In this study, we showed that ANCA-positive LN had worse baseline renal function and a lower proteinuria remission rate than ANCA-negative LN; the decrease in ANCA titers seems to be consistent with an improvement in renal function. However, ANCA positivity was not associated with poor outcomes in LN patients.
The age of onset of ANCA-positive LN patients was higher than that in ANCA-negative LN patients but lower than that in AAV patients. Older people are known to be more susceptible to AAV, but studies have not determined a difference in the age of onset between ANCA-positive and ANCA-negative LN patients [7, 16–18]. ANCA-positive LN patients are more likely to have a worse baseline renal function (lower eGFR) than ANCA-negative LN patients, which is similar with those of previous studies [7, 19]. In addition, the follow-up data showed that ANCA titers seemed to decrease concomitantly with renal function improvement. Therefore, treatment options for LN patients with ANCA-positivity should be carefully chosen. However, the association of ANCA positivity in LN and poor prognosis remains controversial [16–18, 20]. Wang et al. showed that the ANCA-positive group had a lower remission rate than the ANCA-negative group, and that ANCA was an independent risk factor for poor renal outcome . Li et al. reported that ANCA-positive LN patients have advanced renal insufficiency; however, the renal outcomes of ANCA-positive and ANCA-negative LNs were not statistically significant . Our survival analysis suggests that the time to reach the endpoint between the ANCA-positive and ANCA-negative LN patients was not significantly different, although we found a lower proteinuria remission rate in ANCA-positive LN patients than in ANCA-negative LN patients. Another study found that ANCA-positive and negative LN patients had significant improvement in proteinuria during the first six months post-biopsy . It is known that neutrophils, NETosis, and complement play a role in tissue damage in SLE. Neutrophil phenotype and function were significantly abnormal in SLE patients, and neutrophil death was enhanced by apoptosis and NETosis . ANCA has been shown to induce endothelial cell injury by stimulating neutrophils and monocytes to release granules, and ANCA is involved in the acceleration of neutrophil apoptosis . These studies suggest a potential pathogenic role of ANCA in SLE. Therefore, we hypothesized that ANCA can be used as an indicator of SLE disease activity; however, further research will be required to confirm this. Regardless, we recommend that all patients with SLE be dynamically monitored for ANCA during treatment follow-up.
AAV patients have normal complement levels; both ANCA-positive and -negative LN patients have decreased complement levels, whereas ANCA-negative patients have lower C4 levels than ANCA-positive patients. Other reports have shown much lower C3 and C4 levels in ANCA-positive LN patients than in ANCA-negative LN patients . These disparities may be due to the selection of patients with different disease activities. To our knowledge, serum total complement levels may not necessarily reflect complement activation. In general, circulating C3 and C4 levels are normal in AAV . In SLE, low C3 and C4 levels are commonly due to C3 and C4 consumption, which is thought to be associated with activation of the classical complement system by autoantibodies and immune complexes. Previous studies have reported that decreases in serum C3 and C4 levels correlate with SLE disease activity; however, there are several contentions against these results . The main reason was that standard laboratory tests measure the concentration of parental C3 and C4 molecules rather than their activation products, and the acute phase response during inflammation may lead to an increase in C4 and C3 synthesis, which can balance the activation and increased consumption of these proteins.
SLE is a prototypic systemic autoimmune disease characterized by the presence of a variety of autoantibodies, including those directed towards DNA, chromatin, histones, and ribonucleoproteins. Jethwa et al. found that elevated levels of anti-dsDNA antibodies in the serum may result in false-positive results on the MPO-ANCA test due to the charge interaction between DNA and MPO in the serum . We analyzed whether cross-reacting autoantibodies in SLE would result in a correlation between ANCA and autoantibody seropositivity and titer. We found that there was no significant correlation between the ANCA titer and antibody seropositivity in the ANCA-positive SLE group, and elevated ds-DNA levels were not associated with elevated ANCA titer levels. However, the ds-DNA titer positively correlated with the number of autoantibodies. Therefore, we speculated that there was no ANCA cross-reaction in this study, but further experimental verification is still needed.
In our study, we analyzed 16 (34.8%) cases of MPO-ANCA and 30 (65.2%) cases of PR3-ANCA in ANCA-positive SLE patients. Previous epidemiological investigations have shown that MPO-ANCA vasculitis is more common in southern Europe, the southern United States, and Asia, whereas PR3-ANCA vasculitis is more common in Northern Europe, northern North America, and Australia . It is not clear whether this is due to genetic differences or other environmental factors, such as vitamin D levels and sun exposure. In China, the incidence of MPO-ANCA-positive AAV ranges from 80–95% . In a large study, ANCA was detected by indirect immunofluorescence in 16.4% of 566 European SLE patients; ELISA detection of MPO-ANCA and PR3-ANCA accounted for 9.3% and 1.7%, respectively . Li et al. found that 3.5% of 110 Chinese LN patients were ANCA positive; 95 eligible patients were selected, which were comprised of 26 (27.4%) who were PR3-ANCA-positive and 69 (72.6%) who were MPO-ANCA positive . Our study results were inconsistent with those of previous studies in a number of aspects. First, Harper and Savage found that serum MPO-ANCA was more prevalent in older ANCA patients than in younger patients ; a report from a Chinese population study was consistent with the results of this study . In our study, ANCA-positive SLE patients were younger (43.27 ± 13.98 years) than those in previous studies. Second, majority of the patients in our study were treated with glucocorticoids and immunosuppressants for SLE; at the time of kidney biopsy, we also detected ANCA serology. Recent reports indicate that MPO-ANCA-positive LN patients show poor renal function and a high level of activity on renal histological examination [30, 31]. Although the ratio of MPO-positive cases to PR3-positive cases was lower in our study, MPO-ANCA-positive SLE patients were more likely to have worse baseline renal function. The rate of each composite endpoint (death rate and ESRD) of the two groups were not significantly different, probably due to the limitation of the small study population. A Chinese LN cohort showed that MPO-positive patients had worse renal outcomes compared with PR3-positive patients .
ANCA mediates acute injury and induces a chronic response to injury . Our results showed that ANCA-positive LN patients were more likely to have severe interstitial fibrosis and higher chronicity index (CI) scores on renal biopsy specimens than ANCA-negative LN patients. Li et al. found that ANCA may be associated with a higher chronicity index in LN patients , and Pyo et al. discovered that ANCA positivity was associated with the chronic index . These findings support the increase in chronic inflammation of ANCA-positive LNs due to ANCA. Other studies have reported that ANCA-positive LN patients are more likely to have segmental endocapillary hypercellularity [17, 19]. However, in our study, LN was more likely to result in segmental endocapillary hypercellularity in ANCA-positive patients than in ANCA-negative patients, albeit without statistical significance. Further studies are needed to examine the pathogenesis of ANCA in LN.
This study has several limitations. First, this was a single-center study, and the number of patients was not large enough to conduct a multivariable analysis. Second, although we followed-up the change in ANCA titers during the treatment period to determine whether the dynamic evolution of ANCA is related to LN activity, the continuity of variables is not sufficient due to the limitations of a retrospective study design.
In conclusion, our study found that ANCA was associated with a poor baseline renal function in LN and delayed urinary protein remission; however, prospective studies are needed to confirm whether ANCA has a pathogenic effect on LN and to determine the specific mechanisms involved to guide therapy. We also recommend regular monitoring of ANCA titers in patients with ANCA-positive SLE, especially in those with renal improvement.