**4. Discussion**

The most compelling results of the current study are as follows: (1) The HCVuninfected cohort had the lowest cumulative incidence of rheumatic diseases among the 3 cohorts, while indifferent cumulative incidences were identified between the HCV-treated and HCV-untreated cohorts. (2) HCV infection, female gender, baseline age ≥ 49 years, CCI score ≥ 1, baseline COPD and dyslipidemia were associated with increased HRs of rheumatic diseases, while baseline liver cirrhosis, ESRD and DM were associated with

decreased HRs. (3) The HCV-untreated cohort had the highest cumulative incidence of overall mortality at 11 years, while HCV-treated and HCV-uninfected cohorts yielded indifferent mortality rates.

The higher rate of baseline cirrhosis in the HCV-treated than the HCV-untreated cohorts of TNHIRD was coincided with the fact that only patients with significant fibrosis were reimbursed with anti-HCV therapy [28], and the other different baseline variables between these 2 cohorts highlight the idea that patients with comorbidities were ineligible for the interferon-based therapy and had been excluded for anti-HCV therapy. The different rates in baseline variables between HCV-infected and HCV-uninfected cohorts were consistent with the phenomenon that HCV infection elicits many cardiometabolic events and hypolipidemia [1]. Therefore, the baseline comparisons of the 3 cohorts supported the reliability of the data based on TNHIRD.

The fact that the HCV-uninfected cohort had the lowest cumulative incidence of rheumatic diseases, and HCV infection increased the HR of developing rheumatic diseases based on multivariate analyses, endorsed the concept that HCV infection might cause rheumatic diseases, despite the fact that some studies did not support the participation of HCV infection in the pathogenesis of RA [29–31]. However, given that the HRs in developing rheumatic diseases between the HCV-treated and HCV-untreated cohorts were indifferent, the HCV-associated risk of rheumatic diseases might not be attenuated by interferon-based anti-HCV therapy. In particular, cryoglobulinemic vasculitis represents the prototype of HCV-related rheumatic diseases [3]; long-term mixed cryoglobulinemia after SVR is common since cryoglobulin-generating B lymphocytes might have reached an HCV-independent autonomous phase before viral clearance [32]. HCV-associated rheumatic disease therefore might persist despite viral clearance. Moreover, whether interferon-based therapy reduces the risk of RA had remained conflicting [33,34], and interferon-based anti-HCV therapy may work as a "trigger" for RA [35,36] or SLE [37] had been shown in some case reports. Although treatment with interferon-alpha may lead to substantial clinical improvement of HCV-related arthritis even without a complete biochemical or virological response [34], autoimmune diseases indeed occur in 4% to 19% of patients receiving interferon-based anti-HCV therapy and the associated symptoms developed between 2 weeks and 7 years after initiation of therapy [38].The interferonbased anti-HCV therapy thus has been contraindicated for many rheumatologic autoimmune/inflammatory diseases based on the concern of triggering rheumatic diseases. New oral interferon-free combinations of various DAAs offer an opportunity for HCV-infected patients with rheumatic diseases to be cured with a short treatment duration and a low risk of side effects [39]. However, SVR following DAA might lead to immune reconstitution as tuberculosis reactivation had been reported [40]. Whether DAA therapy precisely attenuates the risks of HCV-associated rheumatic disease without introducing other harm as mentioned above [40] demands further investigation.

On the other hand, that female sex and baseline age ≥ 49 years are positively associated with the increased HRs of rheumatic diseases is consistent with the fact that female sex and old age had been identified as risk factors for RA [41]. CCI score ≥ 1 and baseline COPD were associated with increased HRs of rheumatic diseases, which coincides with the fact that comorbidities including respiratory disease were more common in patients with RA at diagnosis than controls [42]. Patients with rheumatic diseases have increased prevalence of metabolic syndrome including dyslipidemia [43], and acute myocardial infarction risk increased by 38% [44] in RA patients might explain why dyslipidemia were associated with increased HR of rheumatic diseases. Of note, the fact that baseline liver cirrhosis, ESRD and DM are associated with reduced HRs of rheumatic diseases is a novel finding. Interestingly, the connections with cirrhosis are variable among different rheumatic diseases. For example, the overall incidences of cirrhosis were reported to be lower in the RA cohort than in the non-RA cohort [45,46], while patients with psoriasis were found to have increased risk of cirrhosis over patients without psoriasis [46].With regard to ESRD and DM, in contrast to their negative associations with the rheumatic disease risks, chronic kidney disease is a common complication of rheumatic diseases [47]; patients undergoing hemodialysis therapy may develop serious rheumatic complications [48], newly diagnosed RA patients are at higher risk of DM [49] and the prevalence of DM is higher in patients with psoriatic arthritis compared with the general population [50]. That rheumatic diseases might be mistaken as ESRD- or DM-related complications in patients with ESRD and DM potentially explains the aforementioned paradox.

Among the 3 cohorts, the HCV-untreated cohort yielded the highest overall mortality, which might be caused by other HCV-associated events such as cirrhosis, HCC or cardiometabolic events [1] other than rheumatic disease-associated complications, since HCV-treated and HCV-uninfected cohorts had indifferent mortalities, although the latter obviously had a lower risk of rheumatic diseases. This phenomenon indicates the importance to prescribe anti-HCV therapy in HCV-infected patients in decreasing overall mortality, regardless of the risk for rheumatic diseases.

There are limitations recognized in the current study. First, because linking the results from TNHIRD to the laboratory results of individual patients was forbidden for privacy protection, the correlation of SVR with rheumatic diseases could not be identified. However, we are confident in the antiviral efficacy in the HCV-treated cohort since interferon-based therapy for HCV infection generally achieves an SVR rate up to 90% in Taiwan [51], where a favorable genetic variation in IFNL3 is prevalent [51]. Second, as mentioned above, interferon-based therapy might elicit rheumatic diseases in SVR patients [35–38] and blunt the impact of viral clearance in attenuating rheumatic disease risks. Third, because most of the rheumatic diseases accounted for the minority of the whole population and our preliminary statistical tests did not show any significance for any individual rheumatic disease, we thus had put all rheumatic disorders together as rheumatic diseases to yield the maximal statistical power. Some specific rheumatic disorders might have different connections with HCV infection or anti-HCV therapy. Anyhow, that SVR did not reduce the incidences of SLE and RA in CHC patients [52] supported our observation. Future prospective studies in other independent large cohorts with identifiable SVR following DAA therapy, subgroup analyses for specific rheumatic disorders and sophisticated molecular investigations are required to elucidate the fundamental mechanisms underlying the findings described here.

Taken together, HCV infection, female sex, baseline age ≥ 49 years, and other comorbidities were associated with risks of rheumatic diseases. Although interferon-based therapy did not attenuate the rheumatic disease risk, it indeed decreased the overall mortality of HCV-infected patients. These findings may merit further study for preventing or treating rheumatic diseases in HCV-infected patients.

**Supplementary Materials:** The following are available online at https://www.mdpi.com/article/ 10.3390/jcm10040817/s1, Figure S1: Steps of the matching process, Figure S2: Comparison of the baseline factors between the HCV-infected cohort (HCV-treated and HCV-untreated cohorts), and HCV-uninfected cohort.

**Author Contributions:** Data curation, Y.-S.L., J.-H.H., M.-Y.C., H.-P.K., R.-N.C.; formal analysis, J.-S.C., H.-P.K., M.-L.C.; funding acquisition, M.-L.C.; investigation, M.-L.C.; methodology, M.-L.C.; supervision, M.-L.C. All authors have read and agreed to the published version of the manuscript.

**Funding:** This study was supported by grants from the Chang Gung Medical Research Program (CMRPG3I0412, CMRPG3K0721 and CMRPG1K0111) and the National Science Council (MOST 108-2314-B-182-051-, MOST 109-2314-B-182-024- and MOST 109-2629-B-182-002-). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. The opinions expressed in this paper are those of the authors and do not necessarily represent those of the Chang Gung Medical Hospital and the National Science Council, Taiwan.

**Institutional Review Board Statement:** The study protocol conformed to the ethical guidelines of the 1975 Declaration of Helsinki and was approved by the local Institutional Review Board.

**Informed Consent Statement:** The need for consent was waived because the national-level data used in this study were de-identified by encrypting personal identification information.

**Acknowledgments:** The authors thank Shu-Chun Chen, Chia-Hui Tsai, Chun-Kai Liang and Shuen-Shian Shiau from the Liver Research Center, Chang Gung Memorial Hospital, Taiwan, for their assistance with data mining.

**Conflicts of Interest:** The authors declare no conflict of interest.
