Improving efficacy of PD-1 blockade in unresponsive lymphoma tumors with in situ vaccination through induction of a highly efficient cross-presenting dendritic cell subset.

Abstract

76 Background: Low-grade non-Hodgkin’s B-cell lymphomas are generally incurable; preliminary results with anti-PD-1 therapy have yielded low response rates. Tumoral DC infiltration correlates with efficacy of checkpoint blockade and tumor-targeted vaccines represent promising, novel treatment strategies to induce anti-tumor T cells. Methods: A20 lymphoma-bearing mice were treated with a PD-1 blocking antibody with an in situ vaccine (ISV) consisting of intratumoral injections of FMS-like tyrosine kinase-3 ligand (Flt3L), local irradiation (XRT) of the tumor and intratumoral injections of the TLR3 agonist poly-ICLC (pIC). Results: Untreated lymphoma tumors contained very low numbers of DC and treatment with anti-PD1 alone did not induce tumor regression or increase survival. Flt3L treatment resulted in a dramatic increase of IRF8+TLR3+ DC at the tumor site, the draining lymph node and the spleen. XRT of A20 cells induced activation of Flt3L-treated splenic DC in vitro and local XRT of the tumor in vivo induced expression of CD103 on infiltrating TLR3+ DC. Local XRT also enhanced uptake of dying tumor cells by DC. Interestingly, tumor antigens were taken up mainly by CD103+ DC and not CD103- subtypes. CD103+ expression distinguishes a subset of migratory cross-presenting DC. Accordingly, CD103+ DC isolated from the tumor induced proliferation of tumor-specific CD8+ T cells more efficiently than CD103- subsets. The combination of Flt3L with XRT and pIC induced tumor-reactive, IFNg-producing T cells, but delayed tumor growth and improved survival only in 40% of mice. ISV also increased expression of PD-L1 on tumor cells and tumor infiltrating DC. Consistently, combination of ISV with PD-1 blockade led to complete tumor regression and increased long-term survival in the majority of mice. PD-1 blockade also increased the number of tumor-reactive T cells and depletion of CD8+ T cells abrogated the anti-tumor effect. Conclusions: In situ vaccination can improve efficacy of anti-PD-1 in checkpoint-unresponsive lymphoma tumors through induction of a cross-presenting DC subset leading to long-term regression of established lymphoma tumors.