Editing T cell genomes without viruses

Researchers used CRISPR and a jolt of electricity to get DNA inside T cells. The glowing ring of green demonstrates how DNA can be targeted to specific structures in a cell. Alex Marson, UCSF, Nature

T cells are an important part of the immune system. They help protect the body from infection and can also help fight cancer. Researchers have been working for years to genetically alter T cells to target specific types of cells for fighting cancer and other diseases. Past methods have used viruses (viral vectors) to insert DNA into T cells. However, the viral vectors used in FDA-approved T cell therapies can’t be used to insert genes into specific genomic sites. Using viral vectors to reprogram T cells can also be a difficult and expensive process.

CRISPR-Cas9 is a genomic engineering technology that allows DNA to be inserted, removed, or changed at particular locations in the genome. It’s one of the faster, cheaper, more accurate, and more efficient genome editing methods. To investigate whether human T cell function can be reprogrammed using CRISPR without viral vectors, a team led by Dr. Alex Marson at the University of California, San Francisco, tested a method called electroporation to deliver the CRISPR-Cas9 system and DNA in human T cells. The research, which was supported by several NIH components, was published in Nature on July 11, 2018.

Electroporation uses a pulse of electricity to get DNA or other molecules inside the cell. The researchers tested and refined their electroporation technique using T cells with a combination of CRISPR-Cas9 ribonucleoprotein and the gene for green fluorescent protein (GFP). They confirmed the presence of GFP in the cells and optimized the technique to minimize the impact on the cells and increase its efficiency.

They next tested their system by targeting different sections of the genome. They fused GFP to different genes. Using confocal microscopy, they tracked the resulting GFP fusion proteins and confirmed that they were all located in their proper, distinct locations within the cell. They found that normal gene regulation mechanisms still worked for at least one of the tested genes.

The team showed that they could simultaneously modify one, two, or three genes at a time. The researchers also analyzed possible off-target effects. They found that the genes rarely reached sites that weren’t specifically targeted. They showed that certain techniques could nearly eliminate these off-target effects.

Finally, the team tested whether the system could correct a specific gene mutation involved in an autoimmune disease. They identified a family with a loss-of-function mutation in the IL-2α receptor, which is important for proper T cell function. Using cells from the family members with the gene mutation, the researchers corrected two specific defects in the gene and showed that the changes improved the receptor’s function in T cells.

The researchers also showed they could replace large sections of DNA in T cells to reprogram the antigen receptor—the part of the T cell that identifies and targets diseased cells. They compared T cells reprogrammed without viral vectors to virally reprogrammed cells in mice. Both types of reprogrammed cells located cancer cells and accumulated in tumors to similar degrees.

“This is a rapid, flexible method that can be used to alter, enhance, and reprogram T cells so we can give them the specificity we want to destroy cancer, recognize infections, or tamp down the excessive immune response seen in autoimmune disease,” Marson says.

—by Tianna Hicklin, Ph.D.