A “Friendly Merger” in the War on Cancer
Authors: Michel Sadelain
Journal: Molecular Therapy (2000)

Oncologists and Molecular Therapists gathered at Memorial Sloan-Kettering Cancer Center (MSKCC) in New York City on January 27, 2000, to discuss “Gene Transfer in the Treatment of Cancer.” Examination of genetic approaches aimed at improving immunotherapy and limiting the toxicity of conventional therapies dominated the day. The friendly merger of immunotherapy and gene transfer technology was apparent in numerous presentations on how to most effectively enlist dendritic cells (DCs) and T lymphocytes in the battle against cancer. Because of their ability to stimulate both CD4+ and CD8+ T lymphocytes (including naive ones), dendritic cells are crucial allies in the fight. However, the best way to utilize DCs remains an open question. Which DCs, and how to recruit and activate them? There are multiple DC subsets. Langerhans cells and dermal/interstitial DCs, for example, differ in their ability to stimulate cytotoxic T lymphocytes (James Young, MSKCC). Furthermore, these DC subsets differentiate from common precursors under the influence of different cytokines and are therefore amenable to comparative studies following their generation in vitro. Ex vivo gene transfer strategies based on oncoretroviral-mediated gene transfer in CD34+ hematopoietic progenitors (Young) or lentiviral-mediated gene transfer in the nondividing blood-derived precursor (Isabelle Rivière, MSKCC) offer ways to introduce in cultured DCs well-defined antigens using vectors that are themselves non-immunogenic. In the clinical setting, this quasi-analytical approach (one type of DC, one antigen) must be complemented by validated assays such as ELISPOT and tetrameter technologies to measure induction of specific immunity accurately (Alan Houghton, MSKCC). How else could one comprehensively define crucial parameters such as the optimal DC functional profile, cell dose, route of administration, and frequency of administration, much less the validation of clinically useful antigens? On the other hand, in vivo approaches offer the advantage of bypassing the relatively cumbersome ex vivo technology and possibly the need to identify tumor antigens. To that end, Ronald Crystal (New York Presbyterian Hospital) presented an interesting approach aiming to attract DCs into established tumors based on adenoviral-mediated transfer of a chemokine gene. He described data that demonstrate that the ensuing influx of dendritic and other cells led to potent induction of tumor-specific cytotoxic T lymphocytes. However, can we trust the dendritic cell to choose—or rather, not to choose—what antigens to present? Excluding antigens created by somatic point mutations, translocations, or aberrant splicing, most tumor antigens are differentiation antigens (Houghton). The induction of immunity against such antigens is de facto a matter of experimental autoimmunity. Is there a hierarchy of tolerance to self-antigens that will allow therapists to establish effective immunity against tumor antigens without causing devastating collateral damage? It's not sure. A safer approach is therefore to carefully select antigens, e.g., gp75/trp1 or prostate-specific membrane antigen (PSMA), and deliver the chosen antigen (s) to DCs recruited to the site of injection. Using plasmid DNA encoding the antigen, Houghton showed that a xenogeneic antigen is often better suited to break immune tolerance than the autologous one (1Bowne W.B. Srinivasan R. Wolchok J.D. Hawkins W.G. Blachere N.E. Dyall R. Lewis J.J. Houghton A.N. Coupling and uncoupling of tumor immunity and autoimmunity.J. Exp. Med. 1999; 190: 1717-1722Crossref PubMed Scopus (213) Google Scholar). However, it is still unknown if plasmid DNA will be potent enough in large animals. The infusion of T lymphocytes also offers a way to control antigen reactivity. In addition, it allows rapid intervention by supplying preformed, expanded effector cells. This is particularly crucial in immunosuppressed patients with EBV-associated lymphoproliferative disease (LPD). Malcolm Brenner (Baylor College of Medicine) and Richard O'Reilly (MSKCC) summarized the spectacular results that can be obtained by this method, which can achieve major regression of disseminated lymphomas with as few as 105 donor T cells per kilogram. However, generation of T cell lines activated on donor-derived antigen-presenting cells but depleted of alloreactive T cells (to prevent graft-versus-host disease, GVHD) is time consuming, labor intensive, and sometimes too late for patients presenting with galloping disease. O'Reilly presented evidence that oncoretroviruses can be used as probes to select antigen-specific T cells within a bulk culture of T cells by virtue of their selective integration in dividing cells (2Koehne G. Gallardo H.F. Sadelain M. O'Reilly R.J. Rapid selection of antigens-specific T lymphocytes by retroviral transduction.Blood. 2000; PubMed Google Scholar). This technology accelerates the generation of safer tumor-reactive T cells. Rivière then showed how genetically modified T cells could be generated, expanded, and purified on a clinical scale in the Gene Transfer and Somatic Cell Engineering Facility at MSKCC. The introduction of the herpes simplex virus thymidine kinase (hsv-tk) gene encoded by the NIT vector (to confer ganciclovir sensitivity) adds another essential layer of safety, which may allow the investigation of this adoptive transfer strategy in HLA-mismatched recipients. Already a 10-year-old idea, the notion that T cells expressing suicide genes can be safely administered in allogeneic recipients still awaits the acid test. Can any success achieved in this one setting extend to other diseases? The EBV-associated LPD is, after all, an exception, expressing copious viral antigens in a tumor cell that is itself a pretty good antigen-presenting cell. Carefully titrated doses of allogeneic T cells can achieve durable remissions without GVHD in chronic myelogenous leukemia (O'Reilly), another condition where donor leukocyte infusion is active. Genetic approaches could ultimately simplify the dosing issues and alleviate safety concerns. However, the next big challenge lies in tackling other diseases that appear more problematic such as acute lymphoblastic leukemia or non-Hodgkin lymphoma (Brenner, O'Reilly) and, eventually, solid tumors (Michel Sadelain, MSKCC). Retrovirally transduced artificial T cell receptors enable the targeting of T cells to tumor antigens such as PSMA, a cell-surface molecule expressed on metastasized prostate cancer cells and the neovasculature of many other cancers (3Gong M. Latouche J.B. Krause A. Heston W.D. Bander N. Sadelain M. Cancer patient T cells genetically targeted to PSMA specifically and efficiently lyse prostate cancer cells.Neoplasia. 1999; 1: 123-127Abstract Full Text PDF PubMed Scopus (201) Google Scholar). The need to sustain the survival, proliferation, and activity of adoptively transferred T cells was also debated at the meeting, with an emphasis on genetic approaches to providing costimulatory signals to T cells. For example, these signals could be provided non-specifically by administering monoclonal antibodies in vivo (Savio Woo, Mount Sinai Medical Center) or rendered tumor-specific by creating antigen-dependent costimulatory receptors (Sadelain). Introducing a novel approach to study adoptive cell therapy and T cell migration to tumor sites, Ronald Blasberg (MSKCC) reviewed noninvasive imaging technology based on visualizing hsv-tk expression in animals (4Sadelain M. Blasberg R.G. Imaging transgene expression for gene therapy.J. Clin. Pharmacol. 1999; 8: 34-39Crossref Scopus (7) Google Scholar). He and other investigators at MSKCC are perfecting the noninvasive imaging of tumor-infiltrating, genetically modified T lymphocytes using γ camera techniques and positron emission tomography. Exploiting drug resistance for the patient's benefit was discussed by Joseph Bertino (MSKCC), who described the availability of newer and better drug resistance genes that protect the hematopoietic tissue from myelotoxic drugs. For example, Bertino and colleagues will soon begin an investigation of a novel dihydrofolate reductase–cytidine deaminase fusion gene conferring high-level resistance to methotrexate and cytarabine (5Sauerbrey A. McPherson H.P. Zhao S.C. Banarejee D. Bertino J.B. Expression of a novel double-mutant dihydrofolate reductase–cytidine deaminase fusion gene confers resistance to both methotrexate and cytosine arabinoside.Hum. Gene Ther. 1999; 10: 249Crossref PubMed Scopus (30) Google Scholar) in patients with relapsed non-Hodgkin lymphoma. However, Brenner noted that we are still waiting for a successful proof-of-concept experiment and that investigators in this arena are still struggling to demonstrate unequivocal in vivo drug selection, let alone marrow protection. Nevertheless, as argued by Bertino, Malcolm Moore (MSKCC), and Lucio Luzzatto (MSKCC), such a demonstration still awaits the development of better vectors, better genes, and better transduction protocols for hematopoietic stem cells. To this end, Moore showed appreciable improvements of retroviral-mediated gene transfer efficiency into CD34+ cells that repopulate long-term NOD-scid/scid mice, emphasizing the importance of optimizing cytokine combinations and using mutant stromal cell lines. There was little at this meeting on targeting of tumors by direct in vivo gene transfer, reflecting the fact that this is a daunting and perhaps unrealistic task if successful therapy requires transduction of a majority of cells within a tumor. As stressed by Woo, combination therapy (using for example a suicide gene to induce apoptotic cell death, a cytokine gene to stimulate T cells via paracrine secretion, and a monoclonal antibody to sustain T cell activation) is of the essence. Yuman Fong (MSKCC) showed how herpes-based vectors could be used to deliver the cytokines and costimulatory molecules, but then made the case for incorporating into our armamentarium the use of replication-competent viruses such as oncolytic, attenuated herpes virus. As pointed out by David Golde (MSKCC) and Brenner in their respective introductory remarks and keynote address, it has taken 20 years to begin to figure out how to effectively use cytokines and monoclonal antibodies in cancer therapy. Will it be any different for the use of gene transfer?