Generation of senescent cell antigen on old cells initiates IgG binding to a neoantigen.

An aging antigen, senescent cell antigen, resides on the 911 amino acid membrane protein band 3. It marks cells for removal by initiating specific IgG autoantibody binding. Band 3 is a ubiquitous membrane transport protein found in the plasma membrane of diverse cell types and tissues, and in nuclear, mitochondrial and Golgi membranes. Band 3 in tissues such as brain performs the same functions as it does in red cells. Senescent cell antigen is generated on brain membranes. Oxidation is a mechanism for generating senescent cell antigen. Neither cross-linking nor hemoglobin appear to play a role in generating senescent cell antigen. Although storage is the only in vitro model that mimics cellular aging in situ, we have discovered three alterations/mutations of band 3 that permit insight into aging in situ. One mutation with an addition to band 3 has normal or decelerated red cell aging. In contrast, another band 3 alteration with a suspected deletion or substitution that renders band 3 more susceptible to proteolysis, shows accelerated aging. The third alteration which is also more susceptible to proteolysis is associated with neurologic defects. Peptide technology was used to map the aging antigenic sites and anion transport sites on band 3 using a competitive inhibition assay and immunoblotting with IgG directed against the aging antigen on old cells. Results indicate that: a) aging antigenic sites reside on human band 3 residues 538-554, and 812-830; b) a putative ankyrin binding region peptide is not involved in senescent cell antigen activity and c) carbohydrate moieties are not required for the antigenicity or recognition of senescent cell antigen since synthetic peptides alone abolish binding of senescent cell IgG to erythrocytes. Peptide residues 588-594 (a 7 amino acid peptide), 822-839 and 869-883 were the most active inhibitors of anion transport (P < or = 0.001 compared to control without peptide). Localization of the active antigenic and transport sites on band 3 molecule facilitates the definition of molecular changes occurring during aging that initiate molecular as well as cellular degeneration. The role of senescent cell antigen and band 3 in brain aging and Alzheimer's disease is discussed.