Multiplex Analysis of Rat Cytokines and Diabetes Biomarkers
Using Bio-Plex Pro™ Rat Cytokine and Diabetes Assays
Doris Yeung, Richard Zimmerman, Amrit Dulat, and Joyce Eldering,
Life Science Group, Bio-Rad Laboratories, Inc., 2000 Alfred Nobel Drive,
Hercules, CA 94547, USA
Introduction
The rat is second only to the mouse as an animal model for
immunological research. Like mice, rats lend themselves to a
variety of laboratory studies (Jennings and Dillehay 2006). In
many rat studies, maximizing assay throughput and reducing
biological sample volume provides significant advantages, as
sample volume is often limited.
Bio-Rad’s newly configured Bio-Plex Pro rat cytokine,
chemokine, and growth factor assays and rat diabetes
assays are magnetic bead–based multiplex assays designed
to meet the small sample volume and high-throughput
demands of research using rat models. These assays permit
the simultaneous measurement of multiple biomarkers in a
single sample using as little as 12.5 µl.
The Bio-Plex Pro rat cytokine and rat diabetes sandwich
immunoassays are optimized for the Bio-Plex suspension
array system and other related platforms using Luminex’s
xMAP technology. The Bio-Plex suspension array system is
more sensitive, has a larger dynamic range, and has a shorter
assay time compared to conventional ELISA. The Bio-Plex
system can simultaneously quantify up to 100 biomarkers
in serum, plasma, cell culture supernatants, and other more
exotic sample matrices. These advantages of the
Bio-Plex Pro assays and Bio-Plex system translate to reduced
labor, improved productivity, and lowered assay cost.
In this work, we have validated the Bio-Plex Pro rat cytokine
and rat diabetes assays according to performance criteria of
life science, preclinical, and pharmaceutical research. These
criteria include assay range, sensitivity, precision, specificity,
and linearity of dilution as well as parallelism to evaluate
robustness in the key sample matrices mentioned above.
The validation studies covered 25 rat cytokine and 5 rat
diabetes markers (Table 1). These markers are available
in a 23-plex, a Th1/Th2 12-plex, or 30 singleplex kits for
customizable configurations tailored specifically to end-user
needs (bulletin 6100).
Method
Bio-Plex Pro rat cytokine and rat diabetes assays employ
a standard sandwich enzyme immunoassay method using
a 96-well plate format. Capture antibodies coupled to
fluorescently colored beads are allowed to react with a sample
containing proteins of interest. After performing a series of
washes to remove unbound analytes, a biotinylated detection
antibody specific for a different epitope on the analyte is
added to the beads. The result is the formation of a sandwich
of antibodies around the specific analyte. The reaction
mixture is detected by the addition of the reporter dye
streptavidin-phycoerythrin (SA-PE), which binds to the sandwich
complexes via the biotinylated detection antibodies. The
contents of each well are drawn up into the Bio-Plex array
reader, which identifies and quantifies each specific reaction
based on bead color and fluorescent signal intensity (Figure 1).
Analytic Performance Characteristics
Assay Precision
Assay precision includes intra-assay %CV and inter-assay %CV.
Intra-assay %CV assesses the variation of median fluorescence
intensity (MFI) of the standard points in three replicate wells
within a representative assay. In contrast, inter-assay %CV
measures the variability of observed standard concentrations
among three independent assays. The mean CV was
calculated from the standard concentrations within the assay
working range (Table 2). The same study was also conducted
in RPMI media and showed comparable or better results
(data not shown).
Assay Accuracy
Assay accuracy (recovery) was calculated as the percentage
of the observed concentration value of a target antigen relative
to the expected value. This was evaluated for each target by
determining the standard curve recovery across an eight-point
standard curve fitted by a five-parameter logistic regression
analysis (Table 3).
Assay Working Range and Sensitivity
Assay working range is defined as a concentration range
between the lowest level of quantification (LLOQ) and the
upper level of quantification (ULOQ) in which the assay is both
precise (intra-assay replicate precision ≤ 20%) and accurate
(standard recovery within 80–120%). The mean LLOQ and
ULOQ from three independent assays were used to derive the
overall assay range in a serum-based matrix. Assay sensitivity
(limit of detection, LOD) was calculated by adding two standard
deviations to the mean background median fluorescence
intensity (MFI) of the blank and calculating the corresponding
concentration in pg/ml. Table 4 summarizes the findings in
both serum and RPMI matrices.
Assay Linearity and Parallelism
Linearity of dilution determines the suitability of a standard
curve for reflecting relative quantities of analytes in a given
matrix. This was examined by spiking recombinant analyte into
rat serum, plasma, or cell culture matrices and performing 1:2
serial dilutions. Each dilution point was assayed in all multiplex
assays and representative singleplex assays. The observed
and expected analyte concentrations (within assay working
ranges) were plotted and the correlation coefficient (R2) values
were generated from linear regression analysis.
Parallelism is a second measure of how well an assay’s
standard curve and the measured analyte in key sample
matrices correlate across the range of the assay. If the
standard curve is a reasonable predictor of target levels in a
given sample, then the standard curve should be parallel to a
curve drawn for a range of target quantities measured in the
sample. In this study, assay parallelism was investigated by
measuring slope differences between spike response curves
in rat serum or plasma and the standard curve in standard
diluent. Spiked standard diluent served as a positive control
to indicate best-case results. Overall, dose-response curve
slopes tended to be ≤ 30% different for spiked serum and
plasma. The exception is PAI-1, which reported a >30%
difference in serum due to its high endogenous levels (Table 5).
Assay Specificity (% Cross-Reactivity)
Assay specificity was examined by subjecting specific test
reagents to single-antigen and single-detection cross-reactivity
studies. The single-antigen study evaluates the specificity of
a capture antibody. This is conducted by testing an individual
antigen in the presence of multiplexed capture beads and
detection antibodies. The single-detection study evaluates
the specificity of the detection antibodies. This is conducted
by testing the individual detection antibody in the presence of
multiplexed antigens and capture beads. Overall the studies
showed that all assays were highly specific at <5% cross-
reactivity (data not shown). The exceptions were IL-12p70,
IL-12p40, MIP-2, and GRO/KC. The IL-12p40 capture and
detection antibodies cross-react with the IL-12p70 antigen,
which contains both p40 and p30 subunits. The capture and
detection antibodies of GRO/KC cross-react with the MIP-2
antigen, which shares 50% homology in amino acid sequence
with GRO/KC. These results suggest that both IL-12p40 and
MIP-2 should be tested as singleplex assays.
Specimen Testing
Samples from key matrices (serum and plasma) were tested
to examine the robustness of the assay ranges. The assays
were able to measure the majority of samples within the
specified working assay range (Table 6).
The expected trends in pooled normal versus disease-
state samples were evident in a set of single-dose
lipopolysaccharide (LPS)-treated rats, in which higher
levels of cytokines were detected in the majority of markers
in both plasma and serum matrices (Table 7).
Agreement with Other Luminex Platforms
To give end users the flexibility of using other Luminex
platforms, the rat assays were also evaluated on both the
Luminex FlexMAP 3D and MAGPIX systems. Samples tested
(n=14) included normal and diseased plasma and serum. The
Bio-Plex 200 platform closely matched both the FlexMAP 3D
and the MAGPIX systems in sample readout, as reported
in the percentage of detectable samples within the working
assay range (Table 8). The exceptions are GM-CSF, GRO/KC,
and GLP-1. These discrepancies were due to minor
fluctuations in standard recovery that resulted in certain sample
measurements falling out of assay range. Overall, the MFI
values are comparable between the Bio-Plex 200 and
MAGPIX systems. The MFI values obtained from FlexMAP
3D were higher due to its enhanced PMT setting. However,
sample concentrations generated with these three instruments
were comparable.
Conclusions
The validation studies presented here demonstrate the
robustness of the 25 rat cytokine and 5 rat diabetes assays.
Assay precision, accuracy, working range, sensitivity, linearity,
parallelism, and specificity were all evaluated. Additionally, the
relevance of the assay working ranges and the equivalence
of the Bio-Plex suspension array system to other Luminex
platforms were confirmed using specimen testing.
References
- Jennings VM and Dillehay DL (2006). Immunology. In The Laboratory Rat,
second edition, Suckow, MA, Weisbroth, SH, Franklin, CL eds. (Academic
Press), pp. 847–864.