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BACKGROUND: Detecting proteins at low concentrations in plasma is crucial for early diagnosis. Current techniques in clinical routine, such as sandwich ELISA, provide sensitive protein detection because of a dependence on target recognition by pairs of antibodies, but detection of still lower protein concentrations is often called for. Proximity ligation assay with rolling circle amplification (PLARCA) is a modified proximity ligation assay (PLA) for analytically specific and sensitive protein detection via binding of target proteins by 3 antibodies, and signal amplification via rolling circle amplification (RCA) in microtiter wells, easily adapted to instrumentation in use in hospitals.

METHODS: Proteins captured by immobilized antibodies were detected using a pair of oligonucleotide-conjugated antibodies. Upon target recognition, these PLA probes guided oligonucleotide ligation, followed by amplification via RCA of circular DNA strands that formed in the reaction. The RCA products were detected by horseradish peroxidase-labeled oligonucleotides to generate colorimetric reaction products with readout in an absorbance microplate reader.

RESULTS: We compared detection of interleukin (IL)-4, IL-6, IL-8, p53, and growth differentiation factor-15 by PLARCA and conventional sandwich ELISA or immuno RCA. PLARCA detected lower concentrations of proteins and exhibited a broader dynamic range compared ELISA and iRCA using the same antibodies. IL-4 and IL-6 were detected in clinical samples at femtomolar concentrations, considerably lower than for ELISA.

CONCLUSIONS: PLARCA offers detection of lower protein levels and increased dynamic ranges compared to ELISA. The PLARCA procedure may be adapted to routine instrumentation available in hospitals and research laboratories.

In this study, levels of inflammatory protein biomarkers in venous plasma, plasma derived from capillary blood from the earlobe, and capillary plasma stored as dried plasma spots (DPS) were compared. Samples from 12 male individuals were assessed with a panel of 92 inflammation-related proteins using multiplex proximity extension assay. Correlations between sample types varied greatly between analytes. A high correlation of rho > 0.8 was observed between capillary plasma and DPS for 32 analytes. At this level of correlation, 13 analytes correlated between venous and capillary plasma and 5 analytes in the comparison of venous blood with DPS.

Since autoantibodies are recognized as valuable biomarkers for clinical diagnostics and prognostics in autoimmune diseases such as Stiff Person Syndrome (SPS) and Type 1 diabetes, detection of such markers at improved sensitivity and specificity could be of significant interest. In addition, as proximity assays have been shown to offer highly sensitive and specific detection of multiple proteins, the technique could be expanded to applications for autoimmunity detection. In the present study, we have applied the newly developed proximity ligation assay with rolling circle amplification (PLARCA), and proximity extension assay (PEA) for the detection of GADA, autoantibodies specific for glutamic acid decarboxylase 65 (GAD65). Through the use of oligonucleotide conjugated autoantigen GAD65 and anti-human antibodies, as proximity probes, we were able to apply these proximity assays to detect GADA in a set of SPS patient samples. In summary, we have applied and established both PLARCA and PEA, as a proof of concept, for the use of the specific and sensitive autoimmune detection.