Сергей Пузанов – Multiple Sclerosis. Everything you need to know (страница 7)

Thus, to trigger multiple sclerosis, the flawed lymphocyte must not only meet its matching epitope but must also find itself in the middle of a wartime environment. Usually, the onset of the disease is linked to a person catching a virus—specifically, the Epstein-Barr virus (EBV), which is considered one of the primary triggers for MS. By adulthood, 90 to 95% of the population is infected, and in most cases, it happens completely without symptoms. The virus multiplies in the lymphoid tissue of the throat, where dendritic cells ambush it. They chop the enemy into fragments (essentially stripping it down for parts) and head to the nearest lymph node. There, they display these pieces on their surface—on MHC molecules—so that passing T-cells can inspect them.

This is where the autoimmune trap springs shut: molecular mimicry. One of the viral fragments, named EBNA1, happens to be identical to a degree of confusion to a myelin peptide called MBP, and this resemblance is a primary trigger for multiple sclerosis. However, for the most part, the issue lies in how the dendritic cell showcases this wreckage: 2022 research proved that a prerequisite for MS is the presence of the DRB1*15:01 gene—which dictates exactly how the dendritic cell presents EBV antigens. Thus, while the fatal process kicks off with the activation of a T-helper cell, the cell itself is entirely blameless. The blame lies with the dendritic cell—it carelessly presented the antigen, botching its primary function. Then again, it didn't do it on purpose: it was born with a flawed MHC-II protein, and the viral fragment simply couldn't sit in it properly. Crucial note: once EBV triggers the primary activation of the T-helper, its role in the development of MS is completely finished. Trying to stop multiple sclerosis by hunting down the virus is entirely pointless.

When a naive T-lymphocyte, whose receptor by a twist of fatal convergence matches this distorted shape, collides with a dendritic cell, it sees a blood enemy on its display stand. Do not forget the massive numbers: a dendritic cell carries up to 200,000 display stands, and the viral wreckage occupies only a few hundred of them. To wake up, the T-cell needs to rack up 200 to 300 receptor hits, and a clever evolutionary trick takes over. An assembly line fires up in the contact zone: a few dozen stands displaying the virus sequentially activate hundreds of T-cell receptors, like a punch card in an old industrial loom. Having collected the magic sum of signals, the lymphocyte activates and begins to multiply: the cell divides every 6 to 8 hours, and in just 4 to 5 days, an army of 10,000 clones is manufactured. The virus retreats into latent standby mode inside the body, while the newborn army of now auto-aggressive T-cells marches out to hunt down enemy agents.

Still, unleashing thousands of militants into the blood is not an absolute death sentence; the body retains a window to correct the mistake. On their way out of the lymph node, the revolutionaries are ambushed by the military police—T-regulatory cells. Their receptors also recognize the body's own tissues, but instead of launching an attack, they deploy protection. The main objective of these "regulators" is to spot an autoimmune mutiny, bind the aggressors, and force them to commit suicide. Furthermore, without a steady feed of inflammatory signals, the militants quickly burn out and perish—the "contraction phase of the immune response" usually wraps up within a couple of weeks. In the vast majority of cases, this security grid works: the clone army is liquidated (or simply dies out) long before reaching the CNS. But if the process is hit by a bad genetic foundation, severe stress, or a profound vitamin D deficiency, the balance of power collapses. The military police fail to contain the uprising, and the surviving militants head down the home stretch—straight toward the border of the brain.

How does a blind T-cell know that behind the blank wall of a blood vessel lies the myelin it's hunting for? It doesn't know a thing—it simply forces its way into any tissue emitting a universal distress signal. Hurtling through the bloodstream, the auto-aggressive clones audit every organ in sequence, and from time to time, they spot invitations to enter: when vessel walls display chemical "velcro"—adhesion molecules—the cell reads it as a plea for intervention. Instant lock-on occurs: the lymphocyte slams on its molecular brakes and begins squeezing through the vessel wall—for instance, into the nervous tissue. What's the funniest part? Even at this exact moment, the immune cell has no idea that the blood-brain barrier hides myelin on the other side. Had the lymphocyte ended up in some other organ, multiple sclerosis would never have started.

The cells of the brain's blood vessels are tightly stitched together, and under normal circumstances, immune cells cannot leak inside. But the surviving militants are activated, which means they are packed with burglary gear. To breach the brain barrier, they carry specialized proteins on their membranes called integrins. T-cells use these to glue themselves to the vessel wall, resisting the powerful torrent of blood flow. Once anchored to the vascular wall, they deploy their chemical weapons—matrix metalloproteinase enzymes. These enzymes dissolve the tight junction proteins between the vessel cells, burning open structural breaches in the BBB. The barrier springs a leak, and the aggressive clones squeeze into the holy of holies of the organism—the tissue of the brain and spinal cord.

Once inside the brain, the T-cells collide with myelin and see that everything around them is occupied by their enemy—the exact match for their receptor. The militants sound a chemical siren, releasing signaling proteins called cytokines. This rallying cry triggers a chain reaction: the brain's local security force, microglia, declares martial law, while heavy artillery—macrophages—pours through the breached barrier from the bloodstream. A ruthless scorched-earth clearance of the territory begins. Macrophages literally scrape the myelin sheath off the nerve fibers, stripping the brain's live electrical cables bare. The organs of the CNS transform into a full-scale combat zone, but the absolute worst unfolds slightly later, as the first wave of the assault winds down.

At this moment, a terrifying mechanism engages: epitope spreading. Myriads of shattered myelin fragments are salvaged by scout cells and put on display. A secondary antigen presentation occurs, completely independent of any virus. The fire rages so intensely that previously hidden layers of insulation are exposed, and the distress signals attract fresh factions of lymphocytes. Auto-reactive cells whose receptors target entirely different myelin peptides join the fray. If the initial breach was launched by clones of just one or two receptor variants, the immune system now recognizes a massive array of targets, drawing hundreds of weapon types into the hotspot. The war becomes entirely self-sustaining, and the shattered blood-brain barrier leaves the gates wide open for anyone to enter.

Uninvited guests rush through the breached doors of the barrier. Absolute chaos erupts: by this stage of the crossfire, the T-lymphocytes targeted specifically at myelin make up no more than 10% of the forces inside the brain. The other 90% are bystander cells that drifted into the hotspot completely by accident. Yet, they don’t just sit on their hands. Blinded by the overwhelming signals of inflammation, these "pedestrians" begin venting toxic enzymes and free radicals, obliterating healthy brain tissue purely by inertia—a phenomenon termed the bystander effect. The catastrophe intensifies with the arrival of B-lymphocytes, which account for 20 to 30% of the cells entering the brain at the height of the disease—many of them hijack the antigen-presenting function, driving completely neutral T-lymphocytes toward aggression. Furthermore, some of these B-lymphocytes, having mutated into plasma cells, set up autonomous factories inside the nervous tissue and stamp out billions of antibodies—which yield the oligoclonal bands that doctors find when analyzing cerebrospinal fluid. The most insane part is that only a handful of these antibodies are targeted at myelin; the other millions of copies are manufactured against completely random antigens. The oligoclonal bands in the CSF of MS patients consist mostly of antibodies against random background noise.

What does the destruction of myelin lead to? Nerve insulation is engineered like a multi-layered pastry, with the fiber wrapped in a membrane dozens of layers deep—and macrophages methodically gnaw away at this armor, layer by layer. As long as a fraction of the winding remains intact, the cable functions. But when the destruction hits a critical threshold—less than 3 layers of myelin—the body's electrical grid surrenders. Normally, a signal shoots down an axon at speeds up to 100 meters per second, but on a stripped section, this mechanism shatters, and the impulse speed drops 50- or even 100-fold. The signal no longer flies—it barely crawls, and frequently shorts out along the way, failing to reach its destination. To make matters worse, the bare wires begin to cross-talk, bleeding false impulses into neighboring fibers. It is these systemic wiring failures that translate hidden molecular chaos into the terrifying symptoms of multiple sclerosis.