Abstract BACKGROUNDS: Host tumor immune microenvironment has been shown to play a critical role on the outcome of the breast cancer patient. Among the immune cells, tumor infiltrating lymphocytes (TILs) were shown to be a predictive biomarker for the efficacy of chemotherapy by clinical trials. Triple negative breast cancer (TNBC) is known to have high infiltration of TILs compared with other subtypes. TILs were reported to be associated with the response to doxorubicin and cyclophosphamide, and with better survival in breast cancer patients. Recently, there are accumulating evidences that organoids generated from patient tumors are useful as models for drug sensitivity pre-clinical trials. Here, we hypothesized that co-culture of TILs with organoids will mimic human tumor immune microenvironment and enhance the drug sensitivity. MATERIAL AND METHODS: GSE22226 that contains both clinical data and gene expression profile of untreated tumors was used as the breast cancer patient cohort that underwent neoadjuvant chemotherapy (NAC). Relative fractions of 22 types of infiltrating immune cells were estimated from tumor gene expression with the CIBERSORT method. Organoids of the same tumors grown as mouse xenografts were generated using standard methods. Organoids (1000/well of 96-w plate) were treated with doxorubicin (0.01-100 µM) for 2 days before addition of TILs of same patient (1-16 x103 per well). Cytotoxicity was assessed after another two days with the CellTiter-Glo 3D cell viability assay. RESULTS: We first assessed the clinical relevance of TILs in response outcome of NAC by utilizing the previously reported cohort, in which Adriamycin, Cyclophosphamide and Taxane were used. We found that the number of TILs were significantly higher in the patients that achieved pathological complete response (pCR) when compared with the ones that did not. We established 18 PDX models all of which are from TNBC and we successfully expanded TILS 7 of these patients. We derived 8 organoids from the patients. We performed the experiment of adding TILs to the organoids. TILs effectively killed organoids when TILs were cultured at 1:8 or 1:16 ratio. Furthermore, the killing of organoids by doxorubicin was enhanced by the addition of TILs. CONCLUSION: Amount of TILs associated with better outcome in NAC cohort. In pre-clinical setting, addition of TIL intensified the sensitivity of organoids to doxorubicin. Citation Format: Yoshihisa Tokumaru, Masanori Oshi, Eriko Katsuta, Nobuhisa Matsuhashi, Manabu Futamura, Kazuhiro Yoshida, Kazuaki Takabe. Organoid and TIL co-culture system models the human tumor immune microenvironment and drug sensitivity [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 317.