Light chain proximal tubulopathy (LCPT) is an uncommon form of renal disease associated with dysproteinemias. It is characterized by intracytoplasmic deposition of crystallized mostly kappa monoclonal light chains in proximal tubules (PTs). Crystals are located within lysosomes by electron microscopy (EM). Rare lambda LCPT cases without crystals by EM were described. Retrospectively, we reviewed clinical, light microscopic (LM), immunofluorescence (IF), and EM findings in 9 cases) (8 males, 1 female; mean age 57 years (38–81)) with multiple myeloma. LM showed abundant cytoplasmic droplets in PT cells in all cases. Droplets were also present in the podocytes, endothelial and parietal cells in one case. IF revealed staining of crystals with kappa in 3 and lambda in 6. EM showed electron dense rectangular, rhomboid, or needle shaped crystals in PT cells in 3 cases (33%), one of which had crystals in podocytes and interstitial cells. Six lambda LCPT cases showed no crystals by EM (67%). This may reflect differences in the physicochemical properties of light chains. The mechanisms of crystal accumulation in these cells and the significance of this finding are unknown. 1. Introduction The kidney is affected in a variety of dysproteinemias, the pathogenesis, and the morphology, which varies depending on the etiology [1]. The common morphological presentation of the affected kidney includes myeloma cast nephropathy, monoclonal immunoglobulin deposition disease, and amyloidosis. Another unique entity which is less frequently reported as case reports and small case series is light chain proximal tubulopathy (LCPT) [2]. The first description of LCPT causing Fanconi syndrome (FS) with needle shape crystals by EM found in the PT epithelial cell cytoplasm was reported in 1957 [3]. Subsequently less than 100 cases of LCPT described in the English language literature as case reports and small case series [4–6]. The largest series with 17 cases was reported by Maldonado et al. in 1975 [7]. In LCPT the excessive light chains (LCs), mostly kappa type by IF, are excreted through the kidney and are reabsorbed in the PT cells leading to tubular damage, less frequently resulting in acquired FS. The association of the LCPT with lambda LC is rarely described in the English language literature [6, 8]. The excessive LC absorbed by the cytoplasm of PT cells result in formation of crystalline structures which can be detected by IF and EM. The crystals are electron dense usually rhomboid, square, or rectangular in configuration and found in the lysosomes of PT cells by EM. Rarely,
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