Expanding the Clinical Spectrum of Myositis with Prominent B-Cell Aggregates (BCM)

Objectives Myositis with prominent B-cell aggregates (BCM) is an uncommon finding on muscle biopsy. The initial clinical phenotype of BCM was described in a series of 10 cases as brachio-cervical inflammatory myopathy (ie, predominantly upper more than lower extremity and neck extensor weakness) associated with other autoimmune diseases including rheumatoid arthritis.[1] Since then, only a handful of cases have been reported, including patients with an inclusion body myositis (IBM)-like phenotype. Considerable knowledge gaps remain for this entity.[2,3] Methods We aimed to describe the clinical and serological characteristics of subjects in the Canadian Inflammatory Myopathy Study (CIMS) cohort with prominent B-cell aggregates (BCM) on muscle biopsy (≥30 CD20+/aggregate). A retrospective study was performed comparing myositis cases and controls with and without BCM on muscle biopsy, respectively. Controls were classified according to clinico-sero-pathological features, as dermatomyositis (DM), overlap myositis (OM) and IBM. Results In this series of 70 subjects, 23 had BCM, 21 had DM, 17 had OM, and 9 had IBM (Table 1). There were more men in the BCM group compared to the DM group (6/23 vs 1/21), and BCM cases were older than OM cases (mean age 53 vs 46 years). BCM subjects had both upper (83%) and lower (78%) extremity weakness, with upper extremity being weaker than lower extremities in 41% of cases. A greater proportion of BCM patients were weaker proximally (52%) compared to IBM patients (22%). Neck flexor weakness was frequent (74%), while neck extensor weakness was uncommon (12%). Most BCM subjects (91%) had associated autoimmune disease: 14 had systemic sclerosis, 5 rheumatoid arthritis, and 1 patient had masticatory muscle weakness with anti-AChR antibodies. Extra-muscular features found in BCM patients included inflammatory arthritis (50%), Raynaud’s phenomenon (30%), DM rash (26%) and interstitial lung disease (22%). The most common myositis-associated autoantibodies in the BCM group were anti-PM-Scl (7/23), -Ku (3/23), -Ro52 (3/23), -CENP (2/23), -RF/-CCP (2/23) and -Mi-2 (1/23). Five BCM subjects had no myositis-specific or myositis-associated antibodies. For treatment, 39% of BCM subjects received rituximab, compared to 18% in OM and 10% in DM. Table 1. Baseline characteristics of 70 autoimmune myositis subjects Conclusion In this largest series of BCM reported to date, we found similarities (concomitant autoimmune diseases) and differences (muscle weakness distribution) with previously reported cases. BCM is a distinct histopathological entity found in several myositis subsets (OM, DM), and the presence of prominent B-cell aggregates on muscle biopsy might provide a potential therapeutic target. [1.] Pestronk A. Arthritis Rheum 2006;54(5):1687-96. [2.] Meyer A. Neuromuscular Disorders 2023;33(2):169-82. [3.] Lucchini M. Neurol Neuroimmunol Neuroinflamm 2021;8(4):e1016.

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