Breaking Entry-and Species Barriers: LentiBOOST® Plus Polybrene Enhances Transduction Efficacy of Dendritic Cells and Monocytes by Adenovirus 5

Due to their ability to trigger strong immune responses, adenoviruses (HAdVs) in general and the serotype5 (HAdV-5) in particular are amongst the most popular viral vectors in research and clinical application. However, efficient transduction using HAdV-5 is predominantly achieved in coxsackie and adenovirus receptor (CAR)-positive cells. In the present study, we used the transduction enhancer LentiBOOST® comprising the polycationic Polybrene to overcome these limitations. Using LentiBOOST®/Polybrene, we yielded transduction rates higher than 50% in murine bone marrow-derived dendritic cells (BMDCs), while maintaining their cytokine expression profile and their capability to induce T-cell proliferation. In human dendritic cells (DCs), we increased the transduction rate from 22% in immature (i)DCs or 43% in mature (m)DCs to more than 80%, without inducing cytotoxicity. While expression of specific maturation markers was slightly upregulated using LentiBOOST®/Polybrene on iDCs, no effect on mDC phenotype or function was observed. Moreover, we achieved efficient HAdV5 transduction also in human monocytes and were able to subsequently differentiate them into proper iDCs and functional mDCs. In summary, we introduce LentiBOOST® comprising Polybrene as a highly potent adenoviral transduction agent for new in-vitro applications in a set of different immune cells in both mice and humans.


Introduction
A variety of gene delivery techniques have been established over the last decades, which can be subdivided into non-viral and viral strategies. The non-viral subclass comprises methods such assuch as the injection of naked DNA, electroporation, gene gun, sonoporation, magnetofection, and lipoplexes, while for the viral subclass lentivirus (LV), herpes simplex virus (HSV), baculovirus, adeno-associated virus (AAV), and human adenovirus (HAdV) have been used for gene transfer [1]. To date, recombinant HAdVs are the most widely used viral vectors for gene therapy, accounting for 18.6% of vectors used in gene and vaccine therapy clinical trials, especially in the context of different cancer types, HIV, tuberculosis, Ebola virus, malaria, influenza, and, lately, SARS-CoV-2 [1][2][3][4]. In contrast to most other viral vaccines, HAdVs trigger both a humoral response as well as a robust (SLBs) on a microcantilever surface confirm an interaction of the poloxamers F68 and F98 with the linear lipid bilayer, resulting in a stronger association of F98, which has a longer PPO core unit compared with F68, with the SLB. Moreover, the poloxamers inhibited lipid diffusion [27].
In the present study, we evaluated the ability of LentiBOOST ® mixed with Polybrene to enhance and optimize the transduction efficacy of murine immature (i)BMDCs and mature (m)BMDCs, as well as of human monocytes and immature (i)DCs and mature (m)DCs, with HAdV-5. For this purpose, we first assessed the optimal transduction conditions, with respect to the used multiplicity of infection (MOI) of the HAdV-5 vector, encoding for the green fluorescent protein (GFP), in combination with LentiBOOST ® /Polybrene, for iBMDCs, mBMDCs, iDCs and mDCs. Using this optimized transduction protocol, we analyzed transduced murine and human DCs further, regarding their survival, expression of typical cell surface markers, secretion of cytokines and their ability to prime naïve allogeneic T cells. Subsequently, we applied our HAdV-5 vector in combination with LentiBOOST ® /Polybrene also to human monocytes, where we observed high transduction rates. Notably, subsequently we were still able to differentiate these transduced monocytes into iDCs and mDCs, displaying a typical phenotype and function, since these generated mDCs were not only able to prime naïve T cells in a mixed lymphocyte reaction (MLR), but could also be electroporated with a mRNA coding for a tumor antigen, or loaded with peptide to induce a tumor antigen-specific T-cell response in vitro.

Mice
C57BL/6 and BALB/c mice were purchased from Charles River/Wiga (Sulzfeld, Germany) and maintained under pathogen-free conditions according to the European Communities Council Directive (86/609/EEC).

Generation of Murine Bone Marrow-Derived DCs
Bone marrow-derived DCs from C57BL/6 mice were generated as described previously [28] and finally resuspended at a density of 2 × 10 6 cells in 10-mL R10 medium consisting of RPMI1640, 1% penicillin/streptomycin/L-glutamine, 2-ME and 10% heatinactivated FBS (GE Healthcare, Chicago, Illinois, United States), additionally supplemented with GM-CSF supernatant (1:10) from a cell line stably transfected with the murine GM-CSF [29]. At days 3 and 6, 10 mL of fresh R10 supplemented with GM-CSF supernatant (1:10) was added, with removing 50% of the old cell culture supernatant at day 6 before. Maturation of BMDCs was induced at day 7 by the addition of 0.1 ng/mL LPS for 20 h. At day 8, cells were used for further experiments.

Generation of Human Monocytes, Monocyte-Derived DCs and T Cells
Human peripheral blood mononucleated cells (PBMCs) of healthy donors were isolated from leucocyte reduction system chambers (LRSCs) using density centrifugation as described previously [30]. For the generation of monocytes, cells were seeded in DC medium consisting of RPMI1640 + 1% penicillin/streptomycin/glutamate + 1% heatinactivated AB serum + 1% HEPES after removing the non-adherent fraction (NAF). NAF was cryopreserved and stored at −80 • C for isolation of T cells for allogeneic MLR or for antigen-specific T-cell priming.

Recombinant Adenoviruses
HAdV-5-Luc1 and HAdV-5-GFP are first-generation clones, E1-and E3-deleted replicationdeficient adenoviral vectors. HAdV-5-Luc1 contains a CMV-firefly luciferase cassette and was kindly provided by D. T. Curiel from the Washington University School of Medicine, MO, USA. HAdV-5-GFP was cloned as follows: a gene cassette containing a CMV-GFP sequence was inserted into pShuttle. Virus genomes were obtained by homologous recombination of the corresponding shuttle plasmids indicated above with pAdEasy-1 in E. coli BJ5183, as described before [31]. Adenovirus particles were produced by transfection of the different PacI-digested pAd vectors into 293 cells using Lipofectamine 2000. All viruses were amplified in 293 cells and purified by two rounds of CsCl density gradient ultracentrifugation. Verification of viral genomes and exclusion of wild-type contamination were performed by PCR. Physical particle concentration (viral particles (vp)/mL) was determined by OD260 reading. Therefore, a serial dilution of virus stock and viral lysis buffer consisting of 10 mM TE and 0.5% SDS was prepared and incubated at 56 • C for 10 min. To determine the OD260, the dilution factor was multiplied with the subtraction of the absorbance at 320 nm from the absorbance of 260 nm. The mean of three consecutive absorptions was then used as OD260. In addition, Infectious particle concentration was determined by TCID50 assay on 293 cells. Measurement of particle concentration for the HAdV-5-Luc1 vector resulted in an OD of 2.06 × 10 12 vp/mL and an TCID50/mL of 3.5 × 10 11 , leading to a ratio of 5.9. Concentration of the HAdV-5-GFP vector was identified as 1.22 × 10 12 vp/mL using OD260 and 7.1 × 10 10 TCID50/mL resulting in a ratio of 17.2. For experiments, TCID50/mL was used as a basis to calculate the multiplicity of infection.

Adenoviral Transduction
Human monocytes were transduced in either 450 µL (six-well plate)/3 mL (cell culture dish) DC medium (for monocytes) or DC medium supplemented with either 1600 U/mL GM-CSF and 500 U/mL IL-4 (for iDCs). Virus suspension was prepared using 0.5 mg/mL LentiBOOST ® + 4 µg/mL Polybrene (Sirion Biotech) or buffer (PBS) only, according to the manufacturer's instructions. Adenovirus was added to the cells at a MOI of 100 or 200 in a volume of 450 µL (six-well plate) or 3 mL (cell culture dish) resulting in a final infection volume of 900 µL or 6 mL, respectively. After 1.5 h of incubation at room temperature on a rocker and 2.5 h at 37 • C/5% CO 2 , 3 mL (six-well)/18 mL (cell culture dish) of DC medium only (for monocytes) or replenished with cytokines (for iDCs) as described before was added per well. Transduced monocytes were either analyzed by flow cytometry 24 h post infection or were differentiated to day 4 iDCs or day 5 mDCs, as described before, and then used for further experiments.
For adenoviral transduction of human DCs, 5 × 10 5 iDCs or mDCs were seeded into a 24-well plate in a volume of 125 µL DC medium supplemented with either 1600 U/mL GM-CSF and 500 U/mL IL-4 (for iDCs) or the maturation cocktail consisting of 400 U/mL IL-1β, 2000 U/mL IL-6, 20 ng/mL TNF-α, and 2 µg/mL PGE2, in addition to IL-4 and GM CSF (for mDCs). Virus suspension was prepared as described before. Virus dilutions were performed in DC medium without additives at different MOIs and added to the cells to a final volume of 250 µL. Afterwards cells were incubated as described above, before 1.5 mL of DC medium replenished with cytokines, was added. Cells were used for further experiments 48 h after transduction.
Immature and LPS-matured murine BMDCs were seeded at a density of 5 × 10 5 cells/well in a 24-well plate in 125 µL of R10 medium. Virus dilutions at different MOIs from 40 to 500 were prepared as described above, using 1 mg/mL LentiBOOST ® and 8 µg/mL Polybrene or PBS only, as described by the manufacturer. After adding 125 µL/well virus suspension, BMDCs were incubated as described above, before 1.5 mL of R10 medium was added. Further analyses of transduced BMDCs were performed 48 h afterwards.

Cell Imaging
Cell imaging was performed using the EVOS FL Fluorescence Microscope according to the manufacturer's instructions.

Dimethylthiazol-Diphenyltetrazolium (MTT) Assay
To perform MTT assays, cells were harvested 48 h after adenoviral transduction. Afterwards, 2 × 10 4 cells per well were seeded in a 96-well flat-bottom plate. Murine BMDCs were cultivated in 100 µL of R10 medium. Human monocytes and monocytederived DCs were cultivated in 100 µL DC medium or DC medium complemented with IL-4 and GM-CSF, respectively, as described before. Cells were incubated for 5 h after adding 10 µL of MTT solution (10 µg/µL in PBS) per well. Formazan crystals were solubilized by adding 100 µL 10% SDS in 0.01 N HCl overnight. Absorbance was measured in duplicates at 570 nm using a Wallac Victor 2 1420 Multilabel Counter (Perkin Elmer, Waltham, MA, USA).

Mixed Lymphocyte Reaction (MLR)
For MLR assays using human matured DCs, cells were co-cultured at different ratios with 2 × 10 6 allogeneic human T cells derived from NAF. Co-cultures were incubated in 96-well flat bottom cell culture plates for 72 h in 200 µL of DC medium. For murine mature BMDCs, titrated numbers of cells were co-cultured in 96-well flat bottom cell culture plates with 4 × 10 5 BALB/c derived spleen cells for 72 h in 200 µL of R10 medium. As a positive control, human T cells or murine splenic cells only were incubated together with CD3/CD28 Dynabeads in 200 µL of corresponding cell culture medium, whereas NAF only was regarded as negative control. After 3 days, cell-free supernatants of DC-T cell co-cultures were collected and stored at −20 • C for further analyses. Cells were pulsed with 3H-thymidine (1 µC/well; PerkinElmer) for 16-20 h to determine T-cell proliferation. Thymidine incorporation was measured using a Wallac 1420 Victor2 Microplate Reader (PerkinElmer).

Cytometric Bead Array
Cell culture supernatants were analyzed using the LEGENDplexTM Human Inflammation Panel 1, or -HU Th Cytokine Panel, or -Mouse Inflammation panel, or -MU Th Cytokine Panel (all BioLegend) according to the manufacturer's instructions.

In-Vitro RNA Transcription and Electroporation of Human DCs
In-vitro transcription of mRNA was performed using the mMESSAGE mMACHINE™ T7 ULTRA Transcription Kit and purified with an RNeasy Kit, according to the manufacturers' protocols. DCs derived from HLA-A2 positive healthy donors were electroporated with 5 µg MelanA mRNA, as described before [33]. As a control, mDCs were electroporated without mRNA as described by Gerer et al. [33].

Statistical Analysis
Statistical calculations were performed using GraphPad Prism 8 software.

LentiBOOST ® in Combination with Polybrene Enables Transduction of Murine BMDCs with HAdV-5
To overcome the low transduction efficacy due to the lack of CAR expression on target cells, or the species-specificity of the HAdV serotype, we examined the transduction agent LentiBOOST ® combined with Polybrene in the context of HAdV transduction, of human and murine primary cells. Therefore, we first transduced murine immature and mature BMDCs with a HAdV-5 coding for GFP at MOIs from 40 to 500, either in the presence or absence of LentiBOOST ® /Polybrene (LeB/PB; Figure 1). As a negative control, cells were either mock-transduced or transduced with a HAdV-5, encoding for the Renilla firefly luciferase (HAdV-5-Luc1). As assessed by flow cytometry 48 h afterwards, percentage of GFP+ iBMDCs increased from 2% to 9% (MOI40), 3% to 27% (MOI100), 5% to 42% (MOI200) and 11% to 54% (MOI500), using LeB/PB ( Figure 1a). Complementing higher transduction rates signal intensity was also increased by using LeB/PB (see Supplementary Figure S1a  In summary, LentiBOOST ® combined with Polybrene breaks the species-specific HAdV-5 infection barriers of murine immature and mature BMDCs without toxic side effects. As MOIs 200 and 500 of HAdV-5-GFP were shown to be most efficient to transduce immature and mature BMDCs, these were used for the following experiments.

LentiBOOST ® /Polybrene Does Not Alter the Phenotype and Function of Murine BMDCs
As DCs rapidly react on environmental changes, we next investigated the influence of the LentiBOOST ® /Polybrene regarding the phenotype and function of immature and mature BMDCs, 48 h after transduction with HAdV-5-Luc1 (MOI500) and HAdV-5-GFP (MOI200 and MOI500), in comparison to cells inoculated with adenovirus alone. To address the functionality of these differentially treated mBMDCs, mixed lymphocyte reactions (MLR) were performed (Figure 3a,b). Here, LentiBOOST ® /Polybrene ("Mock" + LeB/PB) or in combination with HAdV-5-Luc1 (MOI 200 and MOI 500), did not affect the capability of mBMDCs to prime allogeneic T cells. By contrast, transduction of mBMDCs with HAdV-5-GFP in the presence of LentiBOOST ® /Polybrene led to a stronger proliferation of T cells at a DC:T cell ratio of 1:10 (1.3 fold MOI200/1.7 fold MOI500) and 1:33 (1.5 fold MOI200/1.7 fold MOI500). This effect was not observed for HAdV-5-GFP transduced BMDCs missing the LeB/PB. Next, we examined the portfolio of classical T-cell-derived cytokines in the supernatants of the co-cultures. We observed no differences between BMDCs transduced in the absence (−LeB/PB) or presence (+LeB/PB), to induce classical Th1/Th2 associated cytokines IL-2 and IL-6 (   Taken together, phenotype and function of BMDCs are not altered by LentiBOOST ® / Polybrene, although a slight increase (i) in the secretion of pro-inflammatory cytokines by iBMDCs, and (ii) the capability of mBMDCs to prime T cells was observed, partially depending on the transgene encoded by the adenovirus.

LentiBOOST ® /Polybrene Induces Maturation of Immature Human DCs but Does Not Alter DC Function
Next, we analyzed if LentiBOOST ® /Polybrene and the observed enhanced infection rate influence the phenotype and function of human DCs. Thus, iDCs and mDCs, either mock-treated, or transduced with HAdV-5-Luc1 (MOI200), or HAdV-5-GFP (MOI 100 and 200), were analyzed by flow cytometry, 48 h post infection. In contrast to murine iBMDCs, human iDCs exposed to LentiBOOST ® /Polybrene showed a significant upregulation of maturation markers CD83 and CD86, while CD25, CD80 and HLA-DR were not statistically altered (Figure 5a). For HAdV-5-GFP the median was even higher when compared with mock-or HAdV-5-Luc1 controls. Interestingly, we observed almost no differences regarding the cytokine production, except for IL-8, which was clearly upregulated in the presence of the LentiBOOST ® /Polybrene (see Supplementary Figure S8a online). However, inoculation of mDCs with LentiBOOST ® /Polybrene did not alter their expression profiles in comparison to control mDCs (Figure 5b), nor changed their cytokine production (see Supplementary Figure S2b online). Moreover, median fluorescence intensity was strongly increased for all analyzed cell surface markers, including CD80, CD83, CD86 and HLA-DR, when compared with LentiBOOST ® /Polybrene treated iDCs.
Next, to analyze the mDCs-mediated allogeneic T-cell proliferation, MLR assays were performed. Results revealed that neither LentiBOOST ® /Polybrene nor the amount of adenovirus used influenced the T-cell stimulatory capacity of mDCs (Figure 6a,b). When analyzing the supernatants derived from the DC-T-cell co-cultures for their content of classical T-cell-derived cytokines, such as IL-2, IL-6, IFN-γ, IL-5, IL-13 and IL-22, no differences between +LeB/PB and -LeB/PB were observed (Figure 6c-f). However, cocultures of T cells with HAdV-5-GFP-transduced mDCs showed reduced levels of IL-5, IL-6, IL-13 and IL-22, independent of the LentiBOOST ® /Polybrene.

Efficient HAdV-5-Mediated Gene Transfer into Monocytes by LentiBOOST ® /Polybrene Allows for Proper Subsequent iDCs and mDCs Differentiation
Similar to murine BMDCs, human monocytes are poorly permissive to HAdV-5, even at high titers. Hence, we transduced freshly isolated human monocytes (see Supplementary Figure S9a online) with HAdV-5-GFP at a MOI of 100 or 200, either in the absence or presence of LentiBOOST ® /Polybrene. Twenty-four hours afterwards, cells were analyzed by flow cytometry. As depicted in Figure 7a, the transduction efficacy increased from 7% to 36% (MOI100) and 15% to 36% (MOI200), when applying LentiBOOST ® /Polybrene to the cell culture. Notably, mean GFP expression was upregulated from 227 to 3591 (MOI100) and 588 to 4264 (MOI200) (see Supplementary Figure S10  Next, mDCs were generated from these iDCs by adding a maturation cocktail, consisting of IL-1β, IL-6, PGE2 and TNF-α for another 24 h. While GFP expression and survival of those cells was comparable with iDCs shown before (Figure 7d), matured DCs highly upregulated typical DC maturation markers, such as CD25, CD80, CD83, CD86 and HLA-DR (Figure 7e). Importantly, the LentiBOOST ® /Polybrene did not affect this maturation process. However, exposure of monocytes to HAdV-5-GFP alone led to a 2.6-(MOI100) or 2.4 (MOI200)-fold increased expression of CD25 on mDCs, which was not observed when LeB/PB was present during transduction. Moreover, transduction with HAdV-5-GFP affected upregulation of CD83 in a virus concentration dependent manner, which in contrast to CD25 was independent of the usage of LentiBOOST ® /Polybrene. Interestingly, in the supernatants of mock-treated cells, without LeB/PB, we found strongly increased IL-6, TNF-α and MCP-1 concentrations (see Supplementary Figure S12c online).
Finally, we investigated the function of these mDCs, using an allogeneic MLR assay and in addition, we used them to prime autologous T cells, in a tumor-antigen-specific manner ( Figure 8). Regarding MLR assays, mock-and HAdV-5-GFP-treated monocytes −/+ LeB/PB, differentiated into mDCs were co-cultured with allogeneic T cells at different ratios, before T-cell proliferation was assessed by thymidine incorporation (Figure 8a). Although we observed a HAdV-5-GFP-and virus concentration-mediated effect regarding the capacity of mDCs to prime naïve T cells, no LentiBOOST ® /Polybrene dependent alterations were observed. In addition, transduced cells (−/+LeB/PB), were still able to induce a high and efficient T-cell proliferation accompanied by an unaltered IL-2 secretion (Figure 8a,b).
To prime autologous T cells, mDCs were additionally electroporated with RNA coding for the tumor antigen MelanA (MelA) or loaded with a MelA-specific peptide. Survival of DCs four hours after electroporation varied from 40% to 60% (Figure 8c). Next, MelApeptide-loaded, MelA RNA or control electroporated ("no RNA") DCs, were used to stimulate bulk autologous CD8+ T cells for one week, before the fraction of MelA-specific T cells was determined by tetramer staining. As shown in Figure 8d and Figure S5 (see Supplementary Information online), MelA-electroporated and MelA-peptide-loaded DCs induced antigen-specific CD8+ T cells, compared with control DCs ("no RNA"), even when they had been transduced with HAdV-5 and treated with LentiBOOST ® /Polybrene before. Noteworthy, the numbers of MelA+ CD8+ T cells vary in a donor-dependent manner (see Supplementary Figure S5 online), which is rather common when cells derived from different healthy donors are used. However, these data clearly indicate that LentiBOOST ® /Polybrene does not impair the capability of DCs to express, process and present a tumor-specific antigen in an MHC-class I-restricted manner.  In summary, using LentiBOOST ® /Polybrene not only allows for the highly efficient transduction of human monocytes with HAdV-5, but additionally for the differentiation into iDCs and functional mDCs.

Discussion
Using directed viral gene transfer to treat human diseases, gene therapy holds the potential to revolutionize medicine [1]. Currently, HAdV vectors attract tremendous attention in the context of newly developed vaccines against SARS-CoV-2. Amongst those that have been already applied and proven safe and highly effective, some are based on a chimpanzee adenovirus (AZD1222, Jenner Institute/AstraZeneca/University of Oxford), or on human adenovirus types 5 and -26 (Ad5-nCoV, Cansino Biologics/Beijing Institute of Biotechnology; JNJ-78436735 [Ad26]; Janssen Pharmaceutical Companies of Johnson and Johnson) [35]. However, usage of HAdV-5 vectors is limited regarding the transduction of cells lacking appropriate receptors on their cell surface (e.g., monocytes) or speciesspecificity (e.g., murine cells such as BMDCs) [19,36]. In the past, several techniques have been developed to promote virus-mediated gene delivery comprising (i) physical methods, (ii) genetic bioengineering of viruses and (iii) chemical methods, namely material additives [37]. Amongst these, polymers are the most extensively studied delivery systems for viral vectors such as HAdV-19 to increase the efficiency of HAdV-mediated gene transfer into epithelial and endothelial as well as mesenchymal stem cells [38,39]. Besides others, the cationic polymer polybrene is a common enhancer of viral delivery in vitro due to its cost efficiency and its simple and safe handling [37,40]. Poloxamers, on the other hand, are commercially available, FDA-approved, thermoresponsive triblock copolymers, consisting of two blocks of hydrophilic poly(ethylene oxide) (PEO) and one block of hydrophobic poly(propylene oxide) (PPO) [37]. Poloxamer 338 (PEO141-PPO44-PEO141) has been described to efficiently enhance LV delivery in T cells 19. In combination with polybrene, Höfig and colleagues reported a further elevated transduction efficacy, which was explained by the distinct modes of each adjuvant, polybrene-compensating electrostatic repulsion and poloxamer 338 fluidization of the membrane [37,41]. LentiBOOST ® /Polybrene by Sirion Biotech is such a two-component transduction enhancer and has already been used to efficiently promote LV transduction in vitro to generate melanoma-specific human T cells for cancer immunotherapy [42]. In the present study, we used LentiBOOST ® /Polybrene to transduce not only murine BMDCs but also human monocyte-derived DCs as well as monocytes efficiently with HAdV-5 in vitro to overcome the barriers of receptor dependency and species specificity. In accordance with previous published data, we report here that, even at high virus titers (MOI 200 and MOI 500), transduction efficacy for iB-MDCs reached only approx. 5% to 11%, and 13% and 24%, for mBMDCs, respectively. While alternative HAdV receptors VCAM-1 and SR-A are constitutively expressed by bone marrow-derived cells [43,44], MHC class molecules are upregulated upon maturation of BMDCs, which might contribute to the higher infection rate. Nevertheless, a much higher transduction efficacy is the prerequisite of success for most in-vitro and in-vivo approaches. Using LentiBOOST ® /Polybrene, transduction efficacy increased to 54% for immature and mature BMDCs, without any serious toxicity, change in phenotype and function of these cells. Interestingly, in comparison to HAdV-5-Luc1 the HAdV vector containing the green fluorescent protein (GFP) induced maturation of iBMDCs independent of the LentiBOOST ® /Polybrene, while for mBMDCs no differences in the maturation status were observed. Moreover, GFP-transduced cells showed a higher capacity to induce T-cell proliferation. GFP was detected in 1961 as a by-product of the extraction of aequorin from the Auquorea victoria jellyfish [45], and since then it has been developed further and is still widely used to label cells or to track gene expression. Of notice, several papers described an immunogenic effect of GFP, being slightly immunogenic in C57BL/6 mice and exhibiting a higher immunogenicity in BALB/c mice [46][47][48]. However, a later study postulated that when used as an intracellular gene reporter, GFP is processed in the host cell followed by presentation at the cell's surface via MHC class I molecules, leading to an antigen-specific recognition by cytotoxic T-lymphocyte [48]. Similarly, in human DCs, GFP expression was shown to act as an "adjuvant" to enhance T-cell immunity to the melanoma tumor antigen MART1 and to expand multiple CD4+ and CD8+ T-cell clones [49]. For murine DCs transduced with HAdV-5-LucI, encoding for the luciferase gene, no induction of maturation in iBMDCs was observed, despite Limberis et al. describing an immunogenic response of C57BL/6 mice towards luciferase in vivo [50].
Within in this study, human iDCs transduced with HAdV-5-GFP showed only an enhanced expression of maturation markers CD80, CD83, CD86 and HLA-DR in comparison to control cells, whereas no differences were observed for mDCs. Moreover, neither the transfer of GFP into mDCs, nor the use of LentiBOOST ® /Polybrene resulted in an altered capability of mDCs to stimulate T cells. Merely T cells co-cultivated with HAdV-5-GFP-transduced mDCs produced less Th2-cytokines IL-5, IL-6 and IL-13, thereby in part supporting the postulated immunogenicity of GFP for human DCs reported by Francesca Re and colleagues [49].
Although Polybrene has been used to improve LV-mediated gene delivery to murine and human DCs, to the best of our knowledge no publication addressed the maturation status of DCs treated with Polybrene only [51,52]. This applies also to the LentiBOOST ® , the Poloxamer 338, which in this study has been used for the first time in combination with Polybrene to improve HAdV-mediated gene transfer into DCs and monocytes. Notably, LentiBOOST ® /Polybrene increased HAdV-mediated GFP gene transfer into iDCs approximately four-fold (MOI 100) and two-fold (MOI 200), to more than 80% GFP positive cells. Interestingly, this high infection rate is not accompanied by a maturation signal as confirmed by the mock-and HAdV5Luc1 controls.
In a last set of experiments, we efficiently transduced human monocytes with HAdV-5 followed by proper differentiation of these cells into immature and mature DCs. Usually, Adenoviruses infect human CD14+ monocytes ineffectively, mainly due to the lack of appropriate expression of integrins αvβ3 and αvβ5 [53], which are the receptors for HAdV-5 on these cells [54,55]. Using LentiBOOST ® /Polybrene in combination with HAdV-5-GFP increased the number of GFP+ monocytes 5.3-fold (MOI 100) and 2.4-fold (MOI 200) to 36.4 percent. Importantly, this GFP expression was stable during differentiation of monocytes to iDCs and mDCs, thereby allowing for a sustained expression of the delivered gene of interest in transduced DCs.
Finally, we analyzed these transduced monocytes differentiated to mDCs with respect to their functionality. Compared with non-transduced cells, these mDCs retained their full capacity to prime allogeneic T cells, independently of the transduction enhancer substance. Notably, when these mDCs were subjected to electroporation with a tumor mRNA encoding MelanA, they endured this stress test and could be used to prime antigen-specific CD8+ T cells. Although mDCs not transduced with HAdV-5-GFP (+/−LeB/PB)-as monocytes usually induced higher numbers of CD3+ CD8+ MelanA+ T cells-efficiently infected (HAdV5GFP + LeB/PB) and electroporated cells were still functional and elicited sufficient antigen-specific T-cell responses in vitro. However, the capability of T-cell priming varied between donors. This might be due to the different frequencies of MelanA-specific naïve precursors in the blood of PBMCs of healthy donors. Moreover, these differences in T-cell reactivity reveal the heterogeneity of the immunological status of the different donors in general.
Taken together, here we present the transduction enhancer LentiBOOST ® /Polybrene as a promising and valuable tool to increase HAdV transduction efficacy, not only of murine and human DCs but also of human monocytes, without compromising the function of those cells. Therefore, using LentiBOOST ® /Polybrene shows potential in promoting HAdVor LV-mediated viral gene transfer in vitro as well as in vivo for future efficient and safe clinical applications.

Data Availability Statement:
The data presented in this study are available in the article.