(826) ACKR1⁺ Venular Endothelial Cells Orchestrate T Cell Infiltration and Angiogenesis in Solid Tumors

Introduction/Rationale: Effective T cell immunity against solid tumors relies not only on T cell function but also on efficient trafficking into tumor sites. To infiltrate tumors, effector T cells must undergo a multi-step adhesion cascade with endothelial cells (EC) lining tumor blood vessels. While these interactions are well characterized in venular endothelial cells (VEC) of normal tissues, their counterparts within the highly disorganized tumor vasculature remain poorly understood.

Methods: Single-cell RNA sequencing and spatial transcriptomics were used to profile endothelial subsets across murine and human solid tumors. A murine-specific anti-ACKR1 monoclonal antibody and a venular-specific Ackr1-creERT2 mouse model were generated to identify and trace ACKR1⁺ VECs during tumor progression. Adoptive T cell transfer experiments were performed to evaluate spatial localization and trafficking patterns relative to ACKR1⁺ VECs in various tumor models.

Results: A subset of tumor VECs resembling their normal-tissue counterparts was identified, defined by high ACKR1 expression and close spatial association with infiltrating T cells. ACKR1⁺ VECs were enriched in “hot” tumors with robust T cell infiltration but depleted in “cold” tumors, a pattern conserved across murine models and human samples. Spatial mapping revealed that ACKR1⁺ VECs predominantly localize at tumor margins, the primary entry sites for adoptively transferred T cells. Lineage tracing further showed that tumor angiogenesis mostly originates from ACKR1⁺ VECs, underscoring their functional importance.

Conclusion: Peritumoral ACKR1⁺ VECs function as vascular gatekeepers that regulate T cell entry into solid tumors. Their abundance, localization, and maturity determine immune infiltration levels, linking vascular identity to tumor immunogenicity. Targeting this endothelial subset may enhance T cell access and improve immunotherapy outcomes.