IMMU-55. Cerebrospinal fluid reduces the efficacy of T cell responses <i>in vitro</i> to leukemia-targeting immunotherapy

Abstract Leptomeningeal cancer, also known as leptomeningeal disease (LMD), occurs when cancer cells metastasize to the leptomeninges (LM). LMD occurs in &amp;gt;110,000 cancer patients per year in the United States arising from multiple cancer types, including leukemia/lymphomas, solid tumors, and primary brain tumors. Treatment options for LMD are limited, typically ineffective, and come with significant morbidity risks. LMD is rarely curable with a median duration of survival of typically weeks to months. As the landscape of cancer treatments is being dramatically altered by revolutionary immunotherapies like chimeric antigen T-cells (CAR-T), bispecific antibodies, and checkpoint inhibitors, additional research is essential to further optimize these immunotherapies to LMD as lessons learned from their applications in other anatomic locations may not readily translate to the LM. Immune cell biology within the LM microenvironment is unique. As such, we hypothesized that cerebral spinal fluid (CSF) impacts the function and efficacy of immunotherapies in this niche. To understand the differences between T cell function in CSF compared to conventional in vitro T cell conditions, we utilized a T cell stimulation assay combining human peripheral blood T cells, a B-cell acute lymphoblastic leukemia (B-ALL) cell line called Nalm6, and an antiCD3-antiCD19 bispecific T cell engager (BiTE) in either human CSF or conventional T cell media. Surface receptor expression for activation markers, exhaustion markers, and differentiation state was obtained along with intracellular cytokine production and cytotoxicity during the height of the T cell activation. We found that the T cells activated in CSF were less proliferative, exhibited higher apoptosis, had altered stimulation phenotypes, and cleared the leukemia less effectively with the BiTE than in the conventional T cell media. These results demonstrate a need to define mechanisms which will enhance T cell function and survival in CSF to optimize anti-tumor activity by immunotherapy against LMD.