The immunogenicity of antibody aggregates in a novel transgenic mouse model.

PURPOSE: Protein aggregates have been discussed as a potential risk factor related to immunogenicity. Here we developed a novel human IgG transgenic (tg) mouse system expressing a mini-repertoire of human IgG1 antibodies (Abs) for the assessment of immunogenic properties of human mAb preparations. METHODS: Transgenic mice were generated using germline versions of the human Ig heavy chain γ1 (IgH-γ1), and the human Ig light chain (IgL) κ and λ genes. Only the soluble form of human IgH-γ1 was used to avoid expression of the membrane Ig-H chain and concomitant allelic exclusion of endogenous murine Ig genes. IgG1 aggregates were generated by different stress conditions such as process-related, low pH and exposure to artificial light. RESULTS: The expression of human Ig proteins induced immunological tolerance to a broad range of human IgG1 molecules in the tg mice. Immunization with IgG1 aggregates demonstrated that soluble oligomers induced by significant light-exposure and carrying neo-epitopes induced a strong immune response in tg mice. In contrast, Ab aggregates alone and monomers with neo-epitopes were not immunogenic. CONCLUSION: This mouse model is able to recognize immunogenic modifications of human IgG1. While the degree of stress-induced aggregation varies for different mAbs, our findings using a particular mAb (mAb1) demonstrate that non-covalently modified aggregates do not break tolerance, contrary to widely held opinion. The immunogenic potential of soluble aggregates of human IgG strongly depends on the presence of neo-epitopes resulting from harsh stress conditions, i.e. extensive exposure to artificial light.

Structure and function of purified monoclonal antibody dimers induced by different stress conditions.

PURPOSE: To investigate structure and function of different monoclonal antibody (MAb) dimers. METHODS: MAb dimers were induced by process-related, low pH and UV light stress. Dimers were isolated and purified by chromatography and extensively characterized by biochemical, structural and functional methods. RESULTS: Highly purified dimer forms were obtained which enabled detailed characterization. Dimers induced by process stress were associated by a single non-covalent interaction site between two Fab domains in a characteristic "bone-like" structure observed in Transmission Electron Microscopy (TEM). These dimers showed reduced potency and antigen binding affinity. Low pH stress generated more stable but also non-covalently associated dimers without chemical alterations in a typical "closed" conformation according to TEM. These dimer species were more compact and more hydrophobic as dimers induced by process stress. They showed bioactivity and antigen binding affinity similar to the native monomer. Light-induced dimers, exhibiting various different conformations, were the most stable dimers with various chemical modifications leading to a broad range in size, charge and hydrophobicity. These dimers fully lost bioactivity and antigen binding affinity. CONCLUSION: The use of highly purified MAb dimers and a panel of characterizations methods enabled to obtain a clear picture about molecular architecture and function of dimers.

Melittin analogs with high lytic activity at endosomal pH enhance transfection with purified targeted PEI polyplexes.

Melittin-polyethylenimine (PEI) conjugates have been shown to enhance gene transfer efficiency of polyplexes due to their membrane-destabilizing properties. Inherent lytic activity at neutral pH however also provokes high cytotoxicity due to plasma membrane damage. In order to shift the lytic activity towards the endosomal membrane, several melittin analogs were designed. Acidic modification of melittin by replacing neutral glutamines (Gln-25 and Gln-26) with glutamic acid residues greatly improved the lytic activity of C-terminally linked PEI conjugates at the endosomal pH of 5. This activity correlated well with the gene transfer efficiency of polyplexes in four different cell lines. Melittin-PEI conjugates with high lytic activities at endosomal pH were then incorporated into EGF receptor-targeted and polyethylene glycol-shielded polyplexes. The resulting particles had virus-like dimension (150 nm) with a neutral surface charge and were subsequently purified by size exclusion chromatography to remove unbound toxic PEI conjugate. These purified polyplexes mediated EGF-receptor-specific gene transfer with up to 70-fold higher activity compared to the corresponding PEI polyplexes without melittin.

DNA polyplexes based on degradable oligoethylenimine-derivatives: combination with EGF receptor targeting and endosomal release functions.

Combination of the degradable polymeric gene carriers OEI-HD-1 and LT- OEI-HD-1 with an EGF targeting conjugate resulted in strongly (up to 900-fold) enhanced polyplex activity in EGF-receptor rich HUH7 hepatocellular carcinoma cells. The targeting ligand effect was DNA dose dependent, could be blocked by competitive receptor binding with unbound EGF ligand, and was not observed in receptor-negative control cells. Measures which enhance intracellular endosomal escape, either photochemically enhanced intracellular release (PCI) or the incorporation of a novel membrane-active melittin analog NMA-3, further enhanced gene transfer activity of EGF/OEI-HD-1 polyplexes.

Optimizing targeted gene delivery: chemical modification of viral vectors and synthesis of artificial virus vector systems.

In comparison to classical medicines, gene therapy has the potential to mediate the highest possible level of therapeutic specificity. Every normal or diseased cell can switch on or off a gene expression cassette in a tissue-, disease-, and time-dependent fashion, by use of specific transcription factors that are active only in a given unique situation. In practice, we face the problem in realizing the concept: the delivery of nucleic acids into target cells is very ineffective and presents a formidable challenge. Key issues for future developments include improved targeting, enhanced intracellular uptake, and reduced toxicity of gene vectors. The currently used classes of vectors have complementary characteristics, such as high intracellular efficiency of viral vectors on the one hand and low immunogenicity and greater flexibility of nonviral vectors on the other hand. The merge of viral and nonviral vector technologies is highlighted as an encouraging strategy for the future; concepts include chemically modified viral vectors ("chemo-viruses") and synthesis of virus-like systems ("synthetic viruses"). Examples for the development of vectors toward artificial synthetic viruses are presented.

C- versus N-terminally linked melittin-polyethylenimine conjugates: the site of linkage strongly influences activity of DNA polyplexes.

BACKGROUND: One major barrier limiting the transfection efficiency of polyplexes is poor endosomal release, especially when small particles are applied. In an approach to overcome this barrier, covalent attachment of the membrane-active peptide all-(L)-melittin to polyethylenimine (PEI) polyplexes was found to enhance gene transfer efficiency. METHODS: The N-terminus of natural all-(L)- or non-immunogenic all-(D)-melittin was covalently coupled to PEI. In addition, two different all-(D)-melittin conjugates were synthesized, with PEI covalently attached to either the C-terminus (C-mel-PEI) or the N-terminus of melittin (N-mel-PEI). Melittin-PEI polyplexes with particle sizes < 150 nm were generated in HEPES-buffered glucose and tested in transfection experiments. The membrane lytic activities of conjugates and polyplexes were analyzed at neutral and endosomal pH. RESULTS: All-(D)-melittin conjugates mediated enhanced gene expression similar to the natural all-(L)-stereoisomer, with up to 160-fold higher luciferase activity than unmodified PEI. The site of melittin linkage strongly influenced the membrane-destabilizing activities of both conjugates and polyplexes. C-mel-PEI was highly lytic at neutral pH and therefore elevated doses of C-mel-PEI polyplexes induced high toxicity. In contrast, N-mel-PEI was less lytic at neutral pH but retained higher lytic activity than C-mel-PEI at endosomal pH. This apparently promoted better endosomal release of N-mel-PEI polyplexes resulting in efficient gene delivery in different cell lines. CONCLUSIONS: The high potency of C-mel-PEI to destabilize membranes at neutral pH is presumably due to a reported destabilization mechanism proceeding through membrane insertion of the peptide. In contrast, N-mel-PEI is supposed to induce lysis by insertion-independent pore formation according to the toroidal pore model.

Targeting of Polyplexes: Toward Synthetic Virus Vector Systems.

Dominating issues in gene vector optimization are specific in recognizing the target cells and exploiting the proper intracellular trafficking routes. Any progress in this area will result in improved specific gene transfer, reduce the required therapeutic vector doses and, in consequence, lower the overall toxicity to the host. To provide polyplexes with the ability to distinguish between non-target and target cells, cell-binding ligands have been incorporated which recognize target-specific cellular receptors. In addition, polyplex domains with unspecific binding capacity (such as surface charges) have to be shielded or removed. Cell-binding ligands can be small molecules, vitamins, carbohydrates, peptides or proteins such as growth factors or antibodies. Such ligands have been incorporated into polyplexes after chemical conjugation to cationic polymers. The choice of the ligand and physical properties of the DNA formulation strongly influence extracellular routing (circulation in blood, tissue distribution), uptake and intracellular delivery of polyplexes. Recent efforts are discussed that aim at the development of polyplexes into virus-like supramolecular complexes; such particles should undergo structural changes compatible with extracellular and intracellular targeting.

Targeting of polyplexes: toward synthetic virus vector systems.

Dominating issues in gene vector optimization are specific in recognizing the target cells and exploiting the proper intracellular trafficking routes. Any progress in this area will result in improved specific gene transfer, reduce the required therapeutic vector doses and, in consequence, lower the overall toxicity to the host. To provide polyplexes with the ability to distinguish between non-target and target cells, cell-binding ligands have been incorporated which recognize target-specific cellular receptors. In addition, polyplex domains with unspecific binding capacity (such as surface charges) have to be shielded or removed. Cell-binding ligands can be small molecules, vitamins, carbohydrates, peptides or proteins such as growth factors or antibodies. Such ligands have been incorporated into polyplexes after chemical conjugation to cationic polymers. The choice of the ligand and physical properties of the DNA formulation strongly influence extracellular routing (circulation in blood, tissue distribution), uptake and intracellular delivery of polyplexes. Recent efforts are discussed that aim at the development of polyplexes into virus-like supramolecular complexes; such particles should undergo structural changes compatible with extracellular and intracellular targeting.

Toward synthetic viruses: endosomal pH-triggered deshielding of targeted polyplexes greatly enhances gene transfer in vitro and in vivo.

Nonviral vectors should undergo "virus-like" changes compatible with the steps of gene delivery. Poly(ethylene) glycol (PEG) shielding of DNA/polycation polyplexes protects from nonspecific interactions with the extracellular environment. pH-triggered removal of the shield within the endosome may be advantageous. Polycation and PEG were linked via acylhydrazides or pyridylhydrazines. The pyridylhydrazone prepared from polylysine and propionaldehyde-PEG showed the greatest acid-dependent hydrolysis; at pH 5, 37 degrees C for 10 min, 90% hydrolyzed, while at pH 7.4 the half-life was 1.5 h. Particle size and zeta potential measurements of the polyplexes showed complete deshielding within 1 h at pH 5, while at pH 7.4 the shield remained at 4 h, 37 degrees C. For gene transfection a targeting conjugate was also included in the polyplex, transferrin as ligand for K562 and Neuro2A cells and epidermal growth factor for HUH-7 and Renca-EGFR cells. Marker gene expression showed that the reversibly shielded polyplexes exhibited up to 2 log orders of magnitude higher gene expression in vitro and 1 log magnitude higher gene expression in an in vivo mouse model, compared to the stably shielded control polyplexes. Engineering of polyplexes with more dynamic domains is an encouraging new direction in nonviral vector design.

Purification of polyethylenimine polyplexes highlights the role of free polycations in gene transfer.

BACKGROUND: Nonviral vectors based on polyethylenimine (PEI) usually contain an excess of PEI that is not complexed to DNA. Since unbound PEI contributes to cellular and systemic toxicity, purification of polyplexes from unbound PEI is desirable. METHODS: Size exclusion chromatography (SEC) was used to purify PEI polyplexes of free PEI. Transfection properties of purified polyplexes and the effect of free PEI on gene delivery were studied in vitro and in vivo after systemic application into mice. RESULTS: SEC did not change the size and zeta-potential of polyplexes. Independent of the amount of PEI used for complex formation, purified PEI polyplexes had the same final PEI nitrogen/DNA phosphate ratio of 2.5. Notably, purified PEI polyplexes demonstrated low cellular and systemic toxicity. High transfection efficiency was achieved with purified polyplexes at high DNA concentrations (8-15 microg/ml). At low DNA concentrations (2-4 microg/ml) gene transfer with purified particles was less efficient than with polyplexes containing free PEI both in vitro and in vivo. Mechanistic studies showed that free PEI partly blocked cellular association of DNA complexes but was essential for the following intracellular gene delivery. Adding free PEI to cells treated with purified particles with a delay of up to 4 h resulted in significantly enhanced transfection efficiency compared with non-purified particles or purified particles without free PEI. CONCLUSIONS: This study presents an efficient method to remove free PEI from PEI polyplexes by SEC. Our results from transfection experiments demonstrate that free PEI substantially contributes to efficient gene expression but also mediates toxic effects in a dose-dependent manner. Purified polyplexes without free PEI have to be applied at increased concentrations to achieve high transfection levels, but exhibit a greatly improved toxicity profile.