Membranous nephropathy preceding Bing-Neel syndrome: successful treatment with tirabrutinib.

We report a unique case of membranous nephropathy and tubulointerstitial nephritis preceding neurological manifestations of Bing-Neel syndrome, a rare variant of Waldenström macroglobulinemia. A 46-year-old man presented with nephrotic syndrome, and kidney biopsy revealed membranous nephropathy with phospholipase A2 receptor positivity and IgG4 predominance. Despite initial partial remission with corticosteroids and cyclosporine, neurological symptoms emerged alongside nephrotic syndrome relapse two years after onset. Diagnostic workup revealed serum monoclonal IgM kappa, abnormal lymphoplasmacytic cells in bone marrow and cerebrospinal fluid, and MYD88 L265P mutation in the latter, confirming Bing-Neel syndrome. Immunostaining of the initial kidney biopsy demonstrated IgM-kappa-positive lymphoplasmacytic infiltration consistent with the monoclonal protein. After bendamustine-rituximab therapy showed partial effectiveness, tirabrutinib, a Bruton tyrosine kinase inhibitor, achieved complete remission of both renal and neurological manifestations with sustained response over three years. Immunofluorescence staining revealed increased nuclear factor (NF)-κB and STAT3 expression in infiltrating immune cells and elevated NF-κB in surrounding tubular epithelial cells, suggesting activation of these pathways downstream of the MYD88 mutation in kidney pathology. Our findings suggest a potential contribution of Bruton's tyrosine kinase signaling to both kidney and neurological pathology in Bing-Neel syndrome, with tirabrutinib achieving clinical improvement in both organ manifestations.