Cancer immunotherapy targeting murine myeloid cells requires endosomal pattern recognition

Immunotherapy is now an established and efficient treatment option for many cancer patients. However, a proportion of patients still experience poor outcomes due to treatment resistance. Thus, a clear understanding of key mechanisms of resistance is needed for the development of new treatments. Here, we employ mouse models to explore an immunotherapeutic approach based on anti-MARCO (αMARCO) and anti-PD-L1 (αPD-L1) antibody-mediated targeting of tumor-associated macrophages (TAM). We demonstrate that effective immunotherapy relies on a functional endosomal pattern recognition machinery. We determine that endosomal Toll-like receptors (TLR), specifically TLR9, precondition macrophages to respond to αMARCO treatment by regulating the transcription of inflammasome components. Absence of TLRs renders TAMs unresponsive to treatment while retaining an immunosuppressive phenotype. Thus, we uncover the intracellular TLR signalling as a feature of immunotherapy efficacy, required to sensitise TAMs to treatment, and indicate that TLR targeting could be exploited to improve immunotherapeutic outcomes.