Chimeric antigen receptor (CAR) T cell therapy has achieved extraordinary success in
eliminating B cell malignancies; however, so far, it has shown limited efficacy in the treatment of
solid tumors, which is thought to be due to insufficient CAR T cell activation. We hypothesized
that the transcription factor PU.1, a master regulator of innate cell functionality, may augment proinflammatory
CAR T cell activation. T cells were engineered with a CEA-specific CAR together
with the constitutive expression of PU.1. CAR-redirected T cell activation was recorded for canonical
functionality in vitro under conditions of prolonged repetitive antigen exposure. Ectopic PU.1
expression in CAR T cells upregulated the costimulatory receptors CD40, CD80, CD86, and CD70,
which, unexpectedly, did not augment effector functions but hampered the upregulation of 4-1BB,
decreased IL-2 production, reduced CAR T cell proliferation, and impaired their cytotoxic capacities.
Under “stress” conditions of repetitive engagement of cognate tumor cells, CAR T cells with ectopic
PU.1 showed reduced persistence, and finally failed to control the growth of cancer cells. Mechanistically,
PU.1 caused CAR T cells to secrete IFN-β, a cytokine known to promote CAR T cell attrition
and apoptosis. Collectively, PU.1 can polarize the functional capacities of CAR T cells towards
innate cells.