A novel polyclonal antibody-based sandwich ELISA for detection of Plasmodium vivax developed from two lactate dehydrogenase protein segments

Characterization of Escherichia coli Strains for Novel Production of Plasmodium ovale Lactate Dehydrogenase

Malaria is one of the most prevalent diseases worldwide with high incidence and mortality. Among the five species that can infect humans, Plasmodium ovale morphologically resembles Plasmodium vivax, resulting in misidentification and confusion in diagnosis, and is responsible for malarial disease relapse due to the formation of hypnozoites. P. ovale receives relatively less attention compared to other major parasites, such as P. falciparum and P. vivax, primarily due to its lower pathogenicity, mortality rates, and prevalence rates. To efficiently produce lactate dehydrogenase (LDH), a major target for diagnosing malaria, this study used three Escherichia coli strains, BL21(DE3), BL21(DE3)pLysS, and Rosetta(DE3), commonly used for recombinant protein production. These strains were characterized to select the optimal strain for P. ovale LDH (PoLDH) production. Gene cloning for recombinant PoLDH production and transformation of the three strains for protein expression were performed. The optimal PoLDH overexpression and washing buffer conditions in nickel-based affinity chromatography were established to ensure high-purity PoLDH. The yields of PoLDH expressed by the three strains were as follows: BL21(DE3), 7.6 mg/L; BL21(DE3)pLysS, 7.4 mg/L; and Rosetta(DE3), 9.5 mg/L. These findings are expected to be highly useful for PoLDH-specific diagnosis and development of antimalarial therapeutics.

Expression, Purification, and Characterization of Plasmodium vivax Lactate Dehydrogenase from Bacteria without Codon Optimization

Plasmodium vivax is the most widespread cause of malaria, especially in subtropical and temperate regions such as Asia-Pacific and America. P. vivax lactate dehydrogenase (PvLDH), an essential enzyme in the glycolytic pathway, is required for the development and reproduction of the parasite. Thus, LDH from these parasites has garnered attention as a diagnostic biomarker for malaria and as a potential molecular target for developing antimalarial drugs. In this study, we prepared a transformed Escherichia coli strain for the overexpression of PvLDH without codon optimization. We introduced this recombinant plasmid DNA prepared by insertion of the PvLDH gene in the pET-21a(+) expression vector, into the Rosetta(DE3), an E. coli strain suitable for eukaryotic protein expression. The time, temperature, and inducer concentration for PvLDH expression from this E. coli Rosetta(DE3), containing the original PvLDH gene, were optimized. We obtained PvLDH with a 31.0 mg/L yield and high purity (>95%) from this Rosetta(DE3) strain. The purified protein was characterized structurally and functionally. The PvLDH expressed and purified from transformed bacteria without codon optimization was successfully demonstrated to exhibit its potential tetramer structure and enzyme activity. These findings are expected to provide valuable insights for research on infectious diseases, metabolism, diagnostics, and therapeutics for malaria caused by P. vivax.

Enzyme-linked aptamer-based sandwich assay (ELASA) for detecting Plasmodium falciparum lactate dehydrogenase, a malarial biomarker

Herein, we report a sensitive and selective enzyme-linked aptamer-based sandwich assay (ELASA) to detect Plasmodium falciparum lactate dehydrogenase (PfLDH), which is an attractive biomarker for malaria diagnosis and antimalarial medication. We performed the sandwich assay with a single aptamer sequence, called 2008s, owing to the structural properties of the PfLDH tetramer instead of using a conventional sandwich assay with two different aptamers (or antibodies) for capturing and probing a target molecule. First, the biotinylated PfLDH aptamer was linked with immobilized streptavidin on a microwell plate for binding flexibility, and then PfLDH was bound to the aptamer. Next, a horseradish peroxidase-conjugated aptamer of the same sequence was used to analyze PfLDH quantitatively. Using this approach, the limit of detection (LOD) of PfLDH with the naked eye was 100 ng mL^-1, and the LOD and limit of quantification from the absorbance measurements were 34.9 ng mL^-1 and 95.5 ng mL^-1, respectively, based on PfLDH spiked blood samples. Our proposed method selectively binds PfLDH, not human lactate dehydrogenase. Therefore, this method may be a valuable tool for diagnosing, monitoring, and quarantining malaria cases easily and rapidly.

Electrochemical immunosensor for detection of Plasmodium vivax lactate dehydrogenase

BACKGROUND

Malaria is a disease that affects many tropical and subtropical countries, including Brazil. The use of tests for malaria detection is one of the fundamental strategies recommended by the World Health Organization for the control and eradication of the disease. The lack of diagnostic tests leads to an increase in transmission and non-reporting cases.

OBJECTIVES

This work described an electrochemical immunosensor for detecting Plasmodium vivax lactate dehydrogenase antigen (Ag-PvLDH).

METHODS

The device has developed by immobilising egg yolk IgY antibodies (Ab-PvLDH) on a gold electrode surface using cysteamine as linker. The immunosensor fabrication was followed by differential pulse voltammetry, and contact angle measurements were performed to characterise the modified gold electrode surface.

FINDINGS

The results for Ag-PvLDH determination exhibit a linear response at 10-50 µg mL^-1 concentration range, with a limit of detection of 455 ng mL^-1. The excellent selectivity of the device was confirmed.

MAIN CONCLUSIONS

The developed immunosensor showed a good performance, therefore, it can be considered an alternative test to detect malaria caused by P. vivax.

Bacillus subtilis spores as delivery system for nasal Plasmodium falciparum circumsporozoite surface protein immunization in a murine model

Malaria remains a widespread public health problem in tropical and subtropical regions around the world, and there is still no vaccine available for full protection. In recent years, it has been observed that spores of Bacillus subtillis can act as a vaccine carrier and adjuvant, promoting an elevated humoral response after co-administration with antigens either coupled or integrated to their surface. In our study, B. subtillis spores from the KO7 strain were used to couple the recombinant CSP protein of P. falciparum (rPfCSP), and the nasal humoral-induced immune response in Balb/C mice was evaluated. Our results demonstrate that the spores coupled to rPfCSP increase the immunogenicity of the antigen, which induces high levels of serum IgG, and with balanced Th1/Th2 immune response, being detected antibodies in serum samples for 250 days. Therefore, the use of B. subtilis spores appears to be promising for use as an adjuvant in a vaccine formulation.

One-step enzyme-free dual electrochemical immunosensor for histidine-rich protein 2 determination

In the present work, we describe a novel one-step enzyme-free dual electrochemical immunosensor for the determination of histidine-rich protein 2 (Ag-PfHRP2), a specific malaria biomarker. A gold electrode (GE) was functionalized with the PfHRP2 antibody (Ab-PfHRP2) using dihexadecyl phosphate (DHP) polymer as an immobilization platform. The Ab-PfHRP2/DHP/GE sensor was characterized by cyclic voltammetry, electrochemical impedance spectroscopy, Fourier-transform infrared spectroscopy, scanning electron microscopy, and atomic force microscopy. The developed immunosensor was employed for indirect Ag-PfHRP2 determination by differential pulse voltammetry (DPV) and electrochemical impedance spectroscopy (EIS). The linear range was 10-400 ng mL-1 and 10-500 ng mL-1 for EIS and DPV, while the limit of detection was 3.3 ng mL-1 and 2.8 ng mL-1, respectively. The electrochemical immunosensor was successfully applied for Ag-PfHRP2 determination in human serum samples. Its performance was compared with an ELISA test, and good correspondence was achieved. The coefficients of intra- and inter-assay variations were less than 5%. The electrochemical immunosensor is a useful and straightforward tool for in situ malaria biomarker determination.

A First Plasmodium vivax Natural Infection Induces Increased Activity of the Interferon Gamma-Driven Tryptophan Catabolism Pathway

The human immune response that controls Plasmodium infection in the liver and blood stages of the parasite life cycle is composed by both pro- and anti-inflammatory programs. Pro-inflammatory responses primarily mediated by IFN-γ controls the infection, but also induce tolerogenic mechanisms to limit host damage, including the tryptophan (TRP) catabolism pathway mediated by the enzyme Indoleamine 2,3-Dioxygenase (IDO1), an enzyme that catalyzes the degradation of TRP to kynurenines (KYN). Here we assessed total serum kynurenines and cytokine dynamics in a cohort of natural Plasmodium vivax human infection and compared them to those of endemic healthy controls and other febrile diseases. In acute malaria, the absolute free kynurenine (KYN) serum levels and the KYN to TRP (KYN/TRP) ratio were significantly elevated in patients compared to healthy controls. Individuals with a diagnosis of a first malaria episode had higher serum KYN levels than individuals with a previous malaria episode. We observed an inverse relationship between the serum levels of IFN-γ and IL-10 in patients with a first malaria episode compared to those of subjects with previous history of malaria. Kynurenine elevation was positively correlated with serum IFN-γ levels in acute infection, whereas, it was negatively correlated with parasite load and P. vivax LDH levels. Overall, the differences observed between infected individuals depended on the number of Plasmodium infections. The decrease in the KYN/TRP ratio in malaria-experienced subjects coincided with the onset of anti-P. vivax IgG. These results suggest that P. vivax infection induces a strong anti-inflammatory program in individuals with first time malaria, which fades with ensuing protective immunity after subsequent episodes. Understanding the tolerance mechanisms involved in the initial exposure would help in defining the balance between protective and pathogenic immune responses necessary to control infection and to improve vaccination strategies.

Simultaneous Quantification of Plasmodium Antigens and Host Factor C-Reactive Protein in Asymptomatic Individuals with Confirmed Malaria by Use of a Novel Multiplex Immunoassay

Malaria rapid diagnostic tests (RDTs) primarily detect Plasmodium falciparum antigen histidine-rich protein 2 (HRP2) and the malaria-conserved antigen lactate dehydrogenase (LDH) for P. vivax and other malaria species. The performance of RDTs and their utility is dependent on circulating antigen concentration distributions in infected individuals in a population in which malaria is endemic and on the limit of detection of the RDT for the antigens.

Malaria rapid diagnostic tests (RDTs) primarily detect Plasmodium falciparum antigen histidine-rich protein 2 (HRP2) and the malaria-conserved antigen lactate dehydrogenase (LDH) for P. vivax and other malaria species. The performance of RDTs and their utility is dependent on circulating antigen concentration distributions in infected individuals in a population in which malaria is endemic and on the limit of detection of the RDT for the antigens. A multiplexed immunoassay for the quantification of HRP2, P. vivax LDH, and all-malaria LDH (pan LDH) was developed to accurately measure circulating antigen concentration and antigen distribution in a population with endemic malaria. The assay also measures C-reactive protein (CRP) levels as an indicator of inflammation. Validation was conducted with clinical specimens from 397 asymptomatic donors from Myanmar and Uganda, confirmed by PCR for infection, and from participants in induced blood-stage malaria challenge studies. The assay lower limits of detection for HRP2, pan LDH, P. vivax LDH, and CRP were 0.2 pg/ml, 9.3 pg/ml, 1.5 pg/ml, and 26.6 ng/ml, respectively. At thresholds for HRP2, pan LDH, and P. vivax LDH of 2.3 pg/ml, 47.8 pg/ml, and 75.1 pg/ml, respectively, and a specificity ≥98.5%, the sensitivities for ultrasensitive PCR-confirmed infections were 93.4%, 84.9%, and 48.9%, respectively. Plasmodium LDH (pLDH) concentration, in contrast to that of HRP2, correlated closely with parasite density. CRP levels were moderately higher in P. falciparum infections with confirmed antigenemia versus those in clinical specimens with no antigen. The 4-plex array is a sensitive tool for quantifying diagnostic antigens in malaria infections and supporting the evaluation of new ultrasensitive RDTs.

An Anti-proteome Nanobody Library Approach Yields a Specific Immunoassay for Trypanosoma congolense Diagnosis Targeting Glycosomal Aldolase

BACKGROUND

Infectious diseases pose a severe worldwide threat to human and livestock health. While early diagnosis could enable prompt preventive interventions, the majority of diseases are found in rural settings where basic laboratory facilities are scarce. Under such field conditions, point-of-care immunoassays provide an appropriate solution for rapid and reliable diagnosis. The limiting steps in the development of the assay are the identification of a suitable target antigen and the selection of appropriate high affinity capture and detection antibodies. To meet these challenges, we describe the development of a Nanobody (Nb)-based antigen detection assay generated from a Nb library directed against the soluble proteome of an infectious agent. In this study, Trypanosoma congolense was chosen as a model system.

METHODOLOGY/PRINCIPAL FINDINGS

An alpaca was vaccinated with whole-parasite soluble proteome to generate a Nb library from which the most potent T. congolense specific Nb sandwich immunoassay (Nb474H-Nb474B) was selected. First, the Nb474-homologous sandwich ELISA (Nb474-ELISA) was shown to detect experimental infections with high Positive Predictive Value (98%), Sensitivity (87%) and Specificity (94%). Second, it was demonstrated under experimental conditions that the assay serves as test-of-cure after Berenil treatment. Finally, this assay allowed target antigen identification. The latter was independently purified through immuno-capturing from (i) T. congolense soluble proteome, (ii) T. congolense secretome preparation and (iii) sera of T. congolense infected mice. Subsequent mass spectrometry analysis identified the target as T. congolense glycosomal aldolase.

CONCLUSIONS/SIGNIFICANCE

The results show that glycosomal aldolase is a candidate biomarker for active T. congolense infections. In addition, and by proof-of-principle, the data demonstrate that the Nb strategy devised here offers a unique approach to both diagnostic development and target discovery that could be widely applied to other infectious diseases.

Development of a recombinant flagellin based ELISA for the detection of Clostridium chauvoei

Blackleg, an economically important and highly fatal disease of ruminants, is caused by anaerobic bacillus, Clostridium chauvoei. Identification and differentiation of the causative agent is crucial for implementation of therapeutic and control measures in real time. Most of the diagnostic tests available for blackleg are PCR based, and only a couple of serological tests have been reported. In this study, we targeted flagellin, an important immunogenic protein of C. chauvoei, to develop a sandwich ELISA for detection of C. chauvoei. Sequence analysis of flagellin gene of related Clostridium species showed that central region of flagellin gene is unique to C. chauvoei. Hence, we cloned and expressed central region of flagellin in a prokaryotic expression system. Antiserum against recombinant flagellin was generated in rabbits and chickens. A sandwich ELISA was developed, in which rabbit anti-flagellin antibodies were used as capture antibodies and chicken anti-flagellin antibodies as detecting antibodies. The test was specific and sensitive in detection of up to 10(4) CFU/ml of C. chauvoei. This study shows that assay developed can be used for detection of C. chauvoei in suspected samples.

Performance of coumarin-derived dendrimer-based fluorescence-linked immunosorbent assay (FLISA) to detect malaria antigen

BACKGROUND

Due to limitation of conventional malaria diagnostics, including microscopy, polymerase chain reaction (PCR), and enzyme-linked immunosorbent assay (ELISA), alternative accurate diagnostics have been demanded for improvement of sensitivity and specificity.

METHODS

Serially diluted Plasmodium LDH antigens, Plasmodium falciparum-infected human red blood cells (RBC) derived from in vitro culture or patient's samples were used for evaluation of the performance of fluorescence-linked immunosorbent assay (FLISA). Microscopic examination was used to determine parasite density and the performance of FLISA was compared to ELISA. Finally, sensitivity and specificity of FLISA was determined by human specimens infected with P. falciparum, Plasmodium vivax, Toxoplasma gondii, and amoebae.

RESULTS

As a result of FLISA, the fluorescent intensity was highly correlated with antigen amount and FLISA was more sensitive than ELISA. FLISA detected at least 0.01 ng/ml of pLDH antigen, which showed 1,000-fold higher sensitivity than ELISA. In vitro-cultured P. falciparum was detected up to 20 parasite number/μL in FLISA but 5120 parasite number/μLin sandwich ELISA. In vitro P. falciparum-infected RBC number was highly correlated with fluorescent intensity (R2 = 0.979), showing that FLISA was reliable for detection of P. falciparum and available for quantification of parasite numbers. Furthermore, eighteen patient samples infected with P. falciparum (n = 9) and P. vivax (n = 9) showed 100% of sensitivity (18/18). FLISA showed 96.3% of specificity (26/27) because one sample of patient blood infected with T. gondii gave a false positive reactivity among healthy donors (n = 9), T. gondii-infected patients (n = 9), and amoeba-infected patients (n = 9).

CONCLUSION

FLISA has a keen and high performance to detect malaria antigen, suggesting a potential assay as malaria immunodiagnostic.