In vivo administration of a lentiviral vaccine targets DCs and induces efficient CD8(+) T cell responses

An Update on the HIV DNA Vaccine Strategy

In 2020, the global prevalence of human immunodeficiency virus (HIV) infection was estimated to be 38 million, and a total of 690,000 people died from acquired immunodeficiency syndrome (AIDS)–related complications. Notably, around 12.6 million people living with HIIV/AIDS did not have access to life-saving treatment. The advent of the highly active antiretroviral therapy (HAART) in the mid-1990s remarkably enhanced the life expectancy of people living with HIV/AIDS as a result of improved immune functions. However, HAART has several drawbacks, especially when it is not used properly, including a high risk for the development of drug resistance, as well as undesirable side effects such as lipodystrophy and endocrine dysfunctions, which result in HAART intolerability. HAART is also not curative. Furthermore, new HIV infections continue to occur globally at a high rate, with an estimated 1.7 million new infections occurring in 2018 alone. Therefore, there is still an urgent need for an affordable, effective, and readily available preventive vaccine against HIV/AIDS. Despite this urgent need, however, progress toward an effective HIV vaccine has been modest over the last four decades. Reasons for this slow progress are mainly associated with the unique aspects of HIV itself and its ability to rapidly mutate, targeting immune cells and escape host immune responses. Several approaches to an HIV vaccine have been undertaken. However, this review will mainly discuss progress made, including the pre-clinical and clinical trials involving vector-based HIV DNA vaccines and the use of integrating lentiviral vectors in HIV vaccine development. We concluded by recommending particularly the use of integrase-defective lentiviral vectors, owing to their safety profiles, as one of the promising vectors in HIV DNA vaccine strategies both for prophylactic and therapeutic HIV vaccines.

A Single Dose of NILV-Based Vaccine Provides Rapid and Durable Protection against Zika Virus

Zika virus, a member of the Flaviviridae family, is primarily transmitted by infected Aedes species mosquitoes. In 2016, Zika infection emerged as a global health emergency for its explosive spread and the remarkable neurological defects in the developing fetus. Development of a safe and effective Zika vaccine remains a high priority owing to the risk of re-emergence and limited understanding of Zika virus epidemiology. We engineered a non-integrating lentiviralvector(NILV)-based Zika vaccine encoding the consensus pre-membrane and envelope glycoprotein of circulating Zika virus strains. We further evaluated the immunogenicity and protective efficacy of this vaccine in both immunocompromised and immunocompetent mouse models. A single immunization in both mouse models elicited a robust neutralizing antibody titer and afforded full protection against Zika challenge as early as 7 days post-immunization. This NILV-based vaccine also induced a long-lasting immunity when immunized mice were challenged 6 months after immunization. Altogether, our NILV Zika vaccine provides a rapid yet durable protection through a single dose of immunization without extra adjuvant formulation. Our data suggest a promising Zika vaccine candidate for an emergency situation, and demonstrate the capacity of lentiviral vector as an efficient vaccine delivery platform.

Lentiviral-Vector-Based Dendritic Cell Vaccine Synergizes with Checkpoint Blockade to Clear Chronic Viral Infection

Dendritic cell vaccines are a promising strategy for the treatment of cancer and infectious diseases but have met with mixed success. We report on a lentiviral vector-based dendritic cell vaccine strategy that generates a cluster of differentiation 8 (CD8) T cell response that is much stronger than that achieved by standard peptide-pulsing approaches. The strategy was tested in the mouse lymphocytic choriomeningitis virus (LCMV) model. Bone marrow-derived dendritic cells from SAMHD1 knockout mice were transduced with a lentiviral vector expressing the GP33 major-histocompatibility-complex (MHC)-class-I-restricted peptide epitope and CD40 ligand (CD40L) and injected into wild-type mice. The mice were highly protected against acute and chronic variant CL-13 LCMVs, resulting in a 100-fold greater decrease than that achieved with peptide epitope-pulsed dendritic cells. Inclusion of an MHC-class-II-restricted epitope in the lentiviral vector further increased the CD8 T cell response and resulted in antigen-specific CD8 T cells that exhibited a phenotype associated with functional cytotoxic T cells. The vaccination synergized with checkpoint blockade to reduce the viral load of mice chronically infected with CL-13 to an undetectable level. The strategy improves upon current dendritic cell vaccine strategies; is applicable to the treatment of disease, including AIDS and cancer; and supports the utility of Vpx-containing vectors.

Alarmin augments the antitumor immunity of lentiviral vaccine in ectopic, orthotopic and autochthonous hepatocellular carcinoma mice

It is a daunting therapeutic challenge to completely eradicate hepatocellular carcinoma (HCC) from patients. Alpha-fetoprotein (AFP) -based vaccines appear promising, however the efficacy needs to be improved. Methods: Here, we explore if fusing high-mobility group nucleosome binding protein 1 (HMGN1), a potent immunoadjuvant, to AFP (lenti-HA) can augment the antitumor immunity of AFP-expressing lentiviral vector (lenti-AFP), a vehicle extensively employed for genetic immunization with high transduction efficacy and good safety profiles. The antitumor immunity of Lenti-HA was systemically assessed in ectopic, orthotopic and autochthonous HCC models. Results: Lenti-HA elicited strong anti-HCC effects in mice and amplified the antitumor immunity of lenti-AFP by reducing effective dose 6-fold. Importantly, lenti-HA induced a robust antitumor immune response with prolonged survival rate and improved the immune and tumor microenvironment in mice with carcinogen-induced autochthonous HCC. Lenti-HA localized primarily to lymphoid organs with no preference for specific immune cell types. Activated dendritic cells (DCs), particularly CD103+CD11b- DCs, were also actively recruited to lymph nodes in lenti-HA-treated HCC mice. Moreover, lenti-HA-transduced human DCs elicited stronger immune response than lenti-AFP against HCC cells in vitro. Conclusion: Our study demonstrates that HMGN1 augments the antitumor immunity of AFP-expressing lentiviral vaccines in HCC mice and human cells in vitro and thus provides a new therapeutic strategy for HCC.

First-in-Human Treatment With a Dendritic Cell-targeting Lentiviral Vector-expressing NY-ESO-1, LV305, Induces Deep, Durable Response in Refractory Metastatic Synovial Sarcoma Patient

Supplemental Digital Content is available in the text.

Effective induction of antitumor T cells is a pivotal goal of cancer immunotherapy. To this end, lentiviral vectors (LV) are uniquely poised to directly prime CD8 T-cell responses via transduction of dendritic cells in vivo and have shown promise as active cancer therapeutics in preclinical tumor models. However, until now, significant barriers related to production and regulation have prevented their widespread use in the clinic. We developed LV305, a dendritic cell-targeting, integration-deficient, replication incompetent LV from the ZVex platform, encoding the full-length cancer-testis antigen NY-ESO-1. LV305 is currently being evaluated in phase 1 and 2 trials in metastatic recurrent cancer patients with NY-ESO-1 positive solid tumors as a single agent and in combination with anti-PD-L1. Here we report on the first patient treated with LV305, a young woman with metastatic, recurrent, therapy-refractive NY-ESO-1⁺ synovial sarcoma. The patient developed a robust NY-ESO-1-specific CD4⁺ and CD8⁺ T-cell response after 3 intradermal injections with LV305, and subsequently over 85% disease regression that is continuing for >2.5 years posttherapy. No adverse events >grade 2 occurred. This case demonstrates that LV305 can be safely administered and has the potential to induce a significant clinical benefit and immunologic response in a patient with advanced stage cancer.

The Antitumor Effects of Vaccine-Activated CD8+ T Cells Associate with Weak TCR Signaling and Induction of Stem-Like Memory T Cells

To understand why vaccine-activated tumor-specific T cells often fail to generate antitumor effects, we studied two α-fetoprotein-specific CD8⁺ T cells (Tet₄₉₉ and Tet₂₁₂) that had different antitumor effects. We found that Tet₄₉₉ required high antigen doses for reactivation, but could survive persistent antigen stimulation and maintain their effector functions. In contrast, Tet₂₁₂ had a low threshold of reactivation, but underwent exhaustion and apoptosis in the presence of persistent antigen. In vivo, Tet₄₉₉ cells expanded more than Tet₂₁₂ upon reencountering antigen and generated stronger antitumor effects. The different antigen responsiveness and antitumor effects of Tet₂₁₂ and Tet₄₉₉ cells correlated with their activation and differentiation states. Compared with Tet₂₁₂, the population of Tet₄₉₉ cells was less activated and contained more stem-like memory T cells (Tscm) that could undergo expansion in vivo The TCR signaling strength on Tet₄₉₉ was weaker than Tet₂₁₂, correlating with more severe Tet₄₉₉ TCR downregulation. Weak TCR signaling may halt T-cell differentiation at the Tscm stage during immune priming and also explains why Tet₄₉₉ reactivation requires a high antigen dose. Weak TCR signaling of Tet₄₉₉ cells in the effector stage will also protect them from exhaustion and apoptosis when they reencounter persistent antigen in tumor lesion, which generates antitumor effects. Further investigation of TCR downregulation and manipulation of TCR signaling strength may help design cancer vaccines to elicit a mix of tumor-specific CD8⁺ T cells, including Tscm, capable of surviving antigen restimulation to generate antitumor effects. Cancer Immunol Res; 5(10); 908-19. ©2017 AACR.

Dendritic cell-targeted lentiviral vector immunization uses pseudotransduction and DNA-mediated STING and cGAS activation

Dendritic cell (DC) activation and antigen presentation are critical for efficient priming of T cell responses. Here, we study how lentiviral vectors (LVs) deliver antigen and activate DCs to generate T cell immunization in vivo. We report that antigenic proteins delivered in vector particles via pseudotransduction were sufficient to stimulate an antigen-specific immune response. The delivery of the viral genome encoding the antigen increased the magnitude of this response in vivo but was irrelevant in vitro. Activation of DCs by LVs was independent of MyD88, TRIF, and MAVS, ruling out an involvement of Toll-like receptor or RIG-I-like receptor signaling. Cellular DNA packaged in LV preparations induced DC activation by the host STING (stimulator of interferon genes) and cGAS (cyclic guanosine monophosphate-adenosine monophosphate synthase) pathway. Envelope-mediated viral fusion also activated DCs in a phosphoinositide 3-kinase-dependent but STING-independent process. Pseudotransduction, transduction, viral fusion, and delivery of cellular DNA collaborate to make the DC-targeted LV preparation an effective immunogen.

Antigen-presenting cell-targeted lentiviral vectors do not support the development of productive T-cell effector responses: implications for in vivo targeted vaccine delivery

Targeting transgene expression specifically to antigen-presenting cells (APCs) has been put forward as a promising strategy to direct the immune system towards immunity. We developed the nanobody-display technology to restrict the tropism of lentiviral vectors (LVs) to APCs. However, we observed that immunization with APC-targeted LVs (DC2.1-LVs) did not evoke strong antigen-specific T-cell immunity when compared to immunization with broad tropism LVs (VSV.G-LVs). In this study, we report that VSV.G-LVs are more immunogenic than DC2.1-LVs because they transduce stromal cells, which has a role in activating antigen-specific T cells. Moreover, VSV.G-LVs trigger a pro-inflammatory innate immune response through transduction of APCs and stromal cells, while DC2.1-LVs trigger a type I interferon response with anti-viral capacity. These findings question the rationale of targeting LVs to APCs and argue for the development of VSV.G-LVs with an improved safety profile.

Dendritic Cells Cross-Present Immunogenic Lentivector-Encoded Antigen from Transduced Cells to Prime Functional T Cell Immunity

Recombinant lentiviral vectors (LVs) are highly effective vaccination vehicles that elicit protective T cell immunity in disease models. Dendritic cells (DCs) acquire antigen at sites of vaccination and migrate to draining lymph nodes, where they prime vaccine-specific T cells. The potency with which LVs activate CD8+ T cell immunity has been attributed to the transduction of DCs at the immunization site and durable presentation of LV-encoded antigens. However, it is not known how LV-encoded antigens continue to be presented to T cells once directly transduced DCs have turned over. Here, we report that LV-encoded antigen is efficiently cross-presented by DCs in vitro. We have further exploited the temporal depletion of DCs in the murine CD11c.DTR (diphtheria toxin receptor) model to demonstrate that repopulating DCs that were absent at the time of immunization cross-present LV-encoded antigen to T cells in vivo. Indirect presentation of antigen from transduced cells by DCs is sufficient to prime functional effector T cells that control tumor growth. These data suggest that DCs cross-present immunogenic antigen from LV-transduced cells, thereby facilitating prolonged activation of T cells in the absence of circulating LV particles. These are findings that may impact on the future design of LV vaccination strategies.

Recent Advances in Lentiviral Vaccines for HIV-1 Infection

The development of an effective HIV vaccine to prevent and/or cure HIV remains a global health priority. Given their central role in the initiation of adaptive immune responses, dendritic cell (DC)-based vaccines are being increasingly explored as immunotherapeutic strategies to enhance HIV-specific T cells in infected individuals and, thus, promote immune responses that may help facilitate a functional cure. HIV-1-based lentiviral (LV) vectors have inherent advantages as DC vaccine vectors due to their ability to transduce non-dividing cells and integrate into the target cell genomic DNA, allowing for expression of encoded antigens over the lifespan of the cell. Moreover, LV vectors may express additional immunostimulatory and immunoregulatory proteins that enhance DC function and direct antigen-specific T cells responses. Recent basic and clinical research efforts have broadened our understanding of LV vectors as DC-based vaccines. In this review, we provide an overview of the pre-clinical and clinical LV vector vaccine studies for treating HIV to date. We also discuss advances in LV vector designs that have enhanced DC transduction efficiency, target cell specificity, and immunogenicity, and address potential safety concerns regarding LV vector-based vaccines.

LV305, a dendritic cell-targeting integration-deficient ZVex(TM)-based lentiviral vector encoding NY-ESO-1, induces potent anti-tumor immune response

We have engineered an integration-deficient lentiviral vector, LV305, to deliver the tumor antigen NY-ESO-1 to human dendritic cells in vivo through pseudotyping with a modified Sindbis virus envelop protein. Mice immunized once with LV305 developed strong, dose-dependent, multifunctional, and cytotoxic NY-ESO-1-specific cluster of differentiation 8 (CD8) T cells within 14 days post-immunization and could be boosted with LV305 at least twice to recall peak-level CD8 T-cell responses. Immunization with LV305 protected mice against tumor growth in an NY-ESO-1-expressing CT26 lung metastasis model, with the protective effect abrogated upon depletion of CD8 T cells. Adoptive transfer of CD8 T cells, alone or together with CD4 T cells or natural killer cells, from LV305-immunized donor mice to tumor-bearing recipient mice conferred significant protection against metastatic tumor growth. Biodistribution of injected LV305 in mice was limited to the site of injection and the draining lymph node, and injected LV305 exhibited minimal excretion. Mice injected with LV305 developed little to no adverse effects, as evaluated by toxicology studies adherent to good laboratory practices. Taken together, these data support the development of LV305 as a clinical candidate for treatment against tumors expressing NY-ESO-1.

Engineering α-fetoprotein-based gene vaccines to prevent and treat hepatocellular carcinoma: review and future prospects

Activation of a patient's immune system offers an attractive approach to prevent and treat hepatocellular carcinoma (HCC). However, the antitumor efficacy of current HCC vaccines was weak owing to insufficient immune activation of targeting self/tumor antigens. We recently found that epitope-optimized α-fetoprotein effectively activated CD8 T cells and generated potent antitumor effects in the carcinogen-induced autochthonous HCC mouse model. We predict that the same antigen engineering approach of epitope-optimization will enable us to develop effective human vaccines to prevent HCC recurrence after liver resection. The engineered human HCC vaccines may also allow us to identify high-affinity T-cell receptors and antibodies that can be used to reprogram T cells to treat HCC tumors via adoptive transfer.

Anti-melanoma vaccines engineered to simultaneously modulate cytokine priming and silence PD-L1 characterized using ex vivo myeloid-derived suppressor cells as a readout of therapeutic efficacy

Efficacious antitumor vaccines strongly stimulate cancer-specific effector T cells and counteract the activity of tumor-infiltrating immunosuppressive cells. We hypothesised that combining cytokine expression with silencing programmed cell death ligand 1 (PD-L1) could potentiate anticancer immune responses of lentivector vaccines. Thus, we engineered a collection of lentivectors that simultaneously co-expressed an antigen, a PD-L1-silencing shRNA, and various T cell-polarising cytokines, including interferon γ (IFNγ), transforming growth factor β (TGFβ) or interleukins (IL12, IL15, IL23, IL17A, IL6, IL10, IL4). In a syngeneic B16F0 melanoma model and using tyrosinase related protein 1 (TRP1) as a vaccine antigen, we found that simultaneous delivery of IL12 and a PD-L1-silencing shRNA was the only combination that exhibited therapeutically relevant anti-melanoma activities. Mechanistically, we found that delivery of the PD-L1 silencing construct boosted T cell numbers, inhibited in vivo tumor growth and strongly cooperated with IL12 cytokine priming and antitumor activities. Finally, we tested the capacities of our vaccines to counteract tumor-infiltrating myeloid-derived suppressor cell (MDSC) activities ex vivo. Interestingly, the lentivector co-expressing IL12 and the PD-L1 silencing shRNA was the only one that counteracted MDSC suppressive activities, potentially underlying the observed anti-melanoma therapeutic benefit. We conclude that (1) evaluation of vaccines in healthy mice has no significant predictive value for the selection of anticancer treatments; (2) B16 cells expressing xenoantigens as a tumor model are of limited value; and (3) vaccines which inhibit the suppressive effect of MDSC on T cells in our ex vivo assay show promising and relevant antitumor activities.

Mucosal immunization with integrase-defective lentiviral vectors protects against influenza virus challenge in mice

Recent reports highlight the potential for integrase-defective lentiviral vectors (IDLV) to be developed as vaccines due to their ability to elicit cell-mediated and humoral immune responses after intramuscular administration. Differently from their integrase-competent counterpart, whose utility for vaccine development is limited by the potential for insertional mutagenesis, IDLV possess a mutation in their integrase gene that prevents genomic integration. Instead, they are maintained as episomal DNA circles that retain the ability to stably express functional proteins. Despite their favorable profile, it is unknown whether IDLV elicit immune responses after intranasal administration, a route that could be advantageous in the case of infection with a respiratory agent. Using influenza as a model, we constructed IDLV expressing the influenza virus nucleoprotein (IDLV-NP), and tested their ability to generate NP-specific immune responses and protect from challenge in vivo. We found that administration of IDLV-NP elicited NP-specific T cell and antibody responses in BALB/c mice. Importantly, IDLV-NP was protective against homologous and heterosubtypic influenza virus challenge only when given by the intranasal route. This is the first report demonstrating that IDLV can induce protective immunity after intranasal administration, and suggests that IDLV may represent a promising vaccine platform against infectious agents.

Epitope-optimized alpha-fetoprotein genetic vaccines prevent carcinogen-induced murine autochthonous hepatocellular carcinoma

Immunization with effective cancer vaccines can offer a much needed adjuvant therapy to fill the treatment gap after liver resection to prevent relapse of hepatocellular carcinoma (HCC). However, current HCC cancer vaccines are mostly based on native shared-self/tumor antigens that are only able to induce weak immune responses. In this study we investigated whether the HCC-associated self/tumor antigen of alpha-fetoprotein (AFP) could be engineered to create an effective vaccine to break immune tolerance and potently activate CD8 T cells to prevent clinically relevant carcinogen-induced autochthonous HCC in mice. We found that the approach of computer-guided methodical epitope-optimization created a highly immunogenic AFP and that immunization with lentivector expressing the epitope-optimized AFP, but not wild-type AFP, potently activated CD8 T cells. Critically, the activated CD8 T cells not only cross-recognized short synthetic wild-type AFP peptides, but also recognized and killed tumor cells expressing wild-type AFP protein. Immunization with lentivector expressing optimized AFP, but not native AFP, completely protected mice from tumor challenge and reduced the incidence of carcinogen-induced autochthonous HCC. In addition, prime-boost immunization with the optimized AFP significantly increased the frequency of AFP-specific memory CD8 T cells in the liver that were highly effective against emerging HCC tumor cells, further enhancing the tumor prevention of carcinogen-induced autochthonous HCC.

CONCLUSIONS

Epitope-optimization is required to break immune tolerance and potently activate AFP-specific CD8 T cells, generating effective antitumor effect to prevent clinically relevant carcinogen-induced autochthonous HCC in mice. Our study provides a practical roadmap to develop effective human HCC vaccines that may result in an improved outcome compared to the current HCC vaccines based on wild-type AFP.

Human fetal liver cells for regulated ex vivo erythropoietin gene therapy

Possible risks and lack of donor livers limit application of liver transplantation. Liver cell transplantation is, at this moment, not a feasible alternative because engraftment in the liver is poor. Furthermore, there is also shortage of cells suitable for transplantation. Fetal liver cells are able to proliferate in cell culture and could therefore present an alternative source of cells for transplantation. In this study, we investigated the utility of human fetal liver cells for therapeutic protein delivery. We transplanted human fetal liver cells in immunodeficient mice but were not able to detect engraftment of human hepatocytes. In contrast, transplantation of human adult hepatocytes led to detectable engraftment of hepatocytes in murine liver. Transplantation of fetal liver cells did lead to abundant reconstitution of murine liver with human endothelium, indicating that endothelial cells are the most promising cell type for ex vivo liver cell gene therapy. Human liver endothelial cells were subsequently transduced with a lentiviral autoregulatory erythropoietin expression vector. After transplantation in immunodeficient mice, these cells mediated long-term regulation of murine hematocrits. Our study shows the potential of human liver endothelial cells for long-term regulated gene therapy.

Lentiviral vectors for cancer immunotherapy and clinical applications

The success of immunotherapy against infectious diseases has shown us the powerful potential that such a treatment offers, and substantial work has been done to apply this strategy in the fight against cancer. Cancer is however a fiercer opponent than pathogen-caused diseases due to natural tolerance towards tumour associated antigens and tumour-induced immunosuppression. Recent gene therapy clinical trials with viral vectors have shown clinical efficacy in the correction of genetic diseases, HIV and cancer. The first successful gene therapy clinical trials were carried out with onco(γ-)retroviral vectors but oncogenesis by insertional mutagenesis appeared as a serious complication. Lentiviral vectors have emerged as a potentially safer strategy, and recently the first clinical trial of patients with advanced leukemia using lentiviral vectors has proven successful. Additionally, therapeutic lentivectors have shown clinical efficacy for the treatment of HIV, X-linked adrenoleukodystrophy, and β-thalassaemia. This review aims at describing lentivectors and how they can be utilized to boost anti-tumour immune responses by manipulating the effector immune cells.

Diminished Memory T-Cell Expansion Due to Delayed Kinetics of Antigen Expression by Lentivectors

Memory CD8⁺ T lymphocytes play a central role in protective immunity. In attempt to increase the frequencies of memory CD8⁺ T cells, repeated immunizations with viral vectors are regularly explored. Lentivectors have emerged as a powerful vaccine modality with relatively low pre-existing and anti-vector immunity, thus, thought to be ideal for boosting memory T cells. Nevertheless, we found that lentivectors elicited diminished secondary T-cell responses that did not exceed those obtained by priming. This was not due to the presence of anti-vector immunity, as limited secondary responses were also observed following heterologous prime-boost immunizations. By dissecting the mechanisms involved in this process, we demonstrate that lentivectors trigger exceptionally slow kinetics of antigen expression, while optimal activation of lentivector-induced T cells relays on durable expression of the antigen. These qualities hamper secondary responses, since lentivector-encoded antigen is rapidly cleared by primary cytotoxic T cells that limit its presentation by dendritic cells. Indeed, blocking antigen clearance by cytotoxic T cells via FTY720 treatment, fully restored antigen presentation. Taken together, while low antigen expression is expected during secondary immunization with any vaccine vector, our results reveal that the intrinsic delayed expression kinetics of lentiviral-encoded antigen, further dampens secondary CD8⁺ T-cell expansion.

Local administration of TLR ligands rescues the function of tumor-infiltrating CD8 T cells and enhances the antitumor effect of lentivector immunization

Cancer vaccines, to date, have shown limited effect to control the growth of established tumors due largely to effector failure of the antitumor immune responses. Tumor lesion is characterized as chronic indolent inflammation in which the effector function of tumor-infiltrating lymphocytes (TILs) is severely impaired. In this study, we investigated whether the effector function of CD8 TILs could be rescued by converting the chronic inflammation milieu to acute inflammation within tumors. We found that injection of TLR3/9 ligands (polyI:C/CpG) into a tumor during the effector phase of lentivector (lv) immunization effectively rescued the function of lv-activated CD8 TILs and decreased the percentage of T regulatory within the tumor, resulting in a marked improvement in the antitumor efficacy of lv immunization. Mechanistically, rescue of the effector function of CD8 TILs by TLR3/9 ligands is most likely dependent on production, within a tumor, of type-1 IFN that can mature and activate tumor-infiltrating dendritic cells. The effector function of CD8 TILs could not be rescued in mice lacking intact type I IFN signaling. These findings have important implications for tumor immunotherapy, suggesting that type I IFN-mediated activation of tumor-infiltrating dendritic cells within a tumor will most likely restore/enhance the effector function of CD8 TILs and thus improve the antitumor efficacy of current cancer vaccines.

Dendritic cell-directed vaccination with a lentivector encoding PSCA for prostate cancer in mice

Many studies have demonstrated that prostate stem cell antigen (PSCA) is an attractive target for immunotherapy based on its overexpression in prostate tumor tissue, especially in some metastatic tissues. In this study, we evaluated dendritic cell (DC)-directed lentiviral vector (DCLV) encoding murine PSCA (DCLV-PSCA) as a novel tumor vaccine for prostate cancer in mouse models. We showed that DCLV-PSCA could preferentially deliver the PSCA antigen gene to DC-SIGN-expressing 293T cells and bone marrow-derived DCs (BMDCs). Direct immunization with the DCLV-PSCA in male C57BL/6 mice elicited robust PSCA-responsive CD8⁺ and CD4⁺ T cells in vivo. In a transgenic adenocarcinoma mouse prostate cell line (TRAMP-C1) synergetic tumor model, we further demonstrated that DCLV-PSCA-vaccinated mice could be protected from lethal tumor challenge in a prophylactic model, whereas slower tumor growth was observed in a therapeutic model. This DCLV-PSCA vaccine also showed efficacy in inhibiting tumor metastases using a PSCA-expressing B16-F10 model. Taken together, these data suggest that DCLV is a potent vaccine carrier for PSCA in delivering anti-prostate cancer immunity.

A nonintegrative lentiviral vector-based vaccine provides long-term sterile protection against malaria

Trials testing the RTS,S candidate malaria vaccine and radiation-attenuated sporozoites (RAS) have shown that protective immunity against malaria can be induced and that an effective vaccine is not out of reach. However, longer-term protection and higher protection rates are required to eradicate malaria from the endemic regions. It implies that there is still a need to explore new vaccine strategies. Lentiviral vectors are very potent at inducing strong immunological memory. However their integrative status challenges their safety profile. Eliminating the integration step obviates the risk of insertional oncogenesis. Providing they confer sterile immunity, nonintegrative lentiviral vectors (NILV) hold promise as mass pediatric vaccine by meeting high safety standards. In this study, we have assessed the protective efficacy of NILV against malaria in a robust pre-clinical model. Mice were immunized with NILV encoding Plasmodium yoelii Circumsporozoite Protein (Py CSP) and challenged with sporozoites one month later. In two independent protective efficacy studies, 50% (37.5-62.5) of the animals were fully protected (p = 0.0072 and p = 0.0008 respectively when compared to naive mice). The remaining mice with detectable parasitized red blood cells exhibited a prolonged patency and reduced parasitemia. Moreover, protection was long-lasting with 42.8% sterile protection six months after the last immunization (p = 0.0042). Post-challenge CD8+ T cells to CSP, in contrast to anti-CSP antibodies, were associated with protection (r = -0.6615 and p = 0.0004 between the frequency of IFN-g secreting specific T cells in spleen and parasitemia). However, while NILV and RAS immunizations elicited comparable immunity to CSP, only RAS conferred 100% of sterile protection. Given that a better protection can be anticipated from a multi-antigen vaccine and an optimized vector design, NILV appear as a promising malaria vaccine.

IL10 released by a new inflammation-regulated lentiviral system efficiently attenuates zymosan-induced arthritis

Administration of anti-inflammatory cytokines is a common therapeutic strategy in chronic inflammatory diseases. Gene therapy is an efficient method for delivering therapeutic molecules to target cells. Expression of the cell adhesion molecule E-selectin (ESEL), which is expressed in the early stages of inflammation, is controlled by proinflammatory cytokines, making its promoter a good candidate for the design of inflammation-regulated gene therapy vectors. This study describes an ESEL promoter (ESELp)-based lentiviral vector (LV) that drives localized transgene expression during inflammation. Mouse matrigel plug assays with ESELp-transduced endothelial cells showed that systemic lipopolysaccharide (LPS) administration selectively induces ESELp-controlled luciferase expression in vivo. Inflammation-specific induction was confirmed in a mouse model of arthritis, showing that this LV is repeatedly induced early in acute inflammation episodes and is downregulated during remission. Moreover, the local acute inflammatory response in this animal model was efficiently blocked by expression of the anti-inflammatory cytokine interleukin-10 (IL10) driven by our LV system. This inflammation-regulated expression system has potential application in the design of new strategies for the local treatment of chronic inflammatory diseases such as cardiovascular and autoimmune diseases.

Immunoglobulin Fc fragment tagging allows strong activation of endogenous CD4 T cells to reshape the tumor milieu and enhance the antitumor effect of lentivector immunization

A major problem with current cancer vaccines is that the induction of CD8 immune responses is rarely associated with antitumor benefits, mainly owing to multiple immune suppressions in established tumor lesions. In this study, we investigated if and how activation of endogenous CD4 T cells could be achieved to influence the suppressive tumor milieu and antitumor effect. We engineered a lentivector (lv) to express a nominal fusion Ag composed of hepatitis B surface protein and IgG2a Fc fragment (HBS-Fc-lv) to increase the magnitude of CD8 response but, more importantly, to induce effective coactivation of CD4 T cells. We found that, remarkably, immunization with HBS-Fc-lv caused significant regression of established tumors. Immunologic analysis revealed that, compared with HBS-lv without Fc fragment, immunization with HBS-Fc-lv markedly increased the number of functional CD8 and CD4 T cells and the level of Th1/Tc1-like cytokines in the tumor while substantially decreasing the regulatory T cell ratio. The favorable immunologic changes in tumor lesions and the improvement of antitumor effects from HBS-Fc-lv immunization were dependent on the CD4 activation, which was Fc receptor mediated. Adoptive transfer of CD4 T cells from the HBS-Fc-lv-immunized mice could activate endogenous CD8 T cells in an IFN-γ-dependent manner. We conclude that endogenous CD4 T cells can be activated by lv expressing Fc-tagged Ag to provide another layer of help--that is, creating a Th1/Tc1-like proinflammatory milieu within the tumor lesion to boost the effector phase of immune responses in enhancing the antitumor effect.

Measles virus glycoprotein-pseudotyped lentiviral vectors are highly superior to vesicular stomatitis virus G pseudotypes for genetic modification of monocyte-derived dendritic cells

Dendritic cells (DCs) are potent antigen-presenting cells capable of promoting or regulating innate and adaptive immune responses against non-self antigens. To better understand the DC biology or to use them for immune intervention, a tremendous effort has been made to improve gene transfer in these cells. Lentiviral vectors (LVs) have conferred a huge advantage in that they can transduce nondividing cells such as human monocyte-derived DCs (MDDCs) but required high amounts of viral particles and/or accessory proteins such as Vpx or Vpr to achieve sufficient transduction rates. As a consequence, these LVs have been shown to cause dramatic functional modifications, such as the activation or maturation of transduced MDDCs. Taking advantage of new pseudotyped LVs, i.e., with envelope glycoproteins from the measles virus (MV), we demonstrate that MDDCs are transduced very efficiently with these new LVs compared to the classically used vesicular stomatitis virus G-pseudotyped LVs and thus allowed to achieve high transduction rates at relatively low multiplicities of infection. Moreover, in this experimental setting, no activation or maturation markers were upregulated, while MV-LV-transduced cells remained able to mature after an appropriate Toll-like receptor stimulation. We then demonstrate that our MV-pseudotyped LVs use DC-SIGN, CD46, and CD150/SLAM as receptors to transduce MDDCs. Altogether, our results show that MV-pseudotyped LVs provide the most accurate and simple viral method for efficiently transferring genes into MDDCs without affecting their activation and/or maturation status.

A TLR4 agonist synergizes with dendritic cell-directed lentiviral vectors for inducing antigen-specific immune responses

TLR4 agonists can be used as adjuvants to trigger innate immune responses of antigen-presenting cells (APCs) such as dendritic cells (DCs) to enhance vaccine-specific immunity. Adjuvant effects of TLR4 agonists are mediated by downstream signaling controlled by both MyD88 and TRIF adapter proteins. In this study, we investigated the adjuvanting capacity of glucopyranosyl lipid A (GLA), a chemically synthesized TLR4 agonist, to boost antigen-specific immunity elicited by DC-directed lentiviral vectors (DC-LV). We found that stimulation by this agonist in vitro can activate DCs in a TLR4-dependent manner. The agonist can significantly boost DC-LV-induced humoral and cellular immune responses, resulting in better antitumor reactions in response to tumor challenges. We observed that the adjuvant-mediated enhancement of cytotoxic CD8(+) T cell responses is CD4(+) T cell-dependent and determined that in vitro the agonist stimulation involves the participation of both MyD88 and TRIF pathways to activate DCs. In vivo immunization study however revealed that adjuvant effects depend more on the MyD88 signaling as TRIF(-/-) mice but not MyD88(-/-) mice were able to maintain the enhanced CD8(+) T cell responses upon DC-LV immunization. Thus, our study supports the use of this TLR4 agonist as a potent adjuvant candidate for boosting DC-LV immunization.

Development of the Nanobody display technology to target lentiviral vectors to antigen-presenting cells

Lentiviral vectors (LVs) provide unique opportunities for the development of immunotherapeutic strategies, as they transduce a variety of cells in situ, including antigen-presenting cells (APCs). Engineering LVs to specifically transduce APCs is required to promote their translation towards the clinic. We report on the Nanobody (Nb) display technology to target LVs to dendritic cells (DCs) and macrophages. This innovative approach exploits the budding mechanism of LVs to incorporate an APC-specific Nb and a binding-defective, fusion-competent form of VSV.G in the viral envelope. In addition to production of high titer LVs, we demonstrated selective, Nb-dependent transduction of mouse DCs and macrophages both in vitro and in situ. Moreover, this strategy was translated to a human model in which selective transduction of in vitro generated or lymph node (LN)-derived DCs and macrophages, was demonstrated. In conclusion, the Nb display technology is an attractive approach to generate LVs targeted to specific cell types.

Targeting lentiviral vectors for cancer immunotherapy

Delivery of tumour-associated antigens (TAA) in a way that induces effective, specific immunity is a challenge in anti-cancer vaccine design. Circumventing tumour-induced tolerogenic mechanisms in vivo is also critical for effective immunotherapy. Effective immune responses are induced by professional antigen presenting cells, in particular dendritic cells (DC). This requires presentation of the antigen to both CD4(+) and CD8(+) T cells in the context of strong co-stimulatory signals. Lentiviral vectors have been tested as vehicles, for both ex vivo and in vivo delivery of TAA and/or activation signals to DC, and have been demonstrated to induce potent T cell mediated immune responses that can control tumour growth. This review will focus on the use of lentiviral vectors for in vivo gene delivery to DC, introducing strategies to target DC, either targeting cell entry or gene expression to improve safety of the lentiviral vaccine or targeting dendritic cell activation pathways to enhance performance of the lentiviral vaccine. In conclusion, this review highlights the potential of lentiviral vectors as a generally applicable 'off-the-shelf' anti-cancer immunotherapeutic.

Immunization delivered by lentiviral vectors for cancer and infectious diseases

The increasing level of understanding of the lentivirus biology has been instrumental in shaping the design strategy of creating therapeutic lentiviral delivery vectors. As a result, lentiviral vectors have become one of the most powerful gene transfer vehicles. They are widely used for therapeutic purposes as well as in studies of basic biology, due to their unique characteristics. Lentiviral vectors have been successfully employed to mediate durable and efficient antigen expression and presentation in dendritic cells both in vitro and in vivo, leading to the activation of cellular immunity and humoral responses. This capability makes the lentiviral vector an ideal choice for immunizations that target a wide range of cancers and infectious diseases. Further advances into optimizing the vector system and understanding the relationship between the immune system and diseases pathogenesis will only augment the potential benefits and utility of lentiviral vaccines for human health.

Lentivector prime and vaccinia virus vector boost generate high-quality CD8 memory T cells and prevent autochthonous mouse melanoma

Most cancer vaccines, to date, fail to control established tumors. However, their application in preventing tumors is another question that is understudied. In the current study, we investigated the CD8 memory T cell responses of lentivector (lv) immunization and its potential to prevent melanoma using both transplantable B16 tumor and autochthonous melanoma models. We found that lv-expressing xenogenic human gp100 could induce potent CD8 responses that cross-react with mouse gp100. Importantly, the lv-primed CD8 response consisted of a high number of memory precursors and could be further increased by recombinant vaccinia virus vector (vv) boost, resulting in enhanced CD8 memory response. These long-lasting CD8 memory T cells played a critical role in immune surveillance and could rapidly respond and expand after sensing B16 tumor cells to prevent tumor establishment. Although CD8 response plays a dominant role after lv immunization, both CD4 and CD8 T cells are responsible for the immune prevention. In addition, we surprisingly found that CD4 help was not only critical for generating primary CD8 responses, but also important for secondary CD8 responses of vv boost. CD4 depletion prior to lv prime or prior to vv boost substantially reduced the magnitude of secondary CD8 effector and memory responses, and severely compromised the effect of cancer immune prevention. More importantly, the CD8 memory response from lv-vv prime-boost immunization could effectively prevent autochthonous melanoma in tumor-prone transgenic mice, providing a strong evidence that lv-vv prime-boost strategy is an effective approach for cancer immune prevention.

Virus-receptor mediated transduction of dendritic cells by lentiviruses enveloped with glycoproteins derived from Semliki Forest virus

Lentiviruses have recently attracted considerable interest for their potential as a genetic modification tool for dendritic cells (DCs). In this study, we explore the ability of lentiviruses enveloped with alphaviral envelope glycoproteins derived from Semliki Forest virus (SFV) to mediate transduction of DCs. We found that SFV glycoprotein (SFV-G)-pseudotyped lentiviruses use C-type lectins (DC-SIGN and L-SIGN) as attachment factors for transduction of DCs. Importantly, SFV-G pseudotypes appear to have enhanced transduction towards C-type lectin-expressing cells when produced under conditions limiting glycosylation to simple high-mannose, N-linked glycans. These results, in addition to the natural DC tropism of SFV-G, offer evidence to support the use of SFV-G-bearing lentiviruses to genetically modify DCs for the study of DC biology and DC-based immunotherapy.

Lentivector expressing HBsAg and immunoglobulin Fc fusion antigen induces potent immune responses and results in seroconversion in HBsAg transgenic mice

Even though hepatitis B virus(HBV) vaccines effectively prevent new cases of HBV infection, with approximately 350 million patients worldwide, chronic HBV infection remains a major health problem because of the associated complications (such as liver cirrhosis and hepatocellular carcinoma) and the limited treatment options. Immunotherapy has the potential to effectively control HBV replication. In this current study, we found that recombinant lentivectors could induce potent HBV surface antigen (HBsAg) specific T cell responses and humoral immune responses. Tagging the HBsAg with immunoglobulin Fc fragment further substantially increased the HBsAg specific immune responses. Remarkably, the HBS-Fc-lv lentivector could effectively break immune tolerance and induce potent HBsAg specific adaptive immune responses in HBsAg transgenic (Tg) mice with low serum level of HBsAg. More importantly, the induction of HBsAg specific immune responses in Tg mice accompanied seroconversion from HBsAg to anti-HBsAg antibody (anti-HBsAb). Our study demonstrated the potential of utilizing lentivector to treat chronic HBV infection following reduction of viral load with antiviral drug therapy.

Conventional dendritic cells are required for the activation of helper-dependent CD8 T cell responses to a model antigen after cutaneous vaccination with lentiviral vectors

Cutaneous vaccination with lentiviral vectors generates systemic CD8 T cell responses that have the potential to eradicate tumors for cancer immunotherapy. However, although s.c. immunization with <1 million lentiviral particles clearly primes cytotoxic T cells, vaccination with much higher doses has routinely been used to define the mechanisms of T cell activation by lentiviral vectors. In particular, experiments to test presentation of lentiviral Ags by dendritic cells (DC) require injection of high viral titers, which may result in aberrant transduction of different DC populations. We exploited inducible murine models of DC depletion to investigate which DC prime the lentiviral response after s.c. immunization with low doses of lentiviral particles. In this article, we demonstrate that conventional DC are required to present Ag to CD8 T cells in draining lymph nodes. Langerhans cells are not required to activate the effector response, and neither Langerhans cells nor plasmacytoid DC are sufficient to prime Ag-specific T cells. Immunization drives the generation of endogenous long-lived memory T cells that can be reactivated to kill Ag-specific targets in the absence of inflammatory challenge. Furthermore, lentiviral vaccination activates expansion of endogenous CD4 Th cells, which are required for the generation of effector CD8 T cells that produce IFN-γ and kill Ag-specific targets. Collectively, we demonstrate that after cutaneous immunization with lentiviral particles, CD4-licensed lymph node conventional DC present Ag to CD8 T cells, resulting in the generation of protective endogenous antitumor immunity that may be effective for cancer immunotherapy.

Production of lentiviral vectors with enhanced efficiency to target dendritic cells by attenuating mannosidase activity of mammalian cells

Background

Dendritic cells (DCs) are antigen-presenting immune cells that interact with T cells and have been widely studied for vaccine applications. To achieve this, DCs can be manipulated by lentiviral vectors (LVs) to express antigens to stimulate the desired antigen-specific T cell response, which gives this approach great potential to fight diseases such as cancers, HIV, and autoimmune diseases. Previously we showed that LVs enveloped with an engineered Sindbis virus glycoprotein (SVGmu) could target DCs through a specific interaction with DC-SIGN, a surface molecule predominantly expressed by DCs. We hypothesized that SVGmu interacts with DC-SIGN in a mannose-dependent manner, and that an increase in high-mannose structures on the glycoprotein surface could result in higher targeting efficiencies of LVs towards DCs. It is known that 1-deoxymannojirimycin (DMJ) can inhibit mannosidase, which is an enzyme that removes high-mannose structures during the glycosylation process. Thus, we investigated the possibility of generating LVs with enhanced capability to modify DCs by supplying DMJ during vector production.

Results

Through western blot analysis and binding tests, we were able to infer that binding of SVGmu to DC-SIGN is directly related to amount of high-mannose structures on SVGmu. We also found that the titer for the LV (FUGW/SVGmu) produced with DMJ against 293T.DCSIGN, a human cell line expressing the human DC-SIGN atnibody, was over four times higher than that of vector produced without DMJ. In addition, transduction of a human DC cell line, MUTZ-3, yielded a higher transduction efficiency for the LV produced with DMJ.

Conclusion

We conclude that LVs produced under conditions with inhibited mannosidase activity can effectively modify cells displaying the DC-specific marker DC-SIGN. This study offers evidence to support the utilization of DMJ in producing LVs that are enhanced carriers for the development of DC-directed vaccines.

A strategy of tumor treatment in mice with doxorubicin-cyclophosphamide combination based on dendritic cell activation by human double-stranded DNA preparation

BACKGROUND

Immunization of mice with tumor homogenate after combined treatment with cyclophosphamide (CP) and double-stranded DNA (dsDNA) preparation is effective at inhibition of growth of tumor challenged after the treatment. It was assumed that this inhibition might be due to activation of the antigen-presenting cells. The purpose was to develop improved antitumor strategy using mice. We studied the combined action of cytostatics doxorubicin (Dox) plus CP with subsequent dsDNA preparation on tumor growth.

METHODS

Three-month old CBA/Lac mice were used in the experiments. Mice were injected with CP and human dsDNA preparation. The percentage of mature dendritic cells (DCs) was estimated by staining of mononuclear cells isolated from spleen and bone marrow 3, 6, and 9 days later with monoclonal antibodies CD34, CD80, and CD86. In the next set of experiments, mice were given intramuscularly injections of 1-3 × 105 tumor cells. Four days later, they were injected intravenously with 6-6.7 mg/kg Dox and intraperitoneally with 100-200 mg/kg CP; 200 mkg human DNA was injected intraperitoneally after CP administration. Differences in tumor size between groups were analyzed for statistical significance by Student's t-test. The MTT-test was done to determine the cytotoxic index of mouse leucocytes from treated groups.

RESULTS

The conducted experiments showed that combined treatment with CP and dsDNA preparation produce an increase in the total amount of mature DCs in vivo. Treatment of tumor bearers with preparation of fragmented dsDNA on the background of pretreatment with Dox plus CP demonstrated a strong suppression of tumor growth in two models. RLS, a weakly immunogenic, resistant to alkalyting cytostatics tumor, grew 3.4-fold slower when compared with the control (p < 0.001). In experiment with Krebs-2 tumor, only 2 of the 10 mice in the Dox+CP+DNA group had a palpable tumor on day 16. The cytotoxic index of leucocytes was 86.5% in the Dox+CP+DNA group, but it was 0% in the Dox+CP group.

CONCLUSIONS

Thus, the set of experiments we performed showed that exogenous dsDNA, when administered on the background of pretreatment with Dox plus CP, has an antitumor effect possibly due to DC activation.

Blockade of programmed death-1 pathway rescues the effector function of tumor-infiltrating T cells and enhances the antitumor efficacy of lentivector immunization

Despite intensive effort, the antitumor efficacy of tumor vaccines remains limited in treating established tumors regardless of the potent systemic tumor-specific immune response and the increases of tumor infiltration of T effector cells. In the current study, we demonstrated that although lentivector (lv) immunization markedly increased Ag-dependent tumor infiltration of CD8 and CD4 T cells and generated Ag-specific antitumor effect, it simultaneously increased the absolute number of myeloid-derived suppressor cells and regulatory T cells in the tumor lesions. In addition, lv immunization induced expression of programmed death-ligand 1 in the tumor lesions. Furthermore, the tumor-infiltrating CD8 T cells expressed high levels of programmed death-1 and were partially dysfunctional, producing lower amounts of effector cytokines and possessing a reduced cytotoxicity. Together, these immune-suppression mechanisms in the tumor microenvironment pose a major obstacle to effective tumor immunotherapy and may explain the limited antitumor efficacy of lv immunization. The loss of effector function in the tumor microenvironment is reversible, and the effector function of CD8 T cells in the tumor could be partially rescued by blocking programmed death-1 and programmed death-ligand 1 pathway in vitro and in vivo, resulting in enhanced antitumor efficacy of lv immunization. These data suggest that immunization alone may exacerbate immune suppression in the tumor lesions and that methods to improve the tumor microenvironment and to rescue the effector functions of tumor-infiltrating T cells should be incorporated into immunization strategies to achieve enhanced antitumor efficacy.

Dendritic cells for active anti-cancer immunotherapy: targeting activation pathways through genetic modification

Tumour immunotherapy has become a treatment modality for cancer, harnessing the immune system to recognize and eradicate tumour cells specifically. It is based on the expression of tumour associated antigens (TAA) by the tumour cells and aims at the induction of TAA-specific effector T cell responses, whilst overruling various mechanisms that can hamper the anti-tumour immune response, e.g. regulatory T cells (Treg). (Re-) activation of effector T cells requires the completion of a carefully orchestrated series of specific steps. Particularly important is the provision of TAA presentation and strong stimulatory signals, delivered by co-stimulatory surface molecules and cytokines. These can only be delivered by professional antigen-presenting cells, in particular dendritic cells (DC). Therefore, DC need to be loaded with TAA and appropriately activated. It is not surprising that an extensive part of DC research has focused on the delivery of both TAA and activation signals to DC, developing a one step approach to obtain potent stimulatory DC. The simultaneous delivery of TAA and activation signals is therefore the topic of this review, emphasizing the role of DC in mediating T cell activation and how we can manipulate DC for the pill-pose of enhancing tumour immunotherapy. As we gain a better understanding of the molecular and cellular mechanisms that mediate induction of TAA-specific T cells, rational approaches for the activation of T cell responses can be developed for the treatment of cancer.

Induction of Specific CD8 T Cells against Intracellular Bacteria by CD8 T-Cell-Oriented Immunization Approaches

For protection against intracellular bacteria such as Mycobacterium tuberculosis and Listeria monocytogenes, the cellular arm of adaptive immunity is necessary. A variety of immunization methods have been evaluated and are reported to induce specific CD8⁺ T cells against intracellular bacterial infection. Modified BCG vaccines have been examined to enhance CD8⁺ T-cell responses. Naked DNA vaccination is a promising strategy to induce CD8⁺ T cells. In addition to this strategy, live attenuated intracellular bacteria such as Shigella, Salmonella, and Listeria have been utilized as carriers of DNA vaccines in animal models. Vaccination with dendritic cells pulsed with antigenic peptides or the cells introduced antigen genes by virus vectors such as retroviruses is also a powerful strategy. Furthermore, vaccination with recombinant lentivirus has been attempted to induce specific CD8⁺ T cells. Combinations of these strategies (prime-boost immunization) have been studied for the efficient induction of intracellular bacteria-specific CD8⁺ T cells.

Separating lentiviral vector injection and induction of gene expression in time, does not prevent an immune response to rtTA in rats

Background

Lentiviral gene transfer can provide long-term expression of therapeutic genes such as erythropoietin. Because overexpression of erythropoietin can be toxic, regulated expression is needed. Doxycycline inducible vectors can regulate expression of therapeutic transgenes efficiently. However, because they express an immunogenic transactivator (rtTA), their utility for gene therapy is limited. In addition to immunogenic proteins that are expressed from inducible vectors, injection of the vector itself is likely to elicit an immune response because viral capsid proteins will induce “danger signals” that trigger an innate response and recruit inflammatory cells.

Methodology and Principal Findings

We have developed an autoregulatory lentiviral vector in which basal expression of rtTA is very low. This enabled us to temporally separate the injection of virus and the expression of the therapeutic gene and rtTA. Wistar rats were injected with an autoregulatory rat erythropoietin expression vector. Two or six weeks after injection, erythropoietin expression was induced by doxycycline. This resulted in an increase of the hematocrit, irrespective of the timing of the induction. However, most rats only responded once to doxycycline administration. Antibodies against rtTA were detected in the early and late induction groups.

Conclusions

Our results suggest that, even when viral vector capsid proteins have disappeared, expression of foreign proteins in muscle will lead to an immune response.

HIV-1 lentiviral vector immunogenicity is mediated by Toll-like receptor 3 (TLR3) and TLR7

Lentiviral vectors are promising vaccine vector candidates that have been tested extensively in preclinical models of infectious disease and cancer immunotherapy. They are also used in gene therapy clinical trials both for the ex vivo modification of cells and for direct in vivo injection. It is therefore critical to understand the mechanism(s) by which such vectors might stimulate the immune system. We evaluated the effect of lentiviral vectors on myeloid dendritic cells (DC), the main target of lentiviral transduction following subcutaneous immunization. The activation of DC cultures was independent of the lentiviral pseudotype but dependent on cell entry and reverse transcription. In vivo-transduced DC also displayed a mature phenotype, produced tumor necrosis factor alpha (TNF-alpha), and stimulated naive CD8(+) T cells. The lentiviral activation of DC was Toll-like receptor (TLR) dependent, as it was inhibited in TRIF/MyD88 knockout (TRIF/MyD88(-/-)) DC. TLR3(-/-) or TLR7(-/-) DC were less activated, and reverse transcription was important for the activation of TLR7(-/-) DC. Moreover, lentivirally transduced DC lacking TLR3 or TLR7 had an impaired capacity to induce antigen-specific CD8(+) T-cell responses. In conclusion, we demonstrated TLR-dependent DC activation by lentiviral vectors, explaining their immunogenicity. These data allow the rational development of strategies to manipulate the host's immune response to the transgene.

Nonintegrating lentiviral vectors can effectively deliver ovalbumin antigen for induction of antitumor immunity

It has been demonstrated that nonintegrating lentiviral vectors (NILVs) are efficient in maintaining transgene expression in vitro and in vivo. Gene delivery by NILVs can significantly reduce nonspecific vector integration, which has been shown to cause malignant transformation in patients receiving gene therapy for X-linked severe combined immunodeficiency. Strong and sustained immune responses were observed after a single immunization with NILVs carrying viral antigens. However, there is no report to date that evaluates the efficacy of NILVs in inducing antigen-specific antitumor immunity. Using a well-characterized tumor model, we tested in vivo immunization with a self-inactivating lentiviral vector harboring a defective integrase. A high frequency of ovalbumin peptide (OVAp1)-specific CD8(+) T cells and a substantial antibody response were detected in naive mice immunized with an NILV encoding an OVA transgene. Furthermore, this immunization method completely protected the mice against the growth of E.G7 tumor cells expressing the OVA antigen. Thus, this study provides evidence that immunization using NILVs can be a safe and promising approach for exploring cancer immunotherapy.

HIV-1 Gag-specific immunity induced by a lentivector-based vaccine directed to dendritic cells

Lentivectors (LVs) have attracted considerable interest for their potential as a vaccine delivery vehicle. In this study, we evaluate in mice a dendritic cell (DC)-directed LV system encoding the Gag protein of human immunodeficiency virus (HIV) (LV-Gag) as a potential vaccine for inducing an anti-HIV immune response. The DC-directed specificity is achieved through pseudotyping the vector with an engineered Sindbis virus glycoprotein capable of selectively binding to the DC-SIGN protein. A single immunization by this vector induces a durable HIV Gag-specific immune response. We investigated the antigen-specific immunity and T-cell memory generated by a prime/boost vaccine regimen delivered by either successive LV-Gag injections or a DNA prime/LV-Gag boost protocol. We found that both prime/boost regimens significantly enhance cellular and humoral immune responses. Importantly, a heterologous DNA prime/LV-Gag boost regimen results in superior Gag-specific T-cell responses as compared with a DNA prime/adenovector boost immunization. It induces not only a higher magnitude response, as measured by Gag-specific tetramer analysis and intracellular IFN-gamma staining, but also a better quality of response evidenced by a wider mix of cytokines produced by the Gag-specific CD8(+) and CD4(+) T cells. A boosting immunization with LV-Gag also generates T cells reactive to a broader range of Gag-derived epitopes. These results demonstrate that this DC-directed LV immunization is a potent modality for eliciting anti-HIV immune responses.

Lentivector immunization stimulates potent CD8 T cell responses against melanoma self-antigen tyrosinase-related protein 1 and generates antitumor immunity in mice

Recombinant lentivector immunization has been demonstrated to induce potent CD8 T cell responses in vivo. In this study, we investigated whether lentivector delivering a self/tumor Ag, tyrosinase related protein 1 (TRP1), could stimulate effective antitumor T cell responses. We found that immunization with lentivector expressing mutated TRP1 Ag elicited potent CD8 T cell responses against multiple TRP1 epitopes. Importantly, the activated CD8 T cells effectively recognize wild-type TRP1 epitopes. At peak times, as many as 10% of CD8 T cells were effector cells against TRP1 Ag. These cells killed wild-type TRP1 peptide-pulsed target cells in vivo and produced IFN-gamma after ex vivo stimulation. The CD8 T cell responses were long-lasting (3-4 wk). Immunized mice were protected from B16 tumor cell challenge. In a therapeutic setting, lentivector immunization induced potent CD8 T cell responses in tumor bearing mice. The number of infiltrating T cells and the ratio of CD8/CD4 were dramatically increased in the tumors of immunized mice. The tumor-infiltrating CD8 T cells were functional and produced IFN-gamma. The potent CD8 T cell responses stimulated by lentivector immunization eliminated small 3-day s.c. B16 tumors and strongly inhibited the growth of more established 5-day tumors. These studies demonstrate that genetic immunization with lentivector expressing mutated self/tumor Ag can generate potent CD8 T cell immune responses and antitumor immunity that prevent and inhibit B16 tumor growth, suggesting that lentivector immunization has the potential for tumor immunotherapy and immune prevention.

Potent induction of B- and T-cell immunity against human carcinoembryonic antigen-expressing tumors in human carcinoembryonic antigen transgenic mice mediated by direct lentivector injection

The applicability of immunotherapy would be dramatically broadened to a greater number of recipients if direct "off-the-shelf" products could be engineered to engender functionally potent immune responses against true "self"-tumor antigens. This would obviate the need for ex vivo culture of dendritic cells or T cells on a patient-by-patient basis, for example. The carcinoembryonic antigen (CEA) is a glycoprotein expressed in normal gut epithelium that is up-regulated in the majority of colon cancers, non-small cell lung cancers, and half of all breast cancers. Such properties make CEA an excellent and important target for cancer immunotherapy. In this study, we show stabilization of 14-day established s.c. mGC4CEA tumors in human CEA (huCEA) transgenic mice following two direct low-dose injections of 0.15x10(6) transducing units of a lentiviral vector (LV) that directs expression of huCEA (LV-huCEA). This stabilization result was reproducible and detailed analyses including antibody assays, multiplex cytokine analyses on unstimulated splenocytes, lymph node cell characterizations, tetramer staining, and immunofluorescence staining of tumor sections showed that this outcome correlated with both a cellular and humoral immune response. Similar tumor outcomes were not seen when mice were vaccinated with a control LV that engineered expression of enGFP only. The long-term potency of this vaccination strategy was also studied and revealed the requirement for maintenance of tumor antigen-specific immunity for efficient tumor control. These data support the use of direct injections of low doses of LV-huCEA for enhancement of tumor immunotherapy directed against CEA.

Protective antiviral immunity conferred by a nonintegrative lentiviral vector-based vaccine

Lentiviral vectors are under intense scrutiny as unique candidate viral vector vaccines against tumor and aggressive pathogens because of their ability to initiate potent and durable specific immune responses. Strategies that alleviate safety concerns will facilitate the clinical developments involving lentiviral vectors. In this respect, the development of integration deficient lentiviral vectors circumvents the safety concerns relative to insertional mutagenesis and might pave the way for clinical applications in which gene transfer is targeted to non-dividing cells. We thus evaluated the potential use of nonintegrative lentiviral vectors as vaccination tools since the main targeted cell in vaccination procedures is the non-dividing dendritic cell (DC). In this study, we demonstrated that a single administration of nonintegrative vectors encoding a secreted form of the envelope of a virulent strain of West Nile Virus (WNV) induces a robust B cell response. Remarkably, nonintegrative lentiviral vectors fully protected mice from a challenge with a lethal dose of WNV and a single immunization was sufficient to induce early and long-lasting protective immunity. Thus, nonintegrative lentiviral vectors might represent a safe and efficacious vaccination platform for the development of prophylactic vaccines against infectious agents.

Expression of vFLIP in a lentiviral vaccine vector activates NF-{kappa}B, matures dendritic cells, and increases CD8+ T-cell responses

Lentiviral vectors deliver antigens to dendritic cells (DCs) in vivo, but they do not trigger DC maturation. We therefore expressed a viral protein that constitutively activates NF-kappaB, vFLIP from Kaposi's sarcoma-associated herpesvirus (KSHV), in a lentivector to mature DCs. vFLIP activated NF-kappaB in mouse bone marrow-derived DCs in vitro and matured these DCs to a similar extent as lipopolysaccharide; costimulatory markers CD80, CD86, CD40, and ICAM-1 were upregulated and tumor necrosis factor alpha and interleukin-12 secreted. The vFLIP-expressing lentivector also matured DCs in vivo. When we coexpressed vFLIP in a lentivector with ovalbumin (Ova), we found an increased immune response to Ova; up to 10 times more Ova-specific CD8(+) T cells secreting gamma interferon were detected in the spleens of vFLIP_Ova-immunized mice than in the spleens of mice immunized with GFP_Ova. Furthermore, this increased CD8(+) T-cell response correlated with improved tumor-free survival in a tumor therapy model. A single immunization with vFLIP_Ova also reduced the parasite load when mice were challenged with OVA-Leishmania donovani. In conclusion, vFLIP from KSHV is a DC activator, maturing DCs in vitro and in vivo. This demonstrates that NF-kappaB activation is sufficient to induce many aspects of DC maturation and that expression of a constitutive NF-kappaB activator can improve the efficacy of a vaccine vector.

Recombinant lentivector as a genetic immunization vehicle for antitumor immunity

Encouraged by remarkable successes in preventing infectious diseases and by the well-established potential of the immune system for controlling tumor growth, active therapeutic immunization approaches hold great promise for treating malignant tumors. In recent years, engineered recombinant viral vectors have been carefully examined as genetic-immunization vehicles and have been demonstrated to induce potent T-cell-mediated immune responses that can control tumor growth. Very recent efforts suggest that lentivectors possess important advantages over other candidate recombinant viral vectors for genetic immunization. Here, we review the development of recombinant lentivectors and the characteristics of T-cell immune responses elicited by lentivector immunization, including the mechanism of T-cell priming with a focus on the role of skin dendritic cells and potential applications for tumor immunotherapy.

Intratracheal administration of third-generation lentivirus vector encoding MPT51 from Mycobacterium tuberculosis induces specific CD8+ T-cell responses in the lung

The present study evaluates the potential of improved third-generation lentivirus vector with respect to their use as an in vivo-administered T-cell vaccine against tuberculosis. Intratracheal administration of the lentivirus vector encoding MPT51 of Mycobacterium tuberculosis could induce MPT51-specific CD8+ T cells in the mediastinal lymph nodes 2 weeks after the administration. The vaccination could generate MPT51-specific memory CD8+ T cells in the lung, but not in the lymph nodes. Further, a single intratracheal immunization of MPT51 lentiviral vaccine decreased significantly the number of virulent M. tuberculosis in the lung after intratracheal challenge of the bacillus. These findings suggest that intratracheal immunization of the third-generation lentiviral vaccines is a promising vaccination strategy against pulmonary tuberculosis.

Engineered lentivector targeting of dendritic cells for in vivo immunization

We report a method of inducing antigen production in dendritic cells by in vivo targeting with lentiviral vectors that specifically bind to the dendritic cell-surface protein DC-SIGN. To target dendritic cells, we enveloped the lentivector with a viral glycoprotein from Sindbis virus engineered to be DC-SIGN-specific. In vitro, this lentivector specifically transduced dendritic cells and induced dendritic cell maturation. A high frequency (up to 12%) of ovalbumin (OVA)-specific CD8(+) T cells and a significant antibody response were observed 2 weeks after injection of a targeted lentiviral vector encoding an OVA transgene into naive mice. This approach also protected against the growth of OVA-expressing E.G7 tumors and induced regression of established tumors. Thus, lentiviral vectors targeting dendritic cells provide a simple method of producing effective immunity and may provide an alternative route for immunization with protein antigens.

Treatment of pulmonary metastatic tumors in mice using lentiviral vector-engineered stem cells

Active cancer immunotherapy relies on functional tumor-specific effector T lymphocytes for tumor elimination. Dendritic cells (DCs), as most potent antigen-presenting cells, have been popularly employed in clinical and experimental tumor treatments. We have previously demonstrated that lentiviral vector-mediated transgene delivery to DC progenitors, including bone marrow cells and hematopoietic stem cells, followed by transplantation supports systemic generation of great numbers of tumor antigen-presenting DCs. These DCs subsequently stimulate marked and systemic immune activation. Here, we examined whether this level of immune activation is sufficient to overcome tumor-induced tolerogenic environment for treating an established aggressive epithelial tumor. We showed that a combination treatment of granulocyte macrophage-colony stimulating factor and cytosine-phosphate-guanine-containing oligonucleotide stimulated large numbers of tumor antigen-presenting DCs in situ from transgene-modified stem cells. Moreover, these in situ generated and activated DCs markedly stimulated activation of antigen-specific CD4 and CD8 T cells by augmenting their numbers, as well as function, even in a tumor-bearing tolerogenic environment. This leads to significant improvement in the therapeutic efficacy of established pulmonary metastases. This study suggests that lentiviral vector-modified stem cells as DC progenitors may be used as an effective therapeutic regimen for treating metastatic epithelial tumors.