Tumor–Immune Cell Crosstalk Drives Immune Cell Reprogramming Towards a Pro-Tumor Proliferative State Involving STAT3 Activation

<b>Background/Objectives</b>: Tumor-induced immune reprogramming is increasingly recognized as a key mechanism by which cancers evade surveillance and promote disease progression. The interaction between cancer and immune cells within the tumor microenvironment (TME) can drive phenotypic and functional changes in immune populations, facilitating metastasis and immune evasion. <b>Methods</b>: In this study, we used co-culture models to expose THP1 monocytes to triple-negative breast cancer (TNBC) cells, MDA-MB-231 and BT-549, either directly or indirectly via tumor-conditioned media, to mimic tumor-immune cell communication. Transcriptomic and pathway analyses revealed that cancer-exposed monocytes adopt a reprogrammed phenotype marked by activation of pro-tumorigenic signaling pathways, enhanced proliferative capacity, and elevated expression of pro-inflammatory cytokines such as IL6. <b>Results</b>: Functional assays confirmed a significant increase in monocyte proliferation under both direct and indirect tumor exposure. Importantly, we demonstrated that this tumor-driven proliferation of THP1 cells could be suppressed by the STAT3 inhibitor STAT3-IN-12. This highlights the critical role of STAT3 signaling in mediating immune cell transformation and supporting a novel immunomodulatory approach for therapeutic intervention. <b>Conclusions</b>: These findings support the potential for targeting tumor-educated transcriptional programs as a novel immunomodulatory strategy in cancer treatment. Restoring immune cell homeostasis and suppressing pro-tumor phenotypes through pharmacological inhibition of the key signaling nodes such as STAT3 may complement existing cancer therapies. This study provides new insights into immune cell plasticity in cancer and identifies actionable strategies to counteract tumor-driven immune dysregulation.