P4‐201: Beneficial catalytic autoimmunity to beta‐amyloid peptide

We previously reported human catalytic autoantibodies to amyloid β peptide (Aβ). We hypothesize that recognition of electrophilic amyloid epitopes by nucleophilic autoantibodies is an innate immune function that is recruited for catalytic clearance of amyloid deposits associated with aging and Alzheimer's disease (AD). Aβ cleavage was measured by HPLC, acid precipitation, mass spectroscopy or electrophoresis. Electrophilic Aβ (E-Aβ) was prepared by carbonylation with the lipid peroxidation end products 4-hydroxynonenal (HNE)/malonaldehyde (MDA) or phosphonate diester insertion. Covalent immune complexes were analyzed by SDS-electrophoresis. Aβ1-42 aggregates were identified by antibody or Thioflavin-T staining. IgM from healthy human sera, the first antibody class produced during B cell differentiation, catalyzed Aβ cleavage at rates superior to IgGs. Preferential Aβ cleavage by IgMs was also observed for antibodies from the sera and cerebrospinal fluid from patients with AD. Two Aβ cleaving antibody fragments were isolated from a phage library, a heterodimeric V L -V L construct (2E6) and a single domain V L construct (5D3). Treatment with antibody 2E6 induced disappearance of oligomeric and fibrillar Aβ. Intracranial antibody injection in Aβ-overexpressing transgenic mice cleared the Aβ plaques. Traditional antibodies bind antigens at complementarity determining regions (CDRs). The Aβ cleaving antibodies contained CDRs with no or minimum mutations acquired by antigen-driven diversification. Deleting the CDRs did not attenuate Aβ cleavage by antibody 2E6, but the catalytic activity was lost by replacing the framework regions (FRs) with corresponding FRs from a non-catalytic antibody. The FRs are evolutionarily conserved segments important for innate recognition of B cell superantigens without requirement for adaptive immune processes. From protease inhibitor and epitope mapping studies, the catalytic mechanism entails noncovalent binding at the Aβ C terminus followed by nucleophilic peptide bond cleavage. Antibody 2E6 reacted covalently with an electrophilic phosphonate-containing Aβ analog and the naturally-occurring Aβ-HNE/Aβ-MDA analogs (E-Aβ). Monoclonal murine antibodies (MAbs) that cleaved Aβ at low substrate concentrations were identified by immunization with non-electrophilic Aβ. A subset of MAbs induced by immunization with E-Aβ cleaved Aβ robustly. Amplification of the innate noncovalent recognition and catalytic functions of antibodies driven by age/disease-associated Aβ accumulation can remove toxic amyloid deposits.