APOE isotype-dependent regulation of IgG and IgA levels against different IAPP epitopes

The efficient clearance of IAPP oligomers (IAPPo) by autoantibodies is crucial as increased plasma levels of IAPPo can induce microvascular alterations and Alzheimer's disease (AD)-characteristic amyloid-β deposition in the brain. We have recently demonstrated that plasma immunoglobulin (Ig) A levels against IAPPo, but not IgG, are reduced in an Apolipoprotein E (APOE) ε4 allele dose-dependent manner. In this study, we aimed to investigate if this APOE genotype-dependent impact can be explained by differences in IAPP epitope recognition by IgA and IgG. We found that the specificity for IAPP epitopes does not differ between IgG and IgA autoantibodies and that IgG and IgA autoantibodies are directed foremost against the C-terminus of the IAPP. However, IgG autoantibody levels against the N-terminus and midportion of IAPP increased significantly in AD patients with APOE44 compared to controls with APOE33, while the opposite was seen in IgA autoantibody levels. We propose that the IgG and IgA levels against different IAPP epitopes are APOE isotype-dependent, possibly due to differences in cytokine profile between various APOE genotypes or the need for different effector functions of IgG or IgA.