Light Chain Dimers Acquire Amyloidogenic Properties in the Absence of Selection

Abstract

It is well established that immunoglobulin Light (L) chains dimers, also known as Bence-Jones (BJ) proteins, are secreted by normal and malignant plasma cells (PCs) and, when produced in large amounts, can aggregate into amyloid fibrils in target tissues leading to organ failure and to the clinical manifestation of Immunoglobulin Light Chain amyloidosis (AL amyloidosis). The unique identity of the pathogenic L chain belonging to each AL amyloidosis patient, which originates from the mechanisms of generation of antibody diversity, has considerably hindered the identification of the structural determinants of L chains aggregation and consequent clinical manifestations. The aim of my thesis is to examine AL amyloidosis from the B cell perspective, considering that in developing B cell clones, L chain variable (VL) domains undergo a process of somatic mutation and are selected for pairing with variable Heavy (H) chain (VH) domains to form a functional B cell receptor (BCR). Therefore, by comparing the effect of L chains hetero-dimerization with H chains to L chains homo-dimerization, I could show that free L chains, which are not subject to antigen-driven selection, are intrinsically unstable and are characterized by the exposure of hydrophobic patches and free thiols; their instability is substantially increased by somatic mutations, leading to aggregation into Congo Red (CR)- and Thioflavin T (ThT)- positive amyloid fibers. I could also show that free L chains have unique specificities and can bind to cells and to the extracellular matrix, a property that may increase their local concentration and drive the nucleation phase of amyloid aggregation in the target tissues. Taken together, these findings demonstrate that, in the absence of a negative selection mechanism, L chain dimers develop through somatic mutations into rogue proteins, leading to their concentration-dependent aggregation and organ accumulation. Consequently, these results suggest that a plausible explanation of AL amyloidosis pathogenesis could be represented by the deleterious combination of a break of tolerance with a break of protein homeostasis.