mRNA transfection of dendritic cells: synergistic effect of ARCA mRNA capping with Poly(A) chains in cis and in trans for a high protein expression level

Enhancement of dendritic cells transfection in vivo and of vaccination against B16F10 melanoma with mannosylated histidylated lipopolyplexes loaded with tumor antigen messenger RNA

We report the preparation of mannosylated nanoparticles loaded with messenger RNA (mRNA) that enhance the transfection of dendritic cells (DCs) in vivo and the anti-B16F10 melanoma vaccination in mice. Mannosylated and histidylated lipopolyplexes (Man(11)-LPR100) were obtained by adding mannosylated and histidylated liposomes to mRNA-PEGylated histidylated polylysine polyplexes. Upon intravenous injection, ∼9% of the radioactivity of technetium 99 m-labeled lipopolyplexes measured in the liver, spleen, lungs, and kidneys was found in the spleen. We demonstrate that spleen from mice injected with enhanced green fluorescent protein (EGFP) mRNA-loaded Man(11)-LPR100 contained four times more DCs expressing EGFP than that from mice injected with sugar-free LPR100. This better transfection of DCs is correlated with a better inhibition of B16F10 melanoma growth and an increased survival time when mice were immunized with MART-1 mRNA-loaded Man(11)-LPR100. These results indicate that mannosylated and histidylated LPR is an efficient system for the delivery of tumor antigen mRNA in splenic DCs aiming to induce an anticancer immune response.

FROM THE CLINICAL EDITOR

This paper discusses the preparation of mannosylated nanoparticles loaded with messenger RNA that enhance the transfection of dendritic cells (DCs) in vivo and the anti-B16F10 melanoma vaccination in mice. The authors describe an efficient system for the delivery of tumor antigen mRNA in splenic DCs aiming to induce an anticancer immune response.

Delivery of messenger RNA using poly(ethylene imine)-poly(ethylene glycol)-copolymer blends for polyplex formation: biophysical characterization and in vitro transfection properties

Nucleic acid based therapies have so far mainly been focused on plasmid DNA (pDNA), small interfering RNA (siRNA), antisense and immunostimulatory oligonucleotides. Messenger RNA (mRNA) was the subject of only a few studies. The objective of this investigation was the preparation of new composite polyplexes with mRNA consisting of poly(ethylene imine) (PEI) and poly(ethylene imine)-poly(ethylene glycol)-copolymers (PEI-PEG) as blends. These complexes were designed to increase the stability of mRNA, to improve transfection efficiency and to reduce cytotoxicity. Hydrodynamic diameters of the polyplexes were measured by dynamic light scattering, polyplex stability was analyzed by gel retardation assay and transfection efficiency of luciferase (Luc) encoding mRNA was evaluated under in vitro conditions. Most of the polyplexes generated showed small particle sizes <200 nm and positive zeta-potentials of +20 mV to +30 mV. Stable complexes were formed even at low nitrogen to phosphate ratios. Polyplexes with mRNA Luc and blends of low molecular weight PEI(5 kDa) and PEI(25k Da)-PEG(20 kDa)₁-block-copolymer showed protein expression as high as polyplexes with PEI(25 kDa). Moreover, luciferase expression was significantly higher than that obtained with one of the components alone. These results suggest that delivery systems for pulmonary application of mRNA merit further investigation under in vitro and in vivo conditions.

Phosphorothioate cap analogs increase stability and translational efficiency of RNA vaccines in immature dendritic cells and induce superior immune responses in vivo

Vaccination with in vitro transcribed RNA coding for tumor antigens is considered a promising approach for cancer immunotherapy and has already entered human clinical testing. One of the basic objectives for development of RNA as a drug is the optimization of immunobioavailability of the encoded antigen in vivo. By analyzing the effect of different synthetic 5' mRNA cap analogs on the kinetics of the encoded protein, we found that m(2)(7,2'-O)Gpp(S)pG (beta-S-ARCA) phosphorothioate caps, in particular the D1 diastereoisomer, profoundly enhance RNA stability and translational efficiency in immature but not mature dendritic cells. Moreover, in vivo delivery of the antigen as beta-S-ARCA(D1)-capped RNA species is superior for protein expression and for efficient priming and expansion of naïve antigen-specific T cells in mice. Our findings establish 5' mRNA cap analogs as yet another module for tuning immunopharmacological properties of recombinant antigen-encoding RNA for vaccination purposes.

Immunotherapy of cancer with dendritic cells loaded with tumor antigens and activated through mRNA electroporation

Since decades, the main goal of tumor immunologists has been to increase the capacity of the immune system to mediate tumor regression. Considerable progress has been made in enhancing the efficacy of therapeutic anticancer vaccines. First, dendritic cells (DCs) have been identified as the key players in orchestrating primary immune responses. A better understanding of their biology and the development of procedures to generate vast amounts of DCs in vitro have accelerated the development of potent immunotherapeutic strategies for cancer. Second, tumor-associated antigens have been identified which are either selectively or preferentially expressed by tumor cells and can be recognized by the immune system. Finally, several studies have been performed on the genetic modification of DCs with tumor antigens. In this regard, loading the DCs with mRNA, which enables them to produce/process and present the tumor antigens themselves, has emerged as a promising strategy. Here, we will first overview the different aspects that must be taken into account when generating an mRNA-based DC vaccine and the published clinical studies exploiting mRNA-loaded DCs. Second, we will give a detailed description of a novel procedure to generate a vaccine consisting of tumor antigen-expressing dendritic cells with an in vitro superior capacity to induce anti-tumor immune responses. Here, immature DCs are electroporated with mRNAs encoding a tumor antigen, CD40 ligand (CD40L), CD70, and constitutively active (caTLR4) to generate mature antigen-presenting DCs.

Towards mRNA with superior translational activity: synthesis and properties of ARCA tetraphosphates with single phosphorothioate modifications

We describe the chemical synthesis and preliminary biophysical and biochemical characterization of a series of mRNA 5' end (cap) analogs designed as reagents for obtaining mRNA molecules with augmented translation efficiency and stability in vivo and as useful tools to study mRNA metabolism. The analogs share three structural features: (i) 5',5'- bridge elongated to tetraphosphate to increase their affinity to translation initiation factor eIF4E (ii) a single phosphorothioate modification at either the α, β, γ or δ-position of the tetraphosphate to decrease their susceptibility to enzymatic degradation and/or to modulate their interaction with specific proteins and (iii) a 2'-O-methyl group in the ribose of 7-methylguanosine, characteristic to Anti-Reverse Cap Analogs (ARCAs), which are incorporated into mRNA during in vitro transcription exclusively in the correct orientation. The dinucleotides bearing modified tetraphosphate bridge were synthesized by ZnCl(2) mediated coupling between two mononucleotide subunits with isolated yields of 30-65%. The preliminary biochemical results show that mRNAs capped with new analogs are 2.5-4.5 more efficiently translated in a cell free system than m(7)GpppG-capped mRNAs, which makes them promising candidates for RNA-based therapeutic applications such as gene therapy and anti-cancer vaccines.

Ectopic expression of a truncated CD40L protein from synthetic post-transcriptionally capped RNA in dendritic cells induces high levels of IL-12 secretion

BACKGROUND

RNA transfection into dendritic cells (DCs) is widely used to achieve antigen expression as well as to modify DC properties. CD40L is expressed by activated T cells and interacts with CD40 receptors expressed on the surface of the DCs leading to Th1 polarization. Previous studies demonstrated that ectopic CD40L expression via DNA transfection into DCs can activate the CD40 receptor signal transduction cascade. In contrast to previous reports, this study demonstrates that the same effect can be achieved when RNA encoding CD40L is electroporated into DCs as evidenced by secretion of IL-12. To achieve higher levels of IL-12 secretion, a systematic approach involving modification of coding and noncoding regions was implemented to optimize protein expression in the DCs for the purpose of increasing IL-12 secretion.

RESULTS

Site-directed mutagenesis of each of the first five in-frame methionine codons in the CD40L coding sequence demonstrated that DCs expressing a truncated CD40L protein initiated from the second methionine codon secreted the highest levels of IL-12. In addition, a post-transcriptional method of capping was utilized for final modification of the CD40L RNA. This method enzymatically creates a type I cap structure identical to that found in most eukaryotic mRNAs, in contrast to the type 0 cap incorporated using the conventional co-transcriptional capping reaction.

CONCLUSION

The combination of knocking out the first initiation methionine and post-transcriptional capping of the CD40L RNA allowed for approximately a one log increase in IL-12 levels by the transfected DCs. We believe this is a first report describing improved protein expression of post-transcriptionally capped RNA in DCs. The post-transcriptional capping which allows generation of a type I cap may have broad utility for optimization of protein expression from RNA in DCs and other cell types.

Bax mRNA therapy using cationic liposomes for human malignant melanoma

BACKGROUND

Bax is a pro-apoptotic molecule that functions as a tumor suppressor and Bax gene therapy has been examined for various cancers. Gene transfer by mRNA lipofection is more efficient than plasmid DNA lipofection and, in the present study, we examined the anti-tumor effects in human malignant melanoma cells (HMGs) using Bax mRNA lipofection.

METHODS

Bax protein expression, cell growth inhibition, caspase-3 activity and apoptosis were examined in vitro. Liposome-Bax mRNA was applied locally once every 5 days for a total of five times to peripheral HMG tumors transplanted in nude mice. Tumor growth inhibition was evaluated by measuring the tumor volume and apoptosis was detected using a terminal deoxynucleotidyl transferase-mediated dUTP nick end labelling (TUNEL) assay.

RESULTS

Enhanced expression of Bax protein was observed following Bax mRNA transfer and cell survival was 59.8%. Caspase-3 activity and TUNEL-positive cells increased significantly following Bax mRNA lipofection compared to Bax plasmid transfer. In mice, tumor growth increased only slightly during liposome-Bax mRNA administration and the tumor volume on day 30 (10 days after completion of administration) was 36.7% of that in the saline control group. By contrast, Bax plasmid transfection resulted in little change in tumor growth compared to controls.

CONCLUSIONS

Bax mRNA therapy using liposomes has stronger anti-tumor effects than Bax gene therapy using a plasmid, and the results suggest that Bax mRNA lipofection may be a viable treatment for malignant melanoma.

Delivery of tumor-antigen-encoding mRNA into dendritic cells for vaccination

Antigen-loaded dendritic cells (DCs) have been intensively investigated as potential cellular antitumor vaccines. Several recent reports have indicated that loading DCs with whole tumor derived mRNA or defined tumor-antigen-encoding mRNA represents an effective nonviral strategy to stimulate T cell responses both for in vitro and in vivo models. Here, we describe the electroporation method as a tool for introducing in vitro transcribed capped mRNA into human DCs for tumor vaccination. We use MART-1/Melan-A as a model tumor-associated antigen for the generation of a DC-based vaccine against melanoma cancer. In addition to efficient antigen loading, it is important to obtain a maximal number of potent antigen-presenting cells. Another prerequisite for the development of a DC-based cancer vaccine is to obtain mature DCs. In this chapter, we describe the basic techniques required for the successful genetic modification of DCs by using the mRNA electroporation method.

Synthesis and characterization of mRNA cap analogs containing phosphorothioate substitutions that bind tightly to eIF4E and are resistant to the decapping pyrophosphatase DcpS

Analogs of the mRNA cap are widely employed to study processes involved in mRNA metabolism as well as being useful in biotechnology and medicinal applications. Here we describe synthesis of six dinucleotide cap analogs bearing a single phosphorothioate modification at either the alpha, beta, or gamma position of the 5',5'-triphosphate chain. Three of them were also modified with methyl groups at the 2'-O position of 7-methylguanosine to produce anti-reverse cap analogs (ARCAs). Due to the presence of stereogenic P centers in the phosphorothioate moieties, each analog was obtained as a mixture of two diastereomers, D1 and D2. The mixtures were resolved by RP HPLC, providing 12 different compounds. Fluorescence quenching experiments were employed to determine the association constant (K(AS)) for complexes of the new analogs with eIF4E. We found that phosphorothioate modifications generally stabilized the complex between eIF4E and the cap analog. The most strongly bound phosphorothioate analog (the D1 isomer of the beta-substituted analog m(7)Gpp(S)pG) was characterized by a K(AS) that was more than fourfold higher than that of its unmodified counterpart (m(7)GpppG). All analogs modified in the gamma position were resistant to hydrolysis by the scavenger decapping pyrophosphatase DcpS from both human and Caenorhabditis elegans sources. The absolute configurations of the diastereomers D1 and D2 of analogs modified at the alpha position (i.e., m(7)Gppp(S)G and m(2) (7,2'-O )Gppp(S)G) were established as S(P) and R(P) , respectively, using enzymatic digestion and correlation with the S(P) and R(P) diastereomers of guanosine 5'-O-(1-thiodiphosphate) (GDPalphaS). The analogs resistant to DcpS act as potent inhibitors of in vitro protein synthesis in rabbit reticulocyte lysates.

Activation of tumor-specific T cells by dendritic cells expressing the NY-ESO-1 antigen after transfection with the cationic lipophosphoramide KLN5

BACKGROUND

Genetic modification of human monocyte-derived dendritic cells (DC) with cDNA sequences encoding tumor-associated antigens (TAA) is a promising strategy for cancer immunotherapy. The present study aimed to develop a nonviral gene transfer method based on the use of the cationic lipophosphoramide reagent, KLN-5, as an alternative to the commonly used viral vectors.

METHODS

First, the efficiency of KLN5 for gene transfection into DC was investigated using the green fluorescent protein (GFP) reporter gene. The highest transfection efficiency/cell viability ratio was determined by flow cytometry. Next, DC were transfected with a plasmid encoding NY-ESO-1, a TAA expressed in numerous cancers, according to the transfection protocol previously established with the GFP reporter. Transfected DC were then co-cultured with a CD8+ NY-ESO-1 specific HLA-A*02.01 T cell clone to control their ability to correctly process and present the corresponding epitope in the HLA-A*02.01 context. Finally, T cell activation was assessed via flow cytometry-based detection of interferon-gamma production.

RESULTS

An optimal KLN5/plasmid DNA ratio allowing both significant transgene expression and high viability of DC could be determined. Under the established experimental conditions, antigen processing and presentation of the immunodominant (SLLMWITQC(157-165)) epitope in the HLA-A*0201 context was demonstrated by activation of the NY-ESO-1-specific CD8+ T cell clone.

CONCLUSIONS

KLN5-based gene transfection into DC allows the efficient induction of TAA presentation and may thus represent a novel attractive nonviral approach for cancer vaccination.

Vaccination with messenger RNA (mRNA)

Both DNA and mRNA can be used as vehicles for gene therapy. Because the immune system is naturally activated by foreign nucleic acids thanks to the presence of Toll-like Receptors (TLR) in endosomes (TLR3, 7, and 8 detect exogenous RNA, while TLR9 can detect exogenous DNA), the delivery of foreign nucleic acids usually induces an immune response directed against the encoded protein. Many preclinical and clinical studies were performed using DNA-based experimental vaccines. However, no such products are yet approved for the human population. Meanwhile, the naturally transient and cytosolically active mRNA molecules are seen as a possibly safer and more potent alternative to DNA for gene vaccination. Optimized mRNA (improved for codon usage, stability, antigen-processing characteristics of the encoded protein, etc.) were demonstrated to be potent gene vaccination vehicles when delivered naked, in liposomes, coated on particles or transfected in dendritic cells in vitro. Human clinical trials indicate that the delivery of mRNA naked or transfected in dendritic cells induces the expected antigen-specific immune response. Follow-up efficacy studies are on the way. Meanwhile, mRNA can be produced in large amounts and GMP quality, allowing the further development of mRNA-based therapies. This chapter describes the structure of mRNA, its possible optimizations for immunization purposes, the different methods of delivery used in preclinical studies, and finally the results of clinical trial where mRNA is the active pharmaceutical ingredient of new innovative vaccines.

Phosphorothioate cap analogs stabilize mRNA and increase translational efficiency in mammalian cells

Capped RNAs synthesized by in vitro transcription have found wide utility for studying mRNA function and metabolism and for producing proteins of interest. We characterize here a recently synthesized series of cap analogs with improved properties that contain a sulfur substitution for a nonbridging oxygen in either the alpha-, beta-, or gamma-phosphate moieties, m(2) (7,2'-O )Gppp(S)G, m(2) (7,2'-O )Gpp(S)pG, and m(2) (7,2'-O )Gp(S)ppG, respectively. The new compounds were also modified at the 2'-O position of the m(7)Guo to make them anti-reverse cap analogs (ARCAs), i.e., they are incorporated exclusively in the correct orientation during in vitro transcription. Each of the S-ARCAs exists in two diastereoisomeric forms (D1 and D2) that can be resolved by reverse-phase HPLC. A major in vivo pathway for mRNA degradation is initiated by removal of the cap by the pyrophosphatase Dcp1/Dcp2, which cleaves between the alpha- and beta-phosphates. Oligonucleotides capped with m(2) (7,2'-O )Gpp(S)pG (D2) were completely resistant to hydrolysis by recombinant human Dcp2 in vitro, whereas those capped with m(2) (7,2'-O )Gpp(S)pG (D1) and both isomers of m(2) (7,2'-O )Gppp(S)G were partially resistant. Luciferase mRNA capped with m(2) (7,2'-O )Gpp(S)pG (D2) had a t (1/2) of 257 min in cultured HC11 mammary epithelial cells compared with 86 min for m(7)Gp(3)G-capped mRNA. Luciferase mRNAs capped with m(2) (7,2'-O )Gpp(S)pG (D1) and m(2) (7,2'-O )Gpp(S)pG (D2) were translated 2.8-fold and 5.1-fold, respectively, more efficiently in HC11 cells than those capped with m(7)Gp(3)G. The greater yield of protein due to combining higher translational efficiency with longer t (1/2) of mRNA should benefit applications that utilize RNA transfection such as protein production, anti-cancer immunization, and gene therapy.

mRNA-based cancer vaccine: prevention of B16 melanoma progression and metastasis by systemic injection of MART1 mRNA histidylated lipopolyplexes

Immunization with mRNA encoding tumor antigen is an emerging vaccine strategy for cancer. In this paper, we demonstrate that mice receiving systemic injections of MART1 mRNA histidylated lipopolyplexes were specifically and significantly protected against B16F10 melanoma tumor progression. The originality of this work concerns the use of a new tumor antigen mRNA formulation as vaccine, which allows an efficient protection against the growth of a highly aggressive tumor model after its delivery by intravenous route. Synthetic melanoma-associated antigen MART1 mRNA was formulated with a polyethylene glycol (PEG)ylated derivative of histidylated polylysine and L-histidine-(N,N-di-n-hexadecylamine)ethylamide liposomes (termed histidylated lipopolyplexes). Lipopolyplexes comprised mRNA/polymer complexes encapsulated by liposomes. The tumor protective effect was induced with MART1 mRNA carrying a poly(A) tail length of 100 adenosines at an optimal dose of 12.5 microg per mouse. MART1 mRNA lipopolyplexes elicited a cellular immune response characterized by the production of interferon-gamma and the induction of cytotoxic T lymphocytes. Finally, the anti-B16 response was enhanced using a formulation containing both MART1 mRNA and MART1-LAMP1 mRNA encoding the antigen targeted to the major histocompatibility complex class II compartments by the lysosomal sorting signal of LAMP1 protein. Our results provide a basis for the development of mRNA histidylated lipopolyplexes for cancer vaccine.

Effective delivery with enhanced translational activity synergistically accelerates mRNA-based transfection

mRNA instead of DNA provides a new and attractive approach for gene therapy and genetic vaccination. Current technologies for mRNA delivery are based on cationic lipids with DOTAP being the most efficient one. We previously reported on the synthesis of an inorganic-organic hybrid carrier by embedding inorganic nano-particles of carbonate apatite onto liposomal carrier DOTAP and demonstrated its high transfection potency of luciferase mRNA both in mitotic and non-mitotic cells. Here we show that in addition to the carrier design for effective endocytosis and release of mRNA to the cytoplasm, enhancement of mRNA translation efficiency is a prerequisite for maximum protein expression. We used the modified cap analog (ARCA) during in vitro transcription of luciferase DNA for proper cap orientation and demonstrated that transfection with ARCA-mRNA resulted in higher protein expression than the mRNA with usual cap structure for both DOTAP and DOTAP-apatite complex. Secondly, exogenous poly(A) was co-delivered with mRNA by the DOTAP-apatite, resulting in very significant expression compared to mRNA delivery only. Finally, when combined both of the effects of smart carrier and the modifications at mRNA translational level, a notable enhancement (100 times) was achieved as compared to the existing DOTAP-based liposome technology. Our findings, therefore, unveiled a novel approach that an effective delivery system can be developed by the improvement of the gene expression level in combination with the enhancement of the carrier potency.

Dendritic cells transfected with interleukin-12 and tumor-associated antigen messenger RNA induce high avidity cytotoxic T cells

Dendritic cells (DC) transfected with messenger RNA (mRNA) encoding tumor-associated antigens (TAA) are able to induce potent tumor-specific T-cell responses directed to a broad spectrum of tumor-associated epitopes. The in vitro generation of DC possessing all the features crucial for the induction of type 1 immune responses, such as mature state, migratory potential and interleukin-12 (IL-12p70) production is complicated. Particularly migratory potential is inversely correlated with IL-12p70 production after maturation with prostaglandin E2 (PGE2), which is included in maturation cocktails currently used in most vaccination trials. Here, we show that transfection of PGE2 matured DC with a single mRNA strain encoding for ubiquitin followed by a TAA which was linked to IL-12 by a self-cleaving 2A sequence, produced biological active IL-12p70 and were able to present the transfected TAA up to 72 h after transfection. Furthermore, use of the anti-reverse cap analog for in vitro transcription of the IL-12 mRNA enabled constitutive IL-12p70 production for up to 5 days. These transfected mature DC migrated efficiently towards lymph node derived chemokines. DCs constitutively expressing IL-12p70, generate TAA-specific cytotoxic T cells with an high functional avidity, independent of CD4+ T-cell help.