The picture changed under inflammatory conditions. The engineered mice showed a markedly reduced inflammatory response. Their blood vessels activated less strongly, and far fewer immune cells migrated from the bloodstream into surrounding tissue.

"Our experiments show that Tie1 cleavage is not required for the normal development or maintenance of blood vessels," first author Dr. Miki Kamiyama said. "Rather, it enables blood vessels to respond specifically to inflammatory signals."

What does this mean for vascular biology?

The findings separate two functions of angiopoietin-Tie signaling. Vessel-stabilizing and growth-promoting activity remains intact when Tie1 stays whole on the cell surface. Receptor cleavage is what unlocks the pro-inflammatory response.

The cleavage works like a toggle that turns on the pathway's inflammatory mode without affecting its vessel-maintenance role. Cells without Tie1 cleavage stay in growth-and-maintenance mode even when inflammation signals arrive.

"The discovery shows how a single signaling pathway can regulate different functions of the vasculature," Singhal said. "Tie1 cleavage creates a functional separation between the regulation of vascular growth and vascular inflammation."

Are blood vessels passive barriers or active controllers?

The study reframes blood vessel cells as active regulators, not a passive barrier. They control when and under which conditions immune cells leave the bloodstream and enter inflamed tissue. Tie1 cleavage is an early vascular event that lets circulating immune cells enter inflamed tissue.

"The discovery shows that the different functions of the endothelium can be regulated more precisely and independently than we previously thought," said lead scientist Hellmut Augustin. "It raises new questions about how such functional separation is organized in other endothelial processes."

What comes next?

The work opens new questions for the field. If a single cleavage event separates growth from inflammation in this pathway, similar separations may exist elsewhere in endothelial biology. The team did not test therapeutic inhibitors, so any clinical application remains speculative. As Augustin noted, the discovery raises new questions about functional separation in other endothelial processes.

The paper, "Tie1 ectodomain cleavage governs vascular responses to inflammatory stimulation but is dispensable for angiogenic signaling," was published in the 2026 volume of the Journal of Clinical Investigation. First author Miki Kamiyama and colleagues describe the work in detail (DOI: 10.1172/jci204085).