Clinical Relevance of Anti‐Ku Autoantibody in Idiopathic Inflammatory Myopathies

To date, studies evaluating anti-Ku antibodies in idiopathic inflammatory myopathies (IIMs) remain limited and are mostly confined to isolated case reports or small case series, mainly focusing on cases of myositis-systemic sclerosis overlap. Moreover, the presence of anti-Ku autoantibody is clinically significant because it may present as an anti-synthetase syndrome (ASyS)-like clinical condition, which can be associated with a more delicate prognosis, thus requiring constant medical follow-up. The presence of anti-Ku autoantibody is clinically significant because it may present with an ASyS-like phenotype, including interstitial lung disease (ILD) requiring surveillance, even in the absence of classical anti-aminoacyl-tRNA synthetase antibodies. This ASyS-like presentation can be associated with a more complex prognosis, necessitating close monitoring and tailored therapeutic strategies. However, the clinical phenotype of anti-Ku antibodies in isolated IIM (without overlap syndromes) remains poorly characterized. Therefore, we aimed to investigate the clinical significance of anti-Ku antibodies in a large, single-center, retrospective cohort comprising 682 Brazilian patients with IIM, classified according to the 2017 European League Against Rheumatism/American College of Rheumatology (EULAR/ACR) criteria for IIM [1], and followed at our tertiary care center between 2000 and 2025. Detection of anti-Ku autoantibodies was performed using a commercial line blot assay (Euroimmun, Lübeck, Germany), following the manufacturer's instructions. Result interpretation was based on the methodology established in a previous study [2], which shows that only moderate and strong reactivity results (intensity ≥ 2+ on the manufacturer's visual scale) should be considered positive, while weak or borderline results (1+) were classified as negative to minimize false positives. Clinical features were defined as follows: fever (body temperature > 37.8°C documented in medical records); arthritis (joint swelling, warmth, or effusion on physical examination); ILD (confirmed by HRCT); Raynaud's phenomenon (episodic, cold- or stress-triggered digital color changes). All clinical assessments were performed by rheumatologists with expertise in inflammatory myopathies. Patients (n = 247) with IIM overlapping with other systemic autoimmune diseases (e.g., rheumatoid arthritis, Sjögren's disease, systemic sclerosis, or systemic lupus erythematosus), IIM associated with malignancy, or unavailable serum samples were excluded, resulting in a final cohort of 435 patients. Anti-Ku autoantibodies were identified in 13 (3.0%) of the 435 patients with defined IIM. Among these 13 patients, four exhibited additional myositis-specific or myositis-associated autoantibodies, including anti-PM/Scl-75 (n = 1), anti-PL-7 (n = 1), and anti-Ro-52 (n = 2). All anti-Ku-positive patients were female (100%), and most were of White ethnicity (76.9%). The median age at IIM diagnosis was 44.0 years (interquartile range [IQR]: 42.0–57.0). The initial clinical diagnoses included polymyositis in 69.2% and dermatomyositis in 30.8% of the patients. Clinical and laboratory characteristics of these patients are summarized in Table 1. Among the analyzed cases, 76.9% presented with fever, whereas 46.2% exhibited joint involvement. The same proportion (46.2%) had pulmonary involvement, characterized by ILD. All six patients with ILD had high-resolution computed tomography (HRCT)-confirmed nonspecific interstitial pneumonia (NSIP) pattern, characterized by ground-glass opacities, fine reticulation, and lower-lobe and peripheral predominance. Muscle biopsy was performed in seven of the 13 patients, with histopathological findings including: one case with extensive endomysial necrosis; four cases with endomysial inflammatory infiltrates and fiber necrosis; and two cases with perivascular and perimysial inflammatory infiltrates, one of which also showed perifascicular atrophy. The median follow-up duration was 116.0 months (IQR: 65.0–122.0). At the last clinical evaluation, 69.2% of patients were receiving immunosuppressive therapy, and 38.5% remained on glucocorticoids. No patients developed incident malignancies during follow-up. One patient (7.7%) died during follow-up due to a severe infection, likely related to immunosuppression and the underlying disease. In the subgroup of patients with concomitant autoantibodies, one had anti-PL-7, typically associated with ASyS [3, 4], while another had anti-PM/Scl, an autoantibody also associated with ASyS according to Barbosa et al. [5]. Apart from these cases, all remaining patients were positive exclusively for anti-Ku and displayed clinical features closely resembling those of ASyS. To date, few studies have analyzed anti-Ku-positive IIM patients. Existing reports have primarily associated anti-Ku antibodies with overlap syndromes, particularly those involving systemic sclerosis [6-9]. However, the present analysis specifically excluded overlap cases (including systemic sclerosis, rheumatoid arthritis, Sjögren's disease, and systemic lupus erythematosus) to avoid potential confounding factors, thereby allowing a clearer evaluation of the anti-Ku-specific phenotype (Table 1). A limitation of the present study is its retrospective, single-center design. Nevertheless, the data were obtained from electronic medical records with standardized and parameterized data, reinforcing the credibility of the data collected. Another limitation is the unavailability of confirmatory autoantibody tests using orthogonal techniques (e.g., ELISA). Recent literature suggests that commercial line blot assays may have suboptimal specificity for anti-Ku antibodies [10], potentially resulting in false-positive results. However, we applied stringent interpretation criteria (moderate to strong reactivity only, intensity ≥ 2+) following the manufacturer's instructions to enhance specificity. Additionally, the small anti-Ku-positive sample size limits statistical power. The intentional exclusion of overlap syndromes, while allowing clearer phenotypic characterization, limits the generalizability of our findings to the broader anti-Ku population, which frequently includes overlap cases. Finally, potential tertiary referral bias may have influenced the clinical spectrum observed. In conclusion, anti-Ku-positive patients in our nonoverlapping IIM cohort exhibited a distinctive clinical profile characterized by frequent fever (76.9%), arthritis (46.2%), ILD (46.2%), and Raynaud's phenomenon (30.8%). This constellation of features shares similarities with ASyS, suggesting that anti-Ku positivity may warrant clinical monitoring strategies analogous to those employed in ASyS, particularly regarding pulmonary and musculoskeletal manifestations, even in the absence of classical anti-aminoacyl-tRNA synthetase antibodies. It is important to note that our findings represent the nonoverlap anti-Ku IIM subset and may not be generalizable to the broader anti-Ku population, which frequently includes overlap syndromes, particularly with systemic sclerosis. Further comparative studies are needed to definitively establish whether anti-Ku represents a distinct clinical subset or a phenotypic variant within the IIM spectrum. Rafaella do Amaral Barbosa: study design, manuscript writing, and critical revision. Samuel Katsuyuki Shinjo: study design, manuscript writing, and critical revision. This work was supported by the Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP), 2025/06363-5, Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), 301500/2022-3, and Faculdade de Medicina da USP, or School of Medicina, São Paulo University. The authors declare no conflicts of interest. The data that support the findings of this study are available on request from the corresponding author. The data are not publicly available due to privacy or ethical restrictions.