Association of Elevated α₁-Antichymotrypsin With Cognitive Impairment in a Prospective Study of the Very Old

Several risk factors have been implicated in the development of Alzheimer's disease, including the apolipoprotein E (APOE) ε4 allele and advanced age (1). The presence of the acute-phase protein α₁-antichymotrypsin (ACT) in senile plaques, inflammatory cytokines in activated microglia, and complement components around dystrophic neurites and neurofibrillary tangles suggests an immune response in the brains of patients with Alzheimer's disease (2, 3). Elevated ACT concentrations have been widely (4, 5) but not universally (6) reported in patients with Alzheimer's disease. Although ACT is present in the peripheral circulation and is synthesized in the liver, central nervous system synthesis may account for ACT in the CSF and neuropil (7). Serum levels of ACT do not correlate with CSF levels of ACT in patients with Alzheimer's disease (8); elevated serum ACT may indicate a systemic inflammatory state in Alzheimer's disease (3). The purpose of this study was to determine whether elevated concentrations of serum acute-phase proteins, specifically ACT and α₂-macroglobulin (MAC), are associated with cognitive impairment in individuals who are at elevated risk for Alzheimer's disease because of their advanced age.

METHOD

Study participants resided in a chronic care facility. All residents receive annual medical evaluations and routine blood tests (e.g., CBC, SMA, urinalysis). The primary care physicians document medical diagnoses on a monthly basis in all patients' charts and update them as clinically indicated. As part of an ongoing study of dementia, research staff administer the Mini-Mental State (9) annually to all consenting residents and perform chart reviews to record medical conditions at the closest time point prior to Mini-Mental State administration. At the time of this study, 403 residents had blood results, at least one Mini-Mental State, and a chart review. For subjects with more than one assessment, initial assessments were used for analyses.

Residual sera from consecutive routine blood tests were available for analyses. Residual sera were stored at –20°C before antigen-competition, antibody-capture, enzyme-linked immunosorbent assays for ACT and MAC were conducted (4). The median effective concentrations for the ACT and MAC assays were 70 and 131 mg/dl, respectively, and the between-plate coefficients of variation were 11.9% and 14.2%, respectively. Correlation coefficients for identical samples run twice were r=0.89 for ACT (F=125.97, df=1,34, p<0.0001) and r=0.77 for MAC (F=49.79, df=1,34, p<0.0001). The coefficients of variation for sample duplicates were 3.5% for both ACT and MAC. The coefficients of variation for duplicate samples run at two dilutions were 11.8% for ACT and 20.0% for MAC.

APOE genotypes were determined by restriction isotyping (10). Pearson correlation coefficients were used for correlational analyses. Mann-Whitney U tests were used to compare means.

RESULTS

Concentrations of ACT (mean=125.5 mg/dl, SD=59.3) and MAC (mean=423.9 mg/dl, SD=172.4) were higher in this group of nursing home residents (N=403) than in a normal adult population, but this was not unexpected in elderly subjects sampled during routine clinic visits. Residents with the following ranges of laboratory values had significantly higher ACT concentrations than those without these abnormalities: creatinine greater than 1.5 (z=2.66, N=403, p=0.008), WBC count greater than 10,000 (z=3.74, N=403, p=0.0002), erythrocyte segmentation rate greater than 75 (z=3.21, N=403, p=0.001), and hematocrit less than 30 (z=3.04, N=403, p=0.002). After cases with any of these abnormalities were excluded, 202 subjects remained. Ninety-three of these subjects had a Mini-Mental State within 6 months of blood tests; these subjects were used for subsequent analyses. APOE genotypes were available for 87 of the 202 subjects.

The 93 subjects with recent Mini-Mental State results were divided into two groups based on their scores: 60 subjects had scores of 20 or less and 33 had scores higher than 20. The dementia diagnoses based on comprehensive chart review for the subjects with Mini-Mental State scores of 20 or less were Alzheimer's disease (N=51), dementia with cerebrovascular disease (N=7), and dementia with parkinsonism (N=2). The two Mini-Mental State groups did not differ in age (mean=86.2 years, SD=8.4, versus mean=86.5 years, SD=7.7, respectively) (z=0.04, N=93, p>0.10) or percent female (82% versus 72%, respectively) (χ²=1.17, df=1, p>0.10). These two groups also did not differ in race, religion, or occupation. Patients with Mini-Mental State scores of 20 or less had higher ACT concentrations than those with Mini-Mental State scores greater than 20 (z=2.70, N=93, p=0.007) (table 1). MAC concentrations were not significantly different between the two Mini-Mental State groups (z=0.36, N=93, p>0.10). Significant correlations were observed between ACT concentrations and Mini-Mental State scores (r=–0.39, df=91, p<0.0001). Patients with at least one APOE ε4 allele (N=22) had nonsignificantly lower Mini-Mental State scores than those with no APOE ε4 alleles (N=65) (mean=13.6, SD=9.6, versus mean=16.5, SD=8.3, respectively) (z=1.28, N=87, p>0.10). ACT levels of individuals with or without an APOE ε4 allele were not significantly different (z=1.76, N=87, p=0.08). No significant relationship was found between MAC concentrations and Mini-Mental State scores (r=0.01, df=91, p>0.10) or presence or absence of APOE ε4 (z=0.74, N=87, p>0.10).

DISCUSSION

Our finding of a relationship between cognitive impairment and ACT, but not MAC, in a medically stable elderly population extends previous reports of elevated concentrations of ACT in Alzheimer's disease (4, 5). The ACT marker of the acute-phase response is consistently noted in cerebral tissues of patients with Alzheimer's disease (3). The finding of elevated serum ACT suggests that a systemic inflammatory state is associated with cognitive impairment in this population and makes it seem less likely that ACT in cerebral cortical plaques is the result of incidental and inconsequential binding interactions with plaque constituents.

Elevated ACT concentrations cannot serve as a diagnostic marker for dementia or Alzheimer's disease, but the assessment of ACT may provide a surrogate marker for the effectiveness of anti-inflammatory treatment regimens (11). Multicenter trials of immunomodulating agents in Alzheimer's disease have been initiated (12). These clinical developments underscore the need for more basic studies focused on the role of acute-phase proteins and other immune system components, like cytokines and complement proteins, in the pathogenesis of Alzheimer's disease.

TABLE 1

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