Pro-inflammatory S100 Proteins Found to Play Role in AAV
S100A8/A9 and S100A12 may serve as biomarkers for disease severity
Elevated levels of certain pro-inflammatory proteins were found in the bloodstream and urine of people with ANCA-associated vasculitis (AAV), and correlated with disease severity, a study shows.
Findings suggest that two of these proteins — S100A8/A9 and S100A12 — from the S100 family of proteins, play a role in AAV development and may serve as biomarkers to assess disease severity.
The study, “The potential pathogenic roles of S100A8/A9 and S100A12 in patients with MPO-ANCA-positive vasculitis,” was published in the journal BMC Immunology.
In AAV, the binding of self-reactive antibodies to two proteins — myeloperoxidase (MPO) and proteinase 3 — within neutrophils abnormally activates these immune cells, causing them to attack blood vessels.
Activated neutrophils can cause damage directly by releasing toxic granules and other substances, but also by secreting signaling molecules that trigger the immune system, leading to further inflammation and blood vessel injury.
Damage-associated molecular patterns (DAMPs) are molecules within cells that are released in response to damage or infection. DAMPs are a type of danger signal that, once released, promotes a noninfectious inflammatory response.
What are S100 proteins?
The S100 family of proteins are DAMPs released by neutrophils, two of which (S100A8/A9 and S100A12) have been shown to stimulate the release of pro-inflammatory signals from other cells. Reports also suggest that elevated levels of these DAMPs are found in the bloodstream of AAV patients. However, their role in AAV has not been clarified.
To shed light on this matter, researchers at the Tianjin Medical University General Hospital in China collected blood and urine samples from 42 MPO-positive AAV patients, of whom 34 had active disease and eight were in remission. As a comparison, samples from 10 healthy adult controls were also evaluated.
Blood tests confirmed the presence of significantly higher levels of S100A8/A9 and S100A12 in patients diagnosed with active AAV compared with those who were in remission or the controls. Similar results were found in urine samples for S100A8/A9, but not S100A12.
Among the 34 individuals with active AAV, elevated S100A8/A9 positively correlated with high S100A12 in both blood and urine samples. S100A8/A9 levels in the blood also correlated with anti-MPO antibody levels and the inflammatory/autoimmune markers C-reaction protein, erythrocyte sedimentation rate, and rheumatoid factor. S100A12 levels in the bloodstream were also closely correlated with anti-MPO levels.
Although no relationship was seen between DAMPs and Birmingham vasculitis activity scores in blood samples, urinary S100A8/A9 was associated with scores, as well as anti-MPO levels.
To investigate further, the researchers exposed neutrophils isolated from healthy individuals to increasing doses of an anti-MPO antibody. This triggered the release of increasing levels of both S100A8/A9 and S100A12, demonstrating that “MPO-ANCA could stimulate the release of S100A8/A9 and S100A12 in a concentration-dependent manner,” the researchers wrote.
Anti-MPO exposed neutrophils also activated two pathways essential to neutrophil function and survival, called TLR4 and RAGE. Further, S100A8/A9 and S100A12 boosted the migration of activated neutrophils and extended their lifespan.
Because interleukin-1-beta (IL-1-beta), a potent pro-inflammatory immune signaling protein, is implicated in AAV, the team investigated whether S100A8/A9 and S100A12 influenced its release from activated neutrophils.
Results showed that activated neutrophils released more IL-1-beta than a non-ANCA antibody, which was enhanced with S100A8/A9 or S100A12 exposure. In addition, TLR4 and RAGE pathway blockers suppressed the level of IL-1-beta release from cells.
Activating neutrophils further stimulated the secretion of proteins involved in the complement pathway — part of the immune system that amplifies the ability of antibodies and immune cells to clear microbes. Complement protein release was further enhanced with S100A8/A9 or S100A12, but suppressed with the addition of TLR4 and RAGE blockers.
Additional experiments confirmed that S100A8/A9 and S100A12 exerted pro-inflammatory effects through the MAPK/NF-kappaB pathway, which is known to contribute to an inflammatory response. TLR4 and RAGE blockers reduced the activation of this pathway, suggesting that “the activation of the [MAPK/NF-kappaB] pathway by S100A8/A9 and S100A12 in AAV was mediated, at least in part, by the TLR4/RAGE,” the team noted.
“[Blood] and urine levels of S100A8/A9 and S100A12 in patients with active MPO-ANCA-positive vasculitis were elevated and correlated with the severity of the disease,” the researchers concluded. “Besides, S100A8/A9 and S100A12 might take part in the pathogenesis [development] of the disease.”
“S100A8/A9 and S100A12 [may] serve as markers for assessing the disease severity,” they added.