Myeloid reprogramming by poly(I:C) recruits progenitor-exhausted CD8+ T cells and sensitizes rhabdoid tumors to PD-1 blockade

Immune exclusion remains a major barrier to effective immunotherapy in solid tumors. Given the abundance and plasticity of tumor-associated macrophages (TAMs) in many tumors, including pediatric tumors, we investigated whether TLR3 activation could reprogram them to facilitate immune access. Single-cell and spatial profiling in a mouse model of rhabdoid tumors showed that they are dominated by TLR3-expressing TAMs, whose depletion delays tumor growth. Treatment with the TLR3 agonist poly(I:C) promotes immune cell infiltration, including progenitor-exhausted CD8+ T cells, by multiple mechanisms including the reduction and reprogramming of immunosuppressive TAMs, promoting nitric-oxide production by peritumoral macrophages, and inducing CXCL9/10 production. Combined poly(I:C) and PD-1 blockade elicited durable, complete tumor rejection. Human macrophages from tumor biopsies showed conserved TLR3 responsiveness, underscoring translational potential. These findings uncover a mechanism by which TLR3-driven myeloid reprogramming transforms immune-excluded tumors into checkpoint-responsive ones, revealing a therapeutic path to overcome resistance to PD-1 blockade.