Significance of Anti-Neutrophil Cytoplasmic Antibodies in Systemic Sclerosis

Jayne Moxey; Molla Huq; Susanna Proudman; Joanne Sahhar; Gene-Siew Ngian; Jenny Walker; Gemma Strickland; Michelle Wilson; Laura Ross; Gabor Major; Janet Roddy; Wendy Stevens; Mandana Nikpour

Disclosures

Arthritis Res Ther. 2019;21(57) 

In This Article

Discussion

The prevalence of ANCA in this large SSc cohort was 8.9%, which is within the range of 0–12% reported in previous studies.[7,10,11] Previous studies have found anti-MPO to be the predominant ANCA subtype in SSc. Comparatively, there was a higher prevalence of anti-PR3 in our cohort (15.5% of ANCA-positive patients) rather than anti-MPO (11.2% of ANCA-positive patients).

Given the known association between penicillamine therapy and the development of ANCA antibodies,[10,16–19] it is notable that in this modern era cohort, relatively few patients (8.3%) had received penicillamine. In addition, there was no significant difference in exposure to penicillamine between ANCA positive and ANCA negative, anti-MPO positive and negative, and anti-PR3 positive and negative groups. This suggests that exposure to penicillamine is not responsible for the difference in ANCA status between these groups.

Overlap syndrome with AAV in our cohort was rare, with only 3 cases of AAV (0.23% of the entire cohort, 2.6% of ANCA-positive patients). This low prevalence of AAV is comparable to a previous study of 2200 SSc patients, which reported a prevalence of AAV of only 0.40%.[12] The reason for the significant majority of ANCA-positive SSc patients not manifesting vasculitis is unclear. Theories for this discordance are that the presence of ANCA is an epiphenomenon of SSc, that ANCA present in SSc may have poor affinity and avidity for the epitope and are therefore less pathogenic, that there may be false positive ANCA results or that ANCA has been detected prior to the future onset of AAV.[8]

There were several differences in demographics between ANCA-positive and ANCA-negative patients. ANCA-positive patients were more likely to be Asian, and this difference was also observed in the anti-PR3-positive group compared to the anti-PR3-negative cohort. A higher proportion of male patients were found in the anti-PR3-positive group, which is notable given that SSc male patients have been shown to have a higher prevalence of ILD and increased mortality.[26]

Limited disease predominated in our entire cohort (74.8%) and in both the ANCA-positive and ANCA-negative groups. Despite a predominance of lcSSc in all groups, there was a significantly higher prevalence of anti-Scl-70 in the ANCA-positive group compared to the ANCA-negative group, the anti-MPO-positive group compared to the anti-MPO-negative group, and the anti-PR3-positive group compared to the anti-PR3-negative group. Accordingly, as these antibodies are mutually exclusive, there was also a lower prevalence of anti-centromere antibodies in the ANCA-positive, anti-MPO-positive and anti-PR3-positive groups. There was a significantly lower prevalence of anti-nuclear antibody in the anti-MPO-positive group.

There was a significantly higher prevalence of ILD in the ANCA positive compared to ANCA-negative group and also the anti-PR3-positive group compared to the anti-PR3-negative group. Importantly, in multivariable analysis, the relationship between ANCA and increased prevalence of ILD was independent of anti-Scl-70. To our knowledge, this is the first study to find an independent association of ANCA and ILD in SSc patients in the absence of comorbid AAV. An association between anti-MPO antibodies and ILD in patients with rheumatoid arthritis without comorbid AAV has been reported in a small retrospective analysis, with 3 of 12 (25%) patients with anti-MPO antibodies manifesting ILD compared to 0 of 85 patients without anti-MPO antibodies.[27] The authors suggested subclinical vasculitis might have led to pulmonary fibrosis in these 3 patients.[27] For patients with SSc/AAV overlap, it is estimated that the prevalence of ILD is around 80%, which is far higher than that expected for each condition individually.[12,28] It is suggested that a high susceptibility to ILD in patients with AAV/SSc overlap may be related to an inflammatory insult or due to oxidative stress caused by ANCA in patients with a predisposition to ILD due to their SSc.[12,28] In our study, the underlying cause of the association between ANCA and increased prevalence of ILD in SSc is unclear. Our findings may suggest a pathological role of ANCA in the development of ILD in SSc, perhaps mediated by subclinical vasculopathy; however, it is also possible that ILD itself induces the development of ANCA antibodies. The temporal relationship between the development of ILD and the emergence of ANCA was not explored in this study.

Our study demonstrates an association between ANCA and PE, and the increased prevalence of PE was also observed in the anti-PR3-positive compared to the anti-PR3-negative group. Notably, in multivariable analysis, this association was independent of the presence of anti-phospholipid antibodies. Patients with AAV are known to have an increased prevalence of venous thromboembolism, particularly when AAV is active; however, the underlying cause for this increased risk is unknown.[29] Our findings suggest that ANCA may have a prothrombotic effect even in the absence of AAV.

Previous studies have suggested that patients with comorbid SSc and AAV have a higher prevalence of overlap syndromes with other connective tissue diseases.[14] Our study demonstrates that even in the absence of AAV, ANCA-positive SSc patients have a higher prevalence of overlap features with another connective tissue disease and specifically a higher prevalence of overlap with Sjogren's syndrome. With respect to treatments, ANCA-positive patients were more likely to receive azathioprine and anti-PR3-positive patients were more likely to receive mycophenolate. These findings may be attributable to the increased prevalence of ILD in these groups. Finally, ANCA-positive status was associated with a 1.6 fold increased hazard of mortality identifying a subset of patients with worse prognosis. While cause-specific mortality was not evaluated, the increased mortality associated with ANCA may be related to ILD, thromboembolic complications and multi-organ involvement seen in overlap SSc.

To our knowledge, this is the largest study investigating the clinical associations of ANCA in SSc published to date. This study is also strengthened by a comprehensive evaluation of associations with disease features and mortality using prospectively collected data. However, the results need to be interpreted within the limitations of the study design. We were unable to check for all possible confounders such as thyroid and liver disease that are known to be associated with ANCA, as information regarding these conditions is not collected in the cohort study. Whilst data are collected prospectively at each annual study visit, this analysis was performed retrospectively, and some data were missing for some patients. In particular, only 67.5% of patients had anti-RNA polymerase III antibodies tested. However, for the remainder of variables, the percentage missing data was below 10%. Even though the study included a large number of patients, the overall prevalence of each of anti-PR3 and anti-MPO was low, limiting the power of the statistical analyses to draw robust conclusions. Lack of data on serial measurement of anti-MPO and anti-PR3 titre is also a limitation, as is lack of confirmation of all ENA and anti-dsDNA testing using immunoprecipitation or Western blot, and the Farr radioimmunoassay, respectively.

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