The human body produces T cells to recognize and fight disease. Each T cell has a unique T cell receptor (or TCR) on its surface that surveils small fragments of proteins presented by other cells. Upon detecting evidence of cancer or infection, a subset of T cells binds the diseased cells and orchestrates their elimination. When tumors and infections cannot be eradicated naturally, researchers employ immunotherapies to boost the immune system’s effectiveness.
By inserting genes encoding a tumor-specific T cell receptor (TCR) into a patient’s T cells, researchers can engineer a large population of T cells to target tumor cells. This approach, called T cell receptor gene therapy, has yielded clinical successes where conventional cancer treatments have failed. However, T cell receptor gene therapy is not without risk. The introduced receptor can become tangled with the resident receptor in each engineered T cell, causing some of these cells to attack healthy cells. A new technique developed by Caltech researchers prevents this from happening, increasing the safety of T cell receptor gene therapy.
The technique, called domain swapping, was developed in the laboratory of David Baltimore, president emeritus and the Robert Andrews Millikan Professor of Biology. A paper describing the findings appears in the November 8 issue of the journal eLife.
The specificity of the T cell receptor (TCR) in each T cell results from the pairing of two protein chains–called an alpha chain and a beta chain–each of which has constant domains (shared between all TCRs) and variable domains (unique to each T cell). Normally, each T cell encodes only one alpha chain and one beta chain, which pair to form a single TCR. In T cell receptor gene therapy, the introduction of genes encoding a tumor-reactive TCR results in T cells that express two alpha chains and two beta chains, with four possible pairings. This non-physiological situation poses a risk of autoimmunity.
The group’s solution was to generate hybrid genes encoding T cell receptor (TCR) chains with their alpha and beta constant domains swapped in a compensatory fashion. When correctly paired, these domain-swapped TCRs retain all of the domains necessary to function. Indeed, the group found that domain-swapped TCRs and unmodified TCRs both function in human T cells, and they prevented tumor growth in mice to a similar extent. However, whereas unmodified TCRs mispaired with resident TCR chains in both mouse and human T cells, and caused autoimmunity in mice, domain-swapped TCRs did not.
In addition to preventing mispairing, domain-swapped TCRs highlight a surprising robustness to the function of the TCR complex. The Caltech group teamed with Mike Kuhns at the University of Arizona to determine that domain-swapped TCRs assemble in a similar manner to unmodified TCRs despite significant structural rearrangement of the constituent protein chains. Domain-swapped TCRs may be useful tools for further study of the structure and function of the TCR complex.
Finally, in collaboration with Wolfgang Uckert at the Max Delbrück Center for Molecular Medicine in Berlin, the researchers showed that domain-swapped TCRs were expressed at higher levels on the T cell surface when the resident TCR genes were silenced.
“Our paper focuses on the increased safety afforded by domain-swapping, but combining these two solutions may result in a therapy with improved safety and efficacy compared to current practice,” Bethune says.
Citation: Bethune, Michael T., Marvin H. Gee, Mario Bunse, Mark S. Lee, Eric H. Gschweng, Meghana S. Pagadala, Jing Zhou et al. “Domain-swapped T cell receptors improve the safety of TCR gene therapy.” eLife 5 (2016): e19095.
Research funding: National Institutes of Health, Prostate Cancer Foundation, Jane Coffin Childs Postdoctoral Fellowship, Pew Charitable Trusts.
Adapted from press release by California Institute of Technology.