In a fibrotic liver, as in a city after a long siege, neighborhoods appear where life has stalled: working cells are replaced by scar tissue, and hiding inside it are senescent macrophages. These cells have stopped dividing but have not retired. They release substances that push their neighbors to build even more scar and drive off immune patrols. Drugs against them exist, but they act like carpet bombing: they kill both senescent and healthy cells, causing severe side effects.
The authors of the study found a way to strike with precision. On the surface of harmful macrophages in fibrotic liver and lung tissue sits the protein P-selectin, like an identification badge on a door. They found a key to fit it: fucoidan, a polysaccharide from brown algae that binds tightly to this protein. From fucoidan they made nanoparticles and loaded them with two types of drugs: the senolytic navitoclax, which kills senescent cells, and the senomorphic dBET6, which suppresses their harmful secretions. The particles were administered to mice with liver or lung fibrosis. They accumulated specifically in the diseased organ and, within it, in P-selectin-positive senescent macrophages, leaving other cells untouched.
The result was twofold. First, the nanoparticles reduced collagen deposition and the number of activated fibroblasts just as well as the free drug, but the severe side effects disappeared: the drop in neutrophils and platelets, weight loss, and death of the animals. Second, after the senescent macrophages were eliminated, dendritic cells and T lymphocytes returned to the tissue, and the profile of signaling molecules shifted from anti-inflammatory to immunostimulatory. In models of liver and lung cancer grown on a fibrotic substrate, the nanoparticles combined with immunotherapy caused lasting tumor shrinkage and extended the lifespan of the mice, whereas without the nanoparticles the tumors did not respond to treatment.
Single-cell sequencing showed that the target is a distinct population of macrophages combining features of senescence and lipid-associated macrophages. These cells release immunosuppressive molecules and live next to activated fibroblasts, crowding out T cells. They turned out to be the siege commanders, holding scar tissue in a state of war with immunity. By striking them precisely, the authors lifted the blockade and allowed immune cells to return to the city.
So far this is work in mice, but it opens a direct path to clinical trials. If biopsies from patients with liver or lung fibrosis reveal the same P-selectin-positive senescent macrophages, they could be selected for therapy. And in oncology, the nanoparticles could broaden the group of patients who benefit from immunotherapy by removing the fibrotic barrier. The sniper is ready; all that remains is to test it in humans.
Fucoidan is a polysaccharide from brown algae that has been used in traditional medicine in East Asia for centuries. Now it serves as a guidance system for nanoparticles because it binds tightly to P-selectin on the cell surface.
Not published on arXiv. Here are its page, the PDF and the formalization: you can check it yourself, not only read our account.