3D Tumor microenvironment interaction reveals AP‐1 complex regulation and contact‐mediated reprogramming of bone marrow stromal cells in chronic lymphocytic leukemia

Chronic lymphocytic leukemia (CLL) cells actively reprogram their tumor microenvironment (TME) to promote drug resistance and tumor progression. Tumor cell survival critically depends on heterotypic communication with benign cells in the microenvironment, particularly bone marrow-derived stromal cells (BMSCs). Our three-dimensional (3D) approach allows us to investigate spatially defined, mutual direct cell-cell interactions between CLL B cells, autologous T cells, and BMSCs, forming complex scaffold-like structures reminiscent of in vivo conditions. Here, we observe that CLL B cells localized in the core regions of 3D structures upregulate the AP-1 transcription factor complex, which confers significant protection against therapy-induced cell death. Additionally, regulatory T cells (T_reg) and follicular T helper cells (T_fH) are more abundant in the core regions. CLL B cells, in turn, induce contact-mediated reprogramming of BMSCs, resulting in a novel, distinct BMSC population with inflammation, bone immune evasion, and cancer-associated fibroblast-like features. Our findings reveal critical mechanisms by which CLL cells exploit the TME to drive pathogenesis and therapeutic resistance using realistic 3D setups, highlighting the potential of targeting AP-1 and the stromal compartment in future treatment strategies.