Neuroimmunology · reanalysis of public data
The Smoldering Rim
What a multiple-sclerosis lesion is made of, cell by cell — and why its slow-burning edge resists our best drugs.
Multiple sclerosis is usually pictured as a disease of relapses — flares that come and go. But much of the long-term disability comes from something quieter and more relentless: lesions that never fully switch off. At the heart of one of these chronic active, or “smoldering,” lesions is a thin ring of ongoing inflammation — the rim — that slowly eats outward into healthy tissue for years. It is invisible to the drugs that tame relapses, and understanding what it’s built from is one of the more consequential open problems in the field.
A lesion begins around a small vein. Immune cells cross the blood–brain barrier there and strip the myelin — the insulation around nerve fibers. The center burns out and goes quiet; the rim, the border with still-healthy tissue, stays inflamed and advances. It’s so characteristic that the vein running through the middle of a lesion — the “central vein sign” — is increasingly used to tell MS lesions apart from look-alikes, and the iron that accumulates in the rim’s immune cells makes it visible on MRI as a “paramagnetic rim.”
So what, exactly, is at the rim? I took a public single-nucleus dataset of MS brain tissue and counted. Here is the census — every cell type, placed where it lives in a cross-section of the lesion, with the numbers taken from the active edge itself.
Three things stand out. First, the lesion is overwhelmingly a story about oligodendrocytes — the myelin-making cells — and their loss: they fall from the great majority of the tissue in healthy white matter to a demyelinated void in the core. Second, the rim is where the immune cells and reactive astrocytes pile up; their fraction surges precisely at the advancing edge. Third — and this is the part that matters for treatment — the immune cells doing the damage are mostly the brain’s own resident microglia, not cells freshly arriving from the blood.
The actors, and what each is doing
Microglia and macrophages are the effectors. At the rim they become iron-laden and inflamed, producing reactive oxygen species that demyelinate the next ring of tissue — and dying myelin releases more iron, which re-activates the next ring of microglia. It’s a self-sustaining loop, which is why the rim travels: each layer of destruction manufactures the fuel and the trigger for the layer just beyond it.
The rim isn’t repeated invasion from outside. It’s a fire that keeps its own embers — a loop of iron and inflammation that lives inside the brain.
Oligodendrocytes are the victims and, grimly, the fuel: they are the brain’s main iron store, so their death is what feeds the loop. OPCs, the precursor cells that should repair the myelin, are recruited but blocked from finishing the job in the hostile environment — so the damage isn’t undone. Astrocytes turn reactive, wall off the dead core, and egg the microglia on. Neurons — really the axons passing through — are the ultimate casualties: stripped of insulation, they run out of energy and degenerate, and that is what disability is made of. And cuffed around the central vein sit the lymphocytes (cytotoxic T cells, and antibody-making plasma cells) and the vascular cells that form the vessel the whole lesion is built around.
Why the drugs don’t reach it
The most effective MS therapies deplete B cells from the blood, and they work beautifully on relapses. But the smoldering rim runs on a loop that is sealed inside the central nervous system, sustained by resident microglia and iron rather than by cells trafficking from the periphery. A drug that clears the blood simply can’t get to it. That gap — great control of relapses, continued silent progression — is one of the central frustrations of modern MS care, and it’s exactly the compartment this kind of cell-by-cell map is trying to make legible, so we can ask which of the rim’s own control switches might be coaxed back on.