Tips from the Bench: Transfection Tip

Co-transfection Protocol for Multiple Plasmid DNAs

Co-transfection of multiple plasmid DNAs is a technique increasingly employed by biology researchers. A few applications that popularly utilize co-transfection are virus production, protein-protein interaction studies, stable cell line generation, or simple addition of reporter DNA constructs to normalize experimental output. One example utilizing co-transfection for virus production is illustrated below.

High Titer Virus Production Using TransIT-2020

Rescue of Influenza Virus by Reverse Transfection With TransIT®-2020 Transfection Reagent. 293T and MDBK cells are reverse transfected with up to 10 separate influenza virus rescue plasmids per experiment. Reagent:total DNA complexes are prepared with TransIT®-2020 Transfection Reagent at a 3:1 ratio of reagent to total DNA. Virus-containing supernatants are harvested 48-96 hr post-transfection. Titers from transfected cells can be as high as 107 pfu/ml. Rescued virus is then amplified on MDBK cells to increase titer and total yield. Viral stocks are subsequently titered by plaque assay on MDCK cells. Titers from representative rescue experiments with different viral strains and reassortants are shown. This experimental strategy is adapted from Neumann, et al. 2005 PNAS.

Data courtesy of Andrew Mehle, University of Wisconsin-Madison

All of the TransIT® broad-spectrum and cell-type specific DNA transfection reagents can be used for co-transfecting multiple plasmids with the following considerations. Additionally, general user protocol guidelines such as as optimal cell confluence, high quality endotoxin-free DNA, etc. should be observed.

  • Ratio of Transfection Reagent: Total DNA: It is critical to maintain the same amount of total DNA as the user protocol recommends and to divide the total DNA into a mixture of the different plasmids needed. Depending on the application, the ratio of each plasmid may need further optimization to achieve the best experimental results. This might differ significantly depending on the promoter elements of each plasmid and the inherently variable expression of different proteins. For example, during stable cell line generation, a plasmid mixture containing 5 to 10 parts gene expression plasmid and 1 part antibiotic selection marker plasmid is generally sufficient to ensure that the selected cells will express both the gene of interest and the selection marker.
  • Premixing of Nucleic Acids: It is also important to prevent preferential complexation of one plasmid DNA over the other(s). This can be accomplished by completely premixing the plasmid DNAs before adding the transfection reagent. The order of addition of different plasmid DNAs should not affect on the transfection outcome if the plasmid mixture is mixed completely before forming the transfection complex.

Outlined below is an easy-to-follow co-transfection protocol for multiple plasmids using TransIT-X2® Dynamic Delivery System in a 6-well format. For other tissue culture formats, refer to the user protocol. This protocol can be used when using other DNA transfection reagents such as TransIT®-2020 and TransIT®-LT1; depending on the transfection reagent and the cell type used for co-transfection, the ratio of the transfection reagent: total DNA may need to be re-optimized.

Transient DNA Co-Transfection Protocol in a 6-Well Plate using TransIT-X2® System

A. Plate cells

  1. Approximately 18-24 hours before transfection, plate cells using the following guidelines. For most cell types, cultures should be ≥80% confluent at the time of transfection. For adherent cells, plate cells at a density of 2-6 × 105 cells/well. For suspension cells, plate cells at a density of 8-10 × 105 cells/well.
  2. Incubate the cells overnight.

B. Prepare co-transfection complexes (Immediately before transfection)

  1. Warm TransIT-X2® to room temperature and vortex gently before using.
  2. Place 250 µl of Opti-MEM® I Reduced-Serum Medium in a sterile tube.
  3. Add 2.5 µg total plasmid DNA. Plasmid DNAs can be added sequentially depending on the optimized ratio. For example, if generating stable cell lines using an antibiotic selection marker on a separate plasmid than the gene of interest, add 2.25 µg plasmid DNA containing the "gene of interest" first to Opti-MEM® I Reduced-Serum Medium; pipet gently to mix completely. Then, add 0.25 µg of the plasmid DNA containing the "antibiotic selection marker" to this mixture; pipet gently to mix completely.
  4. Add 7.5 µl of TransIT-X2®. Pipet gently to mix completely. For further optimization of your cell type, test additional amounts of TransIT-X2® as per the user protocol. For cell type specific protocol recommendations, please refer to the TransIT-X2® protocol optimization card.
  5. Incubate at room temperature for 15-30 minutes to allow sufficient time for complexes to form.

C. Distribute the complex mixture to cells in complete growth medium

  1. Add the co-transfection complexes (prepared in Step B) drop-wise to different areas of the wells.
  2. Gently rock the culture vessel back-and-forth and from side-to-side to evenly distribute the co-transfection complexes.
  3. Incubate for 24-72 hours or as required. It is not necessary to replace the complete growth medium with fresh medium.
  4. Harvest cells and assay for target and/or reporter gene expression.

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