Cytokine-mediated inhibition of fibrillar amyloid-β peptide degradation by human mononuclear phagocytes

Vaccination therapy of AD animal models and patients strongly suggests an active role of brain mononuclear phagocytes in immune-mediated clearance of amyloid-beta peptides (A beta) in brain. Although A beta uptake by macrophages can be regulated by pro- and anti-inflammatory cytokines, their effects on macrophage-mediated A beta degradation are poorly understood. To better understand this mechanism of degradation, we examined whether pro- and anti-inflammatory cytokines affect the degradation of A beta using primary cultured human monocyte-derived macrophages (MDM) and microglia using pulse-chase analysis of fibrillar and oligomer I-125-A beta 40 and A beta 42. Initial uptake of fibrillar A beta 40 and A beta 42 was 40% and its degradation was saturated by 120 h in both MDM and microglia, compared with an initial uptake of oligomeric A beta less than 0.5% and saturation of degradation within 24 h. IFN-gamma increased the intracellular retention of fibrillar A beta 40 and A beta 42 by inhibiting degradation, whereas IL-4, IL-10, and TGF-beta 1, but not IL-13 anti IL-27, enhanced degradation. Fibrillar A beta degradation in MDM is sensitive to lysosomal and insulin degrading enzyme inhibitors but insensitive to proteasomal and neprilysin inhibitors. IFN-gamma and TNF-alpha directly reduced the expression of insulin degrading enzyme and chaperone molecules (heat shock protein 70 and heat shock cognate protein 70), which are involved in refolding of aggregated proteins. Coculture of MDM with activated, but not naive T cells, suppressed A beta degradation in MDM, which was partially blocked by a combination of neutralizing Abs against proinflammatory cytokines. These data suggest that proinflammatory cytokines suppress A beta degradation in MDM, whereas select anti-inflammatory and regulatory cytokines antagonize these effects.