A small cluster of damaged immune cells can do outsized harm. That is the picture emerging from new research at UCLA, where scientists identified a rogue population of macrophages, the tissue-patrolling immune cells that normally clean up debris and help maintain order. In aging livers, and in livers damaged by fatty liver disease, those cells can slip into a senescent state. They stop dividing, refuse to die and begin sending out inflammatory signals that disturb the tissue around them. “Senescent cells are fairly rare, but think of them like a broken-down car on the 405,” said Anthony Covarrubias, senior author of the study and a member of the Eli and Edythe Broad Center of Regenerative Medicine and Stem Cell Research at UCLA. “Just one stalled car can back up traffic for miles. Now imagine five or ten of them slowly accumulating. That’s what these cells do to a tissue: even a small number causes enormous disruption.” For years, scientists argued over whether macrophages could truly become senescent at all. Part of the problem was that …