5 August 2011. Though Alzheimer’s is not typically considered an immune disease, serum autoantibodies just may hold promise as a blood-based AD diagnostic, suggests a study published August 3 in PLoS ONE. Using protein microarrays to probe human blood samples, Robert Nagele of the University of Medicine and Dentistry of New Jersey in Stratford and colleagues identified 10 autoantibody markers that distinguished AD from control patients with 96 percent sensitivity and 92.5 percent specificity. These numbers are “really quite impressive—by far the best anybody has reported to date,” Tom Kodadek of the Scripps Research Institute, Jupiter, Florida, told ARF. However, he and others noted that, while the data point to the presence of AD biomarkers in human sera, a reliable AD blood test is still quite a way off.
Prior work by Nagele’s group indicates that the vast majority of people—young and old, diseased and healthy—have brain-reactive autoantibodies in their serum (Levin et al., 2010). The researchers had their first hunch that autoantibodies might play a role in AD when they noticed that many of the surviving neurons in their immunohistochemical studies on AD brains were loaded with immunoglobulins. “This was a consistent feature in regions with AD pathology,” Nagele said. He figured the antibodies came from the blood and reached brain neurons through a disrupted blood-brain barrier—an idea suggested by earlier studies on neuronal death in AD brain (see D’Andrea, 2003). And “when antibodies bind to cell surfaces, it’s not a good thing. They tend to clump things together and immobilize them,” Nagele said. If this applied to signaling receptors, neuronal function would be compromised, prompting Nagele to wonder whether autoantibodies might, in fact, trigger neurodegenerative disease. His lab showed recently that serum autoantibodies can increase intraneuronal Aβ deposition in adult mouse neurons in vitro (Nagele et al., 2011).
To get an idea what the immunoglobulins recognize in AD brains, first author Eric Nagele and colleagues used microarrays to screen human serum samples for autoantibodies against some 9,500 proteins—roughly a third of the human proteome. Eric Nagele, son of Robert, is a medical student at the University of Medicine and Dentistry of New Jersey, and a scientist at Durin Technologies, Inc., which his father founded last year. The team analyzed commercially supplied sera from 50 AD (early- and late-stage) and 40 non-demented controls (young and old).
Half of the AD and control samples served as the training set, from which 451 autoantibodies came up more frequently in the AD group. The researchers chose the 10 showing the greatest difference between AD and controls, and verified these markers in an independent analysis with the remaining 25 AD and 20 control samples. AD samples were also compared against sera from Parkinson's disease and breast cancer patients, with accuracy rates of the 10 markers exceeding 90 percent in all cases. Dot blot analysis confirmed that the two strongest AD-specific antigens were present at lower titers in the control sera.
“The data are very promising for a specific biomarker for AD in peripheral blood,” noted Kaj Blennow of the University of Gothenburg in Sweden. “But much work is needed before we have such a test in our hands.” For starters, the findings must be confirmed in independent AD and control populations, which is challenging in a biomarker field plagued by methodological variability (see ARF related news series) and “notoriously difficult” for plasma markers, Blennow wrote. Other groups have already jumped into the fray (see
ARF related news story on Ray et al., 2007). Power3 Medical Products in The Woodlands, Texas, announced validation testing of its NuroPro Blood Test more than two years ago (ARF related news story), but did not respond when contacted for this story.
Using microarrays that run $2,000 a pop, the current study was expensive, Robert Nagele said—hence, the limited sample sizes. In addition, the samples could have been better characterized, noted Anne Fagan of Washington University School of Medicine, St. Louis, Missouri. Because the sera came from commercial suppliers rather than controlled research cohorts, no information was given on how the samples were collected or processed, or whether the patients took medications or had brain amyloid, for example. Given that up to a third of non-demented elderly show Aβ deposition (Aizenstein et al., 2008), some controls in the current study would be expected to have AD pathology, possibly even mild cognitive impairment, leading to questions about whether the identified autoantibodies are markers of AD dementia or AD pathology, Fagan said.
Furthermore, the current study does not say “how early the biomarkers come up, how sensitively we can measure these in early AD,” said Kodadek, who used a library of synthetic antigens to capture disease-specific antibodies (ARF related news story on Reddy et al., 2011). Early detection is important in light of new AD diagnostic criteria that reflect growing evidence that disease begins years before the appearance of obvious symptoms (ARF related news story on
McKhann et al., 2011). Robert Nagele said his team would love to tap into existing longitudinal cohorts and track people before and after they show signs of cognitive decline. Their microarray studies thus far, some yet to be published, suggest that autoantibodies may not only serve diagnostic markers for AD, but also PD and non-neurodegenerative diseases. “If I were a betting man, I’d say there’s a common phenomenon here. These markers are detecting the presence of disease,” Robert Nagele told ARF.—Esther Landhuis.
Nagele E, Han M, DeMarshall C, Belinka B, Nagele R. Diagnosis of Alzheimer’s disease based on disease-specific autoantibody profiles in human sera. PLoS ONE. 3 Aug 2011. Abstract