Despite the impressive rates of clinical response to programmed death 1 (PD-1) blockade in multiple cancers, the majority of patients still fail to respond to this therapy. The CT26 tumor in mice showed similar heterogeneity, with most tumors unaffected by anti-PD-1. As in humans, response of CT26 to anti-PD-1 correlated with increased T-and B-cell infiltration and IFN expression. We show that intratumoral injection of a highly interferogenic TLR9 agonist, SD-101, in anti-PD-1 nonresponders led to a complete, durable rejection of essentially all injected tumors and a majority of uninjected, distant-site tumors. Therapeutic efficacy of the combination was also observed with the TSA mammary adenocarcinoma and MCA38 colon carcinoma tumor models that show little response to PD-1 blockade alone. Intratumoral SD-101 substantially increased leukocyte infiltration and IFN-regulated gene expression, and its activity was dependent on CD8 + T cells and type I IFN signaling.Anti-PD-1 plus intratumoral SD-101 promoted infiltration of activated, proliferating CD8 + T cells and led to a synergistic increase in total and tumor antigen-specific CD8 + T cells expressing both IFN-γ and TNF-α.Additionally, PD-1 blockade could alter the CpG-mediated differentiation of tumor-specific CD8 + T cells into CD127 low KLRG1 high shortlived effector cells, preferentially expanding the CD127 high KLRG1 low long-lived memory precursors. Tumor control and intratumoral T-cell proliferation in response to the combined treatment is independent of T-cell trafficking from secondary lymphoid organs. These findings suggest that a CpG oligonucleotide given intratumorally may increase the response of cancer patients to PD-1 blockade, increasing the quantity and the quality of tumor-specific CD8 + T cells.PD-1 blockade | TLR9 agonist | multifunctional CD8+ T cells

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e14550 Background: CAR T cell therapy holds enormous promise for many cancer types but its application may be limited by serious toxicities. To lower this hurdle, our aim is to engineer tunable cell therapies. One of our approaches includes a “ON-switch” chimeric antigen receptor (Wu et al., Science 2015) that requires the administration of a small molecule acting as a dimerizing agent between one polypeptide chain containing the antigen recognition domain and half of an inducible heterodimerization system and another polypeptide chain containing the second half of the inducible heterodimerization motif, the CD3ζ chain and a costimulatory motif. Using an FDA approved small molecule drug, we evaluate the reversibility of ON-switch CAR T cells in preclinical models. Methods: First, we evaluated the proliferation, cytotoxicity and cytokine production of several ON-switch constructs in human primary T cells. Next, to address the reversibility of the ON-switch (ON→OFF→ON), we performed a series of co-culture experiments where the small molecule drug was added to tumor cells and ON-switch CAR T cells, then washed out, and then re-introduced back into the co-cultures. We compared CAR T cell mediated killing and cytokine production from the On-switch CAR T cells relative to a canonical CAR T control. Results: Our On-switch CAR T cells were shown to proliferate, secrete cytokines as well as mediate dose dependent cytotoxicity in the presence of the small molecule drug. Importantly, in the presence of antigen but in absence of the small molecule drug we did not measure any significant functional activity in our ON-switch CARs. Additonally, following the removal of the small molecule drug over a period several days we did not observe any significant CAR mediated cytotoxicity. Following the subsequent re-addition of the small molecule, we observed further CAR T cell mediated cytotoxicity against tumor cells. Conclusions: These results show that the small molecule inducible On-switch CARs maintain functional activity as well as reversibility allowing for the tunable control of a CAR T cell.