In the present study, we demonstrated the usefulness of serum TIMP-1 levels as a predictive biomarker of relapse and sustained remission in AAV patients with MPA and GPA during maintenance therapy. The main results obtained were as follows: 1) TIMP-1 was a more useful marker of disease activity than CRP and MPO-ANCA, distinguishing mildly or highly active AAV from remission 6 months after the initiation of treatment; 2) elevated TIMP-1 levels in patients in remission were associated with relapse and/or difficulty reducing the GC dosage; 3) approximately 30% of patients in remission with a serum TIMP-1 level of ≥ 150 ng/mL relapsed after 6 to 12 months, whereas the majority of patients with a TIMP-1 level < 150 ng/mL sustained remission for at least 12 months; and 4) a serum TIMP-1 level < 150 ng/mL reflects complete remission, while ≥ 150 ng/mL indicates subclinical inflammation.
Relapse is common in AAV and requires repeated remission-induction therapy . Although the definition of clinical remission may appear to be straightforward, many patients in remission still exhibit persistent inflammatory and immunological activities. BVAS is a common clinical indicator for evaluating disease activity in AAV, and remission is defined as a BVAS score of 0 . However, difficulties are associated with evaluating disease activity using BVAS. For example, if patients with persistent inflammation do not exhibit any overt clinical signs, BVAS will be 0. Persistent lung nodules and elevated creatinine levels are not included in scoring from the second evaluation, and other clinical signs that persist for more than 3 months are regarded as damage and often not included in the score. These issues suggest that true remission may not be achieved in many patients with BVAS 0 and persistent inflammation may increase the risk of subsequent relapse. Therefore, more sensitive biomarkers are needed to establish whether disease activity is truly suppressed or completely abrogated instead of relying on the clinical definition of remission.
ANCA is often used to evaluate disease activity; however, its utility for predicting relapse is limited [2, 7, 23–27]. Large cohorts identified proteinase-3 (PR3)-ANCA, but not MPO-ANCA, as an independent risk factor for relapse [23–26]. In the serial analysis of PR3-ANCA in the RAVE study, an elevated PR3-ANCA titer was a poor predictor of subsequent relapse within the overall population, but was predictive of relapse in patients with renal disease or alveolar hemorrhage . In two prospective Japanese cohorts, the reappearance of MPO-ANCA was significantly associated with relapse . However, among 25 relapsed patients after negative conversion of MPO-ANCA, 6 (24%) patients did not experience reappearance of MPO-ANCA. On the other hand, among 76 patients without negative conversion of MPO-ANCA, 60 patients (79%) maintained remission. Based on these findings, ANCA shows limited utility for predicting relapse and is rarely used as a predictor of sustained remission.
Previous studies investigated predictive markers of relapse other than ANCA. In the RAVE study, an increase in serum calprotectin levels by 2 or 6 months was predictive of relapse by 18 months in PR3-ANCA-positive AAV patients treated with rituximab. However, this marker was not useful in patients treated with oral cyclophosphamide . The CD8 T-cell transcription signature was associated with subsequent relapse in AAV or systemic lupus erythematosus, but requires further prospective validation .
In the two large cohort studies, the Japanese RemIT-JAV-RPGN study (our previous study)  and RAVE study , TIMP-1 levels were a useful biomarker for distinguishing mildly or highly active AAV from remission regardless of differences in the population, such as AAV type, ANCA type, and treatment. In our previous study, TIMP-1 correlated with the total BVAS score before treatment, and was less likely to be elevated in bacterial infections than CRP. In contrast, CRP, ESR, and MPO-ANCA were unable to distinguish patients with mildly active AAV from those in remission. These findings suggest that TIMP-1 is a superior biomarker to CRP, ESR, and MPO-ANCA for monitoring the disease activity of AAV. However, in both studies, TIMP-1 was only evaluated at two points, before and 6 months after the initiation of remission-induction therapy.
In the present study, we examined whether TIMP-1 levels are clinically useful as a predictor of relapse and sustained remission during maintenance therapy. Based on the results of an 18-month follow-up in the RemIT-JAV-RPGN cohort study, elevated TIMP-1 levels at 6 months were associated with relapse and/or difficulty reducing GC from 6 to 18 months. In the MAAV-EU study, TIMP-1 levels were significantly higher in relapsed patients 6 months before relapse than in sustained remission patients. Based on these findings, many patients with relapse have elevated TIMP-1 levels 6 months before relapse. However, elevated TIMP-1 levels were observed in some patients in sustained remission.
Since the cut-off point for sustained remission without difficulty reducing GC was 148 ng/mL, patients in remission were divided into two groups of TIMP-1 levels, ≥ 150 ng/mL and < 150 ng/mL, and their relationship with clinical outcomes was examined. In the analysis of both studies, approximately 30% of patients in remission with a TIMP-1 level ≥ 150 ng/mL relapsed after 6 to 12 months. More importantly, the majority of patients with a TIMP-1 level < 150 ng/mL remained in remission for at least 12 months. Therefore, a serum TIMP-1 level < 150 ng/mL reflects complete remission, whereas ≥ 150 ng/mL indicates subclinical inflammation. Moreover, TIMP-1 levels are more useful as a predictor of sustained remission than of relapse. During maintenance therapy, TIMP-1 levels < 150 ng/mL are an important indicator of the long-term maintenance of remission. Thus, in patients with AAV, the maintenance of a TIMP-1 level < 150 ng/mL may allow for the tapering or cessation of treatment.
TIMP-1 is expressed by various cell types in most human tissues. It is an endogenous inhibitor of matrix metalloproteinases (MMPs) and its production increases in response to an increase in MMPs. In addition, TIMP-1 has MMP-independent roles as a cytokine. It is a well-known regulator of extracellular matrix turnover, tissue remodeling, inflammation, cell growth, apoptosis, and cellular behavior in tissue [29–31]. Therefore, TIMP-1 levels may more accurately reflect the condition of inflammatory sites than CRP; however, the underlying mechanisms have not yet been elucidated. The normalization of TIMP-1 levels may indicate that vascular inflammation has completely subsided. On the other hand, serum levels of TIMP1 were previously reported to be elevated in patients with various cancers, myocardial infarction, ischemic stroke, and sepsis [29–36]. Therefore, it is important to note that TIMP-1 may not serve as a predictor of sustained remission in AAV patients with these disorders. We expect TIMP-1 levels to become a predictor of sustained remission during maintenance therapy and contribute to treatment strategies for AAV.