In situ vaccination improves efficacy of PD-1 blockade in unresponsive lymphoma tumors through induction of a highly efficient cross-presenting dendritic cell subset expressing TLR3

Abstract

Abstract B cell lymphomas are generally incurable, with low response rates to anti-PD1 therapy. Here, we tested the combination of in situ vaccination with PD-1 blockade in a pre-clinical mouse model. A20 lymphoma-bearing mice were treated with a PD1 blocking antibody with or without an in situ vaccine combining intratumoral (it) injections of FMS-like tyrosine kinase-3 ligand (Flt3L), poly-ICLC (pIC) and local irradiation (XRT) of the tumor. Untreated lymphoma tumors contained few dendritic cells (DC) and anti-PD1 treatment did not induce tumor regression. Flt3L treatment resulted in a dramatic increase of IRF8+TLR3+ DC in the tumor. 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 TLR3+ DC. Local XRT also enhanced uptake of dying tumor cells by it DC. Importantly, tumor antigens were taken up mainly by CD103+ DC and not CD103-DC. Accordingly, CD103+ DC isolated from the tumor induced proliferation of tumor-specific CD8+ T cells more efficiently than CD103-subsets. Combination of Flt3L with XRT and pIC induced tumor-reactive T cells, and delayed tumor growth and improved survival in 40% of mice. In situ vaccination also increased expression of PD-L1 on tumor cells and it DC. Consistently, combination of in situ vaccination with anti-PD1 led to complete tumor regression and increased long-term survival in the majority of mice. PD1 blockade also increased the number of tumor-reactive T cells and depletion of CD8+ T cells abrogated the anti-tumor effect. In conclusion, in situ vaccination combining Flt3L, local XRT and pIC improves efficacy of anti-PD1 in checkpoint-unresponsive lymphoma tumors through induction of a highly efficient cross-presenting DC subset.