TLR9 Agonists Potentiate Adoptive T Cell Therapy in Cancer through a B Cell–CD2 Costimulatory Axis

Adoptive T cell transfer (ACT) therapy offers curative potential for some patients with cancer. Toll-like receptor (TLR) agonists improve the efficacy of ACT therapy, and elucidating the underlying mechanism of potency could help determine the best way to maximize the benefits of TLR agonists. Here, we identified an innate-adaptive circuit in which TLR9-activated B cells augment CD8+ T cell fitness and antitumor activity through CD2-dependent costimulation. Among multiple TLR agonists tested, class B CpG uniquely programmed murine and human CD8⁺ T cells for superior effector differentiation, metabolic fitness, and antitumor control. Disruption of CD2 signaling blunted the benefits of TLR9 agonism, including impaired glycolytic capacity and reduced tumor control. Independently, blocking other costimulatory molecules, such as CD86, CD80, CD28 or ICOS, did not impair the antitumor activity of CpG-conditioned T cells. Gain of function experiments revealed that CD2 stimulation recapitulated the effect of TLR9 agonism, bolstering the effector function of tumor infiltrating lymphocytes and CAR T cells. Consistent with these findings, elevated CD2 expression in human tumors correlated with improved overall survival across multiple cancer patient cohorts, underscoring the clinical importance of this signaling cue. Together, these data uncover a non-canonical B cell-CD2 costimulation axis through which TLR9 agonists potentiate ACT, revealing a targetable pathway to overcome resistance to cell therapy in solid tumors.