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Detecting viral antigens at low concentrations in field samples can be crucial for early veterinary diagnostics. Proximity ligation assays (PLAs) in both solution and solid-phase formats are widely used for high-performance protein detection in medical research. However, the affinity reagents used, which are mainly poly- and monoclonal antibodies, play an important role in the performance of PLAs. Here, we have established the first homogeneous and solid-phase proximity-dependent DNA aptamer ligation assays for rapid and accurate detection of Newcastle disease virus (NDV). NDV is detected by a pair of extended DNA aptamers that, upon binding in proximity to proteins on the envelope of the virus, are joined by enzymatic ligation to form a unique amplicon that can be sensitively detected using real-time PCR. The sensitivity, specificity, and reproducibility of the assays were validated using 40 farm samples. The results demonstrated that the developed homogeneous and solid-phase PLAs, which use NDV-selective DNA aptamers, are more sensitive than the sandwich enzymatic-linked aptamer assay (ELAA), and have a comparable sensitivity to real-time reverse transcription PCR (rRT-PCR) as the gold standard detection method. In addition, the solid-phase PLA was shown to have a greater dynamic range with improved lower limit of detection, upper- and lower limit of quantification, and minimal detectable dose as compared with those of ELAA and rRT-PCR. The specificity of PLA is shown to be concordant with rRT-PCR.

We have redesigned probes for in situ proximity ligation assay (PLA), resulting in more efficient localized detection of target proteins. In situ PLA depends on recognition of target proteins by pairs of antibody-oligonucleotide conjugates (PLA probes), which jointly give rise to DNA circles that template localized rolling circle amplification reactions. The requirement for dual recognition of the target proteins improves selectivity by ignoring any cross-reactivity not shared by the antibodies, and it allows detection of protein-protein interactions and post-translational modifications. We herein describe an improved design of the PLA probes -UnFold probes - where all elements required for formation of circular DNA strands are incorporated in the probes. Premature interactions between the UnFold probes are prevented by including an enzymatic "unfolding" step in the detection reactions. This allows DNA circles to form by pairs of reagents only after excess reagents have been removed. We demonstrate the performance of UnFold probes for detection of protein-protein interactions and post-translational modifications in fixed cells and tissues, revealing considerably more efficient signal generation. We also apply the UnFold probes to detect IL-6 in solution phase after capture on solid supports, demonstrating increased sensitivity over both normal sandwich enzyme-linked immunosorbent assays and conventional PLA assays.

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.

Extracellular vesicles (EVs) are continuously released by most cells, and they carry surface markers of their cells of origin. Found in all body fluids, EVs function as conveyers of cellular information, and evidence implicates them as markers of disease. These characteristics make EVs attractive diagnostic targets. However, detection and characterization of EVs is challenging due to their small size. We've established a method, called ExoPLA, that allows individual EVs to be detected and characterized at high specificity and sensitivity. Based on the in situ proximity ligation assay (in situ PLA), proximal oligonucleotide-conjugated antibodies bound to their targets on the surfaces of the EVs allow formation of circular products that can be fluorescently labeled by rolling circle amplification. The intense fluorescent signals produced in this assay allow detection and enumeration of individual EVs by flow cytometry. We describe the procedures for ExoPLA, along with expected results and troubleshooting.

Improved diagnosis, prognosis and disease follow-up is a fundamental procedure and a constant challenge in medicine.  Among the different molecular biomarkers, proteins are the essential regulatory component in blood; hence, by developing enhanced specific and sensitive molecular tools will gives great insight into the different processes in disease treatment.  In this thesis, we build on the proximity ligation assay to develop and apply new adaptable methods to facilitate protein detection.

In paper I, I present a variant of the proximity ligation assay (we call PLARCA) using micro titer plate for detection and quantification of protein using optical density as readout in the fluorometer. PLARCA detected femtomolar levels of these proteins in patient samples, which was considerably below the detection threshold for ELISA.

In paper II, we developed and adapted a new method into the in situ PLA methods for detection and identification of extracellular vesicles (EVs) using flow cytometry as readout (a method we call ExoPLA).  We identified five target proteins on the surface of the Evs and using three colors, we identified the EV using flow cytometer.

In paper III, we aim to improve the efficiency of in situ PLA by creating and developing new designs and versions of the assay we called Unfold probes Through comparison of detection of protein using in situ PLA versus Unfold probes, we observed considerable decrease in non-specific signals, and also a lower detection threshold.

In paper IV, we describe the development of a solid phase proximity extension (sp-PEA) assay for protein detection and quantification. We compared detection of IL-8, TNF-alpha, IL-10 and IL-6 using spPEA and PEA; spPEA demonstrations over 2 orders of magnitudes in the lower detection concentrations by decreased in background noise.

Proximity ligation assays are a group of protein detection techniques in which reagents with affinity for target proteins, typically antibodies, are coupled to short strands of DNA. DNA-modified affinity reagents are combined in assays constructed such that the coordinated binding of individual target molecules or complexes of interacting proteins by two or more of the reagents, followed by DNA ligation and/or polymerization reactions, gives rise to amplifiable DNA reporter strands. Proximity ligation assays have been shown to exhibit excellent sensitivity in single and multiplexed protein assays for individual or interacting proteins, both in solution and in situ. This unit describes procedures for developing solid-phase proximity ligation assays for soluble proteins using either real-time PCR or DNA sequencing as the readout. In addition, critical steps for assay optimization are discussed.