When Autoimmunity Meets Malignancy: A Rare Case of Concomitant Systemic Lupus and Multiple Myeloma.

The association between systemic lupus erythematosus (SLE) and multiple myeloma (MM) is rare, with fewer than 20 cases reported in the literature, raising questions about the link between chronic immune dysregulation and oncogenesis. We report the case of a 43-year-old Malagasy woman in whom a monoclonal β2-globulin spike, incidentally discovered during the workup of pneumonia, led to the diagnosis of MM. The diagnosis of SLE was straightforward, based on evocative clinical features (photosensitive malar rash, inflammatory arthralgia) and positive autoantibodies (antinuclear antibodies at 1:320, native anti-DNA at 861 IU/mL). Anemia (Hb at 10 g/dL), with a drop of more than 2 g/dL from baseline, and a high erythrocyte sedimentation rate (90 mm) prompted a bone marrow aspiration, which revealed 35% dystrophic clonal plasma cells. A diagnosis of kappa light chain MM was made, without bone lesions. This case highlights three major issues: (1) the diagnosis of MM in a young patient with atypical but fulfilled SLiM-CRAB criteria; (2) a pathophysiological mechanism suggesting chronic B-cell stimulation related to SLE, rather than an iatrogenic effect; (3) the potential role of autologous stem cell transplantation (ASCT) in modulating immune response in both conditions. It underscores the need for diagnostic vigilance, better access to advanced biomarkers, and a multidisciplinary approach.