Chimeric Antigen Receptor T-cells versus Solid Tumors

The future of cancer research is targeting, meaning the ability to destroy cancer cells efficiently and with few to no side-effects on normal tissues. Chimeric antigen receptor (CAR) T-cells are a step forward in this regard, and have proven to be an effective treatment for leukemia in trials. Researchers are now attempting to adapt their use to other types of cancer. In this example, the T-cells are engineered to make them more discriminating when targeted at solid tumors:

Many solid cancers have high levels of certain proteins such as ErbB2 and EGFR, which make them suitable targets for anticancer therapies. However, such proteins are also present at low levels in normal cells. Because of this, CAR T cells that are developed to target one of these proteins on tumor cells also recognize and attack normal cells that have the protein, causing severe toxicity.

To develop CAR T cells that can distinguish between cancer and normal cells, researchers first constructed a panel of CARs with the single chain variable fragments (scFv) - the part of the CAR T cell that recognizes the tumor target - using sequences from mutated 4D5 antibodies that had varying affinities to ErbB2, a protein present at high levels in some solid tumors, including breast cancer. Next, they incorporated different scFvs into the CAR backbone or "construct," such that they resulted in a range of CAR T cells - from those that had high affinity to ErbB2 to those that had low affinity to ErbB2. The newly engineered CAR T cells varied in their affinity to ErbB2 by three orders of magnitude. The researchers then conducted a series of experiments to test the functionality of the affinity-tuned CAR T cells and found that high-affinity CAR T cells did not discriminate tumor cells from normal cells and attacked all of them, whereas low-affinity CAR T cells were sensitive to tumor cells that had high levels of ErbB2 and not to normal cells that had low levels of the protein.

Next, they tested the engineered CAR T cells in mice that bore human cells with high levels of ErbB2 on one side of their bodies and human cells with normal levels of ErbB2 on the other side of their bodies. Here again, low-affinity CAR T cells selectively eliminated cells that had high levels of ErbB2 but had no effect on cells that had normal levels of the protein. In order to prove that this technology can be extended to other solid tumor targets, the researchers developed low-affinity CAR T cells targeting EGFR, a protein present in high levels in some lung and colon cancers, among others, and preliminary preclinical results showed that these CAR T cells were able to discriminate between cancer cells and normal cells.



Miki Ando et al. demonstrated the efficacy of suicide gene therapy by introducing inducible caspase-9 (iC9) into iPSCs. Activation of iC9 system in vivo with a specific chemical inducer of dimerization (CID) initiates a caspase cascade that eliminates iPSCs and tumors originated from iPSCs. They introduced this iC9/CID safeguard system into a previously reported iPSC-derived, rejuvenated cytotoxic T lymphocyte (rejCTL) therapy model and confirmed that rejCTLs from iPSCs are expressing high levels of iC9 without disturbing antigen-specific killing activity. iC9-expressing rejCTLs exert antitumor effects in vivo. Upon induction, the iC9 system efficiently leads to apoptosis in rejuvenated CTLs. This safeguard system can eliminate contaminating iPSCs, debulk tumors originated from iPSCs, stop cytokine release syndrome associated with iPSC-derived CTL therapy, and control “on-target, off-tumor toxicities”. It should be applicable to other cell therapies using iPSC-derived cells.
I think that the same principle can be used for the cultivation of human organs in the body of a pig. If we introduce the iC9 gene into the cells of a pig (of those lines that are tolerant to human cells). Then grown up from human iPSCs in the body of the pig human rejuvenated organ can be purified from pig cells via apoptosis.

Posted by: Dmitry Dzhagarov at September 1st, 2015 1:39 PM

Hi Dmitry, I'm not sure how important inducible suicide genes will help in CAR T cell therapies. I think other strategies may win out. (But as you say they could potentially be very useful in the production of human organs inside pigs).

One company whose name I cannot remember at present is producing CAR T cells whose receptors lock onto the back end of a specially engineered antibody, stop administering this antibody and the CAR T cell attack stops.

CytomX is looking to produce antibodies and T cell receptors that are inactivated until they encounter proteases in the tumor micro-envrioment.

Juno are looking at producing inhibitory CARs that prevent the activation of CAR T cells when a secondary healthy antigen is detected.

This low affinity CAR approach is also very interesting though.

Posted by: Jim at September 1st, 2015 5:28 PM

Does anyone know how much of a role tumor produced cytokines play in targeted T-cell inactivation?

Posted by: Dan Knauer at October 25th, 2015 12:58 PM

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