Extracellular Vesicles of Neutrophils May Be Blood Biomarkers for AAV

Extracellular vesicles, or the tiny membrane-bound particles that are released from cells, could be used as a blood biomarker of disease activity in people with ANCA-associated vasculitis (AAV), a new study suggests.

The study, “Microparticles expressing myeloperoxidase as potential biomarkers in anti-neutrophil cytoplasmic antibody (ANCA)-associated vasculitides (AAV),” was published in the Journal of Molecular Medicine.

As part of normal biological processes, including cell activation and death, cells often release tiny sacks of cellular contents — such as proteins, RNA, etc. — that help them send signals to local and distant cells.

But these sacks, also known as extracellular vesicles (EVs), can also derive from cells involved in disease processes, in which case they can also play a role in disease progression.

Neutrophils, a type of immune cell, has been implicated as a driver of AAV, and their extracellular vesicles are thought to carry molecules with pro-inflammatory features. They also carry the myeloperoxidase (MPO) protein, which is known to participate in blood cell damage, the hallmark of AAV.

However, whether neutrophil-derived EVs and the proteins they contain could serve as biomarkers of disease activity remains unknown.

Researchers at the Karolinska Institutet, in Sweden, and at the University of Nis, in Serbia, examined blood samples from 46 people with granulomatosis with polyangiitis or microscopic polyangiitis, the two most common AAV subtypes, as well as from 23 healthy people matched to patients in terms of age and gender.

Among patients, 23 had active disease at the time of sampling, as defined by a Birmingham Vasculitis Activity Score (BVAS) greater than zero. The two groups of patients were fairly similar, though inflammation and corticosteroid use was greater among those with active disease.

Using blood samples from participants, investigators isolated EVs derived from neutrophils (MPO-positive EVs) and looked specifically at EVs that contained any of three different neutrophil-related proteins: pentraxin 3 (PTX3), high-mobility group box 1 (HMGB1), and tumor necrosis factor-like weak inducer of apoptosis (TWEAK).

PTX3 and HMGB1 are specifically associated with inflammatory neutrophil activation, while TWEAK has been linked to kidney damage.

Compared with controls, AAV patients had significantly higher levels of EVs containing PTX3, HMGB1, TWEAK. EVs containing PTX3 were also higher in patients with active disease compared with those in remission, the team found.

Total blood levels of PTX3 were significantly higher in patients compared with controls, and also significantly higher in patients with active disease compared to those in remission. No differences between the patient groups in total blood levels of HMGB1 or TWEAK were seen.

Researchers found that the amount of PTX3 and HMGB1 on EVs significantly correlated with disease activity, as measured by BVAS. In other words, people with higher levels of these proteins in EVs were more likely to have more severe disease. Total blood levels of PTX3 also correlated with disease activity.

“According to our results, determination of HMGB1+ and PTX3+ MPO+ MPs is associated with disease activity in AAV,” the researchers concluded.

This finding suggests that measuring blood levels of these EVs could be useful for assessing disease severity in AAV. Notably, the findings indicate that measuring the levels of biomarkers in EVs could have more prognostic relevance than measuring the same markers in the blood.

“HMGB1-positive MPO+MPs may be a better indicator of disease activity in AAV than HMGB1 levels detected in serum,” the researchers wrote.

“Our study suggests that PTX3 and HMGB1 expressed on MPO+MPs might be used as promising biomarkers reflecting inflammation and disease activity in AAV patients,” the researchers concluded. “Our findings need to be replicated in a larger prospective study to validate these promising biomarkers of disease activity in patients with AAV.”