Listeria-infected macrophages weaken blood vessel barriers to spread infection

Researchers from the Cluster of Excellence CMFI and collaborators have uncovered a key biomechanical mechanism that helps Listeria monocytogenes spread through the body. Their study, now published in Cell Reports, shows that infected macrophages actively weaken the mechanical integrity of endothelial cell layers — the cellular barrier lining blood vessels — to facilitate their own transmigration.

Endothelial cells actively reinforce their barrier

Under normal conditions, endothelial cells (ECs) respond to contact with macrophages by strengthening their barrier. Direct macrophage–endothelial contact increases cellular traction forces and monolayer tension, reinforcing barrier integrity and limiting immune cell passage.

Infection disrupts protective response

However, when macrophages are infected with Listeria monocytogenes, this protective biomechanical response is suppressed. Instead of strengthening, endothelial cells show reduced contractile forces and barrier resistance. As a result, infected macrophages cross the endothelial layer nearly twice as efficiently as uninfected cells.

Importantly, the researchers demonstrate that this effect is largely driven by direct cell–cell contact rather than by soluble inflammatory cytokines. While TNF-α (Tumor necrosis factor alpha) contributes partially to barrier weakening, it cannot fully explain the enhanced transmigration observed during infection. TNF-α is a key pro-inflammatory cytokine of the immune system that promotes inflammation, activates immune cells and regulates programmed cell death (apoptosis).

“Our study shows that Listeria-infected macrophages reshape the mechanical behavior of endothelial cells, and that endothelial forces actively regulate how immune cells cross the vessel wall. Importantly, this process depends on the infection status of the macrophages. An exciting question that emerges is how the physical forces generated by infected macrophages drive their transmigration, and whether this differs from uninfected cells,” says last author Effie Bastounis, Junior Research Group Leader at CMFI.

Confirmed in vivo

Using a zebrafish infection model, the team confirmed that infection increases vascular permeability and accelerates macrophage transmigration in vivo. The ablation, i.e. the removal, of macrophages reduced vascular leakage, supporting the idea that direct macrophage–endothelial interactions are central to the process.

“We observed that infected macrophages can organize local tissue mechanics and coordinate a biomechanical layer of host defense. In this sense, the response to intracellular pathogens is not only biochemical but also fundamentally mechanical”, adds Marie Münkel, former PhD student in the Bastounis Lab and first author.