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

Viral vector- and virus-like particle-based vaccines against infectious diseases: A minireview

To overcome the limitations of conventional vaccines, new platforms for vaccine design have emerged such as those based on viral vectors and virus-like particles (VLPs). Viral vector vaccines are highly efficient and the onset of protection is quick. Many recombinant vaccine candidates for humans are based on viruses belonging to different families such as Adenoviridae, Retroviridae, Paramyxoviridae, Rhabdoviridae, and Parvoviridae. Also, the first viral vector vaccine licensed for human vaccination was the Japanese encephalitis virus vaccine. Since then, several viral vectors have been approved for vaccination against the viruses of Lassa fever, Ebola, hepatitis B, hepatitis E, SARS-CoV-2, and malaria. VLPs are nanoparticles that mimic viral particles formed from the self-assembly of structural proteins and VLP-based vaccines against hepatitis B and E viruses, human papillomavirus, and malaria have been commercialized. As evidenced by the accelerated production of vaccines against COVID-19, these new approaches are important tools for vaccinology and for generating rapid responses against pathogens and emerging pandemic threats.

Development of Virus-like Particle Plant-Based Vaccines against Avian H5 and H9 Influenza A Viruses

Avian influenza A virus (AIV) is a significant cause of mortality in poultry, causing substantial economic loss, particularly in developing countries, and has zoonotic potential. For example, highly pathogenic avian influenza (HPAI) viruses of the H5 subtype have been circulating in Egypt for around two decades. In the last decade, H5N1 viruses of clade 2.2.1 have been succeeded by the antigenically distinct H5N8 clade 2.3.4.4b viruses. Furthermore, H9N2 viruses co-circulate with the H5N8 viruses in Egyptian poultry. It is widely recognised that effective vaccination against IAV requires a close antigenic match between the vaccine and viruses circulating in the field. Therefore, approaches to develop cost-effective vaccines that can be rapidly adapted to local virus strains are required for developing countries such as Egypt. In this project, the haemagglutinin (HA) proteins of Egyptian H5 and H9 viruses were expressed by transient transfection of plants (Nicotiana benthamiana). The formation of virus-like particles (VLPs) was confirmed by transmission electron microscopy. Mice were immunised with four doses of either H5 or H9 VLPs with adjuvant. Antibody and cellular immune responses were measured against the corresponding recombinant protein using ELISA and enzyme-linked immunosorbent assay (ELISpot), respectively. Chickens were immunised with one dose of H5 VLPs, eliciting HA-specific antibodies measured by ELISA and a pseudotyped virus neutralisation test using a heterologous H5 HA. In conclusion, plant-based VLP vaccines have potential for producing an effective vaccine candidate within a short time at a relatively low cost.

Use of lentiviral vectors in vaccination

INTRODUCTION

Lentiviral vectors have emerged as powerful vectors for vaccination, due to their high efficiency to transduce dendritic cells and to induce long-lasting humoral immunity, CD8⁺ T cells, and effective protection in numerous preclinical animal models of infection and oncology.

AREAS COVERED

Here, we reviewed the literature, highlighting the relevance of lentiviral vectors in vaccinology. We recapitulated both their virological and immunological aspects of lentiviral vectors. We compared lentiviral vectors to the gold standard viral vaccine vectors, i.e. adenoviral vectors, and updated the latest results in lentiviral vector-based vaccination in preclinical models.

EXPERT OPINION

Lentiviral vectors are non-replicative, negligibly inflammatory, and not targets of preexisting immunity in human populations. These are major characteristics to consider in vaccine development. The potential of lentiviral vectors to transduce non-dividing cells, including dendritic cells, is determinant in their strong immunogenicity. Notably, lentiviral vectors can be engineered to target antigen expression to specific host cells. The very weak inflammatory properties of these vectors allow their use in mucosal vaccination, with particular interest in infectious diseases that affect the lungs or brain, including COVID-19. Recent results in various preclinical models have reinforced the interest of these vectors in prophylaxis against infectious diseases and in onco-immunotherapy.

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.

Production of Recombinant HIV-1 p24-Nef Protein in Two Forms as Potential Candidate Vaccines in Three Vehicles

BACKGROUND

Different approaches have been investigated to develop a preventive or therapeutic vaccine, although none of them has been fully practical. Therapeutic vaccines against HIV-1 have been studied with the aim of eliminating the virus from reservoir cells with or without HAART (Highly Active Antiretroviral Therapy). Fusion proteins with the most immunogenic features among conserved regions can facilitate this achievement in such a variable virus. To achieve the most immunogenic and also conserved regions, bioinformatics tools are widely used to predict antigens' features before applying them.

OBJECTIVE

This study aimed at the in vitro evaluation of p24 -Nef fusion protein based on the previous in silico design to achieve a potential therapeutic subunit vaccine against HIV-1.

METHODS

The truncated form of p24-Nef using AAY flexible linker and the full protein were expressed and evaluated in the prokaryotic system and confirmed by western blotting. We also used pcDNA3.1 to transfect Lenti-X 293T cells. Moreover, lentiviral vectors were applied to produce recombinant virions harboring the genes of interest and cell transduction.

RESULTS

Both fusion proteins in a truncated and a full form were expressed and confirmed by Anti Nef polyclonal antibody in western blotting. Recombinant virions were generated and transduced Lenti-X 293T cells confirming by immunofluorescence microscope and p24 ELISA assay kit. Transduced cells were analyzed by SDS-PAGE and western blotting, which resulted in approved protein expression.

CONCLUSION

Fusion protein of p24 and Nef is well expressed in eukaryotic cell lines according to its pre-evaluated features by bioinformatics tools.

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.

Recent advances on HIV DNA vaccines development: Stepwise improvements to clinical trials

According to WHO (World Health Organization) reports, more than 770,000 people died from HIV and almost 1.7 million people becoming newly infected in the worldwide in 2018. Therefore, many attempts should be done to produce a forceful vaccine to control the AIDS. DNA-based vaccines have been investigated for HIV vaccination by researches during the recent 20 years. The DNA vaccines are novel approach for induction of both type of immune responses (cellular and humoral) in the host cells and have many advantages including high stability, fast and easy of fabrication and absence of severe side effects when compared with other vaccination methods. Recent studies have been focused on vaccine design, immune responses and on the use of adjuvants as a promising strategy for increased level of responses, delivery approaches by viral and non-viral methods and vector design for different antigens of HIV virus. In this review, we outlined the aforementioned advances on HIV DNA vaccines. Then we described the future trends in clinical trials as a strong strategy even in healthy volunteers and the potential developments in control and prevention of HIV.

An engineered novel lentivector specifically transducing dendritic cells and eliciting robust HBV-specific CTL response by upregulating autophagy in T cells

Dendritic cells (DCs) play a predominant role in initiating cell immune responses. Here we generated a DC-targeting lentiviral vector (LVDC-UbHBcAg-LIGHT) and evaluated its capacity to elicit HBV-specific cytotoxic T lymphocyte (CTL) responses. DC-SIGN-mediated specific transduction using this construct was confirmed in DC-SIGN-expressing 293T cells and ex vivo-cultured bone marrow cells. LVDC-UbHBcAg-LIGHT-loaded DCs were highly effective in inducing HBV-specific CTLs. Mechanistic studies demonstrated autophagy blocking led to a significant increase in apoptosis and obvious inhibition of CD8 + T cells entry into S-phase, correspondingly attenuated LVDC-UbHBcAg-LIGHT-loaded DC-induced T cell responses. This observation was supported by accumulation of pro-apoptotic proteins and the main negative cell cycle regulator-CDKN1B that otherwise would be degraded in activated T cells where autophagy preferentially occured. Our findings revealed an important role of autophagy in the activation of T cells and suggested LVDC-UbHBcAg-LIGHT may potentially be used as a therapeutic strategy to combat persistent HBV infection with higher security.

DNA prime/MVTT boost regimen with HIV-1 mosaic Gag enhances the potency of antigen-specific immune responses

HIV-1 diversity and latent reservoir are the major challenges for the development of an effective AIDS vaccine. It is well indicated that Gag-specific CD8⁺ T cells serve as the dominant host immune surveillance for HIV-1 control, but it still remains a challenge for vaccine design to induce broader and stronger cytotoxic T cell immunity against the virus. Genetic variation of the HIV-1 gag gene across different clades is one of the reasons for the reduction of antigenic epitope coverage. Here, we report an immunization strategy with heterologous vaccines expressing a mosaic Gag antigen aimed to increase antigenic breadth against a wider spectrum of HIV-1 strains. Priming using a DNA vaccine via in vivo electroporation, followed by boosting with a live replication-competent modified vaccinia TianTan (MVTT) vectored vaccine, elicited greater and broader protective Gag-specific immune responses in mice. Compared to DNA or MVTT homologous immunization, the heterologous DNA/MVTT vaccination resulted in higher frequencies of broadly reactive, Gag-specific, polyfunctional, long-lived cytotoxic CD8⁺ T cells, as well as increased anti-Gag antibody titer. Importantly, the DNA/MVTT heterologous vaccination induced protection against EcoHIV and mesothelioma AB1-Gag challenges. In summary, the stronger protective Gag-specific immunity induced by the heterologous regimen using two safe vectors shows promise for further development to enhance anti-HIV-1 immunity. Our study has important implications for immunogen design and the development of an effective HIV-1 heterologous vaccination strategy.

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.

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.

CRACC-targeting Fc-fusion protein induces activation of NK cells and DCs and improves T cell immune responses to antigenic targets

The CD2-like receptor activating cytotoxic cell (CRACC) receptor is a member of the SLAM family of receptors that are found on several types of immune cells. We previously demonstrated that increasing the abundance of the adaptor protein EAT-2 during vaccination enhanced innate and adaptive immune responses to vaccine antigens. Engagement of the CRACC receptor in the presence of the EAT-2 adaptor generally results in immune cell activation, while activating CRACC signaling in cells that lack EAT-2 adaptor inhibits their effector and regulatory functions. As EAT-2 is the only SAP adaptor that interacts with the CRACC receptor, we hypothesized that technologies that specifically modulate CRACC signaling during vaccination may also improve antigen specific adaptive immune responses. To test this hypothesis, we constructed a CRACC-targeting Fc fusion protein and included it in vaccination attempts. Indeed, mice co-vaccinated with the CRACC-Fc fusion protein and an adenovirus vaccine expressing the HIV-Gag protein had improved Gag-specific T cell responses, as compared to control mice. These responses are characterized by increased numbers of Gag-specific tetramer+ CD8+ T cells and increases in production of IFNγ, TNFα, and IL2, by Gag-specific CD8+ T cells. Moreover, our results revealed that use of the CRACC-Fc fusion protein enhances vaccine-elicited innate immune responses, as characterized by increased dendritic cells (DCs) maturation and IFNγ production from NK cells. This study highlights the importance of CRACC signaling during the induction of an immune response generally, and during vaccinations specifically, and also lends insight into the mechanisms underlying our prior results noting EAT-2-dependent improvements in vaccine efficacy.

Lentiviral vectors in cancer immunotherapy

Basic science advances in cancer immunotherapy have resulted in various treatments that have recently shown success in the clinic. Many of these therapies require the insertion of genes into cells to directly kill them or to redirect the host's cells to induce potent immune responses. Other analogous therapies work by modifying effector cells for improved targeting and enhanced killing of tumor cells. Initial studies done using γ-retroviruses were promising, but safety concerns centered on the potential for insertional mutagenesis have highlighted the desire to develop other options for gene delivery. Lentiviral vectors (LVs) have been identified as potentially more effective and safer alternative delivery vehicles. LVs are now in use in clinical trials for many different types of inherited and acquired disorders, including cancer. This review will discuss current knowledge of LVs and the applications of this viral vector-based delivery vehicle to cancer immunotherapy.

Pros and Cons of Antigen-Presenting Cell Targeted Tumor Vaccines

In therapeutic antitumor vaccination, dendritic cells play the leading role since they decide if, how, when, and where a potent antitumor immune response will take place. Since the disentanglement of the complexity and merit of different antigen-presenting cell subtypes, antitumor immunotherapeutic research started to investigate the potential benefit of targeting these subtypes in situ. This review will discuss which antigen-presenting cell subtypes are at play and how they have been targeted and finally question the true meaning of targeting antitumor-based vaccines.

Pseudotyping lentiviral vectors with lymphocytic choriomeningitis virus glycoproteins for transduction of dendritic cells and in vivo immunization

Lentiviral vectors (LVs) are promising delivery systems for gene therapy, and they can be further engineered to increase their potential for effectively delivering transgenes to desired cell populations. Here, we have engineered LVs pseudotyped with envelope glycoproteins derived from lymphocytic choriomeningitis virus (LCMV) for antigen delivery to elicit vaccine-directed immune responses. Two variants, LCMV-WE and LCMV-Arm53b, were evaluated for their ability to mediate LV-based cellular transduction in vitro. LCMV-WE with a leucine residue at position 260 (260L) is known for its high-affinity binding with a cellular receptor, α-dystroglycan (α-DG), whereas LCMV-Arm53b has low-affinity binding resulting from a phenylalanine residue at the same position. In contrast to LCMV-Arm53b, we found that LVs pseudotyped with LCMV-WE could transduce 293T cells and murine dendritic cells much more efficiently based, at least in part, on their favorable interaction with α-DG. In mice, LCMV-WE-bearing LVs encoding a model antigen, invariant chain ovalbumin, could elicit substantial antigen-specific CD8(+) T cell immune response. The response could be further enhanced by a homologous boosting immunization with the same vector. These findings offer evidence to support the potential utilization of LCMV-WE-bearing LVs for vectored vaccines against cancer and infectious diseases.

Generation of a stable packaging cell line producing high-titer PPT-deleted integration-deficient lentiviral vectors

The risk of insertional mutagenesis inherent to all integrating exogenous expression cassettes was the impetus for the development of various integration-defective lentiviral vector (IDLV) systems. These systems were successfully employed in a plethora of preclinical applications, underscoring their clinical potential. However, current production of IDLVs by transient plasmid transfection is not optimal for large-scale production of clinical grade vectors. Here, we describe the development of the first tetracycline-inducible stable IDLV packaging cell line comprising the D64E integrase mutant and the VSV-G envelope protein. A conditional self-inactivating (cSIN) vector and a novel polypurine tract (PPT)-deleted vector were incorporated into the newly developed stable packaging cell line by transduction and stable transfection, respectively. High-titer (~10(7) infectious units (IU)/ml) cSIN vectors were routinely generated. Furthermore, screening of single-cell clones stably transfected with PPT-deleted vector DNA resulted in the identification of highly efficient producer cell lines generating IDLV titers higher than 10(8) IU/mL, which upon concentration increased to 10(10) IU/ml. IDLVs generated by stable producer lines efficiently transduce CNS tissues of rodents. Overall, the availability of high-titer IDLV lentivirus packaging cell line described here will significantly facilitate IDLV-based basic science research, as well as preclinical and clinical applications.

Lentiviral Protein Transfer Vectors Are an Efficient Vaccine Platform and Induce a Strong Antigen-Specific Cytotoxic T Cell Response

To induce and trigger innate and adaptive immune responses, antigen-presenting cells (APCs) take up and process antigens. Retroviral particles are capable of transferring not only genetic information but also foreign cargo proteins when they are genetically fused to viral structural proteins. Here, we demonstrate the capacity of lentiviral protein transfer vectors (PTVs) for targeted antigen transfer directly into APCs and thereby induction of cytotoxic T cell responses. Targeting of lentiviral PTVs to APCs can be achieved analogously to gene transfer vectors by pseudotyping the particles with truncated wild-type measles virus (MV) glycoproteins (GPs), which use human SLAM (signaling lymphocyte activation molecule) as a main entry receptor. SLAM is expressed on stimulated lymphocytes and APCs, including dendritic cells. SLAM-targeted PTVs transferred the reporter protein green fluorescent protein (GFP) or Cre recombinase with strict receptor specificity into SLAM-expressing CHO and B cell lines, in contrast to broadly transducing vesicular stomatitis virus G protein (VSV-G) pseudotyped PTVs. Primary myeloid dendritic cells (mDCs) incubated with targeted or nontargeted ovalbumin (Ova)-transferring PTVs stimulated Ova-specific T lymphocytes, especially CD8(+) T cells. Administration of Ova-PTVs into SLAM-transgenic and control mice confirmed the observed predominant induction of antigen-specific CD8(+) T cells and demonstrated the capacity of protein transfer vectors as suitable vaccines for the induction of antigen-specific immune responses.

IMPORTANCE

This study demonstrates the specificity and efficacy of antigen transfer by SLAM-targeted and nontargeted lentiviral protein transfer vectors into antigen-presenting cells to trigger antigen-specific immune responses in vitro and in vivo. The observed predominant activation of antigen-specific CD8(+) T cells indicates the suitability of SLAM-targeted and also nontargeted PTVs as a vaccine for the induction of cytotoxic immune responses. Since cytotoxic CD8(+) T lymphocytes are a mainstay of antitumoral immune responses, PTVs could be engineered for the transfer of specific tumor antigens provoking tailored antitumoral immunity. Therefore, PTVs can be used as safe and efficient alternatives to gene transfer vectors or live attenuated replicating vector platforms, avoiding genotoxicity or general toxicity in highly immunocompromised patients, respectively. Thereby, the potential for easy envelope exchange allows the circumventing of neutralizing antibodies, e.g., during repeated boost immunizations.

Adenoviral vectors coated with cationic PEG derivatives for intravaginal vaccination against HIV-1

Mucus layer coating the vaginal epithelium represents a barrier for intravaginally delivered recombined adenoviral (rAd) vectors, but it could be overcome by proper polyethylene glycol (PEG) modification. Here we synthesized two cationic PEG derivatives, amino-(EO)n/(AGE)m-Cyss (APCs). The polymers contained neutral linear PEG (2-5 kDa) to provide a hydrophilic surface and amine pendants to provide positive charge for coating negatively charged rAd by physical adsorption. Given proper molecular composition, the polymer (5k-APC) could coat rAd without causing aggregation, facilitating its mucus penetrating ability and enhancing gene expression both in vitro and in vivo. With HIVgag as the model antigen, the polymer-rAd complexes were administered intravaginally to elicit both systemic and mucosal immune responses. 5k-APC-rAd immunization elicited robust HIVgag-specific cellular responses and also induced higher antigen-specific serum IgG. More importantly, mice immunized with 5k-APC-rAd showed higher level of IgA in vaginal lavage fluid. These findings suggest that 5k-APC-rAd is a promising system for intravaginal immunization against infectious diseases such as HIV within the vaginal tract.

A Rev-Independent gag/pol Eliminates Detectable psi-gag Recombination in Lentiviral Vectors

Lentiviral vectors (LVs) are being developed for clinical use in humans for applications including gene therapy and immunotherapy. A safety concern for use of LVs in humans is the generation of replication-competent lentivirus (RCL), which may arise due to recombination between the split genomes of third-generation LVs. Although no RCL has been detected to date, design optimizations that minimize recombination events between split genome vectors would provide an added safety benefit that may further reduce the risk of RCL formation. Here we describe design elements introduced to the gag/pol plasmid with the intention of eliminating psi-gag recombination between the vector genome and gag/pol. These design changes, consisting of codon optimization of the gag/pol sequence and the deletion of the Rev-responsive element, abrogate the requirement for Rev in expression of Gag protein, thus the resulting gag/pol construct being Rev independent (RI gag/pol). We show that generating vector using the RI gag/pol construct has no effect on particle production or transduction titers. The RI and wild-type gag/pol vectors function equivalently as antigen-specific immunotherapy, potently inducing antigen-specific CD8 T cells that protect against challenge with vaccinia virus. Most importantly, the designed RI gag/pol eliminated detectable psi-gag recombination. Interestingly, we detected recombination between the vector genome and gag/pol from regions without sequence homology. Our findings imply that although unpredictable recombination events may still occur, the RI gag/pol design is sufficient to prevent psi-gag recombination.

Lentiviral vector-based therapy in head and neck cancer (Review)

Head and neck squamous cell carcinoma (HNSCC) is the sixth most common neoplasm worldwide. Despite advances in multimodality treatments involving surgery, radiation and chemotherapy, the five-year survival rate has remained at ~50% for the past 35 years. Therefore, the early detection of recurrent or persistent disease is extremely important. Replication-incompetent HIV-1-based lentiviral vectors have emerged as powerful and safe tools for gene delivery. Commonly, HNSCC is a locoregional disease that presents at or close to the body surface. Thus, HNSCC is amendable to intratumoral injections of gene therapy vectors aimed at correcting defects associated with tumor suppressor genes to induce the direct cytotoxicity of cancer cells or immune modulation to promote antitumor immunity. Current investigations analyzing HNSCC gene mutations and stem cell markers and the cancer immunoediting concept are creating exciting therapeutic opportunities for lentiviral and other gene transfer vectors. The present review reports specific examples of the current applications of lentiviral vectors in HNSCC.

A tetracycline-regulated cell line produces high-titer lentiviral vectors that specifically target dendritic cells

Lentiviral vectors (LVs) are a powerful means of delivering genetic material to many types of cells. Because of safety concerns associated with these HIV-1 derived vectors, producing large quantities of LVs is challenging. In this paper, we report a method for producing high titers of self-inactivating LVs. We retrovirally transduce the tet-off stable producer cell line GPR to generate a cell line, GPRS, which can express all the viral components, including a dendritic cell-specific glycoprotein, SVGmu. Then, we use concatemeric DNA transfection to transfect the LV transfer plasmid encoding a reporter gene GFP in combination with a selectable marker. Several of the resulting clones can produce LV at a titer 10-fold greater than what we achieve with transient transfection. Plus, these viruses efficiently transduce dendritic cells in vitro and generate a strong T cell immune response to our reporter antigen. This method may be a good option for producing strong LV-based vaccines for clinical studies of cancer or infectious diseases.

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.

Antibody-mimetic ligand selected by mRNA display targets DC-SIGN for dendritic cell-directed antigen delivery

Dendritic cell (DC)-based vaccines have shown promise as an immunotherapeutic modality for cancer and infectious diseases in many preclinical studies and clinical trials. Provenge (sipuleucel-T), a DC-based vaccine based on ex vivo-generated autologous DCs loaded with antigens, has recently received FDA approval for prostate cancer treatment, further validating the potential of DC-based vaccine modalities. However, direct antigen delivery to DCs in vivo via DC-specific surface receptors would enable a more direct and less laborious approach to immunization. In this study, the recombinant extracellular domains (ECD) of human and mouse DC-SIGN (hDC-SIGN and mDC-SIGN) were generated as DC-specific targets for mRNA display. Accordingly, an antibody-mimetic library was constructed by randomizing two exposed binding loops of an expression-enhanced 10th human fibronectin type III domain (e10Fn3). After three rounds of selection against mDC-SIGN, followed by four rounds of selection against hDC-SIGN, we were able to evolve several dual-specific ligands, which could bind to both soluble ECD of human and mouse DC-SIGNs. Using a cell-binding assay, one ligand, eFn-DC6, was found to have high affinity to hDC-SIGN and moderate affinity to mDC-SIGN. When fused with an antigenic peptide, eFn-DC6 could direct the antigen delivery and presentation by human peripheral blood mononuclear cell (PBMC)-derived DCs and stimulate antigen-specific CD8(+) T cells to secrete inflammatory cytokines. Taken together, these results demonstrate the utility of mRNA display to select protein carriers for DC-based vaccination and offer in vitro evidence that the antibody-mimetic ligand eFn-DC6 represents a promising candidate for the development of an in vivo DC-based vaccine in humans.

PD1-based DNA vaccine amplifies HIV-1 GAG-specific CD8+ T cells in mice

Viral vector-based vaccines that induce protective CD8+ T cell immunity can prevent or control pathogenic SIV infections, but issues of preexisting immunity and safety have impeded their implementation in HIV-1. Here, we report the development of what we believe to be a novel antigen-targeting DNA vaccine strategy that exploits the binding of programmed death-1 (PD1) to its ligands expressed on dendritic cells (DCs) by fusing soluble PD1 with HIV-1 GAG p24 antigen. As compared with non-DC-targeting vaccines, intramuscular immunization via electroporation (EP) of the fusion DNA in mice elicited consistently high frequencies of GAG-specific, broadly reactive, polyfunctional, long-lived, and cytotoxic CD8+ T cells and robust anti-GAG antibody titers. Vaccination conferred remarkable protection against mucosal challenge with vaccinia GAG viruses. Soluble PD1-based vaccination potentiated CD8+ T cell responses by enhancing antigen binding and uptake in DCs and activation in the draining lymph node. It also increased IL-12-producing DCs and engaged antigen cross-presentation when compared with anti-DEC205 antibody-mediated DC targeting. The high frequency of durable and protective GAG-specific CD8+ T cell immunity induced by soluble PD1-based vaccination suggests that PD1-based DNA vaccines could potentially be used against HIV-1 and other pathogens.

Potentiating functional antigen-specific CD8⁺ T cell immunity by a novel PD1 isoform-based fusion DNA vaccine

Understanding and identifying new ways of mounting an effective CD8⁺ T cell immune response is important for eliminating infectious pathogens. Although upregulated programmed death-1 (PD1) in chronic infections (such as HIV-1 and tuberculosis) impedes T cell responses, blocking this PD1/PD-L pathway could functionally rescue the "exhausted" T cells. However, there exists a number of PD1 spliced variants with unknown biological function. Here, we identified a new isoform of human PD1 (Δ42PD1) that contains a 42-nucleotide in-frame deletion located at exon 2 domain found expressed in peripheral blood mononuclear cells (PBMCs). Δ42PD1 appears to function distinctly from PD1, as it does not engage PD-L1/PD-L2 but its recombinant form could induce proinflammatory cytokines. We utilized Δ42PD1 as an intramolecular adjuvant to develop a fusion DNA vaccine with HIV-1 Gag p24 antigen to immunize mice, which elicited a significantly enhanced level of anti-p24 IgG1/IgG2a antibody titers, and important p24-specific and tetramer⁺CD8⁺ T cells responses that lasted for ≥7.5 months. Furthermore, p24-specific CD8⁺ T cells remain functionally improved in proliferative and cytolytic capacities. Importantly, the enhanced antigen-specific immunity protected mice against pathogenic viral challenge and tumor growth. Thus, this newly identified PD1 variant (Δ42PD1) amplifies the generation of antigen-specific CD8⁺ T cell immunity when used in a DNA vaccine.

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.

Lentiviral vectors: basic to translational

More than two decades have passed since genetically modified HIV was used for gene delivery. Through continuous improvements these early marker gene-carrying HIVs have evolved into safer and more effective lentiviral vectors. Lentiviral vectors offer several attractive properties as gene-delivery vehicles, including: (i) sustained gene delivery through stable vector integration into host genome; (ii) the capability of infecting both dividing and non-dividing cells; (iii) broad tissue tropisms, including important gene- and cell-therapy-target cell types; (iv) no expression of viral proteins after vector transduction; (v) the ability to deliver complex genetic elements, such as polycistronic or intron-containing sequences; (vi) potentially safer integration site profile; and (vii) a relatively easy system for vector manipulation and production. Accordingly, lentivector technologies now have widespread use in basic biology and translational studies for stable transgene overexpression, persistent gene silencing, immunization, in vivo imaging, generating transgenic animals, induction of pluripotent cells, stem cell modification and lineage tracking, or site-directed gene editing. Moreover, in the present high-throughput '-omics' era, the commercial availability of premade lentiviral vectors, which are engineered to express or silence genome-wide genes, accelerates the rapid expansion of this vector technology. In the present review, we assess the advances in lentiviral vector technology, including basic lentivirology, vector designs for improved efficiency and biosafety, protocols for vector production and infection, targeted gene delivery, advanced lentiviral applications and issues associated with the vector system.

PD-1/PD-L1 blockade can enhance HIV-1 Gag-specific T cell immunity elicited by dendritic cell-directed lentiviral vaccines

Exhaustion of CD8(+) T cells and upregulation of programmed death 1 (PD-1), a negative regulator of T cell activation, are characteristic features of individuals chronically infected with human immunodeficiency virus type 1. In a previous study, we showed in mice that a dendritic cell-directed lentiviral vector (DCLV) system encoding the human immunodeficiency virus (HIV)-1 Gag protein was an efficient vaccine modality to induce a durable Gag-specific T cell immune response. In this study, we demonstrate that blocking of the PD-1/PD-1 ligand (PD-L) inhibitory signal via an anti-PD-L1 antibody generated an enhanced HIV-1 Gag-specific CD8(+) immune response following both a single round of DCLV immunization and a homologous prime/boost regimen. The prime/boost regimen combined with PD-L1 blockade generated very high levels of Gag-specific CD8(+) T cells comprising several valuable features: improved ability to produce multiple cytokines, responding to a broader range of Gag-derived epitopes, and long-lasting memory. This enhanced cellular immune response generated by DCLV immunization combined with anti-PD-L1 blockade correlated with improved viral control following challenge with Gag-expressing vaccinia virus. Taken together, our studies offer evidence to support the use of PD-1/PD-L1 blockade as an adjuvant modality to enhance antigen-specific immune responses elicited by T cell-based immunizations such as DCLV.

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.

Construction of stable producer cells to make high-titer lentiviral vectors for dendritic cell-based vaccination

Lentiviral vectors (LVs) enveloped with an engineered Sindbis virus glycoprotein can specifically bind to dendritic cells (DCs) through the surface receptor DC-SIGN and induce antigen expression, thus providing an efficient method for delivering DC-directed vaccines. In this study, we constructed a stable producer line (LV-MGFP) for synthesizing DC-SIGN-targeted HIV-1-based LVs (DC-LVs) encoding green fluorescent protein (GFP) by a concatemeric array transfection technique. We demonstrated that the established stable clones could routinely produce vector supernatants with titers above 10(7) transduction units per milliliter (TU/mL) during a continuous 3-month cell passage. The producer cells were also capable of generating similar titers of DC-LVs in serum-free medium. Moreover, the addition of 1-deoxymannojirimycin (DMJ) enabled the producer cells to manufacture DC-LVs with both improved titers and enhanced potency to evoke antigen-specific CD8(+) T cell responses in mice. The stable lines could accommodate the replacement of the internal murine stem cell virus (MSCV) promoter with the human ubiquitin-C (Ubi) promoter in the lentiviral backbone. The resulting DC-LVs bearing Ubi exhibited the enhanced potency to elicit vaccine-specific immunity. Based on accumulated evidence, our studies support the application of this production method in manufacturing DC-LVs for preclinical and clinical testing of novel DC-based immunization.

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.

Differential immune responses mediated by adenovirus- and lentivirus-transduced DCs in a HER-2/neu overexpressing tumor model

Recent investigations have demonstrated that adenoviral and lentiviral vectors encoding HER-2 can be utilized in cancer immunotherapy. However, it is not known whether both viral systems elicit a similar immune response. Here, we compare the immune response in mice induced by dendritic cells (DCs) infected with either recombinant adenovirus or lentivirus encoding rat HER-2 (rHER-2). Both vaccine types yielded similar control of tumor growth, but we found clear differences in their immune responses 10 days after DC immunization. Adenovirus rHER-2-transduced DCs elicited locally and systemically high frequencies of CD4+ and CD8+ T cells, while lentivirus rHER-2-transduced DCs predominantly led to CD4+ T-cell infiltration at the tumor site. Splenocytes from mice immunized with lentivirus rHER-2-transduced DCs secreted higher levels of interferon (IFN)-γ, mainly by CD4+ T cells, following stimulation by RM-1-mHER-2 tumors. In contrast, the adenovirus vaccinated group exhibited CD4+ and CD8+ T cells that both contributed to IFN-γ production. Besides an established cellular immune response, the rHER-2/DC vaccine elicited a significant humoral response that was highest in the adenovirus group. DC subsets and regulatory T cells in the spleen were also differentially modulated in the two vaccine systems. Finally, adoptive transfer of splenocytes from both groups of immunized mice strongly inhibited in vivo tumor growth. Our results suggest that not only the target antigen but also the virus system may determine the nature and magnitude of antitumor immunity by DC vaccination.

Lentiviral vectors targeted to MHC II are effective in immunization

Abstract vectors (LVs) that are targeted to APC using a chimeric measles virus (MV) hemagglutinin (H). The MV H protein is mutated to prevent binding to MV receptors and incorporates a single-chain antibody that recognizes murine major histocompatibility complex class II (MHC II). This targeted LV is highly efficient in transduction of freshly isolated mouse B cells and dendritic cells. MHC II-positive cells in spleen are transduced after intravenous injection, and a robust immune response to an antigen transgene is generated.

Dendritic cell-directed lentivector vaccine induces antigen-specific immune responses against murine melanoma

Lentivectors are potential vaccine delivery vehicles because they can efficiently transduce a variety of non-dividing cells, including antigen-presenting cells, and do not cause expression of extra viral proteins. To improve safety while retaining efficiency, a dendritic cell (DC)-specific lentivector was constructed by pseudotyping the vector with an engineered viral glycoprotein derived from Sindbis virus. We assessed the level of anti-tumor immunity conferred by this engineered lentivector encoding the melanoma antigen gp100 in a mouse model. Footpad injection of the engineered lentivectors results in the best antigen-specific immune response as compared with subcutaneous and intraperitoneal injections. A single prime vaccination of the engineered lentivectors can elicit a high frequency (up to 10%) of gp100-specific CD8(+) T cells in peripheral blood 3 weeks after the vaccination and this response will be maintained at around 5% for up to 8 weeks. We found that these engineered lentivectors elicited relatively low levels of anti-vector neutralizing antibody responses. Importantly, direct injection of this engineered lentivector inhibited the growth of aggressive B16 murine melanoma. These data suggest that DC-specific lentivectors can be a novel and alternative vaccine carrier with the potential to deliver effective anti-tumor immunity 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.

Emerging applications of lentiviral vectors in dendritic cell-based immunotherapy

Dendritic cells are professional antigen-presenting cells that initiate, regulate and shape the induction of specific immune responses. The ability to use dendritic cells in the induction of antigen-specific tolerance, antigen-specific immunity or specific differentiation of T-helper subsets holds great promise in dendritic cell-based immunotherapy of various diseases such as cancer, viral infections, allergy, as well as autoimmunity. Replication-incompetent HIV-1-based lentiviral vector is now emerging as a promising delivery system to genetically modify dendritic cells through antigen recognition, costimulatory molecules and/or polarization signals for the manipulation of antigen-specific immunity in vivo. This article discusses some of the recent advances in the uses of lentiviral vectors in dendritic cell-based immunotherapy.

Dendritic cells infected by recombinant rabies virus vaccine vector expressing HIV-1 Gag are immunogenic even in the presence of vector-specific immunity

Dendritic cells (DC) are the most potent antigen presenting cells whose ability to interact with T cells, B cells and NK cells has led to their extensive use in vaccine design. Here, we designed a DC-based HIV-1 vaccine using an attenuated rabies virus vector expressing HIV-1 Gag (RIDC-Gag). To test this, BALB/c mice were immunized with RIDC-Gag, and the primary, secondary as well as humoral immune responses were monitored. Our results indicate that RIDC-Gag stimulated HIV-1 Gag-specific immune responses in mice. When challenged with vaccinia virus (VV) expressing HIV-1 Gag, they elicited a potent Gag-specific recall response characterized by CD8+ T cells expressing multiple cytokines that were capable of specifically lysing Gag-pulsed target cells. Moreover, RIDC-Gag also enhanced CD8+ T cell responses via a homologous prime-boost regimen. These results show that a DC-based vaccine using live RV is immunogenic and a potential candidate for a therapeutic HIV-1 vaccine.

Vaccines delivered by integration-deficient lentiviral vectors targeting dendritic cells induces strong antigen-specific immunity

We report a study of an integration-deficient lentiviral vector (IDLV) enveloped with a Sindbis virus glycoprotein mutant (SVGmu) capable of selectively binding to dendritic cells (DCs) for its potential as a vaccine carrier. The in vitro assays showed that the D64V point mutation in the catalytic domain of HIV-1 integrase efficiently inhibited the integration of the transgene upon vector transduction, while the targeting specificity of the vector to preferentially transduce and mediate durable expression in DCs was maintained. Substantial immune responses in C57BL/6 mice and complete protection against a challenge with the C57BL/6 thymoma EG.7 tumor expressing a delivered ovalbumin (OVA) antigen in mice have been achieved through the direct injection of the DC-directed IDLV encoding OVA. Thus, this DC-directed IDLV system represents a promising and efficient vector platform with remarkably improved safety for the future development of DC-based immunotherapy.

Impairment of in vitro generation of monocyte-derived human dendritic cells by inactivated human immunodeficiency virus-1: Involvement of type I interferon produced from plasmacytoid dendritc cells

In an attempt to simplify the protocol of DC generation in vitro, studies conducted herein show that functional DCs could be generated from bulk peripheral blood mononuclear cells (PBMCs) in media containing GM-CSF and IL-4. Interestingly, when PBMCs, but not purified monocytes, were exposed to either CCR5- or CXCR4-tropic inactivated HIV-1 isolates (iHIV-1) at the initiation of the culture, DC yields were significantly reduced in a dose-dependent manner because of monocyte apoptosis. Similar impairment of DC generation was noted using type I IFNs and poly IC not only in cultures of PBMCs but also using highly enriched monocytes. This effect was reversed by antihuman type I IFN receptor, but not by anti-FasL, anti-TRAIL, anti-TNF, or a mixture of these antibodies. iHIV-1-exposed PBMCs, but not monocytes, produced high levels of IFN-alpha but not IFN-beta. PBMCs depleted of CD123(+) plasmacytoid DCs produced low levels of IFN-alpha and were resistant to iHIV-1-mediated DC impairment. Interestingly, exogenously added TNF reversed the impairment by iHIV-1 in the PBMC cultures. In conclusion, the present results indicate that iHIV-1 impairs the in vitro generation of functional DCs from PBMCs through the induction of IFN-alpha from plasmacytoid DCs in a CD4-dependent fashion in the absence of TNF.