Development of a nanobody tagged with streptavidin-binding peptide and its application in a Luminex fluoroimmunoassay for alpha fetal protein in serum

Nanobody in a Double "Y"-Shaped Assembly: A Promising Candidate for Lateral Flow Immunoassays

Derived from camelid heavy-chain antibodies, nanobodies (Nbs) are the smallest natural antibodies and are an ideal tool in biological studies because of their simple structure, high yield, and low cost. Nbs possess significant potential for developing highly specific and user-friendly diagnostic assays. Despite offering considerable advantages in detection applications, knowledge is limited regarding the exclusive use of Nbs in lateral flow immunoassay (LFIA) detection. Herein, we present a novel double "Y" architecture, achieved by using the SpyTag/SpyCatcher and Im7/CL7 systems. The double "Y" assemblies exhibited a significantly higher affinity for their epitopes, as particularly evident in the reduced dissociation rate. An LFIA employing double "Y" assemblies was effectively used to detect the severe acute respiratory syndrome coronavirus-2 N protein, with a detection limit of at least 500 pg/mL. This study helps broaden the array of tools available for the development of Nb-based diagnostic techniques.

Nanobody based immunoassay for alpha fetal protein detection using streptavidin-conjugated polymerized horseradish peroxidase for signal amplification

The enzyme-linked immunosorbent assay (ELISA) offers several advantages, including simple operation, high throughput, and low cost, making it an ideal immunoassay method for efficient screening of disease-related biomarkers in clinical samples. However, the traditional colorimetric ELISA has relatively low sensitivity, which promotes the continuous emergence of various novel signal amplification technologies. In this work, we fused the AFP-specific nanobody (A1) with the streptavidin-binding peptide (SBP) to develop a fusion protein (A1-SBP) as biorecognition element in a colorimetric ELISA for detecting AFP. Besides, to further improve the sensitivity of the traditional colorimetric ELISA, the streptavidin-conjugated polymerized horseradish peroxidase (SA-PolyHRP) were selected as a detection probe for signal amplification. The proposed signal enhancement strategy demonstrated a limit of detection (LOD) of 0.597 ng/mL for the SA-polyHRP-based ELISA, which is 7.67-fold lower than that of the traditional SA-HRP-based ELISA without additional steps. Furthermore, the proposed SA-polyHRP-based ELISA showed a good correlation with the detection of clinical samples using the Roche E601 chemiluminescence immunoassay analyzer. Therefore, the proposed signal enhancement strategy is an attractive approach for improving the sensitivity of immunoassay without requiring additional steps.

Generation of a nanobody-alkaline phosphatase heptamer fusion for ratiometric fluorescence immunodetection of trace alpha fetoprotein in serum

Alpha fetoprotein (AFP) is an important biomarker for diagnosis of hepatocellular carcinoma (HCC). Whereas, it is always a challenge to detect trace AFP in serum. In this work, a ratiometric fluorescence enzyme immunoassay (RFEIA) was developed using nanobody-alkaline phosphatase (Nb-AP) heptamer and MnFe layered double hydroxides nanoflakes (MnFe LDH) for ultrasensitive detection of AFP. The Nb-AP heptamer (Nb-C4bpα-AP) was constructed by fusion expression of Nb, AP, and α-chain of C4 binding protein (C4bpα), where the C4bpα contributed to multimerization through self-assembly. The dual functional Nb-C4bpα-AP can recognize AFP, dephosphorylate ascorbic acid-2-phosphate (AAP) into ascorbic acid (AA), and thus tune the MnFe LDH-mediated ratiometric fluorescence, which was generated from the oxidization of MnFe LDH on o-phenylenediamine (OPD) and the catalyzation of MnFe LDH on the cyclization reaction between AA and OPD. By integration of Nb-C4bpα-AP, MnFe LDH, AAP, and OPD, the RFEIA showed a limit of detection of 0.013 ng/mL with good selectivity, accuracy and precision. Furthermore, results of clinical serum samples tested by the RFEIA were well confirmed by the automated chemiluminescence immunoassay analyzer. Thus, this work demonstrated that the Nb-C4bpα-AP is a robust immunoreagent and the developed RFEIA could be a very promising tool for diagnosis of HCC.

Generation of a nanobody-alkaline phosphatase fusion and its application in an enzyme cascade-amplified immunoassay for colorimetric detection of alpha fetoprotein in human serum

Sensitive detection of liver disease biomarkers can facilitate the diagnosis of primary hepatoma and other benign liver diseases, and the alpha fetoprotein (AFP) was selected as the model macromolecule in this work. Herein an enzyme cascade-amplified immunoassay (ECAIA) based on the nanobody-alkaline phosphatase fusion (Nb-ALP) and MnO₂ nanoflakes was developed for detecting AFP. The bifunctional biological macromolecule Nb-ALP serves as the detection antibody and the reporter molecule. The MnO₂ nanoflakes mimic the oxidase for catalyzing the 3,3',5,5'-tetramethylbenzidine (TMB) into the blue oxidized TMB, which has a quantitative signal at the wavelength of 650 nm. Moreover, the Nb-ALP could dephosphorylate the ascorbic acid-2-phosphate (AAP) to form the ascorbic acid (AA) that can disintegrate the nanoflakes to reduce their oxidation capacity with the content decrease of the oxidized TMB. Using the constructed TMB-MnO₂ colorimetric sensing system for Nb-ALP and the optimized experimental parameters, the ECAIA has a limit of detection (LOD) of 0.148 ng/mL which is 18.7-fold lower than that of the p-nitrophenylphosphate (pNPP)-based method (LOD = 2.776 ng/mL). The ECAIA showed good selectivity for AFP with observed negligible cross-reactions with several common cancer biomarkers. The recovery rate for AFP spiked in human serum ranged from 94.8% to 113% with the relative standard deviation from 0.3% to 6.5%. For analysis of the actual human serum samples, a good linear correlation was found between the results tested by the ECAIA and the automatic chemiluminescence analyzer. Thus, the ECAIA was demonstrated to be a promising tool for highly sensitive and selective detection of AFP, providing a reference for analysis of other macromolecule biomarkers.

Phage-mediated double-nanobody sandwich immunoassay for detecting alpha fetal protein in human serum

Alpha fetal protein (AFP) is a significant biomarker of liver cancer. Herein we developed a novel phage-mediated double-nanobody sandwich immunoassay (P-ELISA) for sensitive detection of AFP in serum, where the phage displayed the nanobody for antigen recognition and multiple copies of major coat protein pVIII for signal amplification. The expressed nanobody Nb-A1 and the phage-displayed nanobody phage-A2 served as the capture antibody and detection antibody, respectively. Based on the optimal experimental conditions, the P-ELISA has a half maximal saturation concentration of 24.85 ng mL^-1 and a limit of detection of 0.237 ng mL^-1 for AFP. The P-ELISA is highly selective for AFP and ignorable cross-reactions were observed with other tested cancer biomarkers. After elimination of the matrix effect by 30-fold dilution with 0.5 × PBS, clinical serum samples were analyzed by the P-ELISA. The results correlated well with those of the AFP commercial ELISA kit and the Roche E601 automatic chemiluminescence analyzer. Thus, it showed the potential of the recombinant phage for highly sensitive and selective detection of AFP and provides a novel detection model for the other disease-related biomarkers.