Remote Control CARs to Prevent and Reverse T Cell Exhaustion

A major cause of resistance to CAR T cell therapy is T cell failure, most commonly due to T cell exhaustion. Exhaustion occurs as a result of excessive zeta phosphorylation, either due to exposure to high tumor burdens, and/or the propensity for scFvs contained within CAR receptors to spontaneous tonically signal, due to clustering in the absence of antigen. Current understanding of the biology of human T cell exhaustion remains incomplete. We have used tonically signaling CAR T cells to model human “T cell exhaustion in a dish”. Using drug regulatable destabilizing domains to regulate CAR protein levels, we observe that diminished CAR protein levels prevent antigen induced- and tonic-signaling and therefore can prevent exhaustion in CAR T cells prone to receptor clustering. Rapid drug responses in this model system also allows interrogation of the reversibility of human T cell exhaustion. When tonic CAR signaling is prevented in T cells that have already acquired characteristics of T cell exhaustion, we observe essentially complete reversibility of the exhaustion phenotype and marked improvement in function. Functional rejuvenation induced via remote control of CAR protein levels is significantly more potent than that induced by PD-1 blockade. These results suggest that regulatable CAR expression systems could prevent and/or reverse T cell exhaustion by attacking the “root cause”, namely excessive or prolonged zeta phosphorylation. We posit that remote control of CAR protein levels could provide a mechanism to enhance safety of CAR T cells by diminishing antigen induced signaling in the presence of excession toxicity, but could also enhance efficacy by preventing or reversing T cell exhaustion.