Cancer immunotherapy targeting myeloid cells requires endosomal pattern recognition

Immunotherapy is now an established and efficient treatment option for many cancer patients, but unfortunately, a proportion of the patients still experience poor outcomes due to resistance to treatment. Thus, there is a need to both understand key mechanisms of resistance as well as to develop new treatments. Herein, we explore an immunotherapeutic approach that targets tumor associated macrophages (TAMs) as an alternative to direct T cell targeting. We demonstrate that TAMs need to have a functional endosomal pattern recognition machinery for immunotherapy to be efficient. This is true for TAM targeting using both anti-PD-L1 (αPD-L1) or anti-MARCO (αMARCO) antibodies, and we determine that the absence of endosomal Toll-like receptors (TLRs) makes the TAMs unresponsive to treatment while retaining an immunosuppressive phenotype. For αMARCO treatment, we also find that TLR9 activation to express components of the inflammasome is specifically needed to render TAMs responsive to treatment. This key feature of immunotherapy efficacy, with TAMs being sensitized by the tumor microenvironment via endosomal TLRs, could be exploited to improve immunotherapeutic outcomes.