Alzheimer's disease (AD) is likely associated with systemic immune activation. During immune response, interferon-gamma stimulates indoleamine 2,3-dioxygenase (IDO) converting tryptophan to N-formylkynurenine followed by kynurenine in an ensuing step. Thus, IDO activity is estimated by the kynurenine per tryptophan quotient (Kyn/Trp). In 21 patients suffering from AD, in 20 controls of similar age, and in 49 blood donors we measured serum tryptophan and kynurenine concentrations by HPLC. Lower tryptophan concentrations were found in elderly control subjects compared to blood donors (62.1 vs. 73.0 microM, p < 0.005). Tryptophan concentrations tended to be still lower in AD patients (54.4 microM, p = 0.07) compared to elderly controls. Enhanced tryptophan degradation in patients was reflected by significantly increased Kyn/Trp (46.1 vs. 34.1 in elderly controls, p < 0.05). Correlations were found in patients between Kyn/Trp and concentrations of soluble immune markers in serum, i.e., neopterin, interleukin-2 receptor and tumor necrosis factor receptor (all p < 0.001). Increased Kyn/Trp was associated with reduced cognitive performance. Tryptophan degradation due to immune activation may exert impact on the pathogenesis of AD.

Abnormalities of immune system compartments were determined in 12 patients with Huntington's disease (eight males, four females; age 42.4+/-11.7 years) and 11 controls (7 males, 4 females; age 47.0+/-12.0). All patients were free from infectious diseases. Serum concentrations of a panel of serum soluble markers of immune activation were investigated, namely neopterin, 55-kDa-type soluble tumor necrosis factor receptor (sTNF-R), interleukin-2-receptor (sIL-2R), kynurenine, tryptophan, immunoglobulins (Ig) A, M and G as well as routine laboratory tests. Compared to controls, we found significantly higher serum levels of IgA (p<0.01), sTNF-R, sIL-2R, neopterin, and complement component C3 (all p<0.05), and serum tryptophan was decreased (p<0.001). Higher concentrations of circulating immune complexes, cardiolipin antibodies, IgM, neopterin and lower tryptophan were associated with loss of cognitive function as assessed by the mini-mental-test. Five patients died within 1 year after measurements were performed. In these patients IgM, circulating immune complexes and neopterin concentrations were higher compared to survivors and serum tryptophan was lower. The data indicate an activation of various immune system compartments in Huntington's disease and that systemic immunological alterations might be important in the course of the disease.

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Hyperhomocysteinemia is a strong risk factor for atherosclerotic vascular disease, and elevated serum homocysteine is correlated with vitamin B deficiency. In this pilot study, significantly elevated homocysteine levels were found in patients with Alzheimer's disease as well as in patients with vascular dementia, probably indicating similar pathophysiological pathways. We found significant correlations between low folic acid concentrations as well as high homocysteine concentrations and cognitive decline. Supplementation with folic acid may be an inexpensive way to reduce elevated homocysteine levels in demented patients.

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We measured serum neopterin concentrations in 24 patients with Alzheimer's disease (8 males, 16 females; age: 73.1+/-6.2 years; free of any infectious process) and fourteen controls of similar age (4 males, 10 females; age: 69.7+/-8.8 years). Compared to controls, significantly higher concentrations of neopterin (p< 0.01) were found in patients with Alzheimer's disease. Among patients, concentrations of neopterin were higher in those with lower mini-mental-state (p < 0.05), and an inverse correlation existed between mini-mental-state and neopterin concentrations. No such association existed with the duration of the disease. There were also significant correlations between neopterin and serum concentrations of immune activation markers such as soluble tumor necrosis factor (TNF) receptor and soluble interleukin-2 receptor (all p<0.01). Thus, increased concentrations of neopterin in serum of patients with Alzheimer's disease correlate with the severity of dementia. The data imply a chronic state of peripheral immune activation in Alzheimer's disease.

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Summary In patients with different forms of dementia we compared serum and cerebrospinal fluid concentrations of α-tocopherol (vitamin-E), the major circulating lipid-soluble antioxidant, and neopterin, a product released from monocytic cells upon stimulation with interferon-γ. A significant inverse correlation was found between the concentrations of both compounds in the cerebrospinal fluid. The data point to a role of oxidative stress to reduce antioxidant levels in elderly demented patients with neurodegenerative diseases.