Process development and characterization of recombinant nucleocapsid protein for its application on COVID-19 diagnosis

COVID-19 pandemic was caused by the severe acute respiratory syndrome coronavirus 2 (Sars-CoV-2). The nucleocapsid (N) protein from Sars-CoV-2 is a highly immunogenic antigen and responsible for genome packing. Serological assays are important tools to detect previous exposure to SARS-CoV-2, complement epidemiological studies, vaccine evaluation and also in COVID-19 surveillance. SARS-CoV-2 N (r2N) protein was produced in Escherichia coli, characterized, and the immunological performance was evaluated by enzyme-linked immunosorbent assay (ELISA) and beads-based array immunoassay. r2N protein oligomers were evidenced when it is associated to nucleic acid. Benzonase treatment reduced host nucleic acid associated to r2N protein, but crosslinking assay still demonstrates the presence of higher-order oligomers. Nevertheless, after RNase treatment the higher-order oligomers reduced, and dimer form increased, suggesting RNA contributes to the oligomer formation. Structural analysis revealed nucleic acid did not interfere with the thermal stability of the recombinant protein. Interestingly, nucleic acid was able to prevent r2N protein aggregation even with increasing temperature while the protein benzonase treated begin aggregation process above 55 °C. In immunological characterization, ELISA performed with 233 serum samples presented a sensitivity of 97.44% (95% Confidence Interval, CI, 91.04%, 99.69%) and a specificity of 98.71% (95% CI, 95.42%, 99.84%) while beads-based array immunoassay carried out with 217 samples showed 100% sensitivity and 98.6% specificity. The results exhibited an excellent immunological performance of r2N protein in serologic assays showing that, even in presence of nucleic acid, it can be used as a component of an immunoassay for the sensitive and specific detection of SARS-CoV-2 antibodies.

Leptospira interrogans insoluble fraction as a potential antigen source for lateral flow immunochromatography

BACKGROUND

Leptospirosis is an emerging zoonosis that affects humans and animals. Immunochromatography rapid test is widely used for early diagnosis of leptospirosis, but with low sensitivity and specificity.

OBJECTIVES

To evaluate Leptospira interrogans insoluble fraction as a potential antigen source for lateral flow immunochromatography.

METHODS

Insoluble fraction derived from the crude bacterial extract was obtained by serial centrifugation. The polypeptide profile was determined using sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). Immune reactivity of this fraction was assessed by Western Blotting and lateral flow immunochromatography (LFI). It was tested 160 microagglutination test (MAT)-positive sera from patients in the acute phase, 100 MAT-negative sera from patients with acute febrile illness, and 45 patients with other infectious diseases.

FINDINGS

There was a predominance of low molecular mass-polypeptide bands, ranging from 2 to 37 kDa. The antibody reactivity of theses polypeptides was found to range from 13-50%, especially between 10 and 38 kDa. Among MAT-positive sera of patients with leptospirosis in the acute phase, 97% were also positive in LFI, indicating high sensitivity. Among MAT-negative sera, all were negative in LFI, indicating high specificity. Only 2% of cross-reactivity was detected.

CONCLUSION

The insoluble fraction can be a valuable antigen source for development of point-of-care diagnosis test for leptospirosis.

Accuracy of chimeric proteins in the serological diagnosis of chronic chagas disease - a Phase II study

BACKGROUND

The performance of current serologic tests for diagnosing chronic Chagas disease (CD) is highly variable. The search for new diagnostic markers has been a constant challenge for improving accuracy and reducing the number of inconclusive results.

METHODOLOGY/PRINCIPAL FINDINGS

Here, four chimeric proteins (IBMP-8.1 to -8.4) comprising immunodominant regions of different Trypanosoma cruzi antigens were tested by enzyme-linked immunosorbent assay. The proteins were used to detect specific anti-T. cruzi antibodies in the sera of 857 chagasic and 689 non-chagasic individuals to evaluate their accuracy for chronic CD diagnosis. The antigens were recombinantly expressed in Escherichia coli and purified by chromatographic methods. The sensitivity and specificity values ranged from 94.3% to 99.3% and 99.4% to 100%, respectively. The diagnostic odds ratio (DOR) values were 6,462 for IBMP-8.1, 3,807 for IBMP-8.2, 32,095 for IBMP-8.3, and 283,714 for IBMP-8.4. These chimeric antigens presented DORs that were higher than the commercial test Pathozyme Chagas. The antigens IBMP-8.3 and -8.4 also showed DORs higher than the Gold ELISA Chagas test. Mixtures with equimolar concentrations were tested in order to improve the diagnosis accuracy of negative samples with high signal and positive samples with low signal. However, no gain in accuracy was observed relative to the individual antigens. A total of 1,079 additional sera were used to test cross-reactivity to unrelated diseases. The cross-reactivity rates ranged from 0.37% to 0.74% even for Leishmania spp., a pathogen showing relatively high genome sequence identity to T. cruzi. Imprecision analyses showed that IBMP chimeras are very stable and the results are highly reproducible.

CONCLUSIONS/SIGNIFICANCE

Our findings indicate that the IBMP-8.4 antigen can be safely used in serological tests for T. cruzi screening in blood banks and for chronic CD laboratory diagnosis.

Flow Cytometry as a Tool for Quality Control of Fluorescent Conjugates Used in Immunoassays

The use of antibodies in immunodiagnostic kits generally implies the conjugation of these proteins with other molecules such as chromophores or fluorochromes. The development of more sensitive quality control procedures than spectrophotometry is essential to assure the use of better fluorescent conjugates since the fluorescent conjugates are critical reagents for a variety of immunodiagnostic kits. In this article, we demonstrate a new flow cytometric protocol to evaluate conjugates by molecules of equivalent soluble fluorochromes (MESF) and by traditional flow cytometric analysis. We have coupled microspheres with anti-IgG-PE and anti-HBSAg-PE conjugates from distinct manufactures and/or different lots and evaluated by flow cytometry. Their fluorescence intensities were followed for a period of 18 months. Our results showed that there was a great difference in the fluorescence intensities between the conjugates studied. The differences were observed between manufactures and lots from both anti-IgG-PE and anti-HBSAg-PE conjugates. Coefficients of variation (CVs) showed that this parameter can be used to determine better coupling conditions, such as homogenous coupling. The MESF analysis, as well as geometric mean evaluation by traditional flow cytometry, showed a decrease in the values for all conjugates during the study and were indispensable tools to validate the results of stability tests. Our data demonstrated the feasibility of the flow cytometric method as a standard quality control of immunoassay kits.

A multicentric evaluation of the recombinant Leishmania infantum antigen-based immunochromatographic assay for the serodiagnosis of canine visceral leishmaniasis

Background

Visceral leishmaniasis (VL) is a serious public health challenge in Brazil and dogs are considered to be the main urban reservoir of the causative agent. The culling of animals to control VL in some countries makes the accurate diagnosis of canine VL (CVL) essential. Recombinant antigens rLci1A and rLci2B were selected from a cDNA library of Leishmania infantum amastigotes due to their strong potential as candidates in diagnostic testing for CVL. The present multicentric study aimed to evaluate the sensitivity of a prototype test using these antigens (DPP rLci1A/rLci2B) against 154 sera obtained from symptomatic dogs within three endemic areas of VL in Brazil. The specificity was evaluated using 40 serum samples from negative dogs and dogs infected with other pathogens. Sensitivity and specificity rates of DPP rLci1A/rLci2B prototype were compared to rates from other diagnostic tests currently in use by the Brazilian Ministry of Health, including DPP®LVC, EIE®LVC.

Findings

DPP rLci1A/rLci2B prototype offered similar performance to that offered by DPP®LVC rapid test, as follows: sensitivity of 87% (CI 81–91) and 88% (CI 82–93) and specificity of 100% (CI 91–100) and 97% (CI 87–100), respectively for DPP rLci1A/rLci2B and DPP®LVC. When results of these two tests were considered concomitantly, sensitivity increased to 93.5% (CI 89–96).

Conclusions

The recombinant antigens rLci1A and rLci2B represent promising candidates for use in a multi-antigen rapid test for CVL. The inclusion of novel antigens to the DPP rLci1A/rLci2B prototype model could offer additionally enhanced sensitivity to detect animals infected by L. infantum.

[Hematological and histopathological evaluation of BALB/c and C57BL/6 mice exposed to Cytoplasmic Repetitive Antigen and Flagellar Repetitive Antigen recombinant antigens of Trypanosoma cruzi]

The Cytoplasmic Repetitive Antigen and Flagellar Repetitive Antigen recombinant antigens of Trypanosoma cruzi were inoculated into BALB/c and C57BL/6 mice and its effects evaluated at hematological and histopathological levels. The results showed that the histological pattern of the organs and the hematological profile of mice were not modified suggesting that these antigens are not harmful for the animal.

Simple immunoaffinity method to purify recombinant hepatitis B surface antigen secreted by transfected mammalian cells

Purification of recombinant hepatitis B surface antigen (recHBsAg) produced in a stable Chinese hamster ovary (CHO) cell line was evaluated using Linx Affinity Purification System (Invitrogen, USA). To purify HBsAg secreted by this cell line, a murine monoclonal antibody (MAbAH1) raised against native HBsAg was used. The purified AH1MAb was conjugated with phenyldiboronic acid (PDBA) and immobilized on the immunoaffinity chromatographic support. Using an optimized protocol the affinity column was able to purify recHBsAg from supernatant of mammalian cells cultures with more than 80% purity. This method showed to be simple and quicker than the current ultracentrifugation methods. The method is also efficient and economical in obtaining purified recHBsAg.