440 Utilizing Stim-R^TM technology to reduce live feeder cells in the tumor infiltrating lymphocyte culture process

<h3>Background</h3> Adoptive cell therapy with tumor-infiltrating lymphocytes (TIL) can mediate effective responses in solid tumors.^1–3 However, the use of live feeder cells during TIL manufacture may limit the scalability of TIL therapy. Approaches to replace feeder cells with synthetic alternatives could mitigate this limitation. However, the development of a synthetic replacement that can recapitulate the functions of feeder cells has been challenging. Lyell’s Epi-R<b>^TM</b> P2 process is a TIL manufacturing process designed to enrich the TIL product for stem-like CD8+ T cells and putative tumor-reactive clones, which have been associated with positive clinical outcomes.⁴ Here, we describe the development of Stim-R^TM, an antigen-presenting cell biomimetic technology, as a synthetic replacement for feeder cells which can be incorporated in the Epi-R P2 manufacturing process (figure 1A). <h3>Methods</h3> Irradiated feeder cells were analyzed using flow cytometry to assess the expression of T-cell stimulating ligands. Based on these data, Stim-R formulations were designed to present ligands expressed by feeder cells in different combinations and stoichiometries. These formulations were utilized to replace feeder cells in the ‘expansion 2’ step of Lyell’s Epi-R P2 TIL manufacturing process at research scale.⁴ A Stim-R formulation engaging CD3, CD28, CD137, and CD2, designated EXT221, meditated the highest CD8+ TIL expansion. Deeper characterization of TIL products produced from 12 melanoma tumors using either EXT221 or feeder cells was performed. The TIL products were evaluated for the expression of stemness markers (CD62L⁺CD27⁺ and CD39^-CD69^-) using flow cytometry, and retention of putative tumor-reactive clones, assessed as the highest frequency clones from the starting tumor material, using T cell receptor (TCR) sequencing. <h3>Results</h3> Incorporating EXT221 resulted in comparable TIL expansion (figure 1B) and product phenotype as products generated using feeder cells in the Epi-R P2 research scale process. Additionally, TIL products generated with EXT221 were polyclonal and retained a proportion of putative tumor-reactive clones. <h3>Conclusions</h3> A Stim-R formulation engaging CD3, CD28, CD137, and CD2, designated EXT221, was identified based on its ability to mediate robust CD8+ TIL expansion in ‘expansion 2’ of Lyell’s Epi-R P2 process. TIL products generated from 12 melanoma samples at research scale using EXT221 exhibited similar expansion and expression of stemness markers as feeder cell products and retained a proportion of putative tumor-reactive clones. Stim-R is being developed as a replacement for live feeder cells with the potential to overcome scalability limitations in TIL manufacturing and may make TIL therapy more accessible for patients. <h3>Acknowledgements</h3> We acknowledge Claire Toney, Omar Ali, Melissa Bedard, Audrey Garces, Stefan Siebert, Lora Zhao, Stasiu Nowak, and Jen Fleischer for their scientific input and support on this project. <h3>References</h3> Creelan BC, Wang C, Teer JK, <i>et al</i>. Tumor-infiltrating lymphocyte treatment for anti-PD-1-resistant metastatic lung cancer: a phase 1 trial. <i>Nat Med</i> 2021;<b>27</b>:1410–1418. Seitter SJ, Sherry RM, Yang JC, <i>et al</i>. Impact of prior treatment on the efficacy of adoptive transfer of tumor-infiltrating lymphocytes in patients with metastatic melanoma. <i>Clin Cancer Res</i> 2021;<b>27</b>:5289–5298. van den Berg JH, Heemskerk B, van Rooij N, <i>et al</i>. Tumor infiltrating lymphocytes (TIL) therapy in metastatic melanoma: boosting of neoantigen-specific T cell reactivity and long-term follow-up. <i>J Immunother Cancer</i> 2020;<b>8</b>:e000848. Krishna S, Lowery FJ, Copeland AR, <i>et al</i>. Stem-like CD8 T cells mediate response of adoptive cell immunotherapy against human cancer. <i>Science</i> 2020;<b>370</b>(6522):1328–1334.