IL-8: What Is Its Role in Immunity and Inflammation?

Interleukin-8, now formally called CXCL8, is a small signaling protein whose primary job is to recruit and activate neutrophils, the immune system’s rapid-response cells, at sites of infection or tissue damage. It belongs to a family of proteins called chemokines, which act as molecular homing signals that guide immune cells through the body. But IL-8 does far more than summon neutrophils. It promotes the growth of new blood vessels, shapes how wounds heal, influences pain signaling, and plays a complicated role in cancer. That breadth of activity is what makes it one of the most studied chemokines in medicine.

What IL-8 Is and Where It Comes From

IL-8 is a member of the CXC subfamily of chemokines, specifically the ELR-positive group. That designation refers to a short sequence of three amino acids near the front of the protein that is critical for activating its receptors. ELR-positive CXC chemokines are sometimes called neutrophil-activating chemokines because they preferentially target neutrophils.¹ Under normal conditions, IL-8 can exist as either a single unit or a pair of linked copies, and both forms are biologically active.²

A wide range of cell types can produce IL-8. Monocytes, neutrophils, epithelial cells, fibroblasts, endothelial cells, mesothelial cells, and tumor cells all secrete it.³ This means that virtually any tissue in the body can raise an IL-8 alarm when it senses danger. The triggers are what you might expect: bacterial products, viral infection, physical injury, and other inflammatory signals like tumor necrosis factor and interleukin-1 all push cells to ramp up IL-8 production. The sheer number of cell types capable of producing it explains why IL-8 shows up in nearly every inflammatory condition researchers examine.

How IL-8 Guides Neutrophils to Sites of Infection

The classic function of IL-8 is pulling neutrophils out of the bloodstream and directing them toward infected or damaged tissue. When IL-8 is released at an injury site, it does not simply diffuse into the blood. Instead, it binds to sugar-rich molecules called glycosaminoglycans on the surface of blood vessel cells and in the surrounding tissue matrix. This tethering creates a concentration gradient, with the highest levels of IL-8 right at the site of injury and lower levels extending outward. Neutrophils circulating in nearby blood vessels detect that gradient and follow it, crawling along vessel walls until they can squeeze through and migrate toward the source.⁴

The binding to these sugar molecules is not just a passive anchor. Different types of glycosaminoglycans bind IL-8 with slightly different strengths, which helps fine-tune how steep the gradient is and therefore how aggressively neutrophils are recruited.⁵ Once neutrophils arrive, IL-8 does not just wave them in and leave. It actively triggers them to release destructive enzymes and undergo other changes that prepare them to kill pathogens. Injecting IL-8 directly into the skin causes a visible exudation of fluid and a sustained, heavy accumulation of neutrophils at the injection site.⁶

IL-8 can also prompt neutrophils to cast out web-like structures made of DNA and antimicrobial proteins, called neutrophil extracellular traps. These traps snare bacteria and other pathogens, but they can also damage surrounding tissue if the process gets out of control. Research on eye inflammation found that IL-8 triggers this trap formation in a dose-dependent manner, with increasing concentrations producing more traps.⁷ IL-8 therefore has a two-step relationship with neutrophils: first it draws them in, then it can push them toward this more aggressive, potentially self-destructive response at the site where they accumulate.⁸

Two Receptors With Different Jobs

IL-8 acts through two receptors on cell surfaces, called CXCR1 and CXCR2. Both are found on neutrophils and on endothelial cells lining blood vessels, but they do not do the same thing. Experiments on microvascular endothelial cells revealed that IL-8 first activates CXCR1, triggering an early response within the first few minutes that involves the formation of internal structural fibers inside the cell. A few minutes later, CXCR2 takes over and drives a different response involving cell retraction and directed migration. The two receptors use completely different internal signaling cascades to accomplish these tasks.⁹

The distinction matters clinically because drugs that block one receptor do not necessarily block the other. In studies of cell migration, CXCR1 and CXCR2 produced different patterns of a key migration-related protein when exposed to the same concentrations of IL-8.¹⁰ This split in receptor function also shows up in blood vessel growth. When researchers tested which receptor was responsible for IL-8’s ability to stimulate new blood vessel formation in intestinal endothelial cells, CXCR2 turned out to be the one driving that process.¹¹ Drug developers have tried to exploit this receptor-level specificity, and one experimental compound called SCH527123 blocks both CXCR1 and CXCR2 and has shown promise in animal models of liver, pancreatic, and ovarian cancers.¹²

Blood Vessel Growth and Why It Matters

IL-8 is not just an immune signal; it is also a potent promoter of angiogenesis, the formation of new blood vessels. In laboratory experiments, IL-8 concentrations as low as 1 ng/mL produced a maximal movement response in intestinal endothelial cells, comparable to the response triggered by VEGF, one of the body’s most powerful angiogenic factors, at 50 times that concentration.¹¹ IL-8 also directly stimulated endothelial cell survival, proliferation, and the formation of tube-like structures that mimic new blood vessels. When researchers neutralized IL-8 with antibodies, the tube formation stopped.¹³

This angiogenic capability is beneficial during normal tissue repair, when damaged areas need new blood supply. But in the wrong context, it becomes a liability. Tumors need blood vessels to grow beyond a tiny size, and IL-8 is one of the signals that helps tumors build their own blood supply. In the tumor microenvironment, IL-8 promotes angiogenesis and also recruits immunosuppressive cells that shield the tumor from the immune system.¹⁴ This dual role, essential for healing but exploitable by tumors, runs through nearly every aspect of IL-8 biology.

IL-8 in Acute Lung Injury and Sepsis

Some of the most dramatic evidence for IL-8’s clinical importance comes from studies of acute respiratory distress syndrome. In patients with ARDS caused by sepsis, the concentration of IL-8 in lung fluid was roughly six times higher than in ARDS patients without sepsis. Crucially, the IL-8 levels were far higher in the lungs than in the blood, suggesting that the lung tissue itself was the primary source of IL-8 production rather than the bloodstream carrying it in from elsewhere.¹⁵

In children with acute respiratory failure, the pattern is similar. IL-8 levels in blood strongly correlated with how severely ill the child was and how impaired their oxygen levels were. Children who did not survive had IL-8 levels four to twelve times higher than those who did survive, and IL-8 was independently associated with death, the duration of mechanical ventilation, and how long the child stayed in intensive care.¹⁶ Findings like these have made IL-8 a candidate biomarker for identifying patients at the highest risk of dying from sepsis-related lung injury.

The body sometimes complicates matters further by producing antibodies against its own IL-8. In ARDS patients, researchers have found stable immune complexes made of anti-IL-8 antibodies bound to IL-8 deposited directly in lung tissue.¹⁷ Rather than neutralizing IL-8 and calming things down, these complexes may themselves contribute to inflammation by activating immune receptors on lung cells.¹⁸ It is a case where the body’s attempt to regulate a signal backfires and potentially worsens the injury.

Chronic Inflammatory Diseases

IL-8 is not confined to acute crises. In inflammatory bowel disease, researchers used tissue-level imaging to show that IL-8 gene expression was concentrated in areas of the intestinal lining that showed active damage. The number of cells producing IL-8 tracked directly with the severity of inflammation, whether the patient had Crohn’s disease or ulcerative colitis. Areas of the bowel that looked normal under the microscope had minimal IL-8 expression, while badly inflamed areas were saturated with it.¹⁹

In rheumatoid arthritis, the situation takes an unusual twist. Patients develop autoantibodies specifically directed against IL-8, and these antibodies correlate with markers of inflammation and disease severity. Patients who have extra-articular manifestations of rheumatoid arthritis, meaning the disease has spread beyond the joints to affect other organs, tend to have elevated levels of a particular type of these anti-IL-8 antibodies.²⁰ The antibodies form immune complexes with IL-8, and instead of resolving the inflammation as autoantibodies sometimes do, they may perpetuate it by triggering additional immune activation. Researchers have suggested that these anti-IL-8 antibodies could serve as markers for more severe or widespread disease.

The Wound Healing Paradox

IL-8’s relationship with wound healing is genuinely contradictory, and the details matter for understanding why some wounds heal slowly. On one hand, IL-8 is bioactive in wounds and can promote healing. Applying IL-8 topically to human skin grafts in an animal model enhanced the regrowth of the outer skin layer because it increased the number of dividing skin cells.²¹ That sounds straightforwardly beneficial.

On the other hand, elevated IL-8 in wounds is associated with delayed healing. In thermal wounds that were not closing on schedule, IL-8 decreased the replication of keratinocytes, the primary cells that regrow the skin surface, and also inhibited wound contraction at certain concentrations.²² The resolution to this contradiction appears to be dose and timing. At lower concentrations, IL-8 supports the early inflammatory phase that clears debris and stimulates cell division. At the elevated concentrations found in chronic or poorly healing wounds, it starts interfering with the contraction and remodeling processes that close a wound. The same signal that is helpful early on becomes obstructive if it persists too long or climbs too high.

IL-8 and Cancer

Tumors have essentially learned to hijack IL-8 signaling for their own benefit. Within the tumor microenvironment, IL-8 serves multiple pro-tumor functions: stimulating the cancer cells themselves to proliferate, pushing them toward a more mobile and invasive state, building new blood vessels to feed the growing mass, and recruiting immune cells that suppress the body’s anti-tumor response rather than enhancing it.¹⁴

This last point has taken on increasing clinical relevance in the era of immunotherapy. Immune checkpoint drugs work by releasing the brakes on the body’s anti-tumor immune response, but if a tumor has recruited a heavy population of immunosuppressive cells driven by IL-8 signaling, those drugs become less effective. Clinical studies have found that patients with high circulating IL-8 levels tend to respond poorly to checkpoint therapy, and IL-8 levels in the blood may help predict who will and will not benefit from these treatments.¹⁴ In several solid cancers, IL-8 has been explored as both a prognostic marker and a tool for selecting patients who are most likely to benefit from immunotherapy, though standardization of measurement methods remains an obstacle.²³

IL-8 as a Biomarker

Because IL-8 levels rise sharply during active inflammation and correlate with disease severity, researchers have spent years trying to turn it into a practical clinical measurement. During the COVID-19 pandemic, IL-8 levels correlated with disease progression and prognosis better than IL-6, another widely measured inflammatory marker, when tracked over time in the same patients.²⁴ In elderly sepsis patients presenting to the emergency department, IL-8 was an independent predictor of 28-day mortality, and combining IL-8 levels with a clinical severity score improved prediction further.²⁵

Despite all this promise, IL-8 has not become a standard bedside test in most settings. The main hurdles are practical: there is no widely agreed-upon standardized method for measuring IL-8 across different laboratories, and because so many cell types produce it in so many conditions, a single IL-8 level can be hard to interpret without clinical context.²³ A high reading tells you the immune system is activated, but not necessarily why or where.

Connections to Obesity and Pain

IL-8 shows up in some surprising places beyond classic infections and immune diseases. In obese individuals, circulating IL-8 is elevated compared to lean individuals and correlates with insulin resistance and cardiovascular disease markers. The source matters: visceral fat, the deep abdominal fat that wraps around organs, releases roughly four times as much IL-8 as subcutaneous fat, the fat just beneath the skin.²⁶ This disproportionate IL-8 output from visceral fat may be one reason why belly fat is more strongly associated with metabolic disease than fat carried elsewhere.

IL-8 also plays a role in pain that goes beyond its inflammatory effects. Research on chronic low back pain and disc degeneration has found that pain-sensing neurons express CXCR2, one of IL-8’s two receptors. When IL-8 activates CXCR2 on these neurons, it increases their excitability by upregulating certain sodium channels and sensitizing a receptor involved in pain perception. A drug called reparixin, which blocks CXCR1 and CXCR2, reduced pain behavior in animal models, apparently by acting directly on these sensory neurons.²⁷ This suggests that IL-8 is not just a byproduct of the inflammation that causes pain; it is, in some cases, a direct driver of the pain signal itself.

Why Mice Do Not Have IL-8

One significant complication in IL-8 research is that the gene encoding human IL-8 is entirely absent from the mouse and rat genomes. For years, scientists assumed that two other chemokines in rodents filled the same role by binding the mouse version of CXCR2 and attracting immune cells in a similar way. More recent work has shown that IL-8 plays a non-redundant role that those stand-ins do not fully cover: when researchers gave recombinant human IL-8 to mice with bacterial colitis, it contributed to neutrophil movement through the colon wall in ways the mouse substitutes did not replicate.²⁸

This absence has real consequences for drug development. Many anti-inflammatory and anti-cancer therapies are first tested in mouse models, but any drug designed to block IL-8 or its receptors cannot be fully evaluated in a standard mouse because the target does not exist in the same form. Researchers have developed transgenic mice that express human IL-8, and these engineered animals show increased mobilization of immature immune cells, worsened inflammation, and accelerated colon cancer development, providing a closer model of what IL-8 does in humans.²⁸ The ELR motif that defines IL-8’s receptor-binding activity appears to be ancient; evolutionary studies tracing it across fish and primate genomes suggest it arose early in vertebrate evolution, even though rodents later lost the gene.²⁹ The result is a quirk of biology that has slowed translational research for decades.