C42-36 Ocrelizumab and the Hidden Pattern of Organizing Pneumonia

Abstract Introduction High-efficacy disease-modifying therapies have transformed the management of multiple sclerosis (MS). Ocrelizumab, an anti-CD20 monoclonal antibody, effectively suppresses inflammatory activity but has also been linked to rare immune-mediated complications, including organizing pneumonia. As its use expands, awareness of these potential adverse effects is essential. We present a case highlighting the diagnostic challenges of ocrelizumab-associated organizing pneumonia and underscore the need for vigilance in patients with persistent pneumonia-like symptoms unresponsive to antibiotics. Case Presentation A 39-year-old man with relapsing remitting multiple sclerosis on ocrelizumab presented with a one-month history of cough, fevers, night sweats, and 20-lb weight loss. He reported daily marijuana vaping but denied tobacco or other drugs. Initial chest computed tomography (CT) showed diffuse ground glass opacities in the right upper, right middle, and left lower lobes (Figure 1). He was treated for multifocal pneumonia without improvement. A few days later, repeat chest CT showed worsening ground glass opacities, now involving more lobes of the lung. He underwent bronchoscopy with bronchoalveolar lavage which revealed lymphocyte predominant fluid. Broad infectious studies were negative. A multidisciplinary team began to suspect organizing pneumonia and arranged surgical lung biopsy. On day 16, he underwent video assisted thoracoscopic lung biopsy. While awaiting results, prednisone 1mg/kg/day was initiated, leading to rapid defervescence. Pathology later confirmed acute lung injury consistent with organizing pneumonia (OP), most likely secondary to ocrelizumab. Given his improvement in symptoms, he was discharged on a steroid taper and ocrelizumab was withheld indefinitely. Follow-up imaging two months later showed marked improvement, however he had residual left upper lobe fibrosis and volume loss (Figure 2). Discussion Organizing pneumonia secondary to ocrelizumab remains rare and reported only in a few cases. Its presentation often mimics infection, malignancy, or vasculitis, often making diagnosis challenging and delayed. While histologic confirmation is required for a definitive diagnosis of OP, most published cases initiated treatment empirically. OP typically improves with corticosteroid therapy, though additional immunosuppressive agents may be required if symptoms persist or recur. This case highlights the need to recognize organizing pneumonia as a rare complication of ocrelizumab and to consider surgical lung biopsy if the diagnosis remains uncertain. This abstract is funded by: None