Researchers at Stanford Medicine have discovered that the body’s most common white blood cells, immune cells called neutrophils, can make a protein that no one knew they made. This unexpected sighting joins a growing list of hints linking the brain disorder schizophrenia to events happening elsewhere in our bodies.
The findings are summarized in a paper published online May 11. Proceedings of the National Academy of Sciences.
The new focus on neutrophil binding as a source of a protein called C4A joins a long list of other observations that are somewhat puzzling when taken alone. For example, large population genetic studies have identified C4A, which is already known to be produced primarily in the liver, as a significant risk factor for schizophrenia. People with schizophrenia tend to have an increased number of neutrophils in their blood. The most effective drugs for schizophrenia are those that inhibit neutrophils.
By linking these seemingly disparate relationships, we can deepen our understanding of schizophrenia and may ultimately lead to better treatments and diagnostics for the disease. ”
Agnes Kalinowski, MD, Clinical Assistant Professor of Psychiatry and Behavioral Sciences, Stanford University
Mr. Kalinowski is both a researcher and a practicing psychiatrist who specializes in schizophrenia. PNAS Lead author of the study. The lead author is Dr. Alexander Urban, associate professor of psychiatry, behavioral sciences, and genetics.
Schizophrenia affects one in 100 people worldwide, with little variation by geography or ethnicity. Its most prominent symptoms are hallucinations, delusions, and fixations. The basic features of the disease are cognitive impairments, such as the inability to think clearly, poor working memory, and disorganized thinking and behavior.
Current treatments for schizophrenia are palliative, Kalinowski said. They do not stop the progression of the disease or restore motivation or cognitive acuity. “The best drug we currently have is clozapine,” she said. “It reduces symptoms such as hallucinations, fixation, and delusions. It can be life-changing. But there are side effects, ranging from weight gain to risk of heart attack.”
Interestingly, another major side effect of clozapine, or teeth Is it a “side effect”? It depletes the number of circulating neutrophils, she noted.
C4A: Relationship between neutrophils and the brain?
You may not have heard much about neutrophils, but you’ve probably seen them. These make up a good half of the circulating white blood cells in a healthy person.
These short-lived kamikaze warriors lead a tough life. These are the first immune cells to arrive at the scene of a bacterial infection. There they devour microbial pathogens. It emits toxic germicidal substances. It then extrudes a slimy, microbe-trapping web made of long string-like molecules containing its own DNA. Neutrophils routinely die on the job and have a life expectancy of less than a day to a week, but eventually become the main cellular component of pus.
C4A is one of about 50 proteins that are arranged in a tight and complex arrangement in the bloodstream called the complement system. This ancient evolutionary core of our modern immune system, which dates back to sponges approximately 500 million years ago, allows us to rapidly recognize a variety of inflammatory events, such as microbial invasion.
In response, a bucket brigade of events occurs. The constituent proteins are activated one by one, binding to and activating the next protein in the brigade. Activation of C4A stops when a small piece called C4-ana is excised.
Activated complement, the result of this bucket brigade, plays roles such as punching holes in the outer membrane of bacterial cells. Interestingly, this is consistently seen in blood samples from people with schizophrenia.
C4A appears in many other places besides the blood. The brain performs very different abilities. This is thought to be related to a process called synaptic pruning, in which the brain periodically removes contact points between excess nerve cells, called synapses.
A healthy adult brain has an estimated 100 trillion to 500 trillion synapses. In the early stages of brain development, their number increases even more rapidly, resulting in the generation of many unnecessary, redundant, and even obfuscated connections. Similar to how an editor removes irrelevant words from a potentially great story, synaptic pruning typically increases cognitive coherence. It occurs in waves during fetal development, childhood, and adolescence.
However, this can be taken too far. The outermost layer of the brain in schizophrenia, the cerebral cortex, which is important for higher mental functions, has been found to have about 30% fewer synapses than in a healthy brain and is thinner than normal.
Researchers found an association between brain thickness and circulating neutrophil numbers, Kalinowski said.
Major risk factors for schizophrenia
Risk factors for schizophrenia include high levels of psychological stress and severe fever during early childhood (<5 years of age) and daily marijuana use, especially during adolescence.
However, these environmental factors seem small compared to genetic factors.
“Schizophrenia is highly heritable, about 80%,” Kalinowski said. The single largest genetic risk factor involves C4A.
Genes are the recipe for proteins. Our genome typically carries two copies of each gene. One is inherited from the mother and the other from the father. But over the course of evolution, some of our genes undergo mutations, called duplications, which result in some people having more copies of those genes in their genomes.
This is the case for C4A. Some people have two copies of the C4A gene, while others have more than one. The number of copies of the C4A gene in a person’s genome is the strongest common genetic risk factor associated with schizophrenia. No one can say why. However, they found that C4A concentrations in the plasma of schizophrenia patients (the blood contents after resident cells have been removed) matched the number of copies of the C4A gene in their genomes.
The intensity of symptoms in patients with schizophrenia is correlated with C4A levels in the brain, estimated from measurements of C4A levels in the cerebrospinal fluid.
In 2021, Kalinowski and Urban co-authored a paper. translational psychiatry This study shows that there is a strong association between the number of copies of the C4A gene in the genome of schizophrenia patients and the amount of C4-ana (a small fragment of C4A that is excised to initiate C4A activation) in the plasma.
C4-ana can be considered a kind of footprint of the presence of activated C4A.
Neutrophils make C4A
new PNAS In the study, Kalinowski, Urban, and their colleagues examined a large gene expression database. Gene expression analysis measures the strength of each gene’s involvement in the early stages of protein production by individual cells.
Our genome contains approximately 20,000 genes. However, only a select set of genes in a single cell are actively involved in directing protein production, depending on the cell type (nerves, skin, heart, etc.) and cell state (healthy or diseased, excited or peaceful, young or energetic, old or grumpy). The researchers also tested blood samples from 10 anonymous volunteers.
They confirmed that neutrophils can and do produce C4A.
Furthermore, the intensity of neutrophil C4A protein production in schizophrenia patients correlated with clinical measurements. Could C4A from neutrophils from schizophrenia patients enter the brain and activate synaptic pruning in these patients?
“It turns out that neutrophils can be small factories for C4A,” Kalinowski said. “And neutrophils from schizophrenia patients start producing much more CA4 than neutrophils from healthy controls.”
Paradoxically, they are reducing the amount. Although there was no difference in the amount of this protein itself in plasma, few The proportion was higher in neutrophils from schizophrenia patients than in controls. However, levels of C4-ana, an evidential marker of C4A activation, were higher in the plasma of schizophrenia patients.
All of this suggests to Kalinowski that the protein is made in large quantities by neutrophils, but is somehow consumed.
“The neutrophils of schizophrenia patients appear to have used up their initially abundant supply of C4A,” she said. “Somewhere, something is running out of it. We don’t know exactly where it’s happening, but we want to figure it out.”
This study raises the possibility that neutrophils play a direct role in schizophrenia. If so, blocking its activity could impede disease progression.
“Drugs that block neutrophil activation would not need to cross the blood-brain barrier because they act in the peripheral bloodstream,” Kalinowski said.
Kalinowski said evaluating neutrophils could also be part of the diagnosis. “If we can extract a diagnostic signal from something in the blood, combined with clinical information, that will change the game.”
Can testing for elevated neutrophil counts predict the onset of symptoms?
“We still don’t have a complete explanation for schizophrenia, but the jigsaw is being put together,” Kalinowski said. “Knowing where each part will go will help you lock the remaining parts into place more quickly.”
The study was funded by the Stanford University School of Medicine’s Translational Research and Applied Medicine Program and the Stanford Department of Psychiatry and Behavioral Sciences.
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Reference magazines:
Kalinowski, A. Others. (2025). Peripheral complement C4 protein in schizophrenia: association with gene copy number and immune cell subtypes. Proceedings of the National Academy of Sciences. DOI: 10.1073/pnas.2536376123. https://www.pnas.org/doi/10.1073/pnas.2536376123
