Biomedical Engineering Reference
In-Depth Information
3.4.2 Disease-Related Autoantibodies in Serum
Type 1 Diabetes
In type 1 diabetes, the autoimmune response is directed against the insulin-
producing b-cells of the pancreas. Diabetes-related autoantibodies include insulin
autoantibodies (IAA) and antibodies to tyrosine phosphatase-like protein IA-2
[
76
,
127
]. For both autoantibodies, SPR biosensor assays were developed. For IAA
detection, an ''indirect competitive immunoassay'' was performed using biosen-
sors with immobilized insulin. Each serum sample was divided into two portions.
One half was spiked with insulin, similar to the preincubation step in a binding
inhibition assay, as the insulin would block potentially available IAA binding sites.
The other half of the sample was just diluted (buffer), so the sample dilution in
both portions was 1:2. In a measurement cycle, first the sample without insulin was
applied on the biosensor surface. Autoantibody potentially bound on the biosensor
surface was detected with a second antibody to enhance the signal response
(sandwich assay). After a regeneration step, the sample with insulin was applied,
including the subsequent detection with a secondary antibody. The difference
between the signal responses was used as a measure of the IAA concentration in
the sample. The cutoff value of the biosensor was determined to be 6 U/ml, i.e.,
IAA concentrations below that level would be detected as IAA-negative. This
value was reported to be similar to the RIA cutoff value. We think this method
describes an interesting alternative to deal with nonspecific effects, which may
alter from serum to serum. By blockage of potentially present IAA in one half of
the sample with insulin, an inherent reference was created which is consistent with
the respective real sample [
76
]. A more traditional way for biosensor assay was
used for the detection of IA-2 autoantibodies in human serum with QCM and SPR
biosensors. In both cases a synthetic peptide of 15 amino acids derived from IA-2
molecules was immobilized on the biosensor surfaces. Measurements were per-
formed using samples diluted 1:100 and spiked with antibody. A minimal detected
antibody concentration of 0.2 mM in 1% serum was obtained with the SPR bio-
sensor; the QCM biosensor was not sensitive enough [
127
].
Rheumatoid Arthritis
Rheumatoid arthritis is characterized by chronic inflammation of the joints;
sometimes, systemic complications may be involved. Currently, antibodies against
citrullinated peptides (citrulline-containing peptides) are the most specific
serological markers for diagnosing rheumatoid arthritis [
128
]. Carbon nanotubes
were used to detect those antibodies. First, a QCM biosensor was coated
with single-walled carbon nanotubes to allow higher binding capacity owing to
the three-dimensional structure, then citrullinated peptide was immobilized.
Measurements were performed with serum samples diluted 1:300. Serum samples
obtained from rheumatoid arthritis patients could be distinguished from serum
