• autoimmune diseases
• polymyositis
• dermatomyositis
• autoantibodies
• lupus erythematosus, systemic

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.