Amyloid-β(1-42) Fibrillar Precursors Are Optimal for Inducing Tumor Necrosis Factor-α Production in the THP-1 Human Monocytic Cell Line

Pathological studies have determined that fibrillar forms of amyloid-beta protein (A beta) comprise the characteristic neuritic plaques in Alzheimer's disease (AD). These Studies have also revealed significant inflammatory markers such as activated microglia and cytokines surrounding the plaques. Although the plaques are a hallmark of AD, they are only part of an array of A beta aggregate morphologies observed in vivo. Interestingly, not all of these A beta deposits provoke an inflammatory response. Since structural polymorphism is a prominent feature of A beta aggregation both in vitro and in vivo, we sought to clarify which A beta morphology or aggregation species induces the strongest proinflammatory response using human THP-1 monocytes as a model system. An aliquot of freshly reconstituted A beta(1-42) in sterile water (100 mu M, pH 3.6) did not effectively stimulate the cells at a final A beta concentration of 15 mu M. However, quiescent incubation of the peptide at 4 degrees C for 48-96 h greatly enhanced its ability to induce tumor necrosis factor-alpha (TNF alpha) production, the level of which surprisingly declined upon further aggregation. Imaging of the A beta(1-42) aggregation solutions with atomic force microscopy indicated that the best cellular response coincided with the appearance of Fibrillar structures, yet conditions that accelerated or increased the level of A beta(1-42) fibril formation such as peptide concentration, temperature, or reconstitution in NaOH/PBS at pH 7.4 diminished its ability to stimulate the cells. Finally, depiction of the A beta(1-42) solution with an antibody that recognizes fibrillar oligomers dramatically weakened the ability to induce TNF alpha production, and size-exclusion separation of the A beta(1-42) solution provided further characterization of an aggregated species with proinflammatory activity. The findings suggested that an intermediate stage A beta(1-42) Fibrillar precursor is optimal for inducing a proinflammatory response in THP-1 monocytes.