Myeloid-conditioning using Toll-like receptor agonists to restore immune potency against melanoma. (TUM4P.926)

Abstract

Abstract Suppressor cells of myeloid origin contribute to the immune imbalance observed in advanced melanoma. Toll-Like Receptor (TLR) signaling regulates myeloid differentiation and maturation. This study sought to determine the distribution of myeloid suppressor cell subsets in B16F10 melanoma-bearing mice, and modulate their immunosuppressive phenotype by employing a regimen of intracellular TLR agonist(s). Tolerogenic dendritic cells (tDC) and granulocytic myeloid-derived suppressor cells (gMDSC) are primarily observed in the periphery (spleen, PBMC, draining lymph node) of B16F10-bearing mice whereas M2 macrophages (M2) are noted to be intratumoral. Toll-Like Receptor-expression analysis by quantitative PCR showed TLR9 was ubiquitously expressed on tDC, gMDSC and M2, while TLR3 was restricted to tDC and M2. Myeloid conditioning regimens employing TLR3 and 9 agonists were tested in vitro on bone marrow-derived tDC (CD11c+, MHCII+, IL-10+) and M2 (F4/80+, CD206+, Arg1+). Combinations of TLR3+9 agonists or Type A+B TLR9 agonists decreased IL-10/Arg1 mRNA expression while triggered IL-12 expression. In vivo, subcutaneous injection of type A+B TLR9 agonists reduced circulating tDC and gMDSC prevalence by 67% and 82% respectively, intratumoral M2 dropped by 35%. In conclusion, we demonstrate combining type A+B TLR9 agonists promoted immune potency by reducing prevalence of peripheral tDC and gMDSC, as well as tumoral M2 in vivo, and triggering IL-12 production by tDC and M2 in vitro.