Development of diagnostic reagents: raising antibodies against synthetic peptides of PfHRP-2 and LDH using microsphere delivery

Comparison of two malaria multiplex immunoassays that enable quantification of malaria antigens

Background

Immunoassay platforms that simultaneously detect malaria antigens including histidine-rich protein 2 (HRP2)/HRP3 and Plasmodium lactate dehydrogenase (pLDH), are useful epidemiological tools for rapid diagnostic test evaluation. This study presents the comparative evaluation of two multiplex platforms in identifying Plasmodium falciparum with presence or absence of HRP2/HRP3 expression as being indicative of hrp2/hrp3 deletions and other Plasmodium species. Moreover, correlation between the malaria antigen measurements performed at these platforms is assessed after calibrating with either assay standards or international standards and the cross-reactivity among Plasmodium species is examined.

Methods

A 77-member panel of specimens composed of the World Health Organization (WHO) international Plasmodium antigen standards, cultured parasites for P. falciparum and Plasmodium knowlesi, and clinical specimens with mono-infections for P. falciparum, Plasmodium vivax, and Plasmodium malariae was generated as both whole blood and dried blood spot (DBS) specimens. Assays for HRP2, P. falciparum–specific pLDH (PfLDH), P. vivax–specific pLDH (PvLDH), and all human Plasmodium species Pan malaria pLDH (PanLDH) on the Human Malaria Array Q-Plex and the xMAP platforms were evaluated with these panels.

Results

The xMAP showed a higher percent positive agreement for identification of hrp2-deleted P. falciparum and Plasmodium species in whole blood and DBS than the Q-Plex. For whole blood samples, there was a highly positive correlation between the two platforms for PfLDH (Pearson r = 0.9926) and PvLDH (r = 0. 9792), moderate positive correlation for HRP2 (r = 0.7432), and poor correlation for PanLDH (r = 0.6139). In Pearson correlation analysis between the two platforms on the DBS, the same assays were r = 0.9828, r = 0.7679, r = 0.6432, and r = 0.8957, respectively. The xMAP HRP2 assay appeared to cross-react with HRP3, while the Q-Plex did not. The Q-Plex PfLDH assay cross-reacted with P. malariae, while the xMAP did not. For both platforms, P. knowlesi was detected on the PvLDH assay. The WHO international standards allowed normalization across both platforms on their HRP2, PfLDH, and PvLDH assays in whole blood and DBS.

Conclusions

Q-Plex and xMAP show good agreement for identification of P. falciparum mutants with hrp2/hrp3 deletions, and other Plasmodium species. Quantitative results from both platforms, normalized into international units for HRP2, PfLDH, and PvLDH, showed good agreement and should allow comparison and analysis of results generated by either platform.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12936-022-04203-9.

Towards development of aptamers that specifically bind to lactate dehydrogenase of Plasmodium falciparum through epitopic targeting

Background

Early detection is crucial for the effective treatment of malaria, particularly in those cases infected with Plasmodium falciparum. There is a need for diagnostic devices with the capacity to distinguish P. falciparum from other strains of malaria. Here, aptamers generated against targeted species-specific epitopes of P. falciparum lactate dehydrogenase (rPfLDH) are described.

Results

Two classes of aptamers bearing high binding affinity and specificity for recombinant P. falciparum lactate dehydrogenase (rPfLDH) and P. falciparum-specific lactate dehydrogenase epitopic oligopeptide (LDHp) were separately generated. Structurally-relevant moieties with particular consensus sequences (GGTAG and GGCG) were found in aptamers reported here and previously published, confirming their importance in recognition of the target, while novel moieties particular to this work (ATTAT and poly-A stretches) were identified. Aptamers with diagnostically-supportive functions were synthesized, prime examples of which are the aptamers designated as LDHp 1, LDHp 11 and rLDH 4 and rLDH 15 in work presented herein. Of the sampled aptamers raised against the recombinant protein, rLDH 4 showed the highest binding to the target rPfLDH in the ELONA assay, with both rLDH 4 and rLDH 15 indicating an ability to discriminate between rPfLDH and rPvLDH. LDHp 11 was generated against a peptide selected as a unique P. falciparum LDH peptide. The aptamer, LDHp 11, like antibodies against the same peptide, only detected rPfLDH and discriminated between rPfLDH and rPvLDH. This was supported by affinity binding experiments where only aptamers generated against a unique species-specific epitope showed an ability to preferentially bind to rPfLDH relative to rPvLDH rather than those generated against the whole recombinant protein. In addition, rLDH 4 and LDHp 11 demonstrated in situ binding to P. falciparum cells during confocal microscopy.

Conclusions

The utilization and application of LDHp 11, an aptamer generated against a unique species-specific epitope of P. falciparum LDH indicated the ability to discriminate between recombinant P. falciparum and Plasmodium vivax LDH. This aptamer holds promise as a biorecognition element in malaria diagnostic devices for the detection, and differentiation, of P. falciparum and P. vivax malaria infections. This study paves the way to explore aptamer generation against targeted species-specific epitopes of other Plasmodium species.

Global sequence diversity of the lactate dehydrogenase gene in Plasmodium falciparum

BACKGROUND

Antigen-detecting rapid diagnostic tests (RDTs) have been recommended by the World Health Organization for use in remote areas to improve malaria case management. Lactate dehydrogenase (LDH) of Plasmodium falciparum is one of the main parasite antigens employed by various commercial RDTs. It has been hypothesized that the poor detection of LDH-based RDTs is attributed in part to the sequence diversity of the gene. To test this, the present study aimed to investigate the genetic diversity of the P. falciparum ldh gene in Thailand and to construct the map of LDH sequence diversity in P. falciparum populations worldwide.

METHODS

The ldh gene was sequenced for 50 P. falciparum isolates in Thailand and compared with hundreds of sequences from P. falciparum populations worldwide. Several indices of molecular variation were calculated, including the proportion of polymorphic sites, the average nucleotide diversity index (π), and the haplotype diversity index (H). Tests of positive selection and neutrality tests were performed to determine signatures of natural selection on the gene. Mean genetic distance within and between species of Plasmodium ldh was analysed to infer evolutionary relationships.

RESULTS

Nucleotide sequences of P. falciparum ldh could be classified into 9 alleles, encoding 5 isoforms of LDH. L1a was the most common allelic type and was distributed in P. falciparum populations worldwide. Plasmodium falciparum ldh sequences were highly conserved, with haplotype and nucleotide diversity values of 0.203 and 0.0004, respectively. The extremely low genetic diversity was maintained by purifying selection, likely due to functional constraints. Phylogenetic analysis inferred the close genetic relationship of P. falciparum to malaria parasites of great apes, rather than to other human malaria parasites.

CONCLUSIONS

This study revealed the global genetic variation of the ldh gene in P. falciparum, providing knowledge for improving detection of LDH-based RDTs and supporting the candidacy of LDH as a therapeutic drug target.

Adjuvant poly(N-isopropylacrylamide) generates more efficient monoclonal antibodies against truncated recombinant histidine-rich protein2 of Plasmodium falciparum for malaria diagnosis

Adjuvants play an important role in eliciting immune responses and subsequent generation of antibodies with high specificity. Recently, poly(N-isopropylacrylamide) (PNiPAAm), also known as a "smart" polymer, has been proposed as a potential adjuvant for making antibodies and vaccines. This material exhibits efficient delivery, protection against degradation, and preservation of antigen epitopes. In this work, we used both CFA and smart polymer to develop a highly specific murine monoclonal antibody (mAb) against recombinant truncated histidine rich protein2 (HRP2) of Plasmodium falciparum. Our results indicate that the mAbs developed using these adjuvants were able to recognize recombinant HRP2 and native PfHRP2 protein from spent medium. The mAbs generated against recombinant truncated HRP2 showed better sensitivity to the antigen and importantly mAbs generated using PNiPAAm adjuvant were in the range of 10(8)-10(9) M(-1). The mAbs generated using PNiPAAm are very efficient and sensitive in detecting HRP2. To the best of our knowledge, this is the first report of such comparison having been made between these two adjuvants and we propose that the smart polymer has huge potential as an alternative to CFA. Additionally, we discuss the utility of the mAbs generated through PNiPAAm for specific diagnosis of malaria caused by P. falciparum.

Novel monoclonal antibody against truncated C terminal region of Histidine Rich Protein2 (PfHRP2) and its utility for the specific diagnosis of malaria caused by Plasmodium falciparum

An accurate diagnosis of malarial infection is an important element in combating this deadly disease. Malaria diagnostic test including, microscopy and other molecular tests are highly sensitive but too complex for field conditions. Rapid detection tests for P. falciparum infection using monoclonal antibodies (mAbs) against highly polymorphic PfHRP2 (Histidine Rich Protein2) are still most preferred test in field conditions, but with limitations such as specificity, and sensitivity leading to false positive and false negative results. To overcome these limitations, we carried out bioinformatics analysis PfHRP2 and PfHRP3 and found that the C-terminal region of PfHRP2 (~105 amino acids) displayed relatively lower sequence identity with PfHRP3. This C-terminal region of PfHRP2 contained unique peptide repeats and was found to be conserved in various isolates of P. falciparum. Moreover, this region was also found to be highly antigenic as predicted by antigenicity propensity scores. Thus we constructed a cDNA clone of the truncated PfHRP2 (recPfHRP2-T3) coding for C-terminal 105 amino acids and expressed it in E. coli and purified the polypeptide to homogeneity. The purified recPfHRP2-T3 was used as an antigen for development of both polyclonal and monoclonal antibody (mAb). The mAbs b10c1 and Aa3c10 developed against recPfHRP2-T3 was found to efficiently recognize recombinant PfHRP2 but not PfHRP3. In addition, the above mAbs reacted positively with spent media and serum sample of P. falciparum infection recognizing the native PfHRP2. The affinity constant of both the clones were found to be 10(9) M(-1). Quantitatively, both these clones showed ~4.4 fold higher reactivity with P. falciparum infected serum compared to serum from healthy volunteers or P. vivax infected patient samples. Thus these anti-C-terminal PfHRP2 mAbs (Aa3c10 and b10c1) display a very high potential for improvising the existing malarial diagnostic tools for detection of P. falciparum infection especially in areas where PfHRP2 polymorphism is highly prevalent.

New developments in malaria diagnostics: monoclonal antibodies against Plasmodium dihydrofolate reductase-thymidylate synthase, heme detoxification protein and glutamate rich protein

Currently available rapid diagnostic tests (RDTs) for malaria show large variation in sensitivity and specificity, and there are concerns about their stability under field conditions. To improve current RDTs, monoclonal antibodies (mAbs) for novel malaria antigens have been developed and screened for their possible use in new diagnostic tests. Three antigens, glutamate rich protein (GLURP), dihydrofolate reductase-thymidylate synthase (DHFR-TS) and heme detoxification protein (HDP), were selected based on literature searches. Recombinant antigens were produced and used to immunize mice. Antibody-producing cell lines were subsequently selected and the resulting antibodies were screened for specificity against Plasmodium falciparum and Plasmodium vivax. The most optimal antibody couples were selected based on antibody affinity (expressed as dissociation constants, KD) and detection limit of crude antigen extract from P. falciparum 3D7 culture. The highest affinity antibodies have KD values of 0.10 nM ± 0.014 (D5) and 0.068 ± 0.015 nM (D6) for DHFR-TS mAbs, 0.10 ± 0.022 nM (H16) and 0.21 ± 0.022 nM (H18) for HDP mAbs and 0.11 ± 0.028 nM (G23) and 0.33 ± 0.093 nM (G22) for GLURP mAbs. The newly developed antibodies performed at least as well as commercially available histidine rich protein antibodies (KD of 0.16 ± 0.13 nM for PTL3 and 1.0 ± 0.049 nM for C1-13), making them promising reagents for further test development.

Opportunities for improving pLDH-based malaria diagnostic tests

BACKGROUND

Monoclonal antibodies to Plasmodium lactate dehydrogenase (pLDH) have been previously used to format immunochromatographic tests for the diagnosis of malaria. Using pLDH as an antigen has several advantages as a sensitive measure of the presence of parasites within patient blood samples. However, variable results in terms of specificity and sensitivity among different commercially available diagnostic kits have been reported and it has not been clear from these studies whether the performance of an individual test is due simply to how it is engineered or whether it is due to the biochemical nature of the pLDH-antibody reaction itself.

METHODS

A series of systematic studies to determine how various pLDH monoclonal antibodies work in combination was undertaken. Different combinations of anti-pLDH monoclonal antibodies were used in a rapid-test immunochromatographic assay format to determine parameters of sensitivity and specificity with regard to individual Plasmodium species.

RESULTS

Dramatic differences were found in both species specificity and overall sensitivity depending on which antibody is used on the immunochromatographic strip and which is used on the colorimetric colloidal-gold used for visual detection.

DISCUSSION

The results demonstrate the feasibility of different test formats for the detection and speciation of malarial infections. In addition, the data will enable the development of a universal rapid test algorithm that may potentially provide a cost-effective strategy to diagnose and manage patients in a wide range of clinical settings.

CONCLUSION

These data emphasize that using different anti-pLDH antibody combinations offers a tractable way to optimize immunochromatographic pLDH tests.

Anti-peptide antibodies differentiate between plasmodial lactate dehydrogenases

Malaria lactate dehydrogenase, a glycolytic enzyme, is a malaria diagnostic target in lateral flow immunochromatographic rapid diagnostic tests. Recombinant Plasmodium yoelii LDH was cloned into the pET-28a vector, expressed and the expressed protein purified from a Ni-NTA affinity matrix. A pan-malarial LDH antibody directed against a common malaria LDH peptide (APGKSDKEWNRDDLL) and two anti-peptide antibodies, each targeting a unique Plasmodium falciparum (LISDAELEAIFDC) and Plasmodium vivax (KITDEEVEGIFDC) LDH peptide were raised in chickens. The antibodies were affinity purified with the appropriate peptide affinity matrix. The affinity purified anti-peptide antibodies detected recombinant P. falciparum, P. vivax and P. yoelii LDH and native P. falciparum and P. yoelii LDH in western blots and immunofluorescence studies. The pan-malarial antibody detected LDH from the three malaria species in western blots. The species-specific anti-peptide antibodies differentiated between P. falciparum and P. vivax LDH. Affinity purified chicken antibodies against recombinant PfLDH, PvLDH and PyLDH proteins each detected the parent and orthologous proteins with similar titers in an ELISA. The study supports an anti-peptide antibody approach to the development of diagnostic reagents.