Light Chains, Heavy Consequences: De Novo Monoclonal Gammopathy of Renal Significance in a Kidney Allograft Recipient

Introduction: Monoclonal gammopathy of renal significance (MGRS) encompasses a spectrum of non-malignant B-cell or plasma cell proliferative disorders which can lead to kidney injury. Caused by an accumulation of monoclonal immunoglobulins, these proteins can deposit in the kidney, causing nephrotoxicity. Underlying causes involve excessive secretion of vascular endothelial growth factors, autoantibodies against complement components, and activating mechanisms contributing to nephropathy. Light chain proximal tubulopathy (LCPT) is a rare MGRS subtype marked by accumulated monoclonal light chains, typically kappa, in proximal tubule cells leading to tubular dysfunction. Involvement of lambda light chains is relatively uncommon and usually associated with noncrystalline deposits. De novo LCPT in allografts is rare and difficult to diagnose, requiring increased clinical awareness. We present a case of a post-kidney transplant who developed de-novo LCPT four months after receiving a kidney allograft. Case Description: A 78-year-old kidney allograft recipient, transplanted for presumed diabetic and hypertensive kidney disease without biopsy confirmation, presented with signs of kidney injury. Despite an uncomplicated surgery, the post-transplant course was complicated by slow graft function. Persistently elevated serum creatinine (~2.0 mg/dL) and low eGFR (28 mL/min/1.73 m2) prompted a renal ultrasound, showing mildly increased resistive indices and edema. Serum protein electrophoresis showed M protein, and a monoclonal spike with decreased IgA and IgM. There was normal free kappa light chain level, but an elevated free lambda light chain (19.01 mg/dL) and a low kappa/lambda light chain ratio (0.05). Bone marrow biopsy demonstrated CD56 expression consistent with plasma cell neoplasm. Allograft biopsy confirmed multiple myeloma involving the kidneys. The patient was referred to hematology-oncology for further management. Discussion: This case is a rare presentation of de novo LCPT post-kidney transplant. Persistently elevated serum creatinine, proteinuria, and graft dysfunction revealed monoclonal lambda light chain accumulation in the kidney and ultimately led to the diagnosis of multiple myeloma. Since this is nephrotoxic, early diagnosis is critical as targeted therapy involves a complex, prolonged treatment course, and preserving allograft function to improve patient outcome.