The presence of replication-competent adenoviruses (RCAs) in batches of replication-defective adenovirus (Ad) vectors is a major problem for the application of these vectors in gene therapy. RCAs are generated by recombination between sequences in the Ad vector and homologous Ad sequences in the helper cells, resulting in the acquisition by the vector of early region 1. To prevent the formation of RCAs, we have developed helper cell lines, which we named PER, and matched Ad vectors that do not have sequence overlap. PER cells contain the Ad serotype 5 (Ad5) E1A- and E1B-encoding sequences (Ad5 nucleotides 459–3510) under the control of the human phosphoglycerate kinase (PGK) promoter. We demonstrate that PER cells synthesize high levels of the Ad5 E1A and E1B proteins. The yields from PER cells of E1-deleted Ads are similar to those obtained from earlier helper cells, such as 911 and 293 cells. Propagation of matched Ad vectors, which lack Ad5 nucleotides 459–3510, in one of the PER clones, PER.C6, does not result in the generation of RCAs, in contrast to propagation in 293 cells. We conclude that the combination of PER.C6 cells and nonoverlapping El-deleted adenoviral vectors eliminates the problem of RCA generation by homologous recombination, and allows cost-effective production of safe, clinical-grade batches of recombinant Ad vectors. The authors describe a new helper cell line and matched, nonhomologous adenoviral vectors. The absence of homologous sequences prevents the formation of replication-competent adenoviruses as a result of homologous recombination between the vector and the adenovirus sequences in the helper cells. With these new cells, the yields of E1-deleted adenoviral vectors are similar to the yields obtained with the conventional 293 and 911 cell lines. In none of nine batches of adenoviral vectors produced in these new cells could replication-dependent adenovirus be detected. The authors conclude that this new cell–vector combination allows cost-effective production of safe, clinical-grade batches of recombinant adenovirus vectors.

The efficiency and specificity of gene transfer with human adenovirus (hAd)-derived gene transfer vectors would be improved if the native viral tropism could be modified. Here, we demonstrate that the minor capsid protein IX (pIX), which is present in 240 copies in the Ad capsid, can be exploited as an anchor for heterologous polypeptides. Protein IX-deleted hAd5 vectors were propagated in hAd5 helper cells expressing pIX variants, with heterologous carboxyl-terminal extensions of up to 113 amino acids in length. The extensions evaluated consist of alpha-helical spacers up to 75 Å in length and to which peptide ligands were fused. The pIX variants were efficiently incorporated into the capsids of Ad particles. On intact particles, the MYC-tagged-pIX molecules were readily accessible to anti-MYC antibodies, as demonstrated by electron microscopic analyses of immunogold-labeled virus particles. The labeling efficiency improved with increasing spacer length, suggesting that the spacers lift and expose the ligand at the capsid surface. Furthermore, we found that the addition of an integrin-binding RGD motif to the pIX markedly stimulated the transduction of coxsackievirus group B and hAd receptor-deficient endothelioma cells, demonstrating the utility of pIX modification in gene transfer. Our data demonstrate that the minor capsid protein IX can be used as an anchor for the addition of polypeptide ligands to Ad particles.

Specificity is an essential prerequisite for cancer gene therapy. Recently we described that apoptin, a protein of 121 amino acids which is derived from the chicken anemia virus, induces programmed cell death or apoptosis in transformed and malignant cells, but not in normal, diploid cells (Danen-van Oorschot AAAM et al, Proc Natl Acad Sci USA 1997; 94: 5843–5847). This protein has an intrinsic specificity that allows it to selectively kill tumor cells, irrespective of the p53 or Bcl-2 status of these cells. Hence, it is attractive to explore the use of the apoptin gene for therapeutic applications, viz cancer gene therapy. In this paper, we describe the generation and characterization of an adenovirus vector, AdMLPvp3, for the expression of apoptin. Despite the fact that apoptin ultimately induces apoptosis in the helper cells, which are transformed by the adenovirus type 5 early region 1 (E1), the propagation kinetics and yields of AdMLPvp3 are similar to those of control vec- tors. Infection with AdMLPvp3 of normal rat hepatocytes in cell culture did not increase the frequency of apoptosis. In contrast, in the hepatoma cell lines HepG2 and Hep3b, infection with AdMLPvp3, but not with control vectors, led to a rapid induction of programmed cell death. Experiments in rats demonstrated that AdMLPvp3 could be safely administered by intraperitoneal, subcutaneous or intravenous injection. Repeated intravenous doses of AdMLPvp3 were also well tolerated, indicating that the apoptin-expressing virus can be administered without severe adverse effects. In a preliminary experiment, a single intratumoral injection of AdMLPvp3 into a xenogeneic tumor (HepG2 cells in Balb/C^nu/nu mice) resulted in a significant reduction of tumor growth. Taken together, our data demonstrate that adenovirus vectors for the expression of the apoptin gene may constitute a powerful tool for the treatment of solid tumors.

Immunotherapy is showing promise for otherwise incurable cancers. Oncolytic viruses (OVs), developed as direct cytotoxic agents, mediate their antitumor effects via activation of the immune system. However, OVs also stimulate antiviral immune responses, including the induction of OV-neutralizing antibodies. Current dogma suggests that the presence of preexisting antiviral neutralizing antibodies in patients, or their development during viral therapy, is a barrier to systemic OV delivery, rendering repeat systemic treatments ineffective. However, we have found that human monocytes loaded with preformed reovirus–antibody complexes, in which the reovirus is fully neutralized, deliver functional replicative reovirus to tumor cells, resulting in tumor cell infection and lysis. This delivery mechanism is mediated, at least in part, by antibody receptors (in particular FcγRIII) that mediate uptake and internalization of the reovirus/antibody complexes by the monocytes. This finding has implications for oncolytic virotherapy and for the design of clinical OV treatment strategies. Cancer Immunol Res; 6(10); 1161–73. ©2018 AACR.

Hemophilia A is caused by a deficiency of factor-VIII procoagulant (fVIII) activity. The current treatment by frequent infusions of plasma-derived fVIII concentrates is very effective but has the risk of transmittance of blood-borne viruses (human immunodeficiency virus [HIV], hepatitis viruses). Use of recombinant DNA-derived fVIII as well as gene therapy could make hemophilia treatment independent of blood-derived products. So far, the problematic production of the fVIII protein and the low titers of the fVIII retrovirus stocks have prevented preclinical trials of gene therapy for hemophilia A in large-animal models. We have initiated a study of the mechanisms that oppose efficient fVIII synthesis. We have established that fVIII cDNA contains sequences that dominantly inhibit its own expression from retroviral as well as from plasmid vectors. The inhibition is not caused by instability of the fVIII mRNA (t1/2, > or = 6 hours) but rather to repression at the level of transcription. A 305-bp fragment is identified that is involved in but not sufficient for repression. This fragment does not overlap the region recently identified by Lynch et al (Hum Gene Ther 4:259, 1993) as a dominant inhibitor of RNA accumulation. The repression is mediated by a cellular factor (or factors) and is independent of the orientation of the element in the transcription unit, giving the repressor element the hallmarks of a transcriptional silencer.

Background: T-cell-engaging CD3-bispecific antibodies (CD3-bsAbs) are promising modalities for cancer immunotherapy. Although this therapy has reached clinical practice for hematological malignancies, the absence of sufficient infiltrating T cells is a major barrier for efficacy in solid tumors. In this study, we exploited oncolytic reovirus as a strategy to enhance the efficacy of CD3-bsAbs in immune-silent solid tumors. Methods: The mutant p53 and K-ras induced murine pancreatic cancer model KPC3 resembles human pancreatic ductal adenocarcinomas with a desmoplastic tumor microenvironment, low T-cell density and resistance to immunotherapy. Immune-competent KPC3 tumor-bearing mice were intratumorally injected with reovirus type 3 Dearing strain and the reovirus-induced changes in the tumor microenvironment and spleen were analyzed over time by NanoString analysis, quantitative RT-PCR and multicolor flow cytometry. The efficacy of reovirus in combination with systemically injected CD3-bsAbs was evaluated in immune-competent mice with established KPC3 or B16.F10 tumors, and in the close-to-patient human epidermal growth factor receptor 2 (HER2)⁺ breast cancer model BT474 engrafted in immunocompromised mice with human T cells as effector cells. Results: Replication-competent reovirus induced an early interferon signature, followed by a strong influx of natural killer cells and CD8⁺ T cells, at the cost of FoxP3⁺ Tregs. Viral replication declined after 7 days and was associated with a systemic activation of lymphocytes and the emergence of intratumoral reovirus-specific CD8⁺ T cells. Although tumor-infiltrating T cells were mostly reovirus-specific and not tumor-specific, they served as non-exhausted effector cells for the subsequently systemically administered CD3-bsAbs. Combination treatment of reovirus and CD3-bsAbs led to the regression of large, established KPC3, B16.F10 and BT474 tumors. Reovirus as a preconditioning regimen performed significantly better than simultaneous or early administration of CD3-bsAbs. This combination treatment induced regressions of distant lesions that were not injected with reovirus, and systemic administration of both reovirus and CD3-bsAbs also led to tumor control. This suggests that this therapy might also be effective for metastatic disease. Conclusions: Oncolytic reovirus administration represents an effective strategy to induce a local interferon response and strong T-cell influx, thereby sensitizing the tumor microenvironment for subsequent CD3-bsAb therapy. This combination therapy warrants further investigation in patients with non-inflamed solid tumors.