Paratopic specificity of two human monoclonal immunoglobulins M expressing Y7 idiotype

Abstract

We have already characterized Y7 idiotype as natural idiotype by showing its expression on IgM molecules derived form cord sera. The aim of this study was to investigate the binding properties of two human monoclonal IgM antibodies isolated from the sera of patients with Waldenstrom macroglobulinaemia, which express Y7 idiotype. In order to estimate the binding repertoire of these two IgM molecules (IgM DJ and IgM RD) we tested the isolated IgM molecules and their F(ab)2 and Fab fragments over the panel of 23 different protein and non-protein antigens, nine different bacterial strains and cell membranes from neuronal and epithelial tissues. These tests were performed by means of direct and competitive ELISA, immunoblot and immunofluorescence assays. Both antibodies reacted with ssDNA, oligonucleotide fragments, and lactobacteria. IgM DJ showed reactivity with myelin associated glycoprotein (MAG) and blood vessel endothelial cells. Binding for DNA oligonucleotide fragments and lactobacteria was interrupted up to 70% with 0.5 M NaCl, which indicated that these interactions were of low affinity. Avidity dependent binding was also evidenced by use of F(ab)2 fragments. Determined characteristics such as polyreactivity which included autoantigens, low affinity and avidity dependent binding, qualified these two monoclonal IgM as natural autoantibodies. Expression of natural idiotype Y7 on two immunoglobulins M,which possessed natural autoantibody properties, indicated the connection of natural autoantibody specificity and the natural idiotype expression and suggested the involvement of network interactions in selection of malignant B cell clones.