Targeting FAP/CAFs to rewire immune-excluded and resistant tumor niches

Immune checkpoint blockade (ICB) has reshaped cancer therapy, yet many solid tumors remain immune-excluded, with cytotoxic T cells trapped in stromal regions and unable to access malignant nests. In these settings, cancer-associated fibroblasts (CAFs) and their extracellular matrix programs act as active ecosystem regulators that impose physical transport barriers, chemokine "gating" (notably CXCL12-CXCR4), and stromal exclusion signals such as TGF-β, collectively sustaining resistance to both immunotherapy and targeted agents, and contributing to treatment intolerance and rehabilitation-relevant functional burden. Building on this mechanistic blueprint, we review clinically relevant strategies that use fibroblast activation protein (FAP), when sufficiently expressed and spatially relevant, as a tractable stromal address label to rewire resistant niches, ranging from FAP-targeted immunocytokines and conditional costimulation to regional FAP-CAR-T approaches and FAPI-PET-enabled stratification/theranostics. We highlight key safety constraints and failure modes that can limit not only efficacy but also functional recovery, and propose a trial-ready roadmap centered on state-guided selection, mechanism-matched combinations, early pharmacodynamic verification of rewiring, and pragmatic functional endpoints.