Plasmacytoma of the tonsil with AL amyloidosis: evidence of post-fibrillogenic proteolysis of the fibril protein

We report of a 58-year-old Caucasian man who was referred to the University Hospital with a greatly enlarged left tonsil which showed calcifications o n computed-tomography scans. Histopathology revealed a plasmacytoma with se condary AL amyloidosis, ossifications, and multinucleated foreign-body-type giant cells. N-terminal sequencing of amyloid-fibril proteins purified fro m the formalin-fixed tissue showed the presence of two proteins of differen t size; these were of lambda-light-chain origin (subgroup V), measured appr oximately 15.2 kDa and 10.5 kDa, and had identical N-terminal ends (YVLTQPP ). When the amyloid deposits were immunolabeled with a polyclonal antibody directed against lambda light chain, they showed two staining patterns: som e deposits showed intense immunolabeling while others were not immunoreacti ve. Immunostaining of amyloid was completely absent after protease pre-trea tment. Immunoelectron microscopy with gold-labeled secondary antibodies sho wed staining that was spatially related to amyloid fibrils and suggested th at the antibody probably detected the fibril protein. Therefore, our hypoth esis in this case is that the different immunostaining patterns are due to a post-fibrillogenic proteolysis of the fibril protein at the C-terminal en d of the light chain, as indicated by the presence of two differently sized lambda-light-chain fragments with identical N-terminal ends.