Transgene-specific Immune Responses Research Articles

Peak transgene expression after intramuscular immunization of mice with adenovirus 26-based vector vaccines correlates with transgene-specific adaptive immune responses.

Non-replicating adenovirus-based vectors have been broadly used for the development of prophylactic vaccines in humans and are licensed for COVID-19 and Ebola virus disease prevention. Adenovirus-based vectored vaccines encode for one or more disease specific transgenes with the aim to induce protective immunity against the target disease. The magnitude and duration of transgene expression of adenovirus 5- based vectors (human type C) in the host are key factors influencing antigen presentation and adaptive immune responses. Here we characterize the magnitude, duration, and organ biodistribution of transgene expression after single intramuscular administration of adenovirus 26-based vector vaccines in mice and evaluate the differences with adenovirus 5-based vector vaccine to understand if this is universally applicable across serotypes. We demonstrate a correlation between peak transgene expression early after adenovirus 26-based vaccination and transgene-specific cellular and humoral immune responses for a model antigen and SARS-CoV-2 spike protein, independent of innate immune activation. Notably, the memory immune response was similar in mice immunized with adenovirus 26-based vaccine and adenovirus 5-based vaccine, despite the latter inducing a higher peak of transgene expression early after immunization and a longer duration of transgene expression. Together these results provide further insights into the mode of action of adenovirus 26-based vector vaccines.

Novel Combinatorial MicroRNA-Binding Sites in AAV Vectors Synergistically Diminish Antigen Presentation and Transgene Immunity for Efficient and Stable Transduction.

Recombinant adeno-associated virus (rAAV) platforms hold promise for in vivo gene therapy but are undermined by the undesirable transduction of antigen presenting cells (APCs), which in turn can trigger host immunity towards rAAV-expressed transgene products. In light of recent adverse events in patients receiving high systemic AAV vector doses that were speculated to be related to host immune responses, development of strategies to mute innate and adaptive immunity is imperative. The use of miRNA binding sites (miR-BSs) to confer endogenous miRNA-mediated regulation to detarget transgene expression from APCs has shown promise for reducing transgene immunity. Studies have shown that designing miR-142BSs into rAAV1 vectors were able to repress costimulatory signals in dendritic cells (DCs), blunt the cytotoxic T cell response, and attenuate clearance of transduced muscle cells in mice to allow sustained transgene expression in myofibers with negligible anti-transgene IgG production. In this study, we screened individual and combinatorial miR-BS designs against 26 miRNAs that are abundantly expressed in APCs, but not in skeletal muscle. The highly immunogenic ovalbumin (OVA) transgene was used as a proxy for foreign antigens. In vitro screening in myoblasts, mouse DCs, and macrophages revealed that the combination of miR-142BS and miR-652-5pBS strongly mutes transgene expression in APCs but maintains high myoblast and myocyte expression. Importantly, rAAV1 vectors carrying this novel miR-142/652-5pBS cassette achieve higher transgene levels following intramuscular injections in mice than previous detargeting designs. The cassette strongly inhibits cytotoxic CTL activation and suppresses the Th17 response in vivo. Our approach, thus, advances the efficiency of miRNA-mediated detargeting to achieve synergistic reduction of transgene-specific immune responses and the development of safe and efficient delivery vehicles for gene therapy.

AAV8 locoregional delivery induces long-term expression of an immunogenic transgene in macaques despite persisting local inflammation

Adeno-associated virus (AAV) vectors are considered efficient vectors for gene transfer, as illustrated by recent successful clinical trials targeting retinal or neurodegenerative disorders. However, limitations as host immune responses to AAV capsid or transduction of limited regions must still be overcome. Here, we focused on locoregional (LR) intravenous perfusion vector delivery that allows transduction of large muscular areas and is considered to be less immunogenic than intramuscular (IM) injection. To confirm this hypothesis, we injected 6 cynomolgus monkeys with an AAV serotype 8 (AAV8) vector encoding for the highly immunogenic GFP driven by either a muscle-specific promoter (n = 3) or a cytomegalovirus (CMV) promoter (n = 3). We report that LR delivery allows long-term GFP expression in the perfused limb (up to 1 year) despite the initiation of a peripheral transgene-specific immune response. The analysis of the immune status of the perfused limb shows that LR delivery induces persisting inflammation. However, this inflammation is not sufficient to result in transgene clearance and is balanced by resident regulatory T cells. Overall, our results suggest that LR delivery promotes persisting transgene expression by induction of Treg cells in situ and might be a safe alternative to IM route to target large muscle territories for the expression of secreted therapeutic factors.

Orf Virus-Based Vaccine Vector D1701-V Induces Strong CD8+ T Cell Response against the Transgene but Not against ORFV-Derived Epitopes.

The potency of viral vector-based vaccines depends on their ability to induce strong transgene-specific immune response without triggering anti-vector immunity. Previously, Orf virus (ORFV, Parapoxvirus) strain D1701-V was reported as a novel vector mediating protection against viral infections. The short-lived ORFV-specific immune response and the absence of virus neutralizing antibodies enables repeated immunizations and enhancement of humoral immune responses against the inserted antigens. However, only limited information exists about the D1701-V induced cellular immunity. In this study we employed major histocompatibility complex (MHC) ligandomics and immunogenicity analysis to identify ORFV-specific epitopes. Using liquid chromatography-tandem mass spectrometry we detected 36 ORFV-derived MHC I peptides, originating from various proteins. Stimulated splenocytes from ORFV-immunized mice did not exhibit specific CD8+ T cell responses against the tested peptides. In contrast, immunization with ovalbumin-expressing ORFV recombinant elicited strong SIINFEKL-specific CD8+ T lymphocyte response. In conclusion, our data indicate that cellular immunity to the ORFV vector is negligible, while strong CD8+ T cell response is induced against the inserted transgene. These results further emphasize the ORFV strain D1701-V as an attractive vector for vaccine development. Moreover, the presented experiments describe prerequisites for the selection of T cell epitopes exploitable for generation of ORFV-based vaccines by reverse genetics.

Development and Preclinical Evaluation of an Integrase Defective Lentiviral Vector Vaccine Expressing the HIVACAT T Cell Immunogen in Mice

Cellular immune responses play a fundamental role in controlling viral replication and AIDS progression in human immunodeficiency virus (HIV)-infected subjects and in simian immunodeficiency virus (SIV)-infected macaques. Integrase defective lentiviral vector (IDLV) represents a promising vaccine candidate, inducing functional and durable immune responses in mice and non-human primates. Here, we designed HIV- and SIV-based IDLVs to express the HIVACAT T cell immunogen (HTI), a mosaic antigen designed to cover vulnerable sites in HIV-1 Gag, Pol, Vif, and Nef. We observed that HTI expression during lentiviral vector production interfered profoundly with IDLV particles release because of sequestration of both HIV- and SIV-Gag proteins in the cytoplasm of the vector-producing cells. However, modifications in IDLV design and vector production procedures greatly improved recovery of both HIV- and SIV-based IDLV-HTI. Immunization experiments in BALB/c mice showed that both IDLVs elicited HTI-specific T cell responses. However, immunization with HIV-based IDLV elicited also a T cell response toward exogenous HIV proteins in IDLV particles, suggesting that SIV-based IDLV may be a preferable platform to assess the induction of transgene-specific immune responses against rationally designed HIV structural antigens. These data support the further evaluation of IDLV as an effective platform of T cell immunogens for the development of an effective HIV vaccine.

Circumventing cellular immunity by miR142-mediated regulation sufficiently supports rAAV-delivered OVA expression without activating humoral immunity.

Recombinant adeno-associated virus (rAAV)-mediated gene delivery can efficiently target muscle tissues to serve as "biofactories" for secreted proteins in prophylactic and therapeutic scenarios. Nevertheless, efficient rAAV-mediated gene delivery is often limited by host immune responses against the transgene product. The development of strategies to prevent anti-transgene immunity is therefore crucial. The employment of endogenous microRNA (miRNA)-mediated regulation to detarget transgene expression from antigen presenting cells (APCs) has shown promise for reducing immunogenicity. However, the mechanisms underlying miRNA-mediated modulation of anti-transgene immunity by APC detargeting are not fully understood. Using the highly immunogenic ovalbumin (OVA) protein as a proxy for foreign antigens, we show that rAAV vectors containing miR142 binding sites efficiently repress co-stimulatory signals in dendritic cells, significantly blunt the cytotoxic T cell response, allow for sustained transgene expression in skeletal myoblasts, and attenuate clearance of transduced muscle cells in mice. Furthermore, the blunting of humoral immunity against circulating OVA correlates with detargeting of OVA expression from APCs. This demonstrates that incorporating APC-specific miRNA binding sites into rAAV vectors provides an effective strategy for reducing transgene-specific immune response. This approach holds promise for clinical applications where the safe and efficient delivery of a prophylactic or therapeutic protein is desired.

Adenoviral Type 35 and 26 Vectors with a Bidirectional Expression Cassette in the E1 Region Show an Improved Genetic Stability Profile and Potent Transgene-Specific Immune Response.

Genetic vaccines based on replication-incompetent adenoviral (AdV) vectors are currently in clinical development. Monovalent AdV vectors express one antigen from an expression cassette placed in most cases in the E1 region. For many vaccines, inclusion of several antigens is necessary in order to raise protective immunity and/or target more than one pathogen or pathogen strain. On the basis of the current technology, a mix of several monovalent vectors can be employed. However, a mix of the standard monovalent AdV vectors may not be optimal with respect to manufacturing costs and the final dose per vector in humans. Alternatively, a variety of bivalent recombinant AdV vector approaches is described in the literature. It remains unclear whether all strategies are equally suitable for clinical development while preserving all the beneficial properties of the monovalent AdV (e.g., immunogenic potency). Therefore, a thorough assessment of different bivalent AdV strategies was performed in a head-to-head fashion compared with the monovalent benchmark. The vectors were tested for rescue efficiency, genetic stability, transgene expression, and potency to induce transgene-specific immune responses. We report that the vector expressing multiple antigens from a bidirectional expression cassette in E1 shows a better genetic stability profile and a potent transgene-specific immune response compared with the other tested bivalent vectors.

Immunodominance of Adenovirus-Derived CD8+ T Cell Epitopes Interferes with the Induction of Transgene-Specific Immunity in Adenovirus-Based Immunization.

Adenovirus (Ad)-based immunization is a popular approach in vaccine development, and Ad-based vectors are renowned for their potential to induce strong CD8+ T cell responses to the encoded transgene. Surprisingly, we previously found in the mouse Friend retrovirus (FV) model that Ad-based immunization did not induce CD8+ T cell responses to the FV Leader-Gag-derived immunodominant epitope GagL85-93 We show now that induction of GagL85-93-specific CD8+ T cells was highly effective when leader-Gag was delivered by plasmid DNA immunization, implying a role for Ad-derived epitopes in mediating unresponsiveness. By immunizing with DNA constructs encoding strings of GagL85-93 and the two Ad-derived epitopes DNA-binding protein418-426 (DBP418-426) and hexon486-494, we confirmed that Ad epitopes prevent induction of GagL85-93-specific CD8+ T cells. Interestingly, while DBP418-426 did not interfere with GagL85-93-specific CD8+ T cell induction, the H-2Dd-restricted hexon486-494 suppressed the CD8+ T cell response to the H-2Db-restricted GagL85-93 strongly in H-2b/d mice but not in H-2b/b mice. This finding indicates that competition occurs at the level of responding CD8+ T cells, and we could indeed demonstrate that coimmunization with an interleukin 2 (IL-2)-encoding plasmid restored GagL85-93-specific CD8+ T cell responses to epitope strings in the presence of hexon486-494 IL-2 codelivery did not restore GagL85-93 responsiveness in Ad-based immunization, however, likely due to the presence of further epitopes in the Ad vector. Our findings show that seemingly immunodominant transgene epitopes can be dominated by Ad-derived epitopes. These findings underline the importance of thorough characterization of vaccine vectors, and modifications of vectors or immunogens may be required to prevent impaired transgene-specific immune responses.IMPORTANCE Ad-based vectors are widely used in experimental preclinical and clinical immunization studies against numerous infectious agents, such as human immunodeficiency virus, Ebola virus, Plasmodium falciparum, or Mycobacterium tuberculosis Preexisting immunity to Ad-based vectors is widely recognized as a hindrance to the widespread use of Ad-based vectors for immunizations in humans; however, our data show that an immune response to Ad-derived T cell epitopes can also result in loss or impairment of transgene-specific immune responses in prenaive vaccinees due to immune competition. Our results highlight that seemingly immunodominant epitopes may be affected by dominance of vector-derived epitopes, and modifications of the vector design or the immunogens employed in immunization may lead to more effective vaccines.

Effects of transgene expression level per cell in mice livers on induction of transgene-specific immune responses after hydrodynamic gene transfer.

We previously showed that high and sustained transgene expression of antigenic proteins induced transgene-specific immune responses. In the present study, a detailed relationship between the level of transgene expression per cell and immune response after hydrodynamic gene transfer was investigated. Cypridina luciferase (cLuc), a secretory antigenic reporter protein, was selected as a model antigen, and pROSA-cLuc, a plasmid expressing cLuc, was constructed. A fixed dose (30 μg) of pROSA-cLuc was delivered to mice by a single hydrodynamic injection or three injections at 24-h intervals because the number of cells transfected with plasmids is dependent on the number of hydrodynamic injections. Serum cLuc activity, an indicator of the total amount of cLuc transgene expression, was almost equal between these two groups. In contrast, the high-dose single injection induced higher levels of cLuc-specific humoral and cellular immune responses than the three low-dose injections. Moreover, the serum cLuc activity of the high-dose single injection group began to decline ~10 days after injection, whereas the activity remained constant in the three low-dose injection group. These results indicate that it is preferable to reduce the level of transgene expression per cell to avoid induction of the transgene-specific immune response after hydrodynamic gene transfer.

Differential immunogenicity between HAdV-5 and chimpanzee adenovirus vector ChAdOx1 is independent of fiber and penton RGD loop sequences in mice.

Replication defective adenoviruses are promising vectors for the delivery of vaccine antigens. However, the potential of a vector to elicit transgene-specific adaptive immune responses is largely dependent on the viral serotype used. HAdV-5 (Human adenovirus C) vectors are more immunogenic than chimpanzee adenovirus vectors from species Human adenovirus E (ChAdOx1 and AdC68) in mice, though the mechanisms responsible for these differences in immunogenicity remain poorly understood. In this study, superior immunogenicity was associated with markedly higher levels of transgene expression in vivo, particularly within draining lymph nodes. To investigate the viral factors contributing to these phenotypes, we generated recombinant ChAdOx1 vectors by exchanging components of the viral capsid reported to be principally involved in cell entry with the corresponding sequences from HAdV-5. Remarkably, pseudotyping with the HAdV-5 fiber and/or penton RGD loop had little to no effect on in vivo transgene expression or transgene-specific adaptive immune responses despite considerable species-specific sequence heterogeneity in these components. Our results suggest that mechanisms governing vector transduction after intramuscular administration in mice may be different from those described in vitro.

525. CD8+ T Cell Tolerance To Epitopes Downstream of Non-Sense Mutations

Recent success in gene therapy clinical trials have shown great promise towards developing efficacious treatments, however immune responses to the therapeutic protein resulting in transgene expression attenuation or loss still remains a concern. Expression can elicit a CD8+ T cell response against “non-self” regions of the protein as they were not presented during the process of central tolerance. Individuals with genes containing non-sense mutations could be at increased risk of developing a CD8+ T cell response following gene therapy as peptide sequences downstream of the mutation would not have previously been presented to the immune system. The effect of nonsense and missense mutations on the immune response to corrected gene expression was studied using the OTC gene, which is mutated in ornithine transcarbamylase deficiency. The immunodominant epitope SIINFEKL was fused to the C-terminus of OTC to evaluate the effect of mutations on the ability to elicit a transgene-specific immune response, as a surrogate for expression and presentation of SIINFEKL to CD8+ T cells. Using an ex-vivo stimulation assay, CD8+ T cells from OT-1 mice were mixed with fibroblasts transiently transfected with a panel of mutant or wild-type human hOTC-SIINFEKL constructs. T cell activation was determined by IFN-γ ELISPOT assay. Surprisingly, activation of T cells occurred to epitopes in the C-terminal region of genes containing non-sense mutations upstream of the SIINFEKL epitope. The most immunogenic nonsense mutant was selected for further studies in C57BL/6 mice. Packaging of this mutant hOTC-SIINFEKL transgene into highly immunogenic adenovirus 5 (Ad5) elicited an immune response in vivo to the downstream SIINFEKL epitope. To study if expression and presentation of epitopes downstream non-sense mutations to CD8+ T cells could result in tolerance induction all of the non-sense mutant constructs were used to generate AAV8 vectors. C57BL/6 mice were administered the non-immunogenic AAV8 vectors and then subsequently challenged the mice with Ad5 hOTC-SIINFEKL or Ad5 OVA. Mice pre-treated with AAV8 had a drastically reduced CD8+ T cell activation to the SIINFEKL epitope compared to no treatment as measured by flow cytometry and IFN-γ ELISPOT assay. These results suggest that patients with non-sense mutations could have immunological tolerance to the therapeutic protein decreasing the risk of a transgene specific immunologic response.

644. Anterior Chamber Associated Immune Deviation (ACAID) Regulates Anti-Transgene Cellular Immune Responses Following Systemic Gene Transfer

Anterior Chamber-Associated Immune Deviation (ACAID), is an antigen specific peripheral immunomodulatory mechanism described when an antigen is introduced into the anterior segment of a mammalian eye. It is not known if the ACAID mechanism could impact on cellular immune responses induced by gene therapy.Our study was therefore conducted in a murine model to evaluate if ACAID mechanism induced in the eye would induce immune tolerance to the product of a transgene injected by systemic route (i. v.) and if this could impact on the destruction of the transduced cells. To test this hypothesis, we have developed a murine model of ACAID by injecting intracameraly soluble peptides (DBY and UTY) of the HY male antigen into wild type female C57Bl/6 mice (H-2b background), followed by an intravenous injection of a recombinant AAV2/8 vector (rAAV2/8-GFP-HY) containing a transgene coding for a fusion GFP-HY molecule. In control animals (without prior ocular peptide injection), a clear transgene-specific immune response develops following rAAV2/8-GFP-HY injection as measured by spleen IFNγ ELISpot analysis, evaluation of T cell polarization profile by cytokine secretion titration and induction of CD8+ T cell destructive response by in vivo cytotoxic assays. In contrast, the number of IFNγ producing spleen cells was reduced up to 80% in ACAID conditions. In addition, cytokine secretion assays showed an impairment of the Th1 profile in ACAID condition with a reduction in the secretion of IFNγ and TNFα (8 and 2 times lower, respectively). Moreover, a model of in vivo cytotoxic assay suggests that almost all HY-expressing cells can survive in ACAID-induced mice in contrast to the control mice.In conclusion, cytokine and in vivo cytotoxic assays indicate that ACAID seems to be a powerful inhibitory mechanism in the context of systemic gene transfer, limiting cellular immune responses against the transgene and promoting transgene expressing cells survival.

The relative magnitude of transgene-specific adaptive immune responses induced by human and chimpanzee adenovirus vectors differs between laboratory animals and a target species

Adenovirus vaccine vectors generated from new viral serotypes are routinely screened in pre-clinical laboratory animal models to identify the most immunogenic and efficacious candidates for further evaluation in clinical human and veterinary settings. Here, we show that studies in a laboratory species do not necessarily predict the hierarchy of vector performance in other mammals. In mice, after intramuscular immunization, HAdV-5 (Human adenovirus C) based vectors elicited cellular and humoral adaptive responses of higher magnitudes compared to the chimpanzee adenovirus vectors ChAdOx1 and AdC68 from species Human adenovirus E. After HAdV-5 vaccination, transgene specific IFN-γ+ CD8+ T cell responses reached peak magnitude later than after ChAdOx1 and AdC68 vaccination, and exhibited a slower contraction to a memory phenotype. In cattle, cellular and humoral immune responses were at least equivalent, if not higher, in magnitude after ChAdOx1 vaccination compared to HAdV-5. Though we have not tested protective efficacy in a disease model, these findings have important implications for the selection of candidate vectors for further evaluation. We propose that vaccines based on ChAdOx1 or other Human adenovirus E serotypes could be at least as immunogenic as current licensed bovine vaccines based on HAdV-5.

Priming of CD8 T cells by adenoviral vectors is critically dependent on B7 and dendritic cells but only partially dependent on CD28 ligation on CD8 T cells.

Adenoviral vectors have long been forerunners in the development of effective CD8 T cell-based vaccines; therefore, it is imperative that we understand the factors controlling the induction of robust and long-lasting transgene-specific immune responses by these vectors. In this study, we investigated the organ sites, molecules, and cell subsets that play a critical role in the priming of transgene-specific CD8 T cells after vaccination with a replication-deficient adenoviral vector. Using a human adenovirus serotype 5 (Ad5) vector and genetically engineered mice, we found that CD8(+) and/or CD103(+) dendritic cells in the draining lymph node played a critical role in the priming of Ad5-induced CD8 T cell responses. Moreover, we found that CD80/86, but not CD28, was essential for efficient generation of both primary effectors and memory CD8 T cells. Interestingly, the lack of CD28 expression resulted in a delayed primary response, whereas memory CD8 T cells generated in CD28-deficient mice appeared almost normal in terms of both phenotype and effector cytokine profile, but they exhibited a significantly reduced proliferative capacity upon secondary challenge while retaining immediate in vivo effector capabilities: in vivo cytotoxicity and short-term in vivo protective capacity. Overall, our data point to an absolute requirement for professional APCs and the expression of the costimulatory molecules CD80/86 for efficient CD8 T cell priming by adenoviral vectors. Additionally, our results suggest the existence of an alternative receptor for CD80/86, which may substitute, in part, for CD28.

Murine Hemophilia A With Or Without Pre-Existing Anti-Factor VIII Inhibitors Is Partially Corrected By Factor VIII Stored In Platelets After Intraosseous Infusion Of Lentiviral Vectors Into Bone Marrow Without Preconditioning

IntroductionPlatelets may comprise an ideal vehicle for delivering FVIII in hemophilia A (HemA) as FVIII stored in platelet α-granules is protected from neutralization by inhibitory antibodies and, during bleeding, activated platelets locally excrete their contents to promote clot formation. In order to avoid specific challenges posed by ex vivo gene delivery including, in particular, the requirement to pre-condition the subject, we evaluated intraosseous (IO) infusion of self-inactivating lentiviral vectors (LV) for in situ gene transfer into bone marrow cells. In previous studies, we confirmed that hematopoietic stem cells (HSCs) can be efficiently transduced to express GFP after IO administration of LV driven by a MND promoter (M-GFP-LV). MethodsIn the current study, we aimed at limiting transgene expression to the megakaryocyte lineages using IO delivery of 20 µL LV containing either GFP (G-GFP-LV) or a B-domain variant human FVIII (G-F8-LV) gene under the control of megakaryocyte strictly specific promoter glycoprotein 1bα (Gp1bα). ResultsIn M-GFP-LV treated control mice, GFP was detected in 6.4% of HSCs, 3.4% of B220+, and 9.0% of CD11c+ bone marrow cells on day 29. In contrast, in G-GFP-LV (6.0E+08 TU/mL) treated mice, GFP was undetectable in bone marrow HSCs, B220+, CD11c+ or CD11b+ cells. GFP expression level in platelets of G-GFP-LV treated mice was ten folds of that in M-GFP-LV treated mice (0.1% vs 0.01%). It indicated that in platelets, the activity of Gp1bα was stronger than that of MND. More importantly, GFP expression levels were stable over 100 days, suggesting that platelets containing the transgene products did not elicit transgene-specific immune responses. Next, we treated HemA mice with G-F8-LV (6.0 E+07 TU/mL). There was no detectable hFVIII expression in bone marrow HSCs on day 8 or in blood cells (CD3ε+, B220+, CD11c+ or CD11b+) on day 35. However, up to 3% platelets express hFVIII on day 91. These results suggested that HSCs in HemA mice were successfully transduced by G-F8-LV after IO infusion, and in the long term, FVIII was synthesized in megakaryocytes and stored in platelet α-granules. In treated mice, the average percentage of platelets expressing hFVIII was stable at 1-2% from day 27 to day 160. The average FVIII antigen level in platelets on day 112 was 1 mU per 1 × 108 platelets, which was comparable with platelet FVIII in transgenic and ex vivo gene therapy treated mice. We also evaluated LV-treated HemA animals for phenotypic correction of bleeding diathesis by tail clip assay. The blood loss was 41% (n=7), 48% (n=5) and 33% (n=5) compared with control HemA (normalized to 100%), mock treated HemA (∼100%), and wild-type (2.5%) mice on days 35, 118 and 160, respectively. Additionally, there was neither detectable FVIII activity nor anti-FVIII antibodies in blood on day 160, which indicated that there was insignificant leaky expression of FVIII in other cells. Finally, we also infused G-F8-LV into HemA inhibitor mice. Inhibitors were induced by repeated injection of 3U recombinant hFVIII. The average antibody level was 80 Bethesda Unit before IO infusion of the vectors. In G-F8-LV treated mice, the average hFVIII antigen level on day 27 was 0.74 mU per 1 x 108 platelets (n=5). Bleeding assay was performed on day 160. The blood loss of treated mice was significantly reduced compared with untreated HemA mice, indicating that IO infusion of G-F8-LV can overcome anti-FVIII antibodies and correct hemophilia phenotype. ConclusionWe have successfully transduced HSCs in situ by a single infusion of LVs into bone marrow to correct hemophilia A. Gp1bα promoter in lentiviral vectors can specifically direct the transgene expression in mouse platelets. Following IO infusion of G-F8-LV, FVIII stored in platelets can persistently and partially correct the HemA phenotype for at least five months (experimental duration) in mice with and without pre-existing inhibitors. Overall, direct transduction of bone marrow cells targeting platelet-specific FVIII expression may provide an effective therapy to treat severe hemophilia A patients with high-titer inhibitors. Disclosures:No relevant conflicts of interest to declare.

TK-Cells Safety and Efficacy in Allogeneic Hematopoietic Stem Cell Transplantation: Long-Term Clinical and Molecular Assessment in 128 Treated Patients.

Abstract 3061 Background and methods.Suicide gene therapy (SGT) applied to allogeneic hematopoietic stem cell transplantation (allo-HSCT) has been one of the first clinical applications of gene therapy. In allo-HSCT the antileukemic potential mediated by alloreactive lymphocytes towards patient-specific antigens, such as minor and major histocompatibility antigens, is counterbalanced by the Graft-versus-Host-Disease (GvHD). The risk of GvHD increases with the level of HLA disparity between host and donor, and leads to impaired quality of life and reduced survival, particularly in patients (pts) transplanted from HLA-mismatched donors. In this contest SGT has been applied to modulate alloreactivity, by inserting the HSV-tk gene in donor lymphocytes. This suicide gene/prodrug system requires cell cycle for optimal killing. In the context of allo-HSCT, this characteristic ensures a further level of specificity in GvHD control, by allowing to selectively kill highly proliferating alloreactive cells during GvHD, while sparing resting T cells. We assessed long-term safety in TK cells treated pts. Results.Overall 128 pts have been treated worldwide in 10 phase I-II clinical trials with donor lymphocytes expressing the HSV-tk suicide gene, with the purpose of enforcing the graft-versus-tumor (GvT) effect and/or promoting a functional post-transplant immune reconstitution (IR) while allowing control of GvHD. This approach proved highly feasible, safe and effective in promoting a dynamic and patient-specific modulation of alloreactivity. TK cells engrafted in the majority of pts and a clinical benefit, measured as improvement of hematopoietic chimerism, malignant regression and/or IR, was reported for 65 pts (51%). GvHD grade II-IV was observed in 28 pts (22%) and was always rapidly and completely controlled by the activation of suicide machinery.A selected population of 57 pts, treated at San Raffaele Institution, was studied in more detail for long-term analysis: 23 pts received cells to treat relapse occurring after an HLA-identical allo-HSCT (Ciceri F et al, Blood. 2007: 109; 4698–470) and 34 pts to improve IR after haploidentical HSCT (Bonini C et al, Blood. 2002; 100: 115a; Ciceri F, Bonini C et al, Lancet Oncology. 2009; 10: 489–500). No adverse event correlated to the gene transfer procedure was ever reported.Genetically modified cells engrafted in 90% of treated pts and could be detected in vivo, at low frequencies for up to 14 years (y).In the HLA-identical setting 11 pts obtained clinical response of the malignant disease and 3/11 are alive with a median follow-up of 15y. Two pts are in complete remission (CR), while one pt affected by chronic myelomonocytic leukemia relapsed 15y post transplant. This pt was subsequently treated with a second transplant from an unrelated match donor and is now in CR 1 year after transplant.In the haploidentical setting, 25 pts/34 reached the target of IR and 9 are alive and in CR with a median follow-up of 7y. Four out of 9 experienced GVHD in the early phase post IR, none presented signs, symptoms, complications related to GvHD and none needed pharmacological treatment in long-term.According to international guideline for long-term follow-up (Majhail NS et al, BBMT. 2012; 18: 348–371) all pts underwent regular screening and clinical evaluation. No major infections occurred in the late phase, no ocular, oral, respiratory, hepatic, renal, genitourinary, muscle-skeletal or neurological distress. Noteworthy 3 pts developed second cancer (2 skin cancer non-melanoma, 1 endometrial cancer), 2 pt hypothyroidism, 1 pt coronary artery disease and 3 pts metabolic syndrome. No evidence of psychosocial symptoms was reported.Immunity against the transgene was reported in pts who received TK cells late after an HLA-identical allo-SCT. The quality of IR at the time of TK cells administration was the most predictive variable. On the contrary, no immunity against HSV-tk was ever detected in pts who received TK cells after transplantation of haploidentical CD34-selected cells, indicating that the infusion of TK cells to highly immunosuppressed pts is not limited by the development of transgene-specific immune responses. Conclusions.Long-term assessment confirmed the overall high benefit/risk ratio of the TK-cell approach in allo-HSCT.A phase III multicentric, randomized clinical trial sponsored by MolMed, is currently undergoing in the context of haploidentical HSCT. Disclosures:Bordignon:MolMed SpA: Employment. Bonini:MolMed S.p.A.: Consultancy.

Immunogenic Comparison of Chimeric Adenovirus 5/35 Vector Carrying Optimized Human Immunodeficiency Virus Clade C Genes and Various Promoters

Adenovirus vector-based vaccine is a promising approach to protect HIV infection. However, a recent phase IIb clinical trial using the vector did not show its protective efficacy against HIV infection. To improve the vaccine, we explored the transgene protein expression and its immunogenicity using optimized codon usage, promoters and adaptors. We compared protein expression and immunogenicity of adenovirus vector vaccines carrying native or codon usage-optimized HIV-1 clade C gag and env genes expression cassettes driven by different promoters (CMV, CMVi, and CA promoters) and adapters (IRES and F2A). The adenovirus vector vaccine containing optimized gag gene produced higher Gag protein expression and induced higher immune responses than the vector containing native gag gene in mice. Furthermore, CA promoter generated higher transgene expression and elicited higher immune responses than other two popularly used promoters (CMV and CMVi). The second gene expression using F2A adaptor resulted in higher protein expression and immunity than that of using IRES and direct fusion protein. Taken together, the adenovirus vector containing the expression cassette with CA promoter, optimized HIV-1 clade C gene and an F2A adaptor produced the best protein expression and elicited the highest transgene-specific immune responses. This finding would be promising for vaccine design and gene therapy.

Intranasal Administration of Adeno-associated Virus Type 12 (AAV12) Leads to Transduction of the Nasal Epithelia and Can Initiate Transgene-specific Immune Response

A critical aspect in defining the utility of a vector for gene therapy applications is the cell tropism and biodistribution of the vector. Adeno-associated virus type 12 (AAV12) has several unique biological and immunological properties that could be exploited for gene therapy purposes, including a unique cell surface receptor, transduction of epithelial cells, and limited neutralization by pooled human antibodies. However, little is known about its cell tropism and biodistribution in vivo. In vivo biodistribution studies with AAV12 vectors encoding a cytomegalovirus promoted luciferase transgene indicated preferential transduction of the nasal epithelia which was not observed with AAV2-based vectors. Expression peaked 2 weeks postadministration, before decreasing to a persistent level. The level of neutralizing antibodies (Nab) induced was sevenfold lower for AAV12 than for AAV2, an advantage for use in repeat administration. Furthermore, vectors encoding influenza A nucleoprotein (NP), an antigen which has previously been shown to induce immune protection against challenge, resulted in generation of both anti-A/NP antibodies and lung anti-A/NP T cells. Our findings suggest further evaluation of AAV12 as a vector for gene therapy and as a potential nasal vaccine.

In Vivo Expansion of Regulatory T cells With IL-2/IL-2 mAb Complexes Prevents Anti-factor VIII Immune Responses in Hemophilia A Mice Treated With Factor VIII Plasmid-mediated Gene Therapy

Generation of transgene-specific immune responses can constitute a major complication following gene therapy treatment. An in vivo approach to inducing selective expansion of Regulatory T (Treg) cells by injecting interleukin-2 (IL-2) mixed with a specific IL-2 monoclonal antibody (JES6-1) was adopted to modulate anti-factor VIII (anti-FVIII) immune responses. Three consecutive IL-2 complexes treatments combined with FVIII plasmid injection prevented anti-FVIII formation and achieved persistent, therapeutic-level of FVIII expression in hemophilia A (HemA) mice. The IL-2 complexes treatment expanded CD4(+)CD25(+)Foxp3(+) Treg cells five- to sevenfold on peak day, and they gradually returned to normal levels within 7-14 days without changing other lymphocyte populations. The transiently expanded Treg cells are highly activated and display suppressive function in vitro. Adoptive transfer of the expanded Treg cells protected recipient mice from generation of high-titer antibodies following FVIII plasmid challenge. Repeated plasmid transfer is applicable in tolerized mice without eliciting immune responses. Mice treated with IL-2 complexes mounted immune responses against both T-dependent and T-independent neoantigens, indicating that IL-2 complexes did not hamper the immune system for long. These results demonstrate the important role of Treg cells in suppressing anti-FVIII immune responses and the potential of developing Treg cell expansion therapies that induce long-term tolerance to FVIII.

An Improved Bicistronic CD20/tCD34 Vector for Efficient Purification and In Vivo Depletion of Gene-Modified T Cells for Adoptive Immunotherapy

T-cell-based adoptive immunotherapy is widely used to treat graft rejection and relapse after stem cell transplantation (SCT). However, this approach is hampered by a high risk of life-threatening graft-versus-host-disease (GvHD). Clinical trials have demonstrated the value of suicide genes to modify T cells for the effective control of GvHD. Herewith, we show that the combination of a codon-optimized B-cell antigen (CD20op) with a selection marker based on a cytoplasmic truncated version of the human stem cell antigen CD34 (tCD34) allows the generation of highly enriched gene-modified T cells. We demonstrate coordinate co-expression of both transgenes and high expression of CD20op resulting in an increased susceptibility to Rituximab (RTX)-induced cell death. In addition, T cells partially retained their alloreactive potential and their CD4/CD8 ratio after transduction and expansion. Long-lasting transgene expression was sustained in vivo after adoptive transfer into Rag-1(-/-) mice. Moreover, gene-modified T cells were quickly and efficiently depleted from peripheral blood (PB) and secondary lymphoid organs of transplanted animals after RTX treatment. These results warrant further steps toward a clinical application of CD20op as a suicide gene for adoptive immunotherapy.