Manipulation of lipoplex concentration at the cell surface boosts transfection efficiency in hard-to-transfect cells

Purpose: Nonviral DNA delivery remains inefficient, and the barriers limiting transfection are incompletely understood. The study investigates multicomponent cationic liposome/DNA lipoplexes to identify an additional transfection barrier and improve delivery, especially in hard-to-transfect cells.

Hypothesis: If low lipoplex concentration at the cell surface is a key transfection barrier, then increasing the dose/number of multicomponent lipoplexes per cell will boost transfection efficiency without unacceptable toxicity, including in hard-to-transfect T-cell acute lymphoblastic leukemia (T-ALL) cell lines where Lipofectamine-based reagents fail.

Aims: Systematically investigate the mechanism of action of multicomponent (MC) cationic liposome/DNA lipoplexes using Lipofectamine (LFN) as a reference. - Characterize lipoplex size, zeta potential, and internal nanostructure. - Quantify cellular uptake, intracellular trafficking, endosomal release, and lysosomal colocalization. - Test whether increasing lipoplex concentration at the cell surface improves transfection in hard-to-transfect T-ALL cell lines.

Delivery system: Platform: Multicomponent (MC) cationic liposome/DNA lipoplexes. - Lipid composition: DOTAP:DC-Chol:DOPC:DOPE = 1:1:1:1 molar ratio. - Payload: Plasmid DNA encoding GFP (`pGLO`), luciferase (`pGL3`), or Cy3-labeled 2.7 kbp plasmid DNA for imaging. - Charge ratio: Cationic lipid/DNA base ratio, ρ. MC/DNA lipoplexes at ρ = 2 were positively charged, <200 nm, and used for subsequent experiments; SAXS structure also reported at ρ = 3. - Targeting ligand: None. - Reference reagent: Lipofectamine 2000 (LFN).

Approach: In vitro model: Chinese Hamster Ovarian (CHO) cells for mechanistic studies. - Hard-to-transfect model: Human T-ALL cell lines DND41, JURKAT, MOLT3, P12-ICHIKAWA, ALL-SILL, and TALL-1. - Transfection readouts: GFP-positive cell percentage and luciferase activity 48 h post-transfection. - Dose optimization: Preformed MC/DNA lipoplexes tested at 0.5×, 1×, 2×, 5×, and 10× the ordinary dose; for T-ALL lines, the highest non-cytotoxic dose was 5× higher than usual (i.e., 5 µg DNA/well versus 1 µg DNA/well). - Controls: LFN/DNA lipoplexes at matched conditions; untreated cells for viability. - No in vivo experiments are reported.

Key methods: Dynamic light scattering (DLS) and electrophoretic light scattering (ELS) for size and zeta potential. - Synchrotron small-angle X-ray scattering (SAXS) for lipoplex nanostructure. - Fluorescence-activated cell sorting (FACS) for cellular uptake and GFP transfection efficiency. - Laser scanning confocal microscopy (LSCM) in live cells for intracellular trafficking and DNA release. - Lysosensor colocalization for lysosomal localization. - MTT assay for cell viability. - Luciferase assay for average reporter expression per cell population.

Key results: MC cationic liposomes: 78.8 ± 5 nm diameter; zeta potential 27.9 ± 1.1 mV. - MC/DNA lipoplexes at ρ = 2: 164.2 ± 4.3 nm; zeta potential 21.4 ± 2.7 mV. - SAXS showed multilamellar structure with periodicity 6.67 nm and a DNA–DNA correlation peak at 4.27 nm. - In CHO cells, MC/DNA gave ~15% GFP-positive cells versus ~40% for LFN/DNA, but luciferase expression was not significantly different. - Cellular uptake after 3 h: ~12% Cy3-positive cells for MC/DNA versus ~32% for LFN/DNA; however, MC/DNA lipoplexes had higher average fluorescence intensity per particle/cell. - MC/DNA lipoplexes were roughly 10-fold fewer in number than LFN/DNA complexes, but each contained more DNA copies. - Increasing MC/DNA dose produced a linear increase in transfection efficiency. - In hard-to-transfect T-ALL lines, optimized MC/DNA dosing gave up to a 30-fold increase in transfected cells over LFN/DNA; LFN was ineffective (<2% transfected cells in these lines). - MC/DNA lipoplexes showed no discernible toxicity in CHO cells, whereas LFN/DNA was cytotoxic. At the 5× dose, LFN had a negative impact on cell viability.

Interpretation: The authors conclude that low lipoplex concentration at the cell surface is an underestimated transfection barrier. Increasing the dose of low-toxicity MC/DNA lipoplexes improves transfection efficiency and enables efficient delivery in hard-to-transfect T-ALL cell lines where Lipofectamine fails. They suggest this approach may broaden nonviral gene delivery to difficult cell types such as stem cells and primary cells.

Limitations: No in vivo validation; all data are from cell culture. - No active targeting ligand; delivery relies on nonspecific lipoplex–cell interactions. - The optimized protocol requires a 5-fold higher dose than usual, which may raise translation and manufacturing considerations. - Protein corona effects were not explored and are explicitly noted as an important future direction. - Mechanistic studies focused mainly on CHO cells; hard-to-transfect T-ALL lines were used primarily for validation of transfection efficiency and viability.