It was shown previously that an N-terminal fragment (nM60) that encompasses
amino acid residues 1 to 563 of
megalin could induce active
Heymann nephritis (AHN) as efficiently as the native
protein. For delineation of a minimal structure within this fragment that is sufficient to induce AHN, smaller
protein fragments that encompass residues 1 to 236 (L6), 1 to 195 (L5), 1 to 156 (L4), and 1 to 120 (L3), representing successive C-terminal truncations within
ligand-binding repeats of nM60, were cloned and produced in a baculovirus insect cell expression system.
Protein fragments L4, L5, and L6 clearly were glycosylated. All four fragments stimulated proliferation of
megalin-sensitized lymph node cells and induced high-titer anti-
megalin autoantibodies in Lewis rats. A full-blown disease, as assessed by severity of
proteinuria, was observed in rats that were immunized with L6 and L5, whereas animals that were immunized with L4 and L3 developed only mild disease. The
proteinuria levels correlated with staining for
complement (C3, C5b-9) and
IgG1 isotype in glomerular immune deposits. The results suggest that one or more molecular determinants on the region that comprises
amino acid residues 157 to 236 contribute to the induction of a full-blown form of AHN. Study of the structure, conformation, and posttranslational modifications of these determinants could provide greater insight into the molecular correlates of immunopathogenesis in this disease model.