anca - associated vasculitides ( aav ) are autoimmune diseases characterized by inflammation of the blood vessel wall and include microscopic polyangiitis ( mpa ) , granulomatosis with polyangiitis ( gpa , formerly wegener 's ) , and eosinophilic granulomatosis with polyangiitis ( egpa , formerly churg - strauss syndrome ) .
antineutrophil cytoplasmic antibodies ( anca ) can usually be detected in the blood of aav patients .
cytoplasmic anca ( canca ) , mainly directed against proteinase 3 ( pr3-anca ) , predominate in gpa , and perinuclear anca ( panca ) , directed against myeloperoxidase ( mpo - anca ) , in mpa . during the past few decades
, significant advances have been made in understanding the disease pathogenesis and clinical management of aav , with substantial decrease in mortality . during long - term follow - up ,
the clinical status does not always correlate with the serum levels of anca . as there is a need to individualize the treatment ,
the emphasis has been placed on discovering new potential biomarkers of disease activity , which may help us to tailor the treatment better .
interleukin-1 ( il-1 ) cytokines play an important role in immune regulation and inflammatory processes .
il-33 is a recently identified cytokine that is a member of the il-1 family that also consists of il-1 and il-18 .
the extracellular form of il-33 activates a target cell by binding to the t1/st2 receptor and induces th2 associated cytokines but il-33 also functions at intracellular level as a nuclear factor regulating transcription [ 35 ] .
alarmins ,  endogenous danger signals activating the immune system , released by necrotic but not apoptotic cells [ 5 , 6 ] .
st2 , a member of the interleukin-1 receptor family , is selectively expressed on th2 ( t helper type 2 ) cells and mediates important th2 functions .
there are two different forms of st2 : transmembrane form ( st2l ) and secreted soluble form ( sst2 ) .
the il-33/st2 pathway has been the subject of a number of recent studies [ 815 ] .
elevated sst2 levels have been found in patients with several cardiovascular and/or pulmonary diseases ( asthma , acute dyspnoea , and myocardial infarction ) , but also sepsis , and elevated sst2 levels are considered an unfavourable prognostic marker in these patients .
il-33 levels were increased in patients with disease associated with chronic inflammation such as ankylosing spondylitis , rheumatoid arthritis , henoch - schonlein purpura , or multiple sclerosis . in sle patients elevated sst2 levels were recently described to correlate with disease activity .
low levels of il-33 and high levels of sst2 were noted in patients with hiv infection   or amyotrophic lateral sclerosis .
the aim of this study was to determine whether serum levels of il-33 or st2 are markers of disease activity in patients with aav , a disease characterised by endothelial injury and chronic inflammation .
the study included 165 samples from 139 aav patients ( 67 men , 72 women ) recruited from the department of nephrology , general university hospital , prague , czech republic , during a 15-month period .
the disease activity was assessed with bvas ( birmingham vasculitis activity score version 3 ) , with the use of clinical data and laboratory tests routinely performed ( erythrocyturia , proteinuria , and serum levels of anca , crp , and s - creatinine ) . active disease was defined as bvas  1 and remission as bvas = 0 .
relapse was defined as a new / worse disease activity and bvas  1 after previous remission .
persistent activity was defined as no new / worse activity , with persistent bvas  1 . in total ,
38 samples were from patients with active disease ( 16 active newly diagnosed , 11 active with relapse , and 11 persistent activity ) and 127 samples were from patients in remission that were available for examination .
twenty - one patients were sampled repeatedly but within each subgroup all patients were unique .
samples from newly diagnosed patients were collected immediately after the diagnosis of aav had been confirmed but some of the patients had already started immunosuppressive treatment .
the control group contained 62 patients ( 34 men , 28 women ) : 22 healthy individuals and 40 patients with chronic kidney disease ( median of creatinine 168  mol / l in nondialysis patients ; 19x hemodialysis patients , 8x iga nephropathy , 10x polycystic kidney disease , and 3x other ) .
after obtaining blood samples , all patients were routinely followed up in the clinics and clinical activity ( relapses ) and deaths were recorded for the purpose of this study .
serum il-33 and sst2 levels were measured with a commercially available elisa ( enzyme - linked immunosorbent assay ) kit .
the sst2 elisa kit was obtained from mbl international , woburn , ma , usa ( code number 7638 ) .
the chi square test was used for comparison of discrete variables ; if the minimum expected value was less than 5 , the yates correction was applied . for continuous variables , the kruskal - wallis test with multiple comparison or the mann - whitney test in the case of two groups was used .
the subgroup of patients with active newly diagnosed aav had higher median sst2 levels than healthy and renal controls ( median 0.39  ng / ml versus 0.13  ng / ml and p < 0.01 , table 3 , figure 1 ) .
median levels of sst2 in patients with active aav tended to be higher compared to patients in remission but the difference did not reach statistical significance .
subsequent analysis revealed that aav patients with active newly diagnosed disease have higher sst2 levels than aav patients in remission ( p < 0.001 , figure 1 ) . on the contrary sst2 levels in patients with relapse or persistent activity
nine of 12 patients , who were sampled repeatedly in different phases of disease ( active newly diagnosed aav  remission , or remission  relapse ) , had lower levels of sst2 in remission than in active disease stage ( table 4 ) .
levels of il-33 were significantly higher in all aav patients , in active aav , and in aav in remission than in healthy and renal control patients ( p = 0.002 , p = 0.007 , and p = 0.007 , resp . ) .
levels of il-33 tended to be lower in patients with active aav than in patients in remission but the difference did not reach statistical significance ( median il-33 levels = 0  pg / ml in both groups , mean il-33 levels are 13.16  pg / ml in active aav patients versus 41.79  pg / ml in patients with remission , p = 0.119 ) .
no relation to levels of anca , bvas , s - creatinine , crp , and proteinuria was disclosed . in the group of newly diagnosed patients
there was no difference in group with serum creatinine above 300  mol / l ( 8 patients ) and below 300  mol / l ( 8 patients ) ( median st2 levels 0.463  ng / ml and 0.221  ng / ml , resp . , p = 0.667 ) . in the group of newly diagnosed patients
there was no difference in st2 levels between patients who already initiated immunosuppressive treatment ( 9 patients ) and those without treatment ( 7 patients ) at the time of blood sampling ( median st2 levels 0.434  ng / ml and 0.350  ng / ml , resp .
there was no difference in st2 levels in patients with higher bvas ( bvas  15 , 7 patients ) and lower bvas ( bvas < 15 , 9 patients ) ( median st2 levels 0.214  ng / ml and 0.343  ng / ml , resp . ,
all newly diagnosed patients had il-33 levels 0  ng / ml . there was no difference between pr3-anca and mpo - anca but levels of il-33 in patients who suffered from granulomatosis with polyangiitis were higher than il-33 levels in patients with microscopic polyangiitis ( p = 0.012 ) .
levels of il-33 were higher in younger aav patients than older patients ( p = 0.007 ) .
no significant difference in sst2 and il-33 levels was found between patients with and without lung involvement ( median sst2 levels in 0.17  ng / ml and 0.13  ng / ml , resp .
, p = 0.338 , median il-33 levels = 0  pg / ml in both groups ) .
the median length of follow - up ( fu ) after blood collection was 30 ( range 250 ) months . during the fu , 22 patients ( 15.8% ) died and 60 patients ( 43.2% ) , 38 pr3-anca and 22 mpo - anca , suffered from a relapse .
we did not observe any difference in st2 or il-33 levels between patients who relapsed and those without relapse ( median 0.14  ng / ml versus 0.17  ng / ml and p = 0.181 ) and between patients who died and the surviving patients ( median sst2 0.16  ng / ml versus 0.16  ng / ml , p = 0.650 ; median il-33 levels = 0  pg / ml in both groups ) .
elevated levels of sst2 are present in the circulation of patients with various inflammatory diseases .
the proinflammatory cytokine il-33 is the ligand for st2 . to our knowledge , this is the first study to examine the association between serum levels of il-33 and sst2 in aav patients and disease activity .
similar to a previous study in sle patients , we demonstrated higher sst2 levels in active newly diagnosed aav patients compared to healthy controls .
elevated sst2 levels in patients with active vasculitis may suggest a relationship of sst2 with the inflammatory process in aav .
however , while the study in sle showed a correlation of sst2 levels with disease activity , we were not able to demonstrate association of sst2 with any other markers of disease activity , including anca levels , crp , or bvas .
contrary to the levels of serum st2 , there was no difference in il-33 in patients with active aav compared to patients in remission . in theory ,
this might be caused by the lower production and/or secretion of il-33 or by its rapid clearance .
there are a growing number of publications describing the precise function of this intriguing novel cytokine that has attracted a lot of attention lately but the cellular source of il-33 is less clear .
nevertheless , it seems that il-33 is mainly expressed in quiescent endothelial cells and the expression is lost when contact inhibition is disrupted or cells are exposed to proinflammatory il1- or tnf- [ 5 , 7 ] which are processes known to participate in neutrophil priming in early pathogenesis phases of aav . subsequently ,
when remission is achieved and endothelial damage repaired , il-33 levels might increase , which was also noted in this study even though the results did not reach statistical significance .
whether endothelial cells are the dominant il-33 secreting cells in vivo is , however , uncertain .
interestingly , the situation in an immune - complex vasculitis , henoch - schonlein purpura ( hsp ) , was just the opposite ; the authors described elevated il-33 levels in acute hsp that decreased in remission .
we were not able to demonstrate any statistically significant difference in different disease activity subgroups of aav patients in this study , but this might be caused by low sensitivity of the assays used .
another explanation for lack of increase in il-33 levels in active aav might be that il-33 is subject to degradation by caspases released from the apoptotic cells in aav , in accordance with the hypothesis that was suggested for amyotrophic lateral sclerosis .
primed neutrophils undergo apoptosis more readily , at which times they express pr3 and mpo on their surface .
it is possible that degradation of il-33 by proapoptotic caspase-3 released from primed neutrophils in active aav may explain the decreased levels of il-33 . as the serum receptor for il-33 , sst2 molecule ,
was increased in active aav patients , it is conceivable that sst2 acts as a contraregulating agent , regulating inflammatory process .
levels of st2 and il-33 may behave differently in patients with extrarenal involvement , which would require further studies .
a few of the newly diagnosed patients had received immunosuppressive treatment prior to blood collection , which may , in general , influence levels of circulating biomarkers in aav . however , we were not able to find any difference in il-33 levels and st2 levels between the already treated and untreated newly diagnosed patients at the time of blood sampling .
the serum levels of sst2 and particularly il-33 were low and we were not able to detect measurable il-33 levels in any of the control patients .
thus , a more sensitive assay might be needed for routine detection . alternately , intracellular il-33 expression and production may not result in a robust increase in serum il-33 levels and il-33 as with some other cytokines may have a very short half - life .
analysis of tissue il-33 expression , for example , in kidney biopsies might provide important clues to reveal the role of il-33/st2 pathway in the pathogenesis of aav , which was , however , not an objective in this study
. it might be useful to confirm the results in a longitudinal prospective study but the limited number of repeated measurements performed in patients in this study is in accordance with the cross - sectional results .
in conclusion , results of this pilot study suggest that sst2 may be a marker of activity in active newly diagnosed patients with aav .
further studies with serial samples and a prospective follow - up are needed to elucidate the precise role of sst2/il-33 in aav .
 
 objective . 
 st2 , a member of the interleukin-1 receptor family , is selectively expressed on th2 cells and mediates important th2 functions . 
 il-33 is a specific ligand of st2 . 
 the aim of the study was to determine whether serum levels of soluble st2 ( sst2 ) or il-33 predict activity of the disease in patients with anca - associated vasculitides ( aav ) . methods . 
 139 aav patients and 62 controls were studied . il-33 and 
 sst2 in the blood were measured with a commercially available elisa . 
 results . 
 newly diagnosed aav patients had higher sst2 levels than controls ( p < 0.01 ) . 
 levels of sst2 were significantly higher in active newly diagnosed aav patients than in patients with remission ( p < 0.001 ) . 
 il-33 levels were higher in aav patients than in the control groups ( p = 0.002 ) . however , serum il-33 levels were not increased in patients with active aav compared to patients in remission . 
 il-33 levels were higher in patients with granulomatosis with polyangiitis than in patients with microscopic polyangiitis ( p = 0.012 ) . conclusions . 
 serum sst2 , but not serum il-33 , may be a marker of activity in aav patients . 
1. Introduction
2. Patients and Controls
3. Results
4. Discussion
5. Conclusion