TLR7 tolerance is independent of the type I IFN pathway and leads to loss of anti-tumor efficacy in mice

Systemic administration of small molecule toll-like receptor (TLR)-7 agonists leads to potent activation of innate immunity and to the generation of anti-tumor immune responses. However, activation of TLRs with small molecule agonists may lead to the induction of TLR tolerance, defined as a state of hyporesponsiveness to subsequent agonism, which may limit immune activation, the generation of anti-tumor responses and clinical response. Our data reveal that dose scheduling impacts on the efficacy of systemic therapy with the selective TLR7 agonist, 6-amino-2-(butylamino)-9-((6-(2-(dimethylamino)ethoxy)pyridin-3-yl)methyl)-7,9-dihydro-8H-purin-8-one (DSR-6434). In a preclinical model of renal cell cancer, systemic administration of DSR-6434 dosed once weekly resulted in a significant anti-tumor response. However, twice weekly dosing of DSR-6434 led to the induction of TLR tolerance, and no anti-tumor response was observed. We show that TLR7 tolerance was independent of type I interferon (IFN) negative feedback because induction of TLR7 tolerance was also observed in IFN-α/β receptor knockout mice treated with DSR-6434. Moreover, our data demonstrate that treatment of bone marrow-derived plasmacytoid dendritic cells (BM-pDC) with DSR-6434 led to downregulation of TLR7 expression. From our data, dose scheduling of systemically administered TLR7 agonists can impact on anti-tumor activity through the induction of TLR tolerance. Furthermore, TLR7 expression on pDC may be a useful biomarker of TLR7 tolerance and aid in the optimization of dosing schedules involving systemically administered TLR7 agonists.