficacy (nisoldipine CC, n = 69; diltiazem retard, n = 66). During bicycle exercise tolerance tests, time to 1mm ST-segment depression, total exercise time, and time to angina were assessed at baseline and at the end of the treatment period. The number of angina attacks and of consumed nitroglycerin tablets were recorded in weekly diaries. Time to onset of 1mm ST-segment depression increased by 69.4 +/- 100.0 seconds with nisoldipine CC and by 65.9 +/- 87.6 seconds with diltiazem retard. The two treatment regimens were equally effective in time to onset of 1mm ST-segment depression, time to angina pectoris, and in exercise duration. A beneficial effect on angina attacks and nitroglycerin consumption was achieved with both treatments. Patient compliance, as assessed by the number of returned tablets, was high, at over 80%. Six patients withdrew from the treatment because of adverse events. Mild and transient adverse events were reported by 24 patients during treatment. One patient experienced a severe circulatory shock on the combination of diltiazem retard and atenolol. Peripheral oedema and headache were more common on nisoldipine CC. We concluded that the two treatments were equally efficacious and tolerated in patients with stable angina pectoris.

Assessing the impact of a downscaled climate change simulation on the fish fauna in an Inner-Alpine River

This study assesses the impact of a changing climate on fish fauna by comparing the past mean state of fish assemblage to a possible future mean state. It is based on (1) local scale observations along an Inner-Alpine river called Mur, (2) an IPCC emission scenario (IS92a), implemented by atmosphere-ocean global circulation model (AOGCM) ECHAM4/OPYC3, and (3) a model-chain that links climate research to hydrobiology. The Mur River is still in a near-natural condition and water temperature in summer is the most important aquatic ecological constraint for fish distribution. The methodological strategy is (1) to use downscaled air temperature and precipitation scenarios for the first half of the twenty-first century, (2) to establish a model that simulates water temperature by means of air temperature and flow rate in order to generate water temperature scenarios, and (3) to evaluate the impact on fish communities using an ecological model that is driven by water temperature. This methodology links the response of fish fauna to an IPCC emission scenario and is to our knowledge an unprecedented approach. The downscaled IS92a scenarios show increased mean air temperatures during the whole year and increased precipitation totals during summer, but reduced totals for the rest of the annual cycle. These changes result in scenarios of increased water temperatures, an altered annual cycle of flow rate, and, in turn, a 70 m displacement in elevation of fish communities towards the river's head. This would enhance stress on species that rely on low water temperatures and coerce cyprinid species into advancing against retreating salmonids. Hyporhithral river sectors would turn into epipotamal sectors. Grayling (Thymallus thymallus) and Danube salmon (Hucho hucho), presently characteristic for the Mur River, would be superceded by other species. Native brown trout (Salmo trutta), already now under pressure of competition, may be at risk of losing its habitat in favour of invaders like the exotic rainbow trout (Oncorhynchus mykiss), which are better adapted to higher water temperatures. Projected changes in fish communities suggest an adverse influence on salmonid sport fishing and a loss in its high economic value.

Phase I trial of intratumoral administration of reovirus type 3 with radiation in patients with advanced malignancies

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Background: Reoviruses are RNA viruses that replicate preferentially in cells with aberrant Ras-pathway signaling. Preclinical data have shown that the combination of reovirus (REO) and radiation (RT) significantly increases RT-induced cytotoxicity. Methods: Patients (pts) with measurable disease amenable to treatment with localised short-course palliative RT were enrolled in cohorts of 3. In the first stage, pts received local tumour RT (20 Gy in 5 fractions) in combination with two intratumoural injections of escalating REO doses to a maximum of 1010 TCID50. If no dose-limiting toxicity (DLT), pts received local tumour RT (36 Gy in 12 fractions) in combination with two, four or six doses of REO at TCID50. Endpoints were safety, viral replication, immunogenicity and antitumoral activity. Results: Eighteen pts have been treated to date with a variety of solid tumours.Two pts, one at the 1 x 109 dose and one at 1 x 1010 refused to receive the second REO injection due to grade 2 flu-like symptoms. Other toxicities have been mild (grade 1–2) and have included fever, sweating, skin erythema. There has been no evidence of exacerbation of the acute RT reaction. Reverse transcription polymerase chain reaction (RT-PCR) studies of blood, urine, stool and sputum on day 8 post-REO administration were negative for viral shedding for all treated pts. The induction of neutralising anti-REO antibodies was attenuated compared with previous experience in pts receiving intravenous infusions. Three pts (oesophageal ,skin squamous and colorectal carcinoma) had significant partial responses. In addition, the pt. with metastatic oesophageal cancer had a volume reduction of 15% in non-irradiated mediastinal disease for more than 6 months. Intratumoral viral replication by electron microscopy and immunohistochemistry of pre- and post-treatment tumour biopsies was performed. Conclusions: The combination of intratumoral REO and radiation is well tolerated. Promising indicators of synergistic effect are observed. Recruitment is ongoing at the last cohort of the trial.

No significant financial relationships to disclose.

Mechanisms of exercise-induced ST-segment depression in patients without typical angina pectoris

OBJECTIVE

To evaluate if exercise-induced ST-segment depression without typical angina pectoris is related to increases in sympatho-adrenal activity or beta-adrenoceptor sensitivity.

PATIENTS

Thirteen patients (four men) aged 35-62 years with ST-segment depression during exercise but atypical symptoms and normal myocardial scintigraphy, and 13 matched controls.

DESIGN AND INTERVENTIONS

Patients and controls were compared regarding responses with: (i) exercise testing without treatment, (ii) exercise testing following beta-adrenoceptor blockade by propranolol (0.15 mg kg(-1) i.v.), (iii) incremental adrenaline infusions (0.1, 0.2, 0.4 and 0.8 nmol kg(-1) min(-1)) and (iv) adrenaline infusions during alpha-adrenoceptor blockade by phentolamine (0.5 mg min(-1)).

MAIN OUTCOME MEASURES

ST-segment depression and tissue Doppler parameters reflecting contractility.

RESULTS

Exercise lowered the ST-segment by 2.44 mm without and 0.87 mm with beta-adrenoceptor blockade (P < 0.001 for difference) amongst patients, but not amongst controls. Maximal heart rate was slightly higher amongst patients (P < 0.05), despite similar loads and plasma catecholamine responses to exercise in the two groups; this difference disappeared after beta-adrenoceptor blockade with propranolol. ST-segment depression during adrenaline infusion was greater in patients compared with controls (P < 0.01) despite similar increases in heart rate. alpha-Blockade enhanced the ST-segment depression (P < 0.001) and heart rate (P < 0.001) responses to adrenaline infusion more markedly amongst patients. Tissue Doppler imaging showed similar contractility and diastolic relaxation responses of patients and controls to adrenaline, but early diastolic movements did not increase amongst patients after phentolamine (P < 0.01).

CONCLUSIONS

Exercise-induced ST-segment depression in patients with a low likelihood of ischaemic heart disease is related to increased beta-adrenergic sensitivity regarding chronotropic and electrophysiological, but not inotropic responses.

Open label phase II studies of modified vaccinia ankara expressing the tumor antigen 5T4 given in conjunction with IFL and FOLFOX chemotherapy regimens: Final analysis of safety and immunogenicity of MVA 5T4 given before, during and after chemotherapy

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Background: 5T4 is a tumour associated antigen that is widely expressed on the surface of most human adenocarcinomas, including colorectal, but rarely in normal cells. Modified Vaccinia Ankara (MVA) has been employed as a vaccine vector to deliver 5T4. Previously, MVA-5T4 has been evaluated in a phase I/II clinical trial in stage IV colorectal cancer patients. MVA-5T4 was shown to be safe and well tolerated and induced 5T4 specific immune responses in most patients. Furthermore, 5T4 specific antibody titres correlated with clinical benefit. Methods: Two open label phase II clinical trials were initiated in which patients with advanced colorectal cancer received MVA-5T4 in conjunction with either 5-FU/leukovorin and irinotecan (TV2-IFL; n=19 patients) or 5-FU/leukovorin and oxaliplatin (TV2-FOLFOX; n=17 patients). MVA-5T4 was administered up to 6 times, 2 prior to, 2 during and 2 post-chemotherapy. The primary objectives were to assess the safety and immunogenicity of MVA-5T4 given in combination with chemotherapy. Results: Recruitment to both trials is complete and MVA-5T4 was well tolerated in all ITT patients, with no serious adverse events being associated with MVA-5T4. 5T4-specific cellular and humoral immune responses were monitored before, during and after chemotherapy in all 23 per protocol patients (n=12 for TV2-IFL and n=11 for TV2-FOLFOX). Following vaccination, all 23 patients mounted 5T4 cellular and/or humoral responses. Immune responses were detectable during chemotherapy in the majority of patients. IFNγ ELISPOT responses to 5T4 peptides revealed precursor frequencies as high as 1 in 1000 PBMCs. Assessment of clinical responses in all PP patients demonstrated an overall response rate of 65% across both trials. Conclusions: MVA-5T4 is safe and well tolerated when administered in conjunction with IFL and FOLFOX chemotherapy regimens. Furthermore, 5T4 specific immune responses are induced in all per protocol patients and can be boosted or maintained during chemotherapy. Encouraging clinical responses have been observed and 5T4 immune responses shown to correlate with clinical benefit.

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Fusogenic membrane glycoprotein-mediated tumour cell fusion activates human dendritic cells for enhanced IL-12 production and T-cell priming

Fusogenic membrane glycoproteins (FMG) are a family of viral genes that, when expressed in tumour cells, trigger extensive cell to cell fusion and subsequent cell death. Gene therapy approaches using FMG are also potentially immunogenic, since syncitia generated ex vivo can be therapeutic as antitumour vaccines in murine models. This study has addressed the mechanisms responsible for the immunogenicity of FMG-mediated cell death, and its applicability to human immune priming. We show that fusion of human Mel888 melanoma cells following transfection with FMG can reverse the suppressive effects of Mel888 on dendritic cells (DC) phenotype, and potentiate IL-12 production by DC on activation in a cell contact-dependent manner. DC loaded with fusing, but not intact, tumour cells primed a naive, tumour-specific cytotoxic T-cell response, which was MHC class I-restricted and associated with production of high levels of IFNgamma and, later, IL-5. Fusing cells were an effective source of antigen for DC cross-priming and presentation of the melanoma-specific antigen gp100 to a specific T-cell clone. These data show, in a human system, that FMG represent an immunogenic, as well as cytotoxic, gene therapy for cancer, reversing the inhibitory effects of tumour cells on DC to potentiate IL-12 production and naive T-cell priming.

Synchronous bilateral ductal carcinoma in situ of the male breast associated with gynecomastia in a 30-year-old patient following repeated injections of stanozolol

We report a rare case of synchronous bilateral and multifocal ductal carcinoma in situ (DCIS) in a 30-year-old patient operated on for gynecomastia following repeated injections of stanozolol, a non-aromatizable androgen. The familial medical history was negative for breast cancer and work-up of serum hormone levels was normal. The patient underwent a modified radical mastectomy without axilla dissection 6 weeks following the primary procedure and recovered uneventfully. The role of synthetic androgens in the development of male breast neoplasia warrants further scrutiny.

Immunosuppressive effects of radiation on human dendritic cells: reduced IL-12 production on activation and impairment of naive T-cell priming

Dendritic cells (DC) are professional antigen-presenting cells (APC) of the immune system, uniquely able to prime naive T-cell responses. They are the focus of a range of novel strategies for the immunotherapy of cancer, a proportion of which include treating DC with ionising radiation to high dose. The effects of radiation on DC have not, however, been fully characterised. We therefore cultured human myeloid DC from CD14+ precursors, and studied the effects of ionising radiation on their phenotype and function. Dendritic cells were remarkably resistant against radiation-induced apoptosis, showed limited changes in surface phenotype, and mostly maintained their endocytic, phagocytic and migratory capacity. However, irradiated DC were less effective in a mixed lymphocyte reaction, and on maturation produced significantly less IL-12 than unirradiated controls, while IL-10 secretion was maintained. Furthermore, peptide-pulsed irradiated mature DC were less effective at naive T-cell priming, stimulating fewer effector cells with lower cytotoxicity against antigen-specific targets. Hence irradiation of DC in vitro, and potentially in vivo, has a significant impact on their function, and may shift the balance between T-cell activation and tolerization in DC-mediated immune responses.

Radiation-induced cell death and dendritic cells: potential for cancer immunotherapy?

Dendritic cells are key orchestrators of the immune system. There is considerable interest in their use for treating cancer. Whether they initiate an effective cytotoxic response against antigen-bearing cells, or produce tolerance, depends on the context in which those antigens are presented. Ionising radiation, and the cell death it causes, has several properties that may facilitate such an effective response. A range of in-vitro and in-vivo data supports this, although potential problems exist that may require concurrent strategies.

Cell death associated with genetic prodrug activation therapy of colorectal cancer

Genetic prodrug activation therapy (GPAT) is a form of cancer gene therapy that has potential use against tumours such as colorectal malignancy. The characterization of such therapies using laboratory models provides a basis for clinical trials. In this study the gene encoding Herpes Simplex Virus thymidine kinase (HSVtk) was delivered to colorectal tumour cells using an Adenoviral (Ad) vector in vitro. In this way the cells were made susceptible to killing with the prodrug ganciclovir to various degrees depending on cell infectability with Ad. Bystander killing effect appeared minimal both in vitro and when transduced cells were injected in vivo. Mechanisms of cell death, measured in vitro using anti-BrDU (DNA-break labelling) and propidium iodide staining variously showed a combination of apoptosis in the G1 cell cycle phase and late apoptotic or necrotic sub-G1 DNA fragmentation, depending on the tumour cell line. These findings suggest that gene therapy of colorectal cancer by GPAT gives rise to therapeutic forms of direct cell death, but requires improvements in transduction, and possibly immune augmentation.

Enhanced left ventricular endocardial border delineation with an intravenous injection of SonoVue, a new echocardiographic contrast agent: A European multicenter study

The safety and efficacy of SonoVue (also referred to as BR1), a new contrast agent for delineating endocardial border of the left ventricle after intravenous administration, was assessed. Two hundred and eighteen patients with suspected coronary artery disease undergoing fundamental echocardiography for the assessment of left ventricle were enrolled in a prospective multicenter, single blind, cross-over study with random sequence allocation of four different doses of SonoVue. Endocardial border definition in the apical and parasternal views was scored as 0 = not visible, 1 = barely visible, and 2 = well visualized before and after contrast enhancement. Analysis was performed by two pairs of off-site observers. Safety of SonoVue was also assessed. Results of our study indicated that the mean improvements in the endocardial border visualization score were as follows: 3.1 +/- 7.8 (95% CI, 2.5 and 3.7) for 0.5 ml, 3.4 +/- 8.0 (95% CI, 2.8 and 4.0) for 1 ml, 3.4 +/- 7.9 (95% CI, 2.8 and 4.0) for 2 ml, and 3.7 +/- 8.0 (95% CI, 3.1 and 4.3) for 4 ml (P < 0.05 for all doses from baseline). Changes from baseline in endocardial visualization scores were also seen in the apical views (P < 0.05) and they were dose-dependent (P < 0.001). Similar enhancements of endocardial visualization scores were observed in the apical views in patients with suboptimal baseline echocardiographic images. Diagnostic confidence for assigning a score and image quality also were significantly better following contrast enhancement. No significant changes in the laboratory parameters and vital signs were noted following contrast enhancement, and the side effects were minimal. It was concluded that SonoVue is safe and effective in delineating endocardial border, including in patients with suboptimal baseline images.

Apoptosis or necrosis for tumor immunotherapy: what's in a name?

Here we discuss how the mechanisms by which tumor cells are killed in vivo by gene transfer affects their immunogenicity. Our own work has shown that necrotic cell death induces immunological activation signals which recruit, load, activate and mature appropriate subsets of antigen-presenting cells. In contrast, for apoptotic cell death to be immunogenic, signals additional to cell death alone must be provided within the milieu of the dying tumor. Our conclusion is that the immunogenicity of tumor killing is determined by a combination of factors, including the mechanism of killing, the levels of cell death, the local environment that exists within the dying tumor and, as a result, the nature of the immune/scavenger cells which are present at the time of antigen release. Knowledge of how these factors can influence the immune system and lead to the breaking of tolerance to tumor-associated antigens, can potentially be exploited in the design of effective immunotherapies for cancer using gene transfer.

Adoptive transfer of immature dendritic cells with autologous or allogeneic tumor cells generates systemic antitumor immunity

Dendritic cells (DCs) are potent antigen-presenting cells that are capable of priming systemic antitumor immune responses in animal tumor models. However, many of the model tumor systems tested need definition of the specific tumor antigens involved. To use DCs in situations that are more relevant to the majority of human cancers, where the antigens are unknown, we have tested the adoptive transfer of immature DCs in mouse colorectal and melanoma models of varying immunogenicity but with undefined antigens. When DCs admixed with a syngeneic primary tumor inoculum were seeded s.c., the growth of the primary tumor was unchanged; however, if the primary tumor was then surgically excised and the animal was rechallenged with the same tumor, significant protection (75%) was generated when DCs were present in the original primary inoculum of a moderately immunogenic colorectal model (CMT93tk). This effect was not observed when a nonimmunogenic melanoma (B16) was tested in an identical protocol. Next, DCs were injected directly into 6-9-mm established tumors; again, protection (55%) was achieved against a secondary tumor challenge following excision of the primary, but only in the CMT93tk model of moderate immunogenicity. To increase the clinical relevance of this approach still further, we tested irradiated allogeneic K1735 melanoma cells mixed with syngeneic DCs as a vaccine against subsequent challenge with the poorly immunogenic syngeneic melanoma B16. The allogeneic vaccine alone was ineffective, but when admixed with DCs, a significant number of animals rejected a subsequent B16 challenge, suggesting that DCs are able to prime an immune response against melanoma antigens shared between K1735 and B16. The generation of systemic antitumor immunity by adoptive transfer of DCs has significant clinical potential because it is technically straightforward and does not require the definition of specific tumor antigens.

Heat shock protein expression in target cells infected with low levels of replication-competent virus contributes to the immunogenicity of adenoviral vectors

A significant limitation of adenoviral vectors is their associated immunogenicity. Since we, and others, have shown that the immunogenicity of cells can be increased by the induction of heat shock proteins (hsp), and because infection with several viruses induces hsp, we investigated whether the immunogenicity of adenoviral gene transfer might be mediated through induction of hsp expression. Neither plasmid DNA nor a recombinant retroviral vector induced hsp expression in transduced B16 melanoma cells. However, hsp70 was upregulated after infection with two of six adenoviral vectors; this induction of hsp70 did not correlate with the adenoviral transgene or with the viral backbone (Ad2 or Ad5). In previous assays, no replication-competent adenovirus (RCA) had been detected in any of these viruses. However, using sensitive assays for RCA, induction of hsp70 was found to correlate with the transfer of E1A and low levels of RCA. Moreover, target cells expressing hsp70 at levels similar to those induced by RCA infection protected syngeneic mice against rechallenge with parental cells, demonstrating that cells induced to express hsp70 by inadvertant transfer of RCA will become immunogenic. These results reveal a novel mechanism contributing to the immunogenicity of adenoviral vectors. If careful screening for RCA is not used when using laboratory-prepared viral stocks, the validity of the resulting experimental data might be significantly affected, especially when the immune stimulatory effects of the transgene are being studied.