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The translatability of mouse models for the clinical development of cancer immunotherapies remains limited because of substantial differences between the murine and the human immune systems, as well as dissimilarities in the tumor biology. Non-human primates (NHPs) display good homologies towards the human immune system, based on the development of vaccines to several human pathogens. Tumor-bearing monkeys (TBMs) are NHPs that spontaneously developed cancer with progression patterns similar to humans, potentially bridging the experimental gap between early preclinical models and cancer patients treated with immunotherapeutic agents. TBMs are prevalently rhesus macaques (Macaca mulata) and the most common cancer types are of gastrointestinal, urogenital and endocrine origin. Rhesus macaques are genetically similar to humans and share many characteristics of aging. In both, humans and rhesus macaques, cancer incidence increases with age with the greatest incidence in those over 60 years of age and 20 years, respectively. TBMs were recently employed to assess the tumor targeting and the pharmacodynamics of the FAP-expressing tumor stroma-targeted immunocytokine FAP-IL2v (Evers et al, AACR 2014, Abstract 2592) and costimulatory agonist FAP-4-1BBL (Claus et al, Science translational Medicine, 2019). In the latter study, we could show targeting of FAP-4-1BBL to FAP-expressing tumor stroma and lymph nodes in a colorectal cancer-bearing rhesus monkey. These data were the basis to investigate tumor targeting of FAP-4-1BBL in an on-going clinical imaging study. In the present work, we validated further TBMs as translational models for cancer immunotherapy, by performing an imaging/biomarker study in animals exposed to a second FAP-targeted TNFRSF agonist. Two breast cancer-bearing rhesus monkeys (one triple-negative and one Luminal A) were first pre-immunized with a diphtheria/pertussis/tetanus (DTP) vaccine, and then exposed to a single injection of the TNFRSF agonist. In both animals, we could measure a strong systemic immune activation, induction of the TNFRSF agonist in T cells and also tumor regression. These observations validated the applied pre-immunization strategy to induce the TNFRSF protein expression, and confirmed the target to be pursued in the clinical setting. In conclusion, we consider TBMs as valuable translational animal models to generate proof-of-mechanism evidence in small “signal-seeking” preclinical studies. Tumor targeting, biodistribution, peripheral and intra-tumoral pharmacodynamic activity, pharmacokinetics, immunogenicity, intra-tumoral metabolic activity and tumor regression can all be assessed in TBMs. Because of the similar tumor stroma biology shared between humans and rhesus macaques, TBMs are particularly well suited to test FAP-targeting agents. We anticipate that testing cancer immunotherapy compounds in TBMs could be of high predictability for clinical behavior. Citation Format: Maurizio Ceppi, Michael Hettich, Volker Teichgräber, Wouter Driessen, Dietrich Tuerck, Estelle Marrer-Berger, Stefan Evers, Flavio Crameri, Bruno Gomes, Jürgen Bachl, Christian Klein, Christina Claus, Maria Amann, Oliver Krieter, Greg Dugan, David Caudell, Jason Grayson, Sai Kumar Solingapuram Kiran, Mark Cline. Tumor-bearing non-human primates: An unrivaled model for translational cancer immunology research [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 6135.