Introduction

Immunoglobulin-A vasculitis (IgAV), formerly called Henoch-Schönlein purpura (HSP), is an inflammatory small-sized blood vessel disease, more common in children and rarer but more serious in adults1,2,3. The defining pathophysiologic feature of this vasculitis is the IgA1-predominant immune deposits in the vessel walls1. Although IgAV usually involves the skin, joints and the gastrointestinal (GI) tract4,5,6,7,8, nephritis is also common in affected patients and constitutes the most feared complication of this vasculitis1,2,3,4,5. IgAV has a multifactorial aetiology in which genetics plays a relevant role in both the predisposition and clinical course of the disease6,9,10. In this regard, outside the human leukocyte antigen region, cytokines signalling pathway genes constitute a key component of the genetic network underlying IgAV6,11,12,13,14.

Interleukin (IL)-33 is a cytokine that belongs to the IL-1 family15. This molecule exerts its biological functions by binding to its receptor, IL-1 receptor like 1 (IL-1RL1) (also known as suppression of tumorigenicity 2 or ST2), a member of the Toll/IL-1 receptor superfamily16. This binding leads to the activation of mast cells and Th2 lymphocytes and, consequently, to the production of chemokines, pro-inflammatory and Th2-associated cytokines, as well as increased serum immunoglobulin levels15. In accordance with that, a pathogenic role of the IL-33 axis in autoimmunity has been described17,18,19. In addition, several lines of evidence demonstrate that genetic variants located both in the IL33 and IL1RL1 genes are implicated in the increased risk of numerous immune-mediated diseases20,21,22,23,24,25,26,27,28.

Taking into account all these considerations, this study aimed to determine, for the first time, the potential implication of the IL33-IL1RL1 signalling pathway in the pathogenesis of IgAV. For this purpose, we genotyped three tag genetic variants within IL33 and three tag polymorphisms within IL1RL1, which cover the major variability of these loci and that were previously associated with several inflammatory diseases, in the largest series of Caucasian IgAV patients ever assessed for genetic studies.

Patients and methods

Study population

A series of 380 unrelated Spanish patients of European ancestry who fulfilled Michel et al. criteria29 and/or the American College of Rheumatology classification criteria30 for IgAV-HSP and/or the 2012 revised International Chapel Hill Consensus Conference Nomenclature31 definition for IgAV were included in the present study. Centres involved in the recruitment of these patients included Hospital Universitario Marqués de Valdecilla (Santander), Hospital Universitario San Cecilio (Granada), Hospital Universitario de Bellvitge (Barcelona), Hospital Universitario Lucus Augusti (Lugo), Hospital Universitario Central de Asturias (Oviedo), Hospital Universitario Severo Ochoa and Hospital Universitario de La Princesa (Madrid), Hospital Universitario Virgen del Rocío (Sevilla) and Hospital Universitario de Basurto (Bilbao). Information on the main clinical features of these patients is displayed in Table 1. For GI manifestations, bowel angina was considered present if there was diffuse abdominal pain that worsened after meals or bowel ischemia usually with bloody diarrhoea. GI bleeding was defined as the presence of melena, haematochezia, or a positive test for occult blood in the stool. Renal manifestations were defined to be present if at least one of the following findings was observed: haematuria, proteinuria, or nephrotic syndrome at any time over the clinical course of the disease and/or renal sequelae (persistent renal involvement) at last follow-up.

Table 1 Main clinical features of the 380 patients with IgAV included in the study.

In addition, a set of 845 sex and ethnically matched healthy controls without history of cutaneous vasculitis or any other autoimmune disease, constituted by blood donors from Hospital Universitario Marqués de Valdecilla (Santander) and National DNA Bank Repository (Salamanca), was also included in this study.

For experiments involving humans and the use of human blood samples, all the methods were carried out in accordance with the approved guidelines and regulations, according to the Declaration of Helsinki. All experimental protocols were approved by the Ethics Committees of clinical research of Cantabria (for Hospital Universitario Marqués de Valdecilla, Santander), Ethics Committee of clinical research of Granada (for Hospital Universitario San Cecilio, Granada), Ethics Committee of clinical research of Barcelona (for Hospital Universitario de Bellvitge, Barcelona), Ethics Committee of clinical research of Galicia (for Hospital Universitario Lucus Augusti, Lugo), Ethics Committee of clinical research of Asturias (for Hospital Universitario Central de Asturias, Oviedo), Ethics Committee of clinical research of Madrid (for Hospital Universitario Severo Ochoa and Hospital Universitario de La Princesa, Madrid), Ethics Committee of clinical research of Sevilla (for Hospital Universitario Virgen del Rocío, Sevilla) and Ethics Committee of clinical research of Euskadi (for Hospital Universitario de Basurto, Bilbao). Patients with IgAV and healthy controls signed an informed written consent before being included in the study.

Single nucleotide polymorphisms selection and genotyping methods

Three tag genetic variants within IL33 (rs3939286, rs7025417 and rs7044343) and three tag polymorphisms within IL1RL1 (rs2310173, rs13015714 and rs2058660) were selected in this study. Regarding IL33, rs3939286 is situated in an intergenic region close to IL33, whereas rs7025417 and rs7044343 are located in the promoter and in intron 5 of the gene, respectively. With respect to IL1RL1, both rs2310173 and r2058660 are placed close to IL1RL1, while rs13015714 is downstream of the gene. Interestingly, these six polymorphisms map and tag both IL33 and IL1RL1 regions as previously described20. In addition, the IL33 and IL1RL1 genetic variants selected in our study were previously associated with several immune-mediated diseases20,21,22,23,24,25,26,27,28. In particular, IL33 rs3939286 was previously implicated in the development of subclinical atherosclerosis in patients with rheumatoid arthritis (RA)20 as well as in the pathogenesis of inflammatory bowel (IBD) disease21 and asthma22, whereas IL33 rs7025417 was previously associated with coronary artery disease23, and rs7044343 with RA24. Regarding IL1RL1, a relevant role of rs2310173 in ankylosing spondylitis25 and ulcerative colitis26, as well as an association of rs13015714 with celiac disease 27 and IBD21, and an influence of rs2058660 on the development of Crohn’s disease28 were previously reported.

Genomic deoxyribonucleic acid (DNA) from all the individuals included in the study was extracted from peripheral blood using standard procedures.

Patients with IgAV and healthy controls were genotyped for the six polymorphisms mentioned above using predesigned TaqMan 5′ single-nucleotide polymorphism genotyping assays (C___2762168_10 for rs3939286, C__31940410_20 for rs7025417, C__29340326_10 for rs7044343, C___2676437_10 for rs2310173, C__31439507_10 for rs13015714 and C__11487892_10 for rs2058660).

Genotyping was performed in a QuantStudioTM 7 Flex Real-Time polymerase chain reaction system, according to the conditions recommended by the manufacturer (Applied Biosystems, Foster City, CA, USA).

Negative controls and duplicate samples were included to check the accuracy of the genotyping.

Statistical analyses

Genotype data were checked for deviation from Hardy–Weinberg equilibrium (HWE).

Differences in IL33 and IL1RL1 frequencies were evaluated between patients with IgAV and healthy controls and between patients with IgAV stratified according to specific clinical characteristics of the disease (age at disease onset or presence/absence of GI or renal manifestations).

First, each IL33 and IL1RL1 polymorphism was analysed independently. Both genotype and allele frequencies were calculated and compared between the groups mentioned above by chi-square test. Strength of association was estimated using odds ratios (OR) and 95% confidence intervals (CI).

Then, allelic combinations (haplotypes) of both IL33 and IL1RL1 polymorphisms were carried out. Haplotype frequencies were calculated by the Haploview v4.2 software (http://broad.mit.edu/mpg/haploview) and then compared between the groups mentioned above by chi-square test. Strength of association was estimated by OR and 95% CI.

P-values were two-tailed and those lower than 0.05 were considered as statistically significant. All analyses were performed with the STATA statistical software 12/SE (Stata Corp., College Station, TX, USA).

Results

Genotyping quality control

IL33 rs3939286, IL33 rs7025417, IL33 rs7044343, IL1RL1 rs2310173, IL1RL1 rs13015714 and IL1RL1 rs2058660 were in HWE.

The genotyping success rate for each polymorphism evaluated in this study was 99.9% for IL33 rs3939286, 99.4% for IL33 rs7025417, 99.8% for IL33 rs7044343, 100% for IL1RL1 rs2310173, 99.6% for IL1RL1 rs13015714 and 99.8% for IL1RL1 rs2058660.

Genotype and allele frequencies of each IL33 and IL1RL1 polymorphism were similar to that reported for populations of European origin in the 1000 Genomes Project (http://www.internationalgenome.org/).

Differences in IL33 and IL1RL1 genotype and allele frequencies between patients with IgAV and healthy controls

In a first step, we compared genotype and allele frequencies of each IL33 and IL1RL1 variant assessed independently between patients with IgAV and healthy controls.

As shown in Table 2, similar genotype and allele IL33 and IL1RL1 frequencies were observed in patients with IgAV when compared to healthy controls (Table 2).

Table 2 Genotype and allele frequencies of IL33 and IL1RL1 genes in patients with IgAV and healthy controls.

Differences in IL33 and IL1RL1 genotype and allele frequencies between patients with IgAV stratified according to specific clinical characteristics of the disease

Then, potential differences in the genotype and allele frequencies of each IL33 and IL1RL1 genetic variant were evaluated between patients with IgAV stratified according to specific clinical characteristics of the disease.

Given that IgAV is often a benign and self-limited pathology in children and a more severe condition in adults, we assessed if potential differences in IL33 and IL1RL1 could exist in IgAV patients stratified according to the age at disease onset. With this respect, and as shown in Table 3, similar genotype and allele IL33 and IL1RL1 frequencies were disclosed when children (age ≤ 20 years) were compared to adults (age > 20 years) (Table 3).

Table 3 Genotype and allele frequencies of IL33 and IL1RL1 genes in patients with IgAV stratified according to clinical characteristics of the disease.

In addition, we compared genotype and allele frequencies of IL33 and IL1RL1 in patients with IgAV stratified according to the presence/absence of GI or renal manifestations. Accordingly, similar genotype and allele IL33 and IL1RL1 frequencies were disclosed when patients with IgAV who developed GI manifestations were compared to those who did not exhibit these complications (Table 3). This was also the case when patients with IgAV were stratified according to the presence/absence of renal manifestations (Table 3).

Haplotype analyses of IL33 and IL1RL1

In a further step, we also examined whether haplotype frequencies of both IL33 and IL1RL1 differed between patients with IgAV and healthy controls as well as between patients with IgAV stratified according to the specific clinical characteristics of the disease mentioned above.

The haplotype analysis did not yield additional information since IL33 and IL1RL1 haplotypes frequencies were similar in patients with IgAV when compared to healthy controls (Table 4). This was also the case when patients with IgAV were stratified according to the age at disease onset and the presence/absence of GI or renal manifestations (Supplementary Tables S1S3 online).

Table 4 Haplotype analysis of IL33 and IL1RL1 genes between patients with IgAV and healthy controls.

Discussion

Several pieces of evidence clearly indicate that a common genetic background may underlie different inflammatory diseases32,33. In this regard, the IL33-IL1RL1 signalling pathway is described as a shared genetic risk factor for numerous immune-related diseases20,21,22,23,24,25,26,27,28.

Based on these considerations, in this study we aimed to determine whether the IL33-IL1RL1 signalling pathway is also implicated in the pathogenesis of IgAV, a small-vessel vasculitis in which cytokines signalling pathway genes are considered as an essential component in its etiology6,11,12,13,14. For that purpose, six polymorphisms within the IL33-IL1RL1 signalling pathway, which cover most of the variability of both IL33 and IL1RL1 regions and that were previously related to numerous immune-mediated diaseases20,21,22,23,24,25,26,27,28, were evaluated in the largest series of Caucasian patients with IgAV ever assessed for genetic studies. Our results showed no influence of the IL33-IL1RL1 signalling pathway on the susceptibility to IgAV when we studied each of the polymorphisms separately. Moreover, when we analysed IL33 genetic variants as well as IL1RL1 polymorphisms together conforming haplotypes, our results revealed a lack of association between the IL33-IL1RL1 signalling pathway and IgAV susceptibility. Furthermore, no specific association of IL33 and IL1RL1 polymorphisms (assessed independently or combined conforming haplotypes) with clinical features of IgAV was observed in our study, indicating that this pathway does not represent a risk factor for the severity of the disease.

Data on the potential effect of the IL33-IL1RL1 signalling pathway on vasculitis were scarce so far. A study performed in Caucasians evaluated the role of this pathway in the susceptibility to giant cell arteritis (GCA)34, another primary systemic vasculitis that involves large and middle-sized blood vessels31,35. In keeping with our results, no influence of IL33 rs3939286, IL33 rs7044343, IL1RL1 rs2310173, IL1RL1 rs13015714 and IL1RL1 rs2058660 on GCA was detected34. By contrast, a consistent association between IL33 rs7025417 and GCA was found, pointing out this polymorphism as a novel genetic risk factor for GCA susceptibility34. In addition, the potential role of an IL33 polymorphism (rs1342326) in the pathogenesis of Behçet’s disease, a condition affecting blood vessels of all sizes and types31, was assessed in Iranians36. With this respect, an effect of this genetic variant in the susceptibility to this disease was disclosed36.

The results derived from our study are of potential clinical relevance. Vasculitides comprises a heterogeneous group of autoimmune diseases that often have overlapping clinical and pathological manifestations37. Although their complex aetiology is far from being completely understood, differences among them in molecular terms have been postulated38. Our data support this hypothesis since no implication of the IL33-IL1RL1 signalling pathway in the pathogenesis of IgAV in Caucasians was disclosed, unlike GCA34.

In summary, based on a large series of Caucasian patients, our results suggest that the IL33-IL1RL1 signalling pathway does not contribute to the genetic network underlying IgAV.