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We would like to thank Yang et al 1 for their rewarding comment on our work, in which we report that patients harbouring anti-Ku autoantibodies with elevated serum levels of creatine kinase (elevated CK) are at risk of interstitial lung disease (ILD), whereas anti-Ku patients with anti-dsDNA are frequently affected by systemic lupus erythematosus and are at risk of glomerulonephritis.2
Yang et al retrospectively investigated 1214 patients with myositis (defined on Bohan and Peter criteria) in a single Chinese centre. Twenty-one patients (1.7%) had anti-Ku antibodies, defined as a fine speckled pattern seen at immunofluorescence, together with positive commercial assay results.
In accordance with our results, Yang et al found that ILD was a predominant characteristic of anti-Ku patients with myositis (76.2%).
Interestingly, using commercial assays, Yang et al also reported the frequent (48%) coexistence of anti-Ku with myositis-specific or myositis-associated autoantibodies (MSA/MAA). Moreover, as compared with patients with isolated anti-Ku antibodies, a skin rash was more frequent in these patients, as well as better pulmonary functional test results. In our cohort, anti-Ku antibodies were systematically confirmed using an in-house immunodiffusion technique. Apart from anti-Jo1 and anti-U1-RNP, MSA/MAA status was not available in all our anti-Ku patients. However, when searched for (using D-Tek line immunoassay, Mons, Belgium), the result was generally negative and only two anti-Ku patients with elevated CK tested positive for a coexisting MSA/MAA (table 1). None of them had a dermatomyositis rash. False positivity for MSA/MAA has recently been shown to be common when using commercial assays (14%), anti-Ku being the most frequent false positive specificity of the EuroImmun line immunoassay (3%).3 Thus, the important report by Yang et al highlights the diagnostic challenge posed by the ‘anti-Ku syndrome’ in view of the limitations of currently available routine tests.
In this regard, Yang et al additionally described the muscle biopsy findings available in 13 of their 21 anti-Ku patients. Noteworthily, the immune-mediated necrotising myopathy (IMNM) pattern, as defined by the 2017 ENMC criteria, was found in 6/7 (86%) of their patients with isolated anti-Ku versus 1/6 (17%) of their counterparts. Similarly, in our cohort, an IMNM pathological pattern was found in 7 of 8 anti-Ku patients who underwent a muscle biopsy (88%) . The sole muscle lesion found in our remaining patient was patchy (not perifascicular) sarcolemmal major histocompatibility complex class I expression.
Overall, these data emphasise that interstitial lung disease is a predominant feature in anti-Ku patients with myositis and, importantly, highlight that IMNM might be part of this anti-Ku syndrome.
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Handling editor Josef S Smolen
Contributors LS and AM wrote the manuscript, and BN performed the analysis. All authors contributed to the final version of the manuscipt. AM supervised the work.
Funding The authors have not declared a specific grant for this research from any funding agency in the public, commercial or not-for-profit sectors.
Competing interests None declared.
Patient and public involvement Patients and/or the public were not involved in the design, or conduct, or reporting, or dissemination plans of this research.
Provenance and peer review Commissioned; internally peer reviewed.