An immune competent in vitro model of IL2 induced vascular leakage enables mechanistic insights and evaluation of mitigation strategies.

Interleukin-2 (IL2) has potent anticancer properties but its clinical use is limited by severe dose-dependent side effects, notably capillary leak syndrome (CLS) - a life-threatening condition marked by vascular barrier disruption and multi-organ dysfunction. Despite extensive efforts to engineer IL2 variants with improved pharmacokinetics or reduced toxicity, none of the 45 IL2-based agents currently under investigation across 139 trials have secured regulatory acceptance for cancer or autoimmune applications, largely due to unresolved safety concerns and limited efficacy, suggesting an incomplete understanding of IL2-induced CLS pathogenesis. Here, we describe the development of an immune-competent in vitro model of IL2-induced CLS comprising human primary endothelial cells co-cultured with donor-derived peripheral blood mononuclear cells (PBMCs) that recapitulates IL2 dose-dependent endothelial barrier loss. We demonstrate that neither IL2 alone nor cytokines released from IL2-stimulated PBMCs are sufficient to impair barrier integrity and show that direct cell-cell proximity is required for barrier loss and endothelial activation. Proteomic profiling identified key IL2-induced inflammatory, adhesion, and matrix remodeling pathways and pharmacological inhibition of selected targets mitigated IL2-mediated vascular dysfunction. This platform recapitulates key hallmarks of IL2-induced CLS, provides a mechanistically relevant, human-specific tool to study CLS pathogenesis and supports the development of safer IL2-based therapeutics.