Deposition of Lambda Chain Constant Region within AL-Amyloid Lesion

[Introduction]

Diagnosis of AL amyloidosis is dependent on the proof of light chains in amyloid lesions. However, immunostaining does not always successfully prove the presence of light chains in lesions in AL amylidosis patients. Here we report that the constant region of immunoglobulin lambda light chain (IGLC2) is seen in amyloid lesions where no positive signals are found with regular immunostaining.

[Materials and Methods]

Amyloid samples were stained with anti-human lambda light chain antibody (DAKO PO-0130) and analyzed with mass-spectrometry combining laser micro-dissection. Bone marrow samples were obtained from patients with amyloidosis, who gave written informed consent, and were subjected to plasma cell purification using CD138-immunomagnetic beads. Expression of immunoglobulin light chain mRNA was examined with RT-PCR. Anti-human IGLL5 antibody, capable of detecting immunoglobulin light chain constant region 2 (IGLC2) in paraffin embedded samples, was utilized.

[Results and Discussion]

We performed immunostaining for immunoglobulin light chains with 18 samples and found that six and eight cases were positive for kappa and lambda light chains, respectively, whereas light chains were not detected in remaining four cases (immunostaining-negative amyloidosis; INA). However, interestingly, mass spectrometry analysis revealed the presence of IGLC2 in all of the INA cases. RT-PCR analysis revealed the presence of IGLC2 mRNA in plasma cells from such INA cases. Surprisingly, amyloid lesions in all of the INA cases were positively stained with anti-IGLL5 antibody, whereas no staining was found in other samples positively stained with DAKO PO-0130. These observations suggest that the deposition of IGLC2 may cause AL amyloidosis, which otherwise could not be diagnosed with regular immunostaining. Although high dose chemotherapy produced hematological remission, half of such cases died within one year, suggesting irreversible and life-threatening amyloid fibril depositions in critical organs in IGLC2-related cases. We further examined additional twelve cases with AL amyloidosis to determine the incidence of IGLC2-related amyloidosis by immunostaining. With regular immunostaining, kappa and lambda chain were found in three and five cases, respectively. Interestingly, the remaining four cases were negative with regular immunostaining but positive with anti-IGLL5 antibody. Taken these observations together, eight IGLC2-related amyloidosis cases and thirteen lambda type amyloidosis were identified. Thus, the incidence of IGLC2-related amyloidosis should be approximately 38% (8/21) among lambda type AL amyloidosis.

We conclude that diagnosis of IGLC2-related AL amyloidosis was possible only with the use of anti-IGLL5 antibody, but not with regular immunostaining. Given the relatively high incidence and often poor prognosis of IGLC2-related amyloidosis, it is important that this clinical entity is recognized to potentially improve outcomes of treatments. Analysis of mechanisms regulating amyloid formation with IGLC2 peptides is currently underway.