F1 antigenaemia in bubonic plague patients, a marker of gravity and efficacy of therapy

Point-of-care vertical flow immunoassay system for ultra-sensitive multiplex biothreat-agent detection in biological fluids

This paper presents simple, fast, and sensitive detection of multiple biothreat agents by paper-based vertical flow colorimetric sandwich immunoassay for detection of Yersinia pestis (LcrV and F1) and Francisella tularensis (lipopolysaccharide; LPS) antigens using a vertical flow immunoassay (VFI) prototype with portable syringe pump and a new membrane holder. The capture antibody (cAb) printing onto nitrocellulose membrane and gold-labelled detection antibody (dAb) were optimized to enhance the assay sensitivity and specificity. Even though the paper pore size was relaxed from previous 0.1 μm to the current 0.45 μm for serum samples, detection limits as low as 0.050 ng/mL for LcrV and F1, and 0.100 ng/mL for FtLPS have been achieved in buffer and similarly in diluted serum (with LcrV and F1 LODs remained the same and LPS LOD reduced to 0.250 ng/mL). These were 40, 80, and 50X (20X for LPS in serum) better than those from lateral flow configuration. Furthermore, the comparison of multiplex format demonstrated low cross-reactivity and equal sensitivity to that of the singleplex assay. The optimized VFI platform thus provides a portable and rapid on-site monitoring system for multiplex biothreat detection with the potential for high sensitivity, specificity, reproducibility, and multiplexing capability, supporting its utility in remote and resource-limited settings.

Development of a dual antigen lateral flow immunoassay for detecting Yersinia pestis

Background

Yersinia pestis is the causative agent of plague, a zoonosis associated with small mammals. Plague is a severe disease, especially in the pneumonic and septicemic forms, where fatality rates approach 100% if left untreated. The bacterium is primarily transmitted via flea bite or through direct contact with an infected host. The 2017 plague outbreak in Madagascar resulted in more than 2,400 cases and was highlighted by an increased number of pneumonic infections. Standard diagnostics for plague include laboratory-based assays such as bacterial culture and serology, which are inadequate for administering immediate patient care for pneumonic and septicemic plague.

Principal findings

The goal of this study was to develop a sensitive rapid plague prototype that can detect all virulent strains of Y. pestis. Monoclonal antibodies (mAbs) were produced against two Y. pestis antigens, low-calcium response V (LcrV) and capsular fraction-1 (F1), and prototype lateral flow immunoassays (LFI) and enzyme-linked immunosorbent assays (ELISA) were constructed. The LFIs developed for the detection of LcrV and F1 had limits of detection (LOD) of roughly 1–2 ng/mL in surrogate clinical samples (antigens spiked into normal human sera). The optimized antigen-capture ELISAs produced LODs of 74 pg/mL for LcrV and 61 pg/mL for F1 when these antigens were spiked into buffer. A dual antigen LFI prototype comprised of two test lines was evaluated for the detection of both antigens in Y. pestis lysates. The dual format was also evaluated for specificity using a small panel of clinical near-neighbors and other Tier 1 bacterial Select Agents.

Conclusions

LcrV is expressed by all virulent Y. pestis strains, but homologs produced by other Yersinia species can confound assay specificity. F1 is specific to Y. pestis but is not expressed by all virulent strains. Utilizing highly reactive mAbs, a dual-antigen detection (multiplexed) LFI was developed to capitalize on the diagnostic strengths of each target.

Epitope Binning of Novel Monoclonal Anti F1 and Anti LcrV Antibodies and Their Application in a Simple, Short, HTRF Test for Clinical Plague Detection

Mouse monoclonal antibodies were raised against plague disease biomarkers: the bacterial capsular protein fraction 1 (F1) and the low-calcium response—LcrV virulence factor (Vag). A novel tandem assay, employing BioLayer Interferometry (BLI), enabled the isolation of antibodies against four different epitopes on Vag. The tandem assay was carried out with hybridoma supernatants, circumventing the need for antibody purification. The BioLayer assay was further adopted for characterization of epitope-repetitive antigens, enabling the discovery of two unique epitopes on F1. The selected antibodies were purified and applied as “oligo-clonal” reagents for the immuno-detection of both biomarkers. The developed Homogenous Time Resolved Fluorescence (HTRF) tests were short (10 min) and simple (no washing steps), allowing for detection of 10 ng/mL F1 and 2.5 ng/mL Vag. The tests were successfully applied for detection of disease biomarkers produced by various Y. pestis strains during growth in blood culture vials.

Respiratory Infections

Respiratory tract infections (RTIs) are a common health problem of international travelers. Travelers may be at increased risk of RTIs due to travel itself (mingling and close quarters in airports, airplanes, cruise ships, and hotels), and due to unique exposure at travel destinations. The clinical spectrum of RTIs in travelers is broad and includes upper RTIs, pharyngitis, otitis, laryngitis, bronchitis, and pneumonia. Most travelers who acquire an RTI only develop mild disease, and only a minority seek medical attention. All travelers should be up to date on any indicated vaccines based on age and medical condition that prevent RTIs, including influenza, measles, pneumococcal diseases, Haemophilus influenzae b, Neisseria meningitidis, diphtheria, and pertussis.

•

Respiratory tract infections (RTIs) are among the most common illnesses reported by travelers. Most RTIs are viral, involve the upper respiratory tract, and do not require specific diagnosis or treatment.

•

Influenza is often considered the most important travel-related infection. Travelers play an integral role in the yearly and global spread of influenza.

•

Lower RTIs, including pneumonia, often require antimicrobial therapy.

•

High-risk groups such as infants, small children, the elderly, and subjects with chronic tracheobronchial or pulmonary disease are at increased risk of developing severe clinical consequences should infection occur. All international travelers should be immunized for seasonal influenza unless otherwise contraindicated, and travelers should be instructed in hand hygiene and sneeze and cough hygiene.

•

All travelers should be up to date on any indicated vaccines that prevent RTIs, including measles, pneumococcal diseases, Haemophilus influenzae b (Hib), meningococcal disease, diphtheria, and pertussis.

•

Travelers may be at increased risk of geographically restricted RTIs, and clinicians should be familiar with the major manifestations of these illnesses.

Rapid and sensitive detection of Yersinia pestis by lateral-flow assay in simulated clinical samples

Background

Yersinia pestis is a contributing agent to the epidemic disease, plague, which killed an estimated 200 million people during historical times. In this study, a rapid, cheap, sensitive, and specific technique, the lateral flow assay (F1 strips), has been successfully developed to detect this pathogen, by using paired monoclonal antibodies (MAbs) against Y. pestis capsule like fraction 1 (F1) protein. Compared with the polyclonal antibody (PAb) based F1 strips, the Mab-based F1 strips have a remarkable increased detection limitation (10 to 100 folds). Furthermore, besides the limitation and specificity evaluation, the application of this F1 strip on simulated clinical samples indicate the LFA can be a good candidate to detect plague.

Methods

Recombinant F1 antigen was expressed and purified from a series of works. The various anti-F1 monoclonal antibodies generated from hybridoma cells were screened with the ELISA technique. To evaluate the feasibility of this Y. pestis F1 test strip, the F1 protein/Y. pestis was spiked into simulated clinical samples such as human serum, mouse bronchoalveolar lavage fluids, and mouse blood to mimic natural infection status. Additionally, this technique was applied to detect the Y. pestis in the environment-captured rats, to evaluate the practical usefulness of the strips.

Results

By using this MAb-based-LFA technique, 4 ng/ml of recombinant F1-protein and 10³ CFU/ml of Y. pestis could be detected in less than 10 mins, which is at least 10-folds than that of the PAb format. On the other hand, although various Yersinia strains were applied to the strips, only Y. pestis strain showed a positive result; all other Yersinia species did not produce a positive signal, indicating the high efficiency and specificity of the MAb-based F1-strips.

Conclusion

Based on our findings, we suggest that the MAb-format-LFA will be valuable as a diagnostic tool for the detection of Y. pestis. This report shows that the F1 strip is sufficient to support not only the detection of plague in simulated clinical samples, but also it may be a good candidate to meet the epidemiological surveillance during an outbreak of the biological warfare.

Rapid detection of Yersinia pestis recombinant fraction 1 capsular antigen

Yersinia pestis, an infectious bacterium that is a causative agent of plague, a disease which has been shown to be one of the most feared in history and which has caused millions of deaths. The capsule-like fraction 1 (F1) antigen expressed by Y. pestis is a known specific marker for the identification of the bacteria; therefore, the detection of F1 is important for Y. pestis recognition. In this study, a rapid, sensitive, and specific technique, the lateral flow assay (LFA), was successfully developed to detect Y. pestis by the recombinant F1 antigen. The assay that utilized an anti-F1 polyclonal antibody (Pab) to identify the bacteria was based on a double-antibody sandwich format on a nitrocellulose membrane. With the LFA method, 50 ng/ml of recombinant F1 protein and 10(5) CFU/mL of Y. pestis could be detected in less than 10 min. This assay also showed no cross-reaction with other Yersinia spp. or with some selected capsule-producing Enterobacteriaceae strains. Furthermore, detection of Y. pestis in simulated samples has been evaluated. The detection sensitivity of Y. pestis in various matrices was 10(5) CFU/mL, which was identical to that in PBS buffer. The results obtained suggest that LFA is an excellent tool for detection of Y. pestis contamination in an environment and hence can be used to monitor plague diseases when they emerge.

Detection and Isolation of Yersinia pestis Without Fraction 1 Antigen by Monoclonal Antibody in Foods and Water

Most available immunoassays for Yersinia pestis are based on the detection of fraction 1 antigen (F1) when yersiniae are grown at 37°C. A monoclonal antibody (MAb) was developed based on the detection of surface antigens that are not F1. F1-deficient Y. pestis cells were induced and used to immunize BALB/c mice from which MAb (immunoglobulin G1), which specifically recognizes Y. pestis, with or without F1, was obtained. This MAb (6B5) did not cross-react with enteric bacteria, including Yersinia enterocolitica. Enzyme-linked immunosorbent assay results revealed that MAb 6B5 is specific for Y. pestis, with the exception of a minor cross-reaction with Yersinia pseudotuberculosis. Western immunoblot analysis revealed that MAb 6B5 recognizes a Y. pestis outer membrane protein of ca. 30 kDa. Magnetic beads that were coated with MAb 6B5 were compared with beads coated with polyclonal antibody (PAb; rabbit) against Y. pestis for the isolation of Y. pestis in food and water samples by using a PATHATRIX cell concentration apparatus. Enrichment cultures of Y. pestis in different foods by using two different times (6 and 24 h) in brain heart infusion broth at 37°C were evaluated. Results revealed MAb 6B5–coated magnetic beads were equivalent to magnetic beads coated with PAb against Y. pestis A1122 whole cells in concentrating Y. pestis for isolation, especially when samples were enriched for 6 h. However, the selectivity for Y. pestis of the magnetic beads coated with MAb 6B5 was greater than that coated with PAb.

Can sputum collection practices for tuberculosis improve laboratory diagnosis of pneumonic plague in developing countries?

Africa reports 90% of human plague cases yet no guidelines currently exist for the safe and accurate collection of sputum from patients with suspected pneumonic plague. Using existing tuberculosis guidelines as a model and in conjunction with epidemiological and clinical features of pneumonic plague, the authors describe the importance of formal guidelines to enhance sputum collection practices and improve safety in developing countries. A model for suggested guidelines is given.

Characterization of an F1 deletion mutant of Yersinia pestis CO92, pathogenic role of F1 antigen in bubonic and pneumonic plague, and evaluation of sensitivity and specificity of F1 antigen capture-based dipsticks

We evaluated two commercial F1 antigen capture-based immunochromatographic dipsticks, Yersinia Pestis (F1) Smart II and Plague BioThreat Alert test strips, in detecting plague bacilli by using whole-blood samples from mice experimentally infected with Yersinia pestis CO92. To assess the specificities of these dipsticks, an in-frame F1-deficient mutant of CO92 (Δcaf) was generated by homologous recombination and used as a negative control. Based on genetic, antigenic/immunologic, and electron microscopic analyses, the Δcaf mutant was devoid of a capsule. The growth rate of the Δcaf mutant generally was similar to that of the wild-type (WT) bacterium at both 26 and 37 °C, although the mutant's growth dropped slightly during the late phase at 37 °C. The Δcaf mutant was as virulent as WT CO92 in the pneumonic plague mouse model; however, it was attenuated in developing bubonic plague. Both dipsticks had similar sensitivities, requiring a minimum of 0.5 μg/ml of purified F1 antigen or 1 × 10(5) to 5 × 10(5) CFU/ml of WT CO92 for positive results, while the blood samples were negative for up to 1 × 10(8) CFU/ml of the Δcaf mutant. Our studies demonstrated the diagnostic potential of two plague dipsticks in detecting capsular-positive strains of Y. pestis in bubonic and pneumonic plague.

The Yersinia pestis caf1M1A1 fimbrial capsule operon promotes transmission by flea bite in a mouse model of bubonic plague

Plague is a zoonosis transmitted by fleas and caused by the gram-negative bacterium Yersinia pestis. During infection, the plasmidic caf1M1A1 operon that encodes the Y. pestis F1 protein capsule is highly expressed, and anti-F1 antibodies are protective. Surprisingly, the capsule is not required for virulence after injection of cultured bacteria, even though it is an antiphagocytic factor and capsule-deficient Y. pestis strains are rarely isolated. We found that a caf-negative Y. pestis mutant was not impaired in either flea colonization or virulence in mice after intradermal inoculation of cultured bacteria. In contrast, absence of the caf operon decreased bubonic plague incidence after a flea bite. Successful development of plague in mice infected by flea bite with the caf-negative mutant required a higher number of infective bites per challenge. In addition, the mutant displayed a highly autoaggregative phenotype in infected liver and spleen. The results suggest that acquisition of the caf locus via horizontal transfer by an ancestral Y. pestis strain increased transmissibility and the potential for epidemic spread. In addition, our data support a model in which atypical caf-negative strains could emerge during climatic conditions that favor a high flea burden. Human infection with such strains would not be diagnosed by the standard clinical tests that detect F1 antibody or antigen, suggesting that more comprehensive surveillance for atypical Y. pestis strains in plague foci may be necessary. The results also highlight the importance of studying Y. pestis pathogenesis in the natural context of arthropod-borne transmission.

Comparison of hand-held test kits, immunofluorescence microscopy, enzyme-linked immunosorbent assay, and flow cytometric analysis for rapid presumptive identification of Yersinia pestis

An in-house immunochromatographic test, Plague BioThreat Alert test strips, ABICAP columns, enzyme-linked immunosorbent assay, flow cytometry, and immunofluorescence microscopy were compared for the detection of the fraction 1 capsular antigen of Yersinia pestis, using spiked buffer and clinical specimens. Hand-held test kits proved to be excellent benchtop tools.

Evaluation of a standardized F1 capsular antigen capture ELISA test kit for the rapid diagnosis of plague

Rapid detection of soluble F1 capsular antigen in serum, bubo fluid or urine of patients proved to be a valuable tool in the presumptive diagnosis of plague. We evaluated a F1 capsular antigen capture ELISA resembling a commercially available test kit. The minimal detectable concentration was 4 ng/ml. The specificity was 100% when investigating 47 sera from healthy Malagasy subjects and 98.4% when 365 sera from German blood donors were studied. Sensitivity was determined on sera (n=11) and buboes (n=18) from bacteriologically confirmed Malagasy plague patients. Sensitivity was 90.1% for serum and 100% for buboes. A standardized F1 capsular antigen capture ELISA test kit might be well suited for the early detection of plague particularly in non-endemic areas where clinical microbiological laboratories have only limited access to alternative techniques for rapid identification of Yersinia pestis.

Serodiagnosis of human plague by a combination of immunomagnetic separation and flow cytometry

BACKGROUND

Plague is a severe, highly communicable bacterial disease caused by Yersinia pestis. It is still endemic in more than 20 countries worldwide. Although known as a devastating disease for centuries, laboratory confirmation of clinical suspected cases is still problematic. No standardized and internationally approved test system is commercially available. The aim of this study was the introduction and evaluation of a combination of immunomagnetic separation and flow cytometry for the serodiagnosis of human plague.

METHODS

Paramagnetic polystyrene beads were coated with purified F1 capsular antigen (F1 CA) and reacted with sera from plague patients, from 26 laboratory personnel vaccinated against plague and from 102 healthy blood donors (HBD). After incubation with fluorescein isothiocyanate-conjugated anti-human rabbit IgG, particle-associated fluorescence was detected by flow cytometry.

RESULTS

Anti-F1 CA antibodies could be demonstrated in all patients with bacteriologically confirmed plague and in 22 sera (84.6%) from vaccinees. Only one serum in the HBD group showed a weakly positive reaction. The total assay time was less than 2 h.

CONCLUSIONS

Compared with a recently published combination of an anti-F1 CA enzyme-linked immunosorbent assay (ELISA) and immunoblot, the new assay showed the same sensitivity as the ELISA and almost the same specificity (99.0 versus 100%) as the immunoblot. Allowing a rapid, reliable, and quantitative analysis, immunomagnetic separation combined with flow cytometry might replace both conventional immunoassays.

Development and testing of a rapid diagnostic test for bubonic and pneumonic plague

BACKGROUND

Plague is often fatal without prompt and appropriate treatment. It affects mainly poor and remote populations. Late diagnosis is one of the major causes of human death and spread of the disease, since it limits the effectiveness of control measures. We aimed to develop and assess a rapid diagnostic test (RDT) for plague.

METHODS

We developed a test that used monoclonal antibodies to the F1 antigen of Yersinia pestis. Sensitivity and specificity were assessed with a range of bacterial cultures and clinical samples, and compared with findings from available ELISA and bacteriological tests for plague. Samples from patients thought to have plague were tested with the RDT in the laboratory and by health workers in 26 pilot sites in Madagascar.

FINDINGS

The RDT detected concentrations of F1 antigen as low as 0.5 ng/mL in up to 15 min, and had a shelf life of 21 days at 60 degrees C. Its sensitivity and specificity were both 100%. RDT detected 41.6% and 31% more positive clinical specimens than did bacteriological methods and ELISA, respectively. The agreement rate between tests done at remote centres and in the laboratory was 89.8%. With the combination of bacteriological methods and F1 ELISA as reference standard, the positive and negative predictive values of the RDT were 90.6% and 86.7%, respectively.

INTERPRETATION

Our RDT is a specific, sensitive, and reliable test that can easily be done by health workers at the patient's bedside, for the rapid diagnosis of pneumonic and bubonic plague. This test will be of key importance for the control of plague in endemic countries.

Epidemiologic features of four successive annual outbreaks of bubonic plague in Mahajanga, Madagascar

From 1995 to 1998, outbreaks of bubonic plague occurred annually in the coastal city of Mahajanga, Madagascar. A total of 1,702 clinically suspected cases of bubonic plague were reported, including 515 laboratory confirmed by Yersinia pestis isolation (297), enzyme-linked immunosorbent assay, or both. Incidence was higher in males and young persons. Most buboes were inguinal, but children had a higher frequency of cervical or axillary buboes. Among laboratory-confirmed hospitalized patients, the case-fatality rate was 7.9%, although all Y. pestis isolates were sensitive to streptomycin, the recommended antibiotic. In this tropical city, plague outbreaks occur during the dry and cool season. Most cases are concentrated in the same crowded and unsanitary districts, a result of close contact among humans, rats, and shrews. Plague remains an important public health problem in Madagascar, and the potential is substantial for spread to other coastal cities and abroad.

Early diagnosis of bubonic plague using F1 antigen capture ELISA assay and rapid immunogold dipstick

Plague is still prevalent in more than 20 countries. Two F1 antigen diagnostic assays (an immunocapture ELISA and an immunogold chromatography dipstick) were evaluated using bubo aspirates, serum and urine specimens from patients suspected with plague. The specificity of the two F1 assays was found 100%. Using bacteriology as a gold reference diagnostic assay, 52 patients were Yersinia pestis culture positive and 141 negative. The sensitivity of the F1 ELISA test was 100% in bubo, 52% in serum and 58% in urine specimens. In culture negative patients, the F1 antigen could be found in 10% bubo aspirates, 5% serum and 7% urine specimens of culture negative patients for whom a seroconversion for anti-F1 antibodies was also observed. The sensitivity of the dipstick assay was 98% on bubo aspirates specimens. Compared to the ELISA test, the agreement rate was 97.5% and the correlation coefficient tau = 0.90 (p < 10(-3)). In conclusion, the diagnosis of bubonic plague has to be performed on bubo fluid rather than on serum or urine specimens. Both the F1 ELISA and the dipstick assays are valuable tools for an early diagnosis and for the surveillance of plague.

Epidemiological and diagnostic aspects of the outbreak of pneumonic plague in Madagascar

BACKGROUND

Plague is a re-emerging disease and pneumonic plague is the most feared clinical form. We describe a well-documented outbreak of pneumonic plague in Madagascar.

METHODS

Field epidemiological data were collected. Biological tests (microscopy, culture of Yersinia pestis, F1 antigen ELISA and dipstick assays, IgG anti-F1 ELISA) were done on sputum, serum, or necropsy samples. The infection rate among 154 contacts was assessed by anti-F1 serological techniques.

FINDINGS

The index case was a bubonic patient with a secondary lung infection, who contaminated a traditional healer and his family. Funeral ceremonies and attendance on patients contaminated other villagers. In total 18 cases were recorded, and eight died. F1 antigen could be detected in sputum by ELISA and dipstick tests as early as the second day after the onset of the symptoms and also 48 h after treatment. Among the contact population 13 of 154 (8.4%) have been exposed to the plague bacillus (symptomless or latent infections).

INTERPRETATION

The F1 dipstick assay on sputum is an invaluable diagnostic tool for pneumonic plague. Treatment of patients and chemoprophylaxis of contacts were efficient in stopping the epidemic.

Diagnosis of bubonic plague by PCR in Madagascar under field conditions

The diagnostic value of a PCR assay that amplifies a 501-bp fragment of the Yersinia pestis caf1 gene has been determined in a reference laboratory with 218 bubo aspirates collected from patients with clinically suspected plague managed in a regional hospital in Madagascar. The culture of Y. pestis and the detection of the F1 antigen (Ag) by enzyme-linked immunosorbent assay (ELISA) were used as reference diagnostic methods. The sensitivity of PCR was 89% (57 of 64) for the Y. pestis-positive patients, and 80.7% (63 of 78) for the F1 Ag-positive patients. The specificity of PCR for the culture-, F1 Ag-, and antibody-negative patients (n = 105) was 100%. Because in Madagascar most patients with plague are managed and their clinical samples are collected in remote villages, the usefulness of PCR was evaluated for routine diagnostic use in the operational conditions of the control program. The sensitivity of PCR was 50% (25 of 50) relative to the results of culture and 35.2% (19 of 54) relative to the results of the F1 Ag immunocapture ELISA. The specificity of PCR under these conditions was 96%. In conclusion, the PCR method was found to be very specific but not as sensitive as culture or the F1 Ag detection method. The limitation in sensitivity may have been due to suboptimal field conditions and the small volumes of samples used for DNA extraction. This technique is not recommended as a routine diagnostic test for plague in Madagascar.

Current epidemiology of human plague in Madagascar

From 1996 to 1998, 5,965 patients with suspected plague were identified in 38 districts of Madagascar (40% of the total population are exposed). Using standard bacteriology, 917 of them were confirmed or presumptive (C + P) cases. However, more than 2,000 plague cases could be estimated using F1 antigen assay. Two out of the 711 Yersinia pestis isolates tested were resistant to chloramphenicol and to ampicillin (both isolates found in the harbour of Mahajanga). Urban plague (Mahajanga harbour and Antananarivo city) accounted for 37.4% of the C + P cases. Bubonic plague represented 97.2% of the cases, and the lethality rate was still high (20%). In comparing the exposed population, plague was more prevalent in males (M:F sex ratio 1.3:1) and patients under 20 years (2.7% babies under two years). Buboes were mainly localised in the inguinal/femoral regions (55.8%). The epidemiological risk factors are discussed.

The V-antigen of Yersinia is surface exposed before target cell contact and involved in virulence protein translocation

Type III-mediated translocation of Yop effectors is an essential virulence mechanism of pathogenic Yersinia. LcrV is the only protein secreted by the type III secretion system that induces protective immunity. LcrV also plays a significant role in the regulation of Yop expression and secretion. The role of LcrV in the virulence process has, however, remained elusive on account of its pleiotropic effects. Here, we show that anti-LcrV antibodies can block the delivery of Yop effectors into the target cell cytosol. This argues strongly for a critical role of LcrV in the Yop translocation process. Additional evidence supporting this role was obtained by genetic analysis. LcrV was found to be present on the bacterial surface before the establishment of bacteria target cell contact. These findings suggest that LcrV serves an important role in the initiation of the translocation process and provides one possible explanation for the mechanism of LcrV-induced protective immunity.