Humoral and cellular immunologic repertoire in Alzheimer's disease

Identification of Aldolase as a Target Antigen in Alzheimer’s Disease

Alzheimer's disease (AD) is the most common human neurodegenerative disease, leading to progressive cognitive decline and eventually death. The prevailing paradigm on the pathogenesis of AD is that abnormally folded proteins accumulate in specific brain areas and lead to neuronal loss via apoptosis. In recent years it has become evident that an inflammatory and possibly autoimmune component exists in AD. Moreover, recent data demonstrate that immunization with amyloid-beta peptide is therapeutically effective in AD. The nature of CNS Ags that are the target of immune attack in AD is unknown. To identify potential autoantigens in AD, we tested sera IgG Abs of AD patients in immunoblots against brain and other tissue lysates. We identified a 42-kDa band in brain lysates that was detected with >50% of 45 AD sera. The band was identified by mass spectrometry to be aldolase A. Western blotting with aldolase using patient sera demonstrated a band of identical size. The Ab reactivity was verified with ELISAs using aldolase. One of 25 elderly control patients and 3 of 30 multiple sclerosis patients showed similar reactivity (p < 0.002). In enzymatic assays, anti-aldolase positive sera were found to inhibit the enzyme's activity, and the presence of the substrate (fructose 1,6-diphosphate) enhanced Ab binding. Immunization of rats and mice with aldolase in complete Freund's adjuvant was not pathogenic. These findings reveal an autoimmune component in AD, point at aldolase as a common autoantigen in this disease, and suggest a new target for potential immune modulation.

Alzheimer's disease: a review concerning immune response and microischemia

Alzheimer's disease (AD), as we think of it today, is the idiopathic progressive loss of cognitive function over a period of several years. The risk of late onset dementia increases significantly with each decade of life such that half of the population over the age of 80 is vulnerable to this disease (1). We know that proper functioning of the central nervous system is dependent on adequate blood flow to remove harmful metabolic products and supply nutrients such as glucose and oxygen to the brain. It has been suggested that cerebral hypoperfusion causes AD (2). Mean cerebral blood flow decreases with age and with sclerosis of cerebral blood vessels. Blood flow appears to increase in stimulated areas of the brain during different activities. However, there is a derangement of blood flow in disease states; this has been documented in the temporal lobes of AD patients, (3,4). English language journal articles located by a MEDLINE search (1960-1999) were reviewed with consideration to the hypothesis that Alzheimer's disease is an autoimmune disease initiated by low oxygen tension and microischemia. Inflammation is thought to be a known contributor to the pathology of AD (5,6). Recent reports support the concept of autoimmunity as a final common pathway of neuron death, particularly for cholinergic in Alzheimer's disease (6). A model of Alzheimer's disease is proposed and related research and treatment modalities are discussed.

Ascorbate availability and neurodegeneration in amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis is a fatal neurodegenerative disease in which upper and lower motoneurons progressively deteriorate and die. Neuronal damage is most evident in the lower central nervous system, and death generally occurs following central respiratory failure. Proposed and demonstrated mechanisms for amyotrophic lateral sclerosis are diverse, and include altered superoxide dismutase and neurofilament proteins, autoimmune attack, and hyperglutamatergic activity. However, they do not account for the late onset of the disease, its earlier onset in males, and the differential vulnerability of neurons located in the brainstem and spinal cord. It is proposed here that, within the context of a specific defect such as altered superoxide dismutase, age-dependent decline in ascorbate availability triggers the disease. A role for ascorbate, which is found in millimolar levels in neurons, is suggested by a number of consistencies: 1) superoxide radicals being a common substrate for superoxide dismutase and ascorbate; 2) a close association between central nervous system ascorbate levels and injury tolerance; 3) a steady decline in ascorbate plasma levels and cellular availability with age; 4) plasma ascorbate levels being lower in males; 5) an association of ascorbate release with motor activity in central nervous system regions, in vivo; 6) the coupling of brain-cell ascorbate release with glutamate uptake; 7) possible roles for ascorbate modulation of N-methyl-D-aspartate receptor activity; 9) the ability of ascorbate to prevent peroxynitrite anion formation; and 10) evidence supporting the scorbutic guinea pig as a model for amyotrophic lateral sclerosis. Emphasis is placed on the probable competition between superoxide dismutase and ascorbate within the context of a primary defect of metal-binding or metal access in high-concentration proteins such as superoxide dismutase and human heavy neurofilaments. Finally, distinct features of alpha-motoneuronal physiology suggest that cell physiological characteristics such as high metabolic activity and extensive calcium dynamics may render neurons differentially vulnerable in amyotrophic lateral sclerosis.

Molecular aspects of inflammatory and immune responses in Alzheimer's disease

Recent advances indicate numerous molecular and cellular elements of the immune system are involved in the pathogenesis of Alzheimer's disease. Amyloid beta protein deposition induces many molecules associated with a predominantly local inflammatory response within the brain parenchyma. These responses also provoke the release of immune system mediators including cytokines, which all seem largely to be produced by reactive cells such as astrocytes and microglia. Classical acute phase proteins of the pentraxin and serine protease inhibitor (serpin) families as well as a host of complement proteins and some coagulation factor seem the most intrinsically involved. These secreted molecules display variable binding with the amyloidotic lesions. Although our understanding of the molecular specificity and significance of the interaction of these proteins within the lesions is not replete, the development of unique inhibitors of the inflammatory reactions could provide therapeutic strategies to impede the pathogenetic process. Currently, this appears a more viable option than to inhibit amyloid beta production or modify amyloid beta precursor protein processing, an approach which seems more complex.

IL-2 and IL-6 secretion in dementia: correlation with type and severity of disease

The production of interleukin-2 (IL-2) and interleukin-6 (IL-6) by peripheral blood mononuclear cells (MNC) was assessed in patients with Alzheimer's disease (AD) who were subdivided into two groups--mild and moderately-severe--according to the severity of the disease, probable vascular dementia (VaD) patients and elderly control subjects. No differences in IL-2 secretion were found between mild AD patients and controls. However, there was a significant increase in IL-2 production both in the moderately-severe AD group and in the VaD group. IL-6 levels in AD patients of both groups were similar and significantly higher than those of VaD and controls. Our results suggest that increased levels of IL-2-production correlate with severity of the dementia, whereas increased levels of IL-6 production seem to be related to AD and thus may play a role in AD pathogenesis.

The immunopathology of Alzheimer's disease and some related disorders

Current evidence clearly indicates that elements of the immune system are involved in the pathogenesis of the principal lesions characterizing Alzheimer's disease (AD). Findings are in accord with features associated with both the innate and adaptive immune mechanisms involved in a predominantly local inflammatory response within the parenchyma. Many of the features are unique to AD, presumably related to the unusual properties of beta amyloid protein. Remarkably, the brain holds the capacity to produce almost all the immune system mediators which largely seem to be generated by glia comprising both astrocytes and microglia. While a variety of humoral mediators including classical acute phase proteins (and serpins) are increased and released, the complement seems most intrinsically involved. The cellular response is elaborated by microglia which seem the main immunocompetent cells partaking in the response. These appear to function as pluripotent macrophages expressing both classes of MHC antigens. Further characterization of this interesting response to cerebral amyloidosis will be challenging.