Endothelial-leukemia interactions remodel drug responses uncovering T-ALL vulnerabilities.

T-cell acute lymphoblastic leukemia (T-ALL) is an aggressive and often incurable disease. To uncover therapeutic vulnerabilities, we first developed T-ALL patient-derived tumor-xenografts (PDX) and exposed PDX cells to a library of 433 clinical-stage compounds in vitro. We identified 39 broadly active compounds with anti-leukemia activity. Since endothelial cells (ECs) can alter drug responses in T-ALL, we developed an endothelial cells (ECs) / T-ALL co-culture system. We found that ECs provide pro-tumorigenic signals and mitigate drug responses to individual T-ALL PDX. ECs broadly rescued several compounds in most of the models, while other drugs were rescued only in individual PDXs suggesting unique crosstalk interactions and/or intrinsic tumor features. Mechanistically, co-cultured T-ALL and ECs underwent bi-directional transcriptomic changes at the single-cell level, highlighting distinct "education signatures". These changes were linked to a bi-directional regulation of multiple pathways in T-ALL and ECs. Remarkably, in-vitro EC-educated T-ALL cells mirrored ex-vivo splenic T-ALL at the single-cell resolution. Lastly, five effective drugs from the two drug screenings were tested in vivo and shown to effectively delay tumor growth/dissemination and prolonging the overall survival (OS). We anticipate that this T-ALL-EC platform can contribute to elucidating leukemia-microenvironment interactions and identify effective compounds and therapeutic vulnerabilities.