Development of a Novel Double Antibody Sandwich ELISA for Quantitative Detection of Porcine Deltacoronavirus Antigen

Porcine deltacoronavirus (PDCoV) can cause diarrhea and dehydration in newborn piglets. Here, we developed a double antibody sandwich quantitative enzyme-linked immunosorbent assay (DAS-ELISA) for detection of PDCoV by using a specific monoclonal antibody against the PDCoV N protein and an anti-PDCoV rabbit polyclonal antibody. Using DAS-ELISA, the detection limit of recombinant PDCoV N protein and virus titer were approximately 0.5 ng/mL and 10^3.0 TCID₅₀/mL, respectively. A total of 59 intestinal and 205 fecal samples were screened for the presence of PDCoV by using DAS-ELISA and reverse transcriptase real-time PCR (RT-qPCR). The coincidence rate of the DAS-ELISA and RT-qPCR was 89.8%. DAS-ELISA had a sensitivity of 80.8% and specificity of 95.6%. More importantly, the DAS-ELISA could detect the antigen of PDCoV inactivated virus, and the viral antigen concentrations remained unchanged in the inactivated virus. These results suggest that DAS-ELISA could be used for antigen detection of clinical samples and inactivated vaccines. It is a novel method for detecting PDCoV infections and evaluating the PDCoV vaccine.

Possible correlation of electrochemiluminescence based numerical cut off index value with concentration of anti-SARS-CoV-2 antibody: Is it worth reporting?

Background: Many laboratories are reporting a numerical cutoff index value (COI) value for most anti-SARS-CoV-2 qualitative tests. These numerical values in patients' report ultimately created great confusion in the public and physicians, therefore this study was designed to evaluate the correlation of electrochemiluminescence (ECLIA) based numerical COI values with quantitative ELISA of anti-SARS-CoV-2 antibody. Design and Methods: Two hundred and twenty-eight (228) recovered COVID-19 patients were included; their serum samples were analyzed by quantitative ELISA and ECLIA for anti-SARSCOV- 2 antibodies. Results: One hundred and seventy-three (75.8%) patients tested positive by ECLIA and ELISA assay and thirty-seven (6.2%) were tested negative by both methods. A weak positive correlation (r=0.37) was found between numerical COI value of ECLIA with ELISA concentration, which was statistically significant with p<0.001. All values were dispersed on scatter plot and there was no significant linear relationship between ECLIA and ELISA assay. Conclusions: As both testing techniques are base upon the same immunological phenomena of detecting antibodies against nucleocapsid protein. We suggest that COI values are not meant to describe the immunity level of the individuals thus the physicians should not consider it as a quantitative value for antibody levels in COVID-19 patients.

[Development of a sandwich ELISA for detecting 3AB non-structural protein of foot-and-mouth disease virus]

Antigenic purity is important for quality control of the foot-and-mouth (FMD) whole virus inactivated vaccine. The recommended method for evaluation the antigenic purity of FMD vaccine is to check the serum conversion to non-structural protein (NSP) 3AB antibody after 2 to 3 times inoculation of animals with inactivated vaccine. In this study, we developed a quantitative ELISA to detect the amount of residual 3AB in vaccine antigen, to provide a reference to evaluate the antigenic purity of FMD vaccine. Monoclonal antibody (Mab) of NSP 3A and HRP-conjugated Mab of NSP 3B were used to establish a sandwich ELISA to quantify the NSP 3AB in vaccine antigen of FMD. Purified NSP 3AB expressed in Escherichia coli was serially diluted and detected to draw the standard curve. The detectable limit was determined to be the lowest concentration of standard where the ratio of its OD value to OD blank well was not less than 2.0. Results: The OD value was linearly corelated with the concentration of 3AB protein within the range between 4.7 and 600 ng/mL. The correlation coefficient R² is greater than 0.99, and the lowest detectable limit is 4.7 ng/mL. The amount of 3AB protein in non-purified inactivated virus antigen was detected between 9.3 and 200 ng/mL depending on the 12 different virus strains, whereas the amount of 3AB in purified virus antigen was below the lowest detectable limit. The amount of 3AB in 9 batches of commercial FMD vaccine antigens was between 9.0 and 74 ng/mL, whereas it was below the detectable limit in other 24 batches of commercial vaccine antigens. Conclusion: the sandwich ELISA established in this study is specific and sensitive to detect the content of 3AB protein in vaccine antigen of FMD, which will be a useful method for evaluation of the antigenic purity and quality control of FMD inactivated vaccine.

Development of a Novel Double Antibody Sandwich Quantitative Enzyme-Linked Immunosorbent Assay for Detection of Porcine Epidemic Diarrhea Virus Antigen

Porcine epidemic diarrhea virus (PEDV) causes acute diarrhea and dehydration in sucking piglets with a high mortality rate. Here, we developed a double antibody sandwich quantitative enzyme-linked immunosorbent assay (DAS-qELISA) for detection of PEDV using a specific monoclonal antibody against PEDV N protein and anti-PEDV rabbit serum. Using DAS-qELISA, the detection limit of recombinant PEDV N protein and virus titer were approximately 1 μg/L and 10^2.0 TCID₅₀/ml, respectively. A total of 90 intestinal and 237 fecal samples were then screened for the presence of PEDV using DAS-qELISA and reverse transcriptase PCR (RT-PCR). DAS-qELISA had a high specificity of 98.1% and sensitivity of 93.5%. The accuracy rate between DAS-qELISA and RT-PCR was 95.7%. More importantly, the viral antigen concentrations remained unchanged before and after one inactivated vaccine preparation by using the DAS-qELISA. These results suggest DAS-qELISA could be used for antigen detection of inactivated vaccine samples and clinical samples. It is a novel method for diagnosing diseases and evaluation of the PEDV vaccine.

Differential Cell Lysis Among Periodontal Strains of JP2 and Non-JP2 Genotype of Aggregatibacter actinomycetemcomitans Serotype B Is Not Reflected in Dissimilar Expression and Production of Leukotoxin

Leukotoxic potential of Aggregatibacter actinomycetemcomitans strains has been studied by the use of several methods, and results differ depending on the methods used. The aim of the present study was to perform a comprehensive examination of the leukotoxic potential of a collection of A. actinomycetemcomitans strains by use of three quantitative methods, Western blotting, ELISA, and mRNA expression assay and compare these results with previous data obtained by a cell lysis assay. A higher leukotoxic potential among JP2 genotype strains compared to non-JP2 genotype strains of A. actinomycetemcomitans was found by Western blotting, ELISA and mRNA expression assay. Leukotoxicity as determined by cell lysis assay showed a variation among strains examined, not only depending on being part of JP2 genotype vs. non-JP2 genotype group of A. actinomycetemcomitans. The leukotoxicity of A. actinomycetemcomitans strains as determined by cell lysis assay did not correspond to the leukotoxic potential of A. actinomycetemcomitans strains as determined by three quantitative methods. A comparison of the results obtained by ELISA and mRNA expression assay showed a reasonable correlation between these two methods. It seems important to use more than one method to assess the LtxA-related virulence capacity of A. actinomycetemcomitans in order to obtain comprehensive understanding of the leukotoxic potential of A. actinomycetemcomitans strains.

[Preparation of monoclonal antibodies against human interleukin-35 and their application in a quantitative ELISA for interleukin-35 detection]

To establish a quantitative ELISA for human interleukin-35 (IL-35) detection, we cloned cDNAs encoding the 2 subunits IL-27EBI3 and IL-12p35 of IL-35 by RT-PCR and transformed the cDNAs into Escherichia coli BL21 star (DE3) by recombinant DNA technology. IL-27EBI3 and IL-12p35 were expressed as recombinant proteins and used as immunogen to immunize Balb/c mice. Spleen cells from the positive serum mice were isolated and fused with SP-2/0 myeloma cells. We obtained the hybridoma cell lines stably secreting target antibodies by indirect ELISA screening of the cell supernatants with recombinant IL-27EBI3 and IL-12p35 as antigen and consecutive subcloning of the cells in the well with positive supernatant. Following further measurement of supernatant titers of the antibodies and identification of their antigen specificity, we obtained a hybridoma cell line 3B11 that stably secrets antibody against IL-27EBI3 and a hybridoma cell line 3A10 that secrets antibody against IL-12p35. Both monoclonal antibodies (mAbs) were identified as the subtype of IgG1. Finally, using the anti-IL-27EBI3 mAb from 3B11 as the capture antibody and the anti-IL-12p35 mAb from 3A10 as the secondary antibody, we established a quantitative double-antibodies sandwich ELISA for IL-35 detection with streptavidin-biotin amplification system. Results demonstrated that the quantitative assay had a detection range of 3.12-200 pg/mL, a detectability of 1.26 pg/mL, and a crossing-reactive rate of 0.1%. The intra-batch RSD and the inter-batch RSD of the quantitative assay were 5.1%-5.6% and 5.6%-7.2%, respectively, and the fortified recovery was 89%-103%. Therefore, the sandwich ELISA assay for IL-35 meets the qualification of quantitative analysis and laid a stable foundation for the development of quantitative ELISA kit for IL-35 detection.