Research ArticleCAR-T CELLS

Engineering γδT cells limits tonic signaling associated with chimeric antigen receptors

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Science Signaling  10 Sep 2019:
Vol. 12, Issue 598, eaax1872
DOI: 10.1126/scisignal.aax1872

Taming tonic signaling

Adoptive transfer of T cells engineered to express a chimeric antigen receptor (CAR) is an effective therapy for select lymphomas. The potency of this therapy can be limited by antigen-independent (tonic) signaling, which promotes progressive CAR-T cell inactivation. Fisher et al. used mass cytometry to analyze CAR-T cells and found that the process of increasing αβ T cell numbers during CAR-T cell production (expansion) was sufficient to increase tonic signaling in αβ T cells. In contrast, expansion of γδT cells did not alter their basal activity. When these cells were engineered to express a chimeric costimulatory receptor, they specifically recognized transformed, but not healthy, myeloid cell targets. These data demonstrate a strategy for engineering specific antitumor responses free of the complications associated with tonic signaling.


Despite the benefits of chimeric antigen receptor (CAR)–T cell therapies against lymphoid malignancies, responses in solid tumors have been more limited and off-target toxicities have been more marked. Among the possible design limitations of CAR-T cells for cancer are unwanted tonic (antigen-independent) signaling and off-target activation. Efforts to overcome these hurdles have been blunted by a lack of mechanistic understanding. Here, we showed that single-cell analysis with time course mass cytometry provided a rapid means of assessing CAR-T cell activation. We compared signal transduction in expanded T cells to that in T cells transduced to express second-generation CARs and found that cell expansion enhanced the response to stimulation. However, expansion also induced tonic signaling and reduced network plasticity, which were associated with expression of the T cell exhaustion markers PD-1 and TIM-3. Because this was most evident in pathways downstream of CD3ζ, we performed similar analyses on γδT cells that expressed chimeric costimulatory receptors (CCRs) lacking CD3ζ but containing DAP10 stimulatory domains. These CCR-γδT cells did not exhibit tonic signaling but were efficiently activated and mounted cytotoxic responses in the presence of CCR-specific stimuli or cognate leukemic cells. Single-cell signaling analysis enabled detailed characterization of CAR-T and CCR-T cell activation to better understand their functional activities. Furthermore, we demonstrated that CCR-γδT cells may offer the potential to avoid on-target, off-tumor toxicity and allo-reactivity in the context of myeloid malignancies.

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