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138 Background: Adoptive cell therapy (ACT) of autologous tumor-infiltrating lymphocytes (TIL) can produce long lasting treatment responses in patients (pts) with metastatic melanoma. In our initial report of 31 treated pts, we demonstrated overall response rate (ORR) of 48%. Due to the evolution of treatment options for metastatic melanoma with heavy reliance on immunomodulatory therapies, we sought to re-evaluate responses in the era of checkpoint blockade. Methods: Pts receive treatment consisting of 7 days of lymphodepleting chemotherapy consisting of cyclophosphamide and fludarabine. High dose interleukin-2 (720,000 IU/kg IV q 8 hrs up to 15 doses) was infused after TIL infusion. Responses were assessed by imaging per irRC. Results: A total of 74 pts have been treated in our initial TIL study with an updated ORR assessment of 43%. Stratification of pts according to their checkpoint blockade immune-modulator therapy prior to TIL ACT, demonstrated that prior Ipilimumab (Ipi) decreased ORR to 33% compared to 51% in checkpoint naïve pts and decreased overall survival (OS) post-TIL therapy (median 7.7 vs 24.6 months respectively). There were too few pts to assess the impact of anti-PD1 as a single agent. A multiparametric analysis revealed that LDH levels at the time of therapy mainly influences OS and ORR to TIL but still could not invalidate the impact of Ipi prior to TIL ACT. The durability of the response was also found to be different between the 2 groups (30% for Ipi prior and 50% No Ipi prior). This new reality also impacted previously reported parameters that correlated with ORR to TIL ACT such as the expression of B-and-T lymphocyte attenuator (BTLA) on CD8+ TIL. Interestingly, assessment of mutation load (ML) of the tumors prior to TIL ACT showed that the check point naïve group display a high ML ( > 100) within their tumor whereas some patients who had Ipi prior to TIL have a low ML ( < 100). Conclusions: Our preliminary analysis shows that pre-treatment with Ipi decreases ORR to TIL ACT. Understanding how prior ipi modifies TIL, the tumor and microenvironment will help to define the full extent of the impact and how to best treat Ipi refractory pts with TIL. Clinical trial information: NCT00338377.