A few years ago we were dismayed to observe that our CAR targeting the GD2 disialoganglioside clustered, induced a “tonic signal”, and lead to T cell exhaustion and poor activity against osteosarcomas (Long, Nat Med 2015).

We tried to engineer a solution and stumbled upon a mutated high affinity (HA) version of the GD2 CAR that made the undesirable effects worse. Furthermore, this HA CAR consistently induced the hallmark features of exhaustion in healthy, naïve human T cells, in a dish, in 10 days. We realized that our flawed CAR was a window to peer into the mechanism of human T cell exhaustion. Some times “pressure makes diamonds”.

So we run every comparative analysis tool available to determine whether our HA.28.z-CAR T cells displayed the same profile as exhausted T cells described in the LCMV murine model, and in other systems. The results were remarkably comparable. Confident in the ability of our new system to act as a human model, we sought to test the hypothesis that “partnerless NFAT”, a model put forth by the Rao lab, drove the altered transcriptional profile of exhausted T cells.

Initial findings demonstrated that c-JUN overexpression induced transcriptional reprogramming and exhaustion resistance, and seemed to validate the model. But we also saw that a version of c-JUN without transactivation activity also induced resistance. Furthermore, targeting inhibitory transcription factors using CRISPR/Cas9 was equally effective.

These unexpected results suggested that an imbalance between activating and inhibitory programs governs a suppressive transcriptional program that drives exhaustion. Within this milieu, c-JUN overexpression disrupts inhibitory AP-1 complexes, restoring the balance.

Using synthetic biology we created a model that expands the basic biological understanding of human T cell exhaustion, and provides a clinically actionable platform that may improve the efficacy of CAR T cells in humans.

Even among the early adopters, few believed that man-made receptors could wield the remarkable potency we are witnessing against high-burden, rapidly growing, B cell malignancies refractory to all other standard therapies. The success of CAR T cells marks a paradigm-shift in understanding the potential of synthetic biology to cure human disease. From our vantage point, synthetic biology is entering a golden era of clinical translation: both driving the development of novel medicines, and accelerating fundamental research.

We envision clinical testing of exhaustion-resistant, multispecific, logic-gated, regulatable CAR T cells in the near term, and remain optimistic that engineered receptors expressed on immune cells will ultimately prove effective for the treatment of some solid cancers, autoimmune, and infectious diseases.

Blog post written by Elena Sotillo & Crystal Mackall

Illustrated by Llum Dimensions