"The Chicken or the Egg": A Unique Case of Secondary Membranous Nephropathy

Introduction: Follicular dendritic cell sarcoma (FDCS) is extremely rare and very scarce in the mediastinum, with only few cases reported. Pemphigus Vulgaris (PV) is a rare autoimmune blistering disease, rarely associated with FDCS. Myasthenia Gravis (MG) is a neuromuscular disorder that can occur concomitantly with PV. Here we present a unique case of a rare mediastinal FDCS, paraneoplastic PV and MG along with secondary membranous nephropathy (MN) Case Description: 52 year old female diagnosed with oculo-muscular Myasthenia gravis (ACh-R ab+) in 3/2022, during workup was found to have a mediastinal mass, no thymic mass seen. In 8/2022 hospitalized for MG crisis, successfully treated with pyridostigmine, prednisone, plasmapheresis, eventually started on MMF and IVIG therapy. s/p mass resection on 5/2023, biopsy showed FDCS. On 9/2023 complicated with severe stomatitis, bullae like skin lesions in whole body, underwent skin biopsy positive for PV, treated with 4 cycles of PLEX, and 2 doses of Rituximab, at that time was found to have nephrotic proteinuria UACR 5,573 mg/g, UPCR 8,926 mg/g, ANA positive 1:320 and dsDNA positive, kidney function remained stable. On 6/28/ 2024 underwent kidney biopsy that showed early MN, IF negative for full house, Class V LN was ruled out. Serum PLA2R titers and PLA2R, THSDA7 on glomerular tissue were negative, suggestive of secondary MN. After kidney biopsy completed 2 more doses of rituximab, after which proteinuria improved, UACR 52 mg/g, UPCR 165 mg/g Discussion: PV as paraneoplastic syndrome, specifically in relation to MG and FDCS is a complex and rare condition. Both autoimmune conditions, can be associated with secondary MN, yet the co-occurrence of these 3 conditions is unusual. In the literature there are only a few cases of paraneoplastic PV as the first presentation of FDCS, there has been cases of refractory MN in cases of PV treated with standard therapy (steroids+ MM and later azathioprine) that improves only after treatment with Rituximab. In this particular case, treating the cancer with surgical resection did not resolve the paraneoplastic conditions, MN developed 2 years after, MG was refractory to standard treatment, and after Rituximab infusions, Paraneoplastic conditions and Nephrotic proteinuria from secondary MN improved, highlighting the question, which came first, the chicken or the egg?