Circulating immune complexes of Aβ and IgM in plasma of patients with Alzheimer’s disease

It has previously been shown that immune complexes (IC) of a given biomarker with class M immunoglobulins (IgM) provide better performances compared to the unbound biomarker in a number of cancer entities. In the present work, we investigated IC of IgM-Aβ as a potential biomarker for Alzheimer’s disease (AD). Aβ–IgM concentration has been measured in 75 plasma samples from patients with AD, individuals with mild cognitive impairment (MCI), and healthy age- and sex-matched controls (HC). To characterize the fractions associated with Aβ, pooled plasma samples were subjected to gel-filtration analysis. Size-separated fractions were analyzed for the presence of Aβ using a sandwich ELISA assay. A strong reactivity was observed in the high molecular weight IgM (>500 kDa) and 150 kDa (IgG) fractions indicating that blood Aβ is strongly associated with antibodies. Using an ELISA assay detecting Aβ–IgM complexes, we observed that high levels of Aβ–IgMs were detectable in HC and MCI patients; however, there was no significant difference to the AD group.