Meeting Highlights

10.1517/14712598.6.8.839 © 2006 Informa UK Ltd ISSN 1471-2598 839

Gene Therapy

9th American Society of Gene Therapy Annual MeetingPH TanJohn Radcliffe Hospital, Regional Vascular Unit, Nuffield Department of Surgery, John Radcliffe Hospitals NHS Trust, University of Oxford, Headley Way, Oxford, OX3 9DU, UK

The American Society of Gene Therapy, under the presidency of Mark Kay,held its annual meeting in Baltimore, MD, USA this year. This meeting waswell received by academics and industry alike in order to promote research,development and application of gene therapy. Exchange of information,promotion of education and development of clinical translation have beenthe main aims of this society. On face value, the meeting seems to highlightthat gene-based treatment is coming of age. Many drawbacks of gene-basedtreatments were discussed at this meeting with an intense vigour.Immunology has been the major hurdle in restricting the full realisation ofgene therapy in the clinical arena. In summary, the field of gene therapy hasprogressed from its infancy stage of trying to get it to work, to another stagewhere attempts are being made to overcome the problems associated withits application, in particular, in relation to human immune responses to thevector and the transgene. This progress should be celebrated and not treatedas a failure of this form of therapy, as to make this progress is indeed anachievement for any form of therapy.

Keywords: gene therapy, immunology, viral vectors

Expert Opin. Biol. Ther. (2006) 6(8):839-842

1. Introduction

This year, the annual meeting for the American Society of Gene Therapy was heldin the Baltimore Convention Centre, Baltimore, Maryland, USA. It has lived up toits name by attracting delegates not only from the US, but also from other countriesin Europe and Asia. The official opening of the Congress was marked with theGeorge Stamatoyannopoulos lecture. George Stamatiyannopoulos was the founderpresident of this society 9 years ago. This year’s lecture was delivered by D Allis(Rockefeller University, New York, but now based at University of Virginia, USA),entitled ‘Beyond the double helix; reading and writing the histone code’.

This meeting is a celebration of decades of discoveries and application in genetherapy. It has promoted an intense cross-fertilisation of basic science and clinicalmedicine, and is one of the great examples of how a multidisciplinary approachfacilitates progress. It is a privilege to observe a new generation full of enthusiasm, aswell as the collaboration in the field of gene therapy in this meeting.

With the support of the Scientific Advisery Committee, under the directionof the president Mark Kay (Stanford University, CA, USA), the society puttogether an outstanding scientific and educational programme that included allrelevant topics and the latest developments in gene therapy, and pleasedeveryone in the various fields associated with gene therapy. The programmestructure included 14 educational sessions, 17 scientific symposia, 16 workshopsplus oral and poster sessions, and multiple new options, such as ‘choosing acareer path; academia versus biotech?’. Therefore, by its very nature, this report

1. Introduction

2. Important highlights from

the meeting

3. Immunology and gene

therapy applications

4. Expert opinion and conclusion

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9th American Society of Gene Therapy Annual Meeting

840 Expert Opin. Biol. Ther. (2006) 6(8)

merely provides a snap-shot of the meeting. This paperreflects the author’s personal interest, which is immunologyand gene therapy.

2. Important highlights from the meeting

D Allis, in his George Stamatoyannopoulos lecture,demonstrated the role of histones, in particular histone 3, inregulating transgene expression. His group favours the viewthat there exists an epigenetic indexing system, or ‘histonecode’, for our genome, which represents a fundamentalregulatory mechanism that acts outside of the DNA itself.They hypothesise that this ‘code’ impacts on most, if not all,chromatin-templated processes, with far-reachingconsequences for gene therapists. Indeed, in two recentpapers, they identified the key processes in the reading,writing and remodelling of chromatin by histone 3A [1,2].This process of remodelling in histones can activate andrepress transgene expression. It is, therefore, important forgene therapists to engineer a vector to integrate to the siteswhere the histones will promote transgene expression and notto the silencing sites.

Another important activity from the meeting was theeducational sessions. At the conclusion of these sessions, allparticipants were able to:

• discuss key aspects of laboratory and/or clinical research ingene therapy at present

• demonstrate knowledge of methods being used by genetherapy researchers and practitioners

• describe one or more stimulating new areas of inquiry inthe area of gene therapy

• demonstrate familiarity with federal regulation andguidelines for clinical gene transfer studies

There were multiple meet-the-investigator sessions forstudents and young postdoctoral fellows to attend. This oftenresulted in the promotion of dialogue between establishedinvestigators and juniors in the field. Without a shadow ofdoubt, these sessions fostered social networking.

3. Immunology and gene therapy applications

On the whole, this year’s meeting heavily concentrated on therole of immunology in gene therapy. It has recently beenaccepted that the vector used to carry therapeutic genes caninduce host immunity to the vector or even the transgeneitself. Indeed, a clinical trial carried out by K High’s group(The Children’s Hospital of Philadelphia, USA) has shownthat despite adeno-associated virus (AAV) vector withFactor VIII transgene integrating into the host genome, thetransgene expression is transient [3,4]. This is due to thecellular immunity of the AAV caspid, leading to the loss ofcells that express the transgene and vector products. Theobservations have led to renewed efforts by gene therapists tolearn about the power of immunology.

Consistent with previously published work that viralvectors can activate dendritic cells (DCs) through type Iinterferon and phosphatidylinositol 3-kinase pathways [5],H Ertl showed that costimulatory molecule upregulationfollowing adenoviral transduction is indeed dependent onthese pathways [6]. Her group’s data also demonstrated thatAAV (type 2) infection diminished the ability of DCs tostimulate CD8 T cells. The mechanism of how AAV affectsDC function is yet to be fully illustrated. Interestingly, hergroup also showed that prior infection to AVV could preventDC activation following adenoviral infection. It is crucial forher group to investigate the mechanism of how AAV infectioncan tolerise CD8 T cells.

Consistent with a previous study [7], T Flotte (UniversityFlorida, Miami, USA) extended the data from small animalsto large animals. Following AAV administration inhepatitis C-infected chimpanzees, liver biopsies were takenand illustrated a degree of inflammatory changes secondary toimmune activation. Transduction of pancreatic islets alsoillustrated the immunogenicity of vectors in an allogeneictransplantation model with the IL-4 transgene. These dataevidently represent the renewed call for gene therapists toexplore human immunology in order to allow application ofgene therapy in the clinical arena.

I MacLachlan (Protiva Biotherapeutics, Burnaby, BritishColumbia, Canada) presented his group’s latest research,illustrating the innate response to immunostimulatory RNA(isRNA) technology [8,9]. As expected, delivery of shortinterfering RNA (siRNA) can activate Toll-like receptors(TLRs), such as TLR3 and 8 in myeloid DCs, and TLR 7and 9 in plasmacytoid DCs; MacLachlan’s group clearlydemonstrated these findings in non-primate animal models.They went on to confirm that the dose, sequence andchemistry of siRNA and the mode of delivery of isRNAhave a direct effect on siRNA triggering innate immunity.More importantly, they have demonstrated techniques toabrogate the immunogenicity of siRNA technology byeither altering the isRNA itself or modifying the vector usedto carry isRNA.

In a similar tone, S Samakoglu (Sloan-Kettering Institute,New York, USA) presented her data showing that using atissue-specific lentiviral vector and embedded short hairpinRNA (shRNA) within the vector-encoded trangenes permitsthe stable coexpression of a therapeutic gene and a specificsiRNA within the same cell [10]. More importantly, sheshowed that the position of the shRNA is critical to achievehigh levels of transgene expression, effective siRNAgeneration and minimal interferon induction. In addition, shealso characterised the induction of interferon response byPol II- and Pol III-driven isRNA. She also demonstrated thatendogenous microRNA expression might be affected byvector-encoded siRNA-driven target gene silencing. Insummary, Samakoglu’s group has shown that interferonresponse and microRNA perturbation can indeed be avertedwith a better vector developed by her group.

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Expert Opin. Biol. Ther. (2006) 6(8) 841

In the session titled ‘Infectious disease – manipulatingregulatory/suppressor T cells’, M Roncarolo (San RaffaeleTelethon Institute for Gene Therapy [HSR-TIGET], Milan,Italy) presented her group data in collaboration with L Naldini(HSR-TIGET), demonstrating that a lentiviral vector(HIV-pseudotyped with vesicular stomatitis virus) can activateplasmacytoid DCs, but not myeloid DCs. Using an in vivodelivery model, she demonstrated both cellular and humoralresponse to the green fluorescent protein (GFP) transgene.However, when the albumin-specific promoter is utilised, thereis less cellular response. Using GFP-transgenic mice, thetransfer of regulatory T cells (Tregs) (possibly with T cellreceptors specific for GFP peptide) into wild-type mice priorto exposure to the vector fails to attenuate the cellular responseto the GFP transgene. However, antigen-presenting cells, suchas DCs that have been rendered tolerogenic, may represent abetter way to induce tolerance of gene therapy vectors ortransgenes. Roncarolo also demonstrated that there are specifictechniques to tolerise DCs. She then proposed that IL-10treatment, ligation of CD45RO/RB and NVP347 treatmentmight be a useful approach to generate tolerogenic DCs.

D Klatzmann (Hopital Pitie SalPetriere-Batiment, Paris,France) chaired this session and also presented the data fromhis laboratory, showing the role of Tregs in tumoursurveillance [11]. Depletion of Tregs leads to excessive tumourgrowth. It has been shown that Treg growth is faster than thegrowth of naive T cells in a tumour, indicating a survivaladvantage for Tregs over antitumour-specific T cells. Indeed,he presented preliminary data from his Phase I/II clinical trialof three cancer patients with tumour vaccination followed byTreg depletion. C Mackall (National Institutes of Health,Bethesda, USA) presented his research, illustrating theemerging evidence that one of the primary physiological rolesof IL-2 is to generate and maintain Tregs [12]. His groupmonitored Tregs during immune reconstitution in individualswith cancer who did or did not receive IL-2 therapy. Tregsunderwent homeostatic peripheral expansion during immunereconstitution, and in lymphopenic individuals receiving IL-2the Treg compartment was markedly increased. Their datasuggest that IL-2 and lymphopenia are primary modulators ofTreg homeostasis. The session concluded with a talk from apioneering scientist in Tregs, E Shevach (National Institutesof Health, Bethesda, USA). His talk highlighted thetherapeutic potential of Tregs; the use of Tregs may havetherapeutic applications in the treatment of many diseases(transplant rejection or autoimmunity) and also in preventingimmunoreactivity to vectors or transgenes.

The plots for the meeting were thickened as the heavilygunned immunologists were invited to cast their brimminglights on the meeting. The most sentinel talk on fundamentalimmunology was from the ‘Queen’ of T cell costimulation,A Sharpe (Brigham and Women Hospital, Harvard Medical

School, Boston, USA). She gave an impressive talk on thecomplex story of T cell costimulation. Her group and herlong-standing collaborator, G Freeman (Dana-Farber CancerInstitute, Harvard Medical School, Boston, MA, USA),defined the role of B7s-CD28/CTLA4 in T cell activation inthe early 1990s, which has led to the identification of othercostimulatory molecules (such as ICOS/ICOS-L [13] orprogramme death [PD]-1/PD-L1/2 [14]). For example, in arecent research paper, they have identified PD-L2 as a ligandfor PD-1, in addition to its previously identified ligandPD-L1 [14]. They have now progressed to show the role ofPD-1 and PD-L1 in maintaining peripheral tolerance, inparticular the pancreatic beta cells [15]. In a similar context,R Ahmed (National Institutes of Health, Bethesda, USA) incollaboration with A Sharpe (recently published in [16])demonstrated the role of PD-1/PD-L1 interaction inexhausting CD8 T cell clones following chronic viralinnfection [16]. Blockade of PD-1/PD-L1 interaction withmonoclonal antibodies can improve CD8 T cell function [16].These findings highlight a potential strategy that couldencourage the immune system to attenuate its response tovectors and transgenes through the PD-1/PD-L1 pathway.

4. Expert opinion and conclusion

On the whole, I hope that the meeting mirrored Baltimorecity – full of medical and scholarly traditions. Many of theplenary sessions of this meeting reflected the problemsassociated with gene therapy vectors. This report may soundlike a rather negative tone on the prospect of gene therapy. It isimportant for the field to be aware that there is anothersignificant hurdle to cross for gene therapy to be fullyapplicable in the clinical arena: the issue of immunology. I feelthat the field is healthy and is slowly working out its problems.In collaboration with immunologists, I am confident that genetherapists will ultimately overcome this immunological hurdle.It was clearly evident in this meeting that many imminentimmunologists, such as M Roncarolo, are now working in thisfield. This represents a significant milestone.

Immunologists at Hammersmith Hospital (ImperialCollege London, UK) have formally studied the signallingpathways and immunological understanding of gene therapyvectors on endothelial cells [17] and DCs [5]. The results areconsistent with most of the data presented in this meeting.Renewed efforts to circumnavigate the immune activationinduced by gene therapy vectors will represent a future area ofresearch. As clearly suggested at this meeting, the use of Tregsand tolerogenic DCs could probably be potential strategies toinduce tolerance to gene therapy vectors and transgenes.Indeed, recently, the author’s group have also proposed manyways to generate tolerogenic DCs [18-20] that could also beused in the field of gene therapy.

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AffiliationPH Tan1,2

1John Radcliffe Hospital, Regional Vascular Unit, Nuffield Department of Surgery, John Radcliffe Hospitals NHS Trust, University of Oxford, Headley Way, Oxford, OX3 9DU, UKE-mail: p.tan@imperial.ac.uk2Hammersmith Hospital, Department of Immunology, Faculty of Medicine, Imperial College London, Du Cane Road, London, W12 ONN, UK

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