In an ongoing in situ vaccine (ISV) phase I trial, 11 patients with indolent non-Hodgkin’s B cell lymphoma had a single lesion treated with Flt3L, radiation, and poly-ICLC (TLR3 agonist). In mice, Flt3L recruited TLR3+IRF8+ DCs to the tumor, and radiation synergized with poly-ICLC to induce their maturation into cross-presenting CD103+ DCs, mediating the influx and activation of tumor-reactive TILs; similar results were observed in patients. ISV induced regression of treated and untreated distant tumors in patients. In non-responders, ISV greatly increased PD-1 expression on CD8+ T cells, prompting a follow-up trial of combination ISV and anti-PD-1.
Indolent non-Hodgkin's lymphomas (iNHLs) are incurable with standard therapy and are poorly responsive to checkpoint blockade. Although lymphoma cells are efficiently killed by primed T cells, in vivo priming of anti-lymphoma T cells has been elusive. Here, we demonstrate that lymphoma cells can directly prime T cells, but in vivo immunity still requires cross-presentation. To address this, we developed an in situ vaccine (ISV), combining Flt3L, radiotherapy, and a TLR3 agonist, which recruited, antigen-loaded and activated intratumoral, cross-presenting dendritic cells (DCs). ISV induced anti-tumor CD8(+) T cell responses and systemic (abscopal) cancer remission in patients with advanced stage iNHL in an ongoing trial ( NCT01976585 ). Non-responding patients developed a population of PD1(+)CD8(+) T cells after ISV, and murine tumors became newly responsive to PD1 blockade, prompting a follow-up trial of the combined therapy. Our data substantiate that recruiting and activating intratumoral, cross-priming DCs is achievable and critical to anti-tumor T cell responses and PD1-blockade efficacy.