Transmembrane-4 L-six family member-1 (TM4SF1) is a small cell surface glycoprotein that is highly and selectively expressed on endothelial cell and mesenchymal stem cell surfaces. TM4SF1 regulates cellular functions by forming protein complexes called TMED (TM4SF1-enriched microdomains) that either recruit growth-factor activated proteins and internalize them via microtubules to distribute the recruited molecules intracellularly or support the formation of nanopodia for intercellular interactions extracellularly. Through a genetically manipulated mouse model for global Tm4sf1 gene knockout, we demonstrate here that TM4SF1 is essential for blood vessel development. Tm4sf1-null embryos fail to develop blood vessels and experience lethality at E9.5. Tm4SF1-heterozygous embryos are smaller in body size during early embryonic development, and almost half die in utero due to intracranial hemorrhage in the intraventricular and subarachnoid space, which becomes apparent by E17.5. Surviving Tm4SF1-heterozygotes do not display overt phenotypic differences relative to wild type littermates postnatally. Together, these studies demonstrate that TM4SF1, through its molecular facilitator and nanopodia formation roles in TMED, intimately regulates blood vessel formation during embryonic development.

Antibody–drug conjugates (ADCs) are a promising therapeutic modality for treating cancer. TM4SF1 is an integral membrane protein that internalizes from the cell surface along microtubules to the nucleus and is highly expressed on the surface of both tumor endothelium and tumor cells. We previously reported that in human tumor xenografts in mice, an ADC directed to mouse TM4SF1 (2A7A-LP2) effectively regressed tumors through an anti-vascular mechanism, and an ADC directed to human TM4SF1 (v1.10-LP2) effectively regressed tumors through an anti-tumor cell mechanism. In this study, we investigated the actions of the mouse TM4SF1-directed ADC on VEGF-A-provoked angiogenic vessels. We employed an adenovirus expressing mouse VEGF-A¹⁶⁴ (Ad-VEGF-A) to induce surrogate tumor blood vessels in the ears of nude mice. We showed that an immune effector function-ablated ADC, 3m2A7A-LP2, was better tolerated than its parent 2A7A-LP2. Homing of 3m2A7A to Ad-VEGF-A-induced new blood vessels became evident within six hours after intraperitoneal injection. A single dose of 3m2A7A-LP2 at 3 mg/kg disrupted evolving Ad-VEGF-A-provoked blood vessels within forty-eight hours, and three doses of 3m2A7A-LP2 at 48 h intervals caused striking local ear necrosis; in each case, there was no apparent harm to vessels in the corresponding control virus-injected ears and the surrounding tissues of the same mice. Our studies demonstrate that an ADC-directed against mouse TM4SF1 specifically targeted the newly formed blood vessels induced by Ad-VEGF-A at multiple stages of their development. Thus, TM4SF1-directed ADCs, through their ability to target angiogenic vessels, represent an alternative anti-angiogenic approach for treating solid tumors.