Lentivector Targeting to Dendritic Cells
Authors: Alexander Ageichik, Mary K Collins, Marie Dewannieux
Journal: Molecular Therapy (2008)

Development of effective vaccines for the immunotherapy of cancer or persistent infectious disease will require the reactivation of a suboptimal immune response; in the case of cancer immunotherapy this often involves breaking immunologic tolerance to self-antigens expressed by the tumor cells. Dendritic cells (DCs) are the most potent antigen-presenting cells of the immune system, and much effort has therefore been devoted to methods for delivering appropriate antigen to DCs to induce immunization. Initial DC therapies tested in clinical trials used autologous DCs cultured in the laboratory, loaded with antigen, and then re-injected.1Steinman RM Banchereau J Taking dendritic cells into medicine.Nature. 2007; 449: 419-426Crossref PubMed Scopus (1685) Google Scholar However, more cost-effective and efficient protocols will involve direct delivery of antigens to DCs in vivo.2Tacken PJ de Vries IJ Torensma R Figdor CG Dendritic-cell immunotherapy: from ex vivo loading to in vivo targeting.Nat Rev Immunol. 2007; 7: 790-802Crossref PubMed Scopus (609) Google Scholar Targeting of antigens to DCs can be achieved by linking antigens to antibodies that recognize DC surface molecules (e.g., Bonifaz et al.3Bonifaz LC Bonnyay DP Charalambous A Darguste DI Fujii S Soares H et al.In vivo targeting of antigens to maturing dendritic cells via the DEC-205 receptor improves T cell vaccination.J Exp Med. 2004; 199: 815-824Crossref PubMed Scopus (764) Google Scholar) or by targeting viral vectors to DCs so that they then express antigens within the antigen-presenting cell. DCs residing in the skin and lymph nodes constitute the natural detection system for external pathogens and are thus relevant for vaccination. Viral vector targeting to DCs can potentially avoid antigen expression in other tissues, and DCs may recognize the viral vectors as pathogens and become activated, increasing the potency of immunization. Adenoviral vectors are potent immunogens, and it has recently proved possible to engineer adenovirus serotype 5 to improve DC specificity and maintain immunogenicity.4Cheng C Gall JG Kong WP Sheets RL Gomez PL King CR et al.Mechanism of ad5 vaccine immunity and toxicity: fiber shaft targeting of dendritic cells.PLoS Pathog. 2007; 3: e25Crossref PubMed Scopus (67) Google Scholar Targeting of enveloped viruses—for example, using single-chain antibodies fused to murine leukemia virus envelope—has often resulted in relatively inefficient infection because the function of the chimeric envelope protein was compromised to some extent (e.g., Martin et al.5Martin F Neil S Kupsch J Maurice M Cosset F Collins M Retrovirus targeting by tropism restriction to melanoma cells.J Virol. 1999; 73: 6923-6929PubMed Google Scholar). As an alternative solution to this problem, binding-deficient but fusion-competent mutants were isolated from Sindbis and measles viruses and used to retarget binding without affecting the efficiency of fusion.6Ohno K Sawai K Iijima Y Levin B Meruelo D Cell-specific targeting of Sindbis virus vectors displaying IgG-binding domains of protein A.Nat Biotechnol. 1997; 15: 763-767Crossref PubMed Scopus (134) Google Scholar,7Nakamura T Peng KW Harvey M Greiner S Lorimer IA James CD et al.Rescue and propagation of fully retargeted oncolytic measles viruses.Nat Biotechnol. 2005; 23: 209-214Crossref PubMed Scopus (222) Google Scholar In these viruses the binding and fusion functions could be separated because they reside on different envelope glycoproteins. The Sindbis virus envelope has also been used for retargeting lentiviral vectors (lentivectors).8Morizono K Xie Y Ringpis GE Johnson M Nassanian H Lee B et al.Lentiviral vector retargeting to P-glycoprotein on metastatic melanoma through intravenous injection.Nat Med. 2005; 11: 346-352Crossref PubMed Scopus (184) Google Scholar Lentivectors are attractive vaccine vector candidates because they can transduce DCs in vivo without interfering with their function and then induce both cellular and humoral immune responses.9Esslinger C Chapatte L Finke D Miconnet I Guillaume P Lévy F et al.In vivo administration of a lentiviral vaccine targets DCs and induces efficient CD8(+) T cell responses.J Clin Invest. 2003; 111: 1673-1681Crossref PubMed Scopus (134) Google Scholar,10Palmowski MJ Lopes L Ikeda Y Salio M Cerundolo V Collins MK Intravenous injection of a lentiviral vector encoding NY-ESO-1 induces an effective CTL response.J Immunol. 2004; 172: 1582-1587Crossref PubMed Scopus (92) Google Scholar,11He Y Zhang J Mi Z Robbins P Falo LD Jr Immunization with lentiviral vector-transduced dendritic cells induces strong and long-lasting T cell responses and therapeutic immunity.J Immunol. 2005; 174: 3808-3817Crossref PubMed Scopus (128) Google Scholar In a recent article, Yang and collaborators presented an elegant piece of work in which they achieved high targeting specificity in vivo using a lentivector-based vaccine.12Yang L Yang H Rideout K Cho T Joo KI Ziegler L et al.Engineered lentivector targeting of dendritic cells for in vivo immunization.Nat Biotechnol. 2008; 26: 326-334Crossref PubMed Scopus (168) Google Scholar Instead of trying to add a new module to the glycoprotein so as to target the vector, they decided to take advantage of the in vivo tropism of Sindbis virus, which naturally shows preference for DCs. They further restricted infection by inactivating its binding to broadly expressed heparan sulfate molecules. This strategy enabled them to show infection of DCs in culture via DC-SIGN, the other natural receptor of Sindbis virus, which is expressed mainly by DCs. Interestingly, this modified glycoprotein works equally well with mouse and human DC-SIGN molecules, making it an interesting candidate for human therapeutic applications. The in vivo studies in mice showed infection of DCs, with substantially fewer off-target transduction events than with the more commonly used vesicular stomatitis virus G (VSV-G)–pseudotyped lentivectors. Despite reducing the number of in vivo –infected cells, this protocol still resulted in strong immunization against the model antigen ovalbumin, with the induction of both specific CD8+ T cells and an antibody response. In this respect, the authors' results resemble those from our group using a specific promoter to target lentivector-driven antigen expression to DCs in vivo.13Lopes L Dewannieux M Gileadi U Bailey R Ikeda Y Whittaker C et al.Immunization with a lentivector that targets tumor antigen expression to dendritic cells induces potent CD8+ and CD4+ T-cell responses.J Virol. 2008; 82: 86-95Crossref PubMed Scopus (60) Google Scholar From a functional point of view, the surface-targeted in vivo vaccination was strong enough to protect mice against challenge with a tumor expressing ovalbumin, and it also proved efficient in tumor-therapy experiments. These results are highly encouraging for the future of lentivector-based vaccination strategies. However, before they can be used with patients for whom alternative therapies exist, there are safety considerations that must be tackled, such as the possibility of insertional mutagenesis. The reduction in in vivo –transduced cells that can be achieved by efficient targeting of DCs is an important step; however, the in vivo tropism of the targeted Sindbis virus must be further assessed with more sensitive methods than the luciferase-based system used in the present study. It is possible that the background infection of cells lacking DC-SIGN still shown by this system may be improved by further mutagenesis. Furthermore, persistence of transduced DCs or other antigen-presenting cells following vector integration may result in prolonged antigen presentation, the consequences of which are not yet fully understood. The ability of DCs to capture viruses via DC-SIGN and transmit them to other cells must also be evaluated in this system. In the case of HIV, this mechanism results in viral spreading far from the site of initial infection.14de Witte L Nabatov A Geijtenbeek TB Distinct roles for DC-SIGN(+)-dendritic cells and Langerhans cells in HIV-1 transmission.Trends Mol Med. 2008; 14: 12-19Abstract Full Text Full Text PDF PubMed Scopus (97) Google Scholar Despite these technical issues, Yang and collaborators provide a proof-of-principle study of lentivector targeting to DCs, which can lead to improved human vaccination. A strength of their strategy may be that, compared with other lentiviral pseudotypes that have been tested in mice, the targeting via DC-SIGN results in both antigen presentation and concomitant DC activation, demonstrated in mouse DC cultures. We have reported that VSV-G–pseudotyped lentivectors do not activate such mouse DCs unless an intracellular activator of the p38 MAP kinase pathway is co-expressed with the antigen.15Escors D Lopes L Lin R Hiscott J Akira S Davis RJ et al.Targeting dendritic cell signaling to regulate the response to immunization.Blood. 2008; 111: 3050-3061Crossref PubMed Scopus (92) Google Scholar The reason for the difference between Sindbis- and VSV-G–pseudotyped lentivectors in the induction of DC maturation is unknown; it may be hypothesized that attachment of Sindbis glycoproteins to DC-SIGN provides DCs with the maturation signals they require. This could allow initiation of a strong immune response despite low transduction efficiency.