Contribution of human smooth muscle cells to amyloid angiopathy in AL (light-chain) amyloidosis

Abstract

Objective: Amyloid light-chain (AL) amyloidosis is a disease process that often compromises the peripheral vascular system and leads to systemic end-organ dysfunction. Although amyloid formation in vessel walls is a multifaceted process, the assembly of the native light chains (LCs) into amyloid fibrils is central to its pathogenesis. Recent evidence suggests that endocytosis and endolysosomal processing of immunoglobin LCs by host cells is essential to the formation of amyloid fibrils that are deposited in at least some tissues. The aim of this study was to elucidate the role of vascular smooth muscle in amyloid angiopathy.

Methods: Human coronary artery smooth muscle cells (SMCs) were grown on coverslips, four chamber glass slides, and growth factor-reduced Matrigel matrix in the presence of 10 µg/ml of ALs (λ and κ isotypes), nonamyloidogenic LCs, and culture medium (negative control) for 48 and 72 hours. Thereafter, a detailed light microscopic, immunohistochemical, and ultrastructural evaluation was conducted to verify amyloid deposition and characterize the role of SMCs in the formation of amyloid deposits in the various experimental conditions.

Results: Amyloid deposits were detected extracellulary as early as 48 hours after exposure of vascular smooth muscle cells (VSMCs) to AL-LCs (amyloidogenic light chains) as confirmed by affinity to Congo red dye, thioflavin T fluorescence, and transmission electron microscopy. No amyloid was present in the cultures of SMCs treated with medium alone or nonamyloidogenic LCs. SMCs associated with amyloid deposits exhibited CD68, lysosome-associated membrane protein 1-1, and intracellular lambda light chain expression and only focal smooth muscle actin and muscle-specific actin positivity. Electron microscopy revealed these cells to have an expanded mature lysosomal compartment closely associated with deposits of newly formed amyloid fibrils.

Conclusions: The interaction of amyloidogenic LCs with VSMCs is necessary for the formation of amyloid fibrils that are deposited in peripheral vessels. VSMCs participate in the formation of amyloid by the intracellular processing of AL-LCs, which is possible due to their transformation from a smooth muscle to a macrophage phenotype. The formation of amyloid fibrils occurs in the mature lysosomal compartment of transformed cells. The amyloid that is formed is then extruded into the extracellular matrix.

Keywords: Angiopathy; fibrils; light chains; phenotypic transformation; vascular pathology.