Pro-inflammatory cytokines in Alzheimer's disease

Alzheimer's disease (AD), the most common progressive neurodegenerative disorder in the elderly, is clinically characterized by progressive impairment of cognitive functions, including reduced critical capacity, weakness of decision-making and problem orientation, while in the more advanced stages it is accompanied by behavioral disorders and impaired verbal ability. Neuropathological characteristics of the disease are the neurofibrillary tangles (NFT), the neuritic plaques (NP), the prominent synaptic loss and eventually the neuronal loss. The presence of inflammatory process seems to be playing important role in the progression of the disease. This process is directed by the activated glial cells and it leads to the overproduction of acute phase proteins, complement factors activation and induction of inflammatory enzyme systems. These inflammatory factors can contribute to neuronal dysfunction and cell death. Cytokines belong to the acute phase proteins, which are secreted from glial cells. They can either strengthen the inflammatory reaction or suppress it, adjusting the intensity and duration of the immune response. In the category of cytokines belong several interleukins (ILs) and various factors (TNF-α, TGF-β). Interleukins are involved in complex intercellular interactions among neurons, microglia and astrocytes, as well as intracellular signal transduction events, which are necessary to promote the inflammatory cascade characteristic of AD neuropathology. It has been observed that increased levels of pro- inflammatory cytokines, including tumor necrosis factor (TNF), interleukin 1β (IL-1β), interleukin 6 (IL-6) and interferon γ (IFN-γ), may suspend phagocytosis of amyloid Aβ in brains of patients with AD. Thus, it may interfere with the effective removal of plaque from microglia, promote astrogliosis and neural death. Normally, during immune surveillance, a balance is maintained between pro- and antiinflammatory influences. Yet, during AD, the abnormal accumulation of soluble amyloid oligomers triggers excessive release of pro inflammatory factors, such as cytokines and other acute-phase reactants, out of proportion to the regulatory components, such as IL-4, IL-10, receptor antagonists, interleukin inhibitors and others, ultimately leading to neuronal and synaptic injury and loss and cognitive decline. These changes in the brain parenchyma are often accompanied by changes in levels of these inflammatory proteins in peripheral blood. Even though literature is presenting conflicting studies, efforts are being made for the detection of cytokines in peripheral blood and association of their levels with the progression of AD. Inflammatory pathways, involving the signaling of cytokines, could be potential targets for the prevention of AD and the development of new therapies. The aim of the present work is to review studies indicating a correlation between these inflammatory agents and AD pathogenesis.