P-Selectin is a novel immune checkpoint in glioblastoma which can be exploited as a target for drug-delivery

Eilam Yeini ¹ Paula Ofek ¹ Sabina Pozzi ¹ Nitzan Albeck ¹ Dikla Ben-Shushan ¹ Galia Tiram ¹ Sapir Golan ¹ Ron Kleiner ¹ Ron Sheinin ⁶ Sahar Israeli Dangoor ¹ Shlomit Reich-Zeliger ⁷ Rachel Grossman ⁵ Zvi Ram ⁵ Henry Brem ³ Thomas Hyde ⁴ Prerna Magod ² Dinorah Friedmann-Morvinski ² Asaf Madi ⁶ Shiran Ferber ¹ Rainer Haag ⁸ Marcelo Calderón ⁸ Ronit Satchi-Fainaro ¹

¹Physiology and Pharmacology, Sackler Faculty of Medicine, Tel Aviv University, Israel
²Department of Biochemistry and Molecular Biology, George S. Wise Faculty of Life Sciences, Tel Aviv University, Israel
³Department of Neurosurgery, Johns Hopkins University School of Medicine, USA
⁴Department of Psychiatry & Behavioral Science, Johns Hopkins University School of Medicine, USA
⁵Department of Neurosurgery, Tel Aviv Sourasky Medical Center, Israel
⁶Department of Pathology, Sackler Faculty of Medicine, Tel Aviv University, Israel
⁷Department of Immunology, Weizmann Institute of Science, Israel
⁸Institute of Chemistry and Biochemistry, Freie Universität Berlin, Germany

Glioblastoma (GB) is a highly aggressive brain tumor. Current standard-of-care results in a marginal therapeutic outcome, partly due to acquirement of resistance and insufficient blood-brain barrier (BBB) penetration of chemotherapeutics. To circumvent these limitations, we aimed to target P-selectin (SELP), an adhesion molecule which is expressed on inflamed, microvascular endothelial cells. Thus, we conjugated the chemotherapy paclitaxel to a dendritic polyglycerol sulfate (dPGS) nanocarrier. dPGS is able to cross the BBB, bind to SELP, and accumulate selectively in intracranial tumors. We show that dPGS has dual targeting properties, as we found that SELP is not only expressed on tumor endothelium but also on GB cells. This led us to investigate the functional role of SELP in GB progression. As the communication between cancer and immune cells is often mediated by adhesion molecules, we explored whether SELP is involved in GB-microglia interactions. Microglia are macrophages-like cells which serve as the brain resident immune cells, and have been shown to facilitate GB progression. They may possess a pro-inflammatory/anti-tumorigenic phenotype in which they are phagocytic, cytotoxic, and bear antigen-presentation capabilities. In contrast, microglia may also be found in anti-inflammatory/pro-tumorigenic phenotype which is related to tissue repair. This phenotype is characterized by angiogenic properties and the secretion of immunosuppressive cytokines which support tumor development. However, the reciprocal mechanisms by which GB cells alter microglia behavior are not fully understood. We show that SELP mediates microglia-enhanced GB proliferation and invasion by altering microglia activation state. We demonstrate these findings by pharmacological and molecular inhibition of SELP which resulted in reduced tumor growth and prolonged survival in GB mouse models. Our work sheds light on tumor-associated microglia function and the mechanisms by which GB cells suppress the immune system and invade the brain. Taking together, our findings show that targeting SELP-expressing GB cells is efficient both for drug-delivery of potent anti-cancerous drugs, and for interfering with GB-microglia interactions, leading to reduced tumorigenesis and improved anti-tumor immune response.