Serological reactivity to Mycobacterium bovis protein antigens in cattle

Identification of New Mycobacterium bovis antigens and development of a multiplexed serological bead-immunoassay for the diagnosis of bovine tuberculosis in cattle

Serological assays for bovine tuberculosis diagnosis require the use of multiple Mycobacterium bovis specific antigens to ensure the detection of infected animals. In the present study, identification and selection process of antigens, based on data from published proteomic studies and involving the use of bioinformatics tools and an immuno-screening step, was firstly performed for identifying novel antigens that elicit an antibody response in M. bovis infection. Based on this approach, a panel of 10 M. bovis antigens [with known relevance (MPB70, MPB83, MPB70/83, and ESAT6/CFP10) and novel (Mb1961c, Mb1301c, Mb3871, Mb1403, Mb0592, and PE25/PPE41)] were constructed and thenused to develop a new multiplexed serological assay based on Luminex technology. The performance of the Luminex-bTB immunoassay was evaluated using sera from cattle with known tuberculosis status. Among the proteins whose ability to detect bovine tuberculosis was evaluated for the first time, PE25/PPE41 and Mb1403, but not Mb3871, showed good detection capacity. Following multiple antigen combination, the final Luminex-bTB immunoassay included seven antigens (MPB70, MPB83, MPB70/83, ESAT6/CFP10, PE25/PPE41, Mb1403, and Mb0592) and showed better global performance than the immunoassay using the four usual antigens (MPB70, MPB70/83, MPB83 and ESAT6/CFP10). The specificity and sensitivity values were, respectively, of 97.6% and 42.8% when the cut-off of two-positive antigens was used to classify samples as positive. With the use of the more-restrictive criterion of three-positive antigens, the specificity increased to 99.2% but the sensitivity decreased to 23.9%. The analysis of antigen profiles generated with the Luminex-bTB immunoassay showed that mainly serodominant proteins were recognized in samples from infected cattle. The detection of Mb1961c and Mb1301c appeared to be associated with presumed false-positive results. Moreover, sera from cattle originating from bTB-outbreaks but having inconclusive or negative skin test results were identified as positive by the Luminex-bTB immunoassay and showed an antigen pattern associated with M. bovis infection. The Luminex-bTB immunoassay including seven antigens may be useful as adjunct test for the detection of M. bovis-infected herds, and different cut-offs could be applied according to the bovine tuberculosis epidemiological context.

Performance of two commercial serological assays for bovine tuberculosis using plasma samples

In the bovine tuberculosis diagnosis, the use of plasma samples (already available for IFNɣ assays) in serological tests might facilitate the work in the field. Here, the performance of two commercial serological tests (ELISA IDEXX M. bovis Ab test and Enferplex Bovine TB antibody test) were evaluated using plasma samples from cattle in Belgium. Specificity values estimated from 567 plasma samples collected from bTB-free cattle were 98.4% when using the ELISA IDEXX M. bovis Ab test, and were 96.5% and 93.3% when using the high specificity and high sensitivity settings of the Enferplex Bovine TB antibody test, respectively. Sensitivity values were calculated relative to SICCT-positive (N = 117) and IFNɣ-positive (N = 132) animals originating from M. bovis-infected herds. Overall, the multiplexed Enferplex Bovine TB antibody test had better sensitivity (mean: 32.5% and 43.4% for the high specificity and sensitivity settings, respectively) compared to the ELISA IDEXX M. bovis Ab test (mean: 12%). Data obtained from plasma samples in the current study were compared to a previous study using both serological tests with sera. In conclusion, both serological tests showed comparable performance with both matrix; although overall specificity values with the Enferplex Bovine TB antibody test were lower when using plasma samples than sera.

Field evaluation of two commercial serological assays for detecting bovine tuberculosis

Diagnosis of bovine tuberculosis in cattle is challenging due to complex immune host response to infection that limit the performance of available diagnostic tests. In this study, performance of two commercial serological assays developed to detect bovine tuberculosis were evaluated: Enferplex Bovine TB antibody kit including 11 antigens (EnferGroup, Ireland) and IDEXX M. bovis Ab kit (IDEXX, USA). The specificity value obtained with the ELISA IDEXX M. bovis Ab test was 97.1%, whereas it was 97.1% and 95.1% for the high specificity and sensitivity settings, respectively, with the Enferplex Bovine TB antibody kit. The sensitivity of the multiplexed Enferplex Bovine TB antibody test for SICCT-positive animals was higher (N = 172; 51.7% and 58.7% with high specificity and sensitivity settings, respectively) compared to the ELISA IDEXX M. bovis Ab test (sensitivity of 36.6%). "Antigen profiles" generated by the multiplexed Enferplex method showed that five out of 11 antigens present in the test were mostly identified as positive sera in cattle originating from bTB-outbreaks. In comparison, unique profiles appeared to be correlated with false positive results. However additional studies are needed to confirm the observed antigen profiles, and their potential use as an additional diagnostic tool. Serial interpretation of the two serological tests produced higher diagnostic specificity (>99%), reducing false positive results, which is essential for a screening test when the prevalence of bovine tuberculosis is low.

Diagnosis of tuberculosis in wildlife: a systematic review

Animal tuberculosis (TB) is a multi-host disease caused by members of the Mycobacterium tuberculosis complex (MTC). Due to its impact on economy, sanitary standards of milk and meat industry, public health and conservation, TB control is an actively ongoing research subject. Several wildlife species are involved in the maintenance and transmission of TB, so that new approaches to wildlife TB diagnosis have gained relevance in recent years. Diagnosis is a paramount step for screening, epidemiological investigation, as well as for ensuring the success of control strategies such as vaccination trials. This is the first review that systematically addresses data available for the diagnosis of TB in wildlife following the Preferred Reporting Items of Systematic Reviews and Meta-Analyses (PRISMA) guidelines. The article also gives an overview of the factors related to host, environment, sampling, and diagnostic techniques which can affect test performance. After three screenings, 124 articles were considered for systematic review. Literature indicates that post-mortem examination and culture are useful methods for disease surveillance, but immunological diagnostic tests based on cellular and humoral immune response detection are gaining importance in wildlife TB diagnosis. Among them, serological tests are especially useful in wildlife because they are relatively inexpensive and easy to perform, facilitate large-scale surveillance and can be used both ante- and post-mortem. Currently available studies assessed test performance mostly in cervids, European badgers, wild suids and wild bovids. Research to improve diagnostic tests for wildlife TB diagnosis is still needed in order to reach accurate, rapid and cost-effective diagnostic techniques adequate to a broad range of target species and consistent over space and time to allow proper disease monitoring.

Comparing recombinant MPB70/SahH and native 20-kDa protein for detecting bovine tuberculosis using ELISA

Bovine tuberculosis (bTB) is a zoonosis caused by Mycobacterium bovis. Test-and-cull protocols and gross pathological examinations of abattoir animals as well as milk pasteurization have been implemented to prevent the spread of tuberculosis from animals to humans worldwide. Despite the importance of precise and rapid diagnostic tests, conventional methods including intradermal skin tests and γ-interferon assays are limited by the high rate of false-negative results for cattle in the late infectious stage and due to laborious and time-consuming procedures. Therefore, antibody detection methods such as enzyme-linked immunosorbent assay (ELISA) are urgently needed to supplement the established approaches and expand the diagnostic window. This study was conducted to develop a bTB ELISA by evaluating recombinant and native proteins and various assay parameters. We produced recombinant MPB70 and SahH (M70S) and a native 20-kDa protein (20K) and optimized the ELISA protocol. The 20K ELISA showed 94.4% sensitivity and 98.2% specificity with an optimal sample-to-positive ratio cut-off of 0.531. The sensitivity and specificity of M70S ELISA were 94.4% and 97.3%, respectively, with an optimal sample-to-negative ratio cut-off of 1.696. Both assays showed acceptable diagnostic efficiency and could be used for bTB diagnosis in combination with established methods for herd screening and to expand the diagnostic window.

Mycobacterium tuberculosis complex in wildlife: Review of current applications of antemortem and postmortem diagnosis

Tuberculosis (TB) is a chronic inflammatory and zoonotic disease caused by Mycobacterium tuberculosis complex (MTBC) members, which affects various domestic animals, wildlife, and humans. Some wild animals serve as reservoir hosts in the transmission and epidemiology of the disease. Therefore, the monitoring and surveillance of both wild and domestic hosts are critical for prevention and control strategies. For TB diagnosis, the single intradermal tuberculin test or the single comparative intradermal tuberculin test, and the gamma-interferon test, which is regarded as an ancillary test, are used. Postmortem examination can identify granulomatous lesions compatible with a diagnosis of TB. In contrast, smears of the lesions can be stained for acid-fast bacilli, and samples of the affected organs can be subjected to histopathological analyses. Culture is the gold standard test for isolating mycobacterial bacilli because it has high sensitivity and specificity compared with other methods. Serology for antibody detection allows the testing of many samples simply, rapidly, and inexpensively, and the protocol can be standardized in different laboratories. Molecular biological analyses are also applicable to trace the epidemiology of the disease. In conclusion, reviewing the various techniques used in MTBC diagnosis can help establish guidelines for researchers when choosing a particular diagnostic method depending on the situation at hand, be it disease outbreaks in wildlife or for epidemiological studies. This is because a good understanding of various diagnostic techniques will aid in monitoring and managing emerging pandemic threats of infectious diseases from wildlife and also preventing the potential spread of zoonotic TB to livestock and humans. This review aimed to provide up-to-date information on different techniques used for diagnosing TB at the interfaces between wildlife, livestock, and humans.

Current ante-mortem techniques for diagnosis of bovine tuberculosis

Bovine tuberculosis (TB), mainly caused by Mycobacterium bovis, is a zoonotic disease with implications for Public Health and having an economic impact due to decreased production and limitations to the trade. Bovine TB is subjected to official eradication campaigns mainly based on a test and slaughter policy using diagnostic assays based on the cell-mediated immune response as the intradermal tuberculin test and the gamma-interferon (IFN-γ) assay. Moreover, several diagnostic assays based on the detection of specific antibodies (Abs) have been developed in the last few years with the aim of complementing the current diagnostic techniques in the near future. This review provides an overview of the current ante-mortem diagnostic tools for diagnosis of bovine TB regarding historical background, methodologies and sensitivity (Se) and specificity (Sp) obtained in previous studies under different epidemiological situations.

Screening of recombinant proteins as antigens in indirect ELISA for diagnosis of bovine tuberculosis

Bovine tuberculosis is an important infectious disease caused by Mycobacterium bovis, which is responsible for considerable economic losses. This disease constitutes a serious public health problem. Control programs in most countries, including Brazil, are based on the identification and slaughter of infected animals, as defined by the skin tuberculin test, which has its constraints. In the present study, the recombinant proteins CFP-10, ESAT-6, Mb0143, MPB83, PE5, PE13, TB10.4, TB15.3 and a chimera of ESAT-6/MPB70/MPB83 (fusion protein) were tested as ELISA antigens for the diagnosis of bovine tuberculosis. The proteins were produced in Escherichia coli, purified and tested in ELISAs with sera from 126 cattle having tested negative in the comparative intradermal tuberculin test (CITT) and 107 sera from cattle having tested positive in the CITT. Also, 236 sera from two BTB-free beef cattle herds were tested. Among the proteins tested, only the ESAT-6/MPB70/MPB83 chimera demonstrated satisfactory agreement with the CITT (kappa index: 0.688), reflecting in 83.2% sensitivity and 86.5% specificity. The ELISA absorbances of the cattle sera from BTB-free herds showed similar levels to those of CITT positive cattle, probably as the result of successive skin tuberculinizations to define the BTB-free status of the herds. However, the ELISA with the ESAT-6/MPB70/MPB83 chimera was useful to discriminate BTB positive and negative cattle in herds prior to the tuberculin skin test.

Bovine tuberculosis: a review of current and emerging diagnostic techniques in view of their relevance for disease control and eradication

Existing strategies for long-term bovine tuberculosis (bTB) control/eradication campaigns are being reconsidered in many countries because of the development of new testing technologies, increased global trade, continued struggle with wildlife reservoirs of bTB, redistribution of international trading partners/agreements, and emerging financial and animal welfare constraints on herd depopulation. Changes under consideration or newly implemented include additional control measures to limit risks with imported animals, enhanced programs to mitigate wildlife reservoir risks, re-evaluation of options to manage bTB-affected herds/regions, modernization of regulatory framework(s) to re-focus control efforts, and consideration of emerging testing technologies (i.e. improved or new tests) for use in bTB control/eradication programs. Traditional slaughter surveillance and test/removal strategies will likely be augmented by incorporation of new technologies and more targeted control efforts. The present review provides an overview of current and emerging bTB testing strategies/tools and a vision for incorporation of emerging technologies into the current control/eradication programs.

Accuracy of three diagnostic tests for determining Mycobacterium bovis infection status in live-sampled wild meerkats (Suricata suricatta)

Accurate diagnosis of Mycobacterium bovis infection (bovine tuberculosis [bTB]) in live animals is notoriously problematic. The aim of the present study was to evaluate the diagnostic potential of 2 new serologic tests (multiantigen print immunoassay [MAPIA] and lateral flow immunoassay rapid test [RT]) in comparison with mycobacterial culture of tracheal washes for determining M. bovis infection status in a free-ranging population of wild meerkats (Suricata suricatta). During a longitudinal study lasting 2.5 years, 240 individually identifiable meerkats were each sampled up to 8 times under anesthesia every 3 months. Diagnostic accuracy was determined through Bayesian and maximum likelihood estimations of sensitivity, specificity, and likelihood ratios for each diagnostic test when used independently and in parallel to classify the disease status of individual meerkats in the absence of a gold standard. Culture of tracheal washes was highly specific (0.99; 95% confidence interval [CI] = 0.77, 1.00) but of low sensitivity (0.36; 95% CI = 0.24, 0.50) for diagnosing M. bovis-infected individuals. The longitudinal nature of the study with repeated sampling of the same individual animals served simultaneously to improve chances of detecting infection and increase confidence in a negative result in individual animals repeatedly testing negative. Although MAPIA and RT were individually of limited diagnostic use, interpreting the results of these 2 tests in parallel produced estimates of sensitivity (0.83; 95% CI = 0.67, 0.93) and specificity (0.73; 95% CI = 0.62, 0.82) high enough to usefully inform decision making when determining exposure to bTB in wild meerkats and potentially other species in which bTB poses a diagnostic challenge.

Antibody responses of cervids (Cervus elaphus) following experimental Mycobacterium bovis infection and the implications for immunodiagnosis

Captive and free-ranging wildlife animals are implicated in the maintenance and transmission of bovine tuberculosis and therefore pose a significant obstacle to eradication of the disease from domestic livestock. The current antemortem diagnostic method, the intradermal tuberculin skin test, is impractical for routine use with many wild animals. Antibody-based assays are particularly attractive because the animals are handled only once and immediate processing of the sample is not required. This report characterizes the antibody responses of red deer-elk hybrids (Cervus elaphus) against Mycobacterium bovis and subsequently evaluates the diagnostic performance of select antigens in a rapid-test format. Sequential serum samples were collected from 10 animals experimentally infected with M. bovis and 5 noninfected animals over a 7-month period postinfection (p.i.). Samples were evaluated by enzyme-linked immunosorbent assays, immunoblot analyses, and multiantigen print immunoassays for seroreactivity to mycobacterial antigens. Although all infected animals produced antibodies to M. bovis protein antigens, there was significant animal-to-animal variation in the kinetics and magnitudes of responses and the antigens recognized. The most frequently recognized antigens included MPB83, ESAT-6, CFP10, and MPB70. Responses to some antigens, such as MPB83, were consistently detected as early as 4 weeks after inoculation, whereas other antigens were detected only much later (>140 days p.i.). Antibody responses were boosted by injection of tuberculin for intradermal tuberculin skin testing. Comparison of single-antigen (fluorescence polarization assay) with multiantigen (CervidTB STAT-PAK) rapid tests demonstrated that a highly sensitive and specific serodiagnostic test for tuberculosis in cervids will require multiple and carefully selected seroreactive antigens covering a broad spectrum of antibody specificities.

A novel fusion protein-based indirect enzyme-linked immunosorbent assay for the detection of bovine tuberculosis

Enzyme-linked immunosorbent assay (ELISA) for diagnosis of bovine tuberculosis has been widely explored over the years. Three Mycobacterium bovis-specific antigen genes, namely, mpb70, mpb83, and esat-6 were recombined in tandem by spliced overlap extension technology and expressed in Escherichia coli to obtain the fusion protein (rM70-83-E6). Western blot analysis showed that rM70-83-E6 can specifically react with bovine tuberculosis-positive sera but not those from cattle infected with other bovine diseases such as bovine paratuberculosis. An indirect ELISA (iELISA) method was established with rM70-83-E6 as the diagnostic antigen. The diagnostic criteria were determined using 150 serum samples from healthy cattle. Analyses of 85 serum samples from cattle with bovine tuberculosis and 100 serum samples from healthy cattle demonstrated that the sensitivity of the iELISA was 69.4% (59/85) and the specificity was 96.0% (96/100). Moreover, 46 out of 67 purified protein derivative (PPD) skin test-positive samples were also positive by iELISA, giving a positive coincidence of 68.7%, while all 50 PPD skin test-negative samples were negative by iELISA, giving a negative coincidence of 100%. The total coincidence between iELISA and the PPD skin test was 82.1%. This study demonstrated that iELISA using rM70-83-E6 antigen is simple, sensitive and easy to perform and can be used to analysis of a large number of samples for serodiagnosis of bovine tubercuiosis.

Ante mortem diagnosis of tuberculosis in cattle: a review of the tuberculin tests, gamma-interferon assay and other ancillary diagnostic techniques

The early, preclinical stages of bovine TB can be detected in live animals by the use of tests of cellular immunity (the skin, gamma-interferon and lymphocyte transformation tests). Tests of humoral (antibody) immunity, Mycobacterium bovis PCR probes on early tissue cultures or live cattle specimens, and tests based on "electronic nose" technology have been developed more recently. The key measure of diagnostic test accuracy is the relationship between sensitivity and specificity, which determines the false-positive and false-negative proportions. None of the tests currently available for the diagnosis of bovine TB allow a perfectly accurate determination of the M. bovis infection status of cattle. Although various factors can reduce the sensitivity and specificity of the skin tests, these remain the primary ante mortem diagnostic tools for TB in cattle, providing a cost-effective and reliable means of screening entire cattle populations. Despite the inescapable limitations of existing diagnostic tests, bovine TB has been effectively eradicated from many developed countries and regions with the implementation of sound programmes of regular tuberculin skin testing and removal of reactors, coupled with slaughterhouse surveillance for undetected infections, repeat testing and culling of infected herds, cattle movement restrictions to prevent introduction of infected animals and occasional slaughter of entire herds with intractable breakdowns. This is likely to remain the mainstay of bovine TB control programmes for the foreseeable future. Additionally, newer ancillary in vitro diagnostic assays are now available to TB control programme managers to supplement the skin tests in defined circumstances according to the specific disease situation in each country or region. The strategic deployment of ancillary in vitro tests alongside the primary skin tests has enhanced the detection of M. bovis-infected cattle and reduced the number of animals slaughtered as false positives.

Use of rMPB70 protein and ESAT-6 peptide as antigens for comparison of the enzyme-linked immunosorbent, immunochromatographic, and latex bead agglutination assays for serodiagnosis of bovine tuberculosis

Current assays used to detect Mycobacterium bovis infection lack accuracy, especially for recently infected animals, or are impractical for rapid field diagnostic applications. To overcome these limitations with serological assays, a synthetic peptide derived from early secretory antigenic target 6 (ESAT6-p) and a recombinant major secreted immunogenic protein (rMPB70) of M. bovis were used in an enzyme-linked immunosorbent assay (EIA), an immunochromatographic assay (ICGA), and a latex bead agglutination assay (LBAA). Sera from noninfected, M. bovis-infected, or M. avium subsp. paratuberculosis-infected (by natural and experimental routes) animals were evaluated. Receiver operating characteristic analysis comparing optical density values from the EIA with results of bacterial culture or skin test, the reference test, established suitable cutoff values for assessing sensitivity and specificity. The EIA and LBAA, respectively, had sensitivities of 98.6 and 94.8%, specificities of 98.5 and 92.6%, and kappa values of 0.97 and 0.88 with ESAT6-p. The EIA, ICGA, and LBAA, respectively, had sensitivities of 96.8, 83.0, and 86.7%, specificities of 90.1, 99.4, and 97.8%, and kappa values of 0.87, 0.85, and 0.83 with rMPB70. Examination of serial samples of sera collected from experimentally M. bovis-infected cattle and deer revealed that ESAT6-p-specific responses developed early after infection whereas responses to rMPB70 developed later in the course of disease. The advantage of the LBAA and ICGA as initial tests for multiple species is a rapid reaction obtained in 2 to 3 h by LBAA or 20 min by ICGA without species-specific secondary antibodies under field conditions, thus allowing immediate segregation of suspect animals for further testing before culling.

Antibody responses in reindeer (Rangifer tarandus) infected with Mycobacterium bovis

Despite having a very low incidence of disease, reindeer (Rangifer tarandus) are subject to tuberculosis (TB) testing requirements for interstate shipment and herd accreditation in the United States. Improved TB tests are desperately needed, as many reindeer are falsely classified as reactors by current testing procedures. Sera collected sequentially from 11 (experimentally) Mycobacterium bovis-infected reindeer and 4 noninfected reindeer were evaluated by enzyme-linked immunosorbent assay (ELISA), immunoblotting, and multiantigen print immunoassay (MAPIA) for antibody specific to M. bovis antigens. Specific antibody was detected as early as 4 weeks after challenge with M. bovis. By MAPIA, sera were tested with 12 native and recombinant antigens, which were used to coat nitrocellulose. All M. bovis-infected reindeer developed responses to MPB83 and a fusion protein, Acr1/MPB83, and 9/11 had responses to MPB70. Other antigens less commonly recognized included MPB59, ESAT-6, and CFP10. Administration of purified protein derivatives for skin testing boosted serum antibody responses, as detected by each of the assays. Of the noninfected reindeer, 2/4 had responses that were detectable immediately following skin testing, which correlated with pathological findings (i.e., presence of granulomatous lesions yet the absence of acid-fast bacteria). The levels of specific antibody produced by infected reindeer appeared to be associated with disease progression but not with cell-mediated immunity. These findings indicate that M. bovis infection of reindeer elicits an antibody response to multiple antigens that can be boosted by skin testing. Serological tests using carefully selected specific antigens have potential for early detection of infections in reindeer.

Antigen recognition by serum antibodies in white-tailed deer (Odocoileus virginianus) experimentally infected with Mycobacterium bovis

White-tailed deer (Odocoileus virginianus) have emerged as reservoirs of bovine tuberculosis in northern America. For tuberculosis surveillance of deer, antibody-based assays are particularly attractive because deer are handled only once and immediate processing of the sample is not required. Sera collected sequentially from 25 Mycobacterium bovis-infected and 7 noninfected deer were evaluated by enzyme-linked immunosorbent assay (ELISA), immunoblotting, and multiantigen print immunoassay (MAPIA) for immunoglobulin specific to M. bovis antigens. Various routes of experimental M. bovis infection, such as intratonsillar inoculation (n = 11), aerosol (n = 6), and exposure to infected deer (in contact, n = 8), were studied. Upon infection, specific bands of reactivity at approximately 24 to 26 kDa, approximately 33 kDa, approximately 42 kDa, and approximately 75 kDa to M. bovis whole-cell sonicate were detected by immunoblot. Lipoarabinomannan-specific immunoglobulin was detected as early as 36 days postchallenge, and responses were detected for 94% of intratonsillarly and "in-contact"-infected deer. In MAPIA, sera were tested with 12 native and recombinant antigens coated on nitrocellulose. All in-contact-infected (8 of 8) and 10 of 11 intratonsillarly infected deer produced antibody reactive with one or more of the recombinant/native antigens. Responses were boosted by injection of tuberculin for intradermal tuberculin skin testing. Additionally, three of six deer receiving a very low dose of M. bovis via aerosol exposure produced antibody specific to one or more recombinant proteins. M. bovis was isolated from one of three nonresponding aerosol-challenged deer. Of the 12 antigens tested, the most immunodominant protein was MPB83; however, a highly sensitive serodiagnostic test will likely require use of multiple antigens.

Analysis of possible factors affecting the specificity of the gamma interferon test in tuberculosis-free cattle herds

Bovine tuberculosis (TB) is still a zoonotic problem in the world. Despite the fact that eradication programs for bovine TB are being implemented in many countries, it remains a public health problem. These programs are mainly based on a single intradermal tuberculin test using bovine tuberculin purified protein derivative (PPD), isolation, and slaughtering of infected animals. The aim of this study was to assess the specificity of the gamma interferon (IFN-gamma) test in TB-free cattle herds, by using not only Australian tuberculins but also tuberculins produced at our institute, and to correlate the response with the type of production (beef cattle, dairy cattle, and a dual-purpose breed), the housing system, and the age of the animals. We studied 800 animals selected from 20 TB- and paratuberculosis-free herds. The animals were tested in parallel, after stimulation with Australian tuberculins and tuberculins produced at our institute, by using the skin test and two IFN-gamma assays. The results of this trial showed that the specificity of the IFN-gamma test is higher than that of the skin test (96.8%) and ranges from 97.3% (using only Australian tuberculins) to 98.6% (using tuberculins produced at our institute). We found that different categories of cattle could influence the specificity of the skin test but that these differences tended to be reduced in the IFN-gamma assay, especially when Italian PPDs were used.

Association of tuberculin-boosted antibody responses with pathology and cell-mediated immunity in cattle vaccinated with Mycobacterium bovis BCG and infected with M. bovis

Vaccine development and our understanding of the pathology of bovine tuberculosis in cattle would be greatly facilitated by definition of the immunological correlates of protection and/or pathology. In this study we analyzed humoral immune responses in Mycobacterium bovis BCG-vaccinated and control cattle (in particular, the relationship between the intradermal comparative tuberculin skin test and serum immunoglobulin G [IgG] responses) against a range of mycobacterial antigens (MPB59, MPB64, MPB70, MPB83, ESAT-6, CFP-10, Acr1, and PstS-1) by multiantigen print immunoassay and conventional enzyme-linked immunosorbent assay. Following M. bovis infection, the comparative tuberculin skin test strongly boosted IgG, IgG1, and IgG2 antibody responses, particularly against MPB83 and MPB70, in unvaccinated cattle but failed to boost these responses, or did so only weakly, in BCG-vaccinated calves. In addition, the skin test-induced increases in MPB83-specific IgG responses correlated positively with bacterial loads and ESAT-6-induced in vitro gamma interferon responses. In conclusion, both the negative correlation of skin test-enhanced MPB83-specific antibody responses with BCG-induced protection and their positive correlation with bacterial loads can serve as useful markers for vaccine efficacy after challenge.

Use of mycobacterial peptides and recombinant proteins for the diagnosis of bovine tuberculosis in skin test-positive cattle

More accurate tests are required to test cattle which have reacted positively in the tuberculin skin test. For this purpose, a range of mycobacterial antigens, MPB59, MPB64, MPB70, MPB83, ESAT-6 and CFP10, were used either as recombinant proteins or as synthetic peptides in the whole blood interferon-gamma (IFN-gamma) test. Groups of uninfected cattle with typical 'non-specificity' problems were targeted, in particular animals with skin tuberculosis, animals vaccinated against Johne's disease and animals that were positive in the standard purified protein derivative (PPD)-based IFN-gamma test. The two study groups consisted of 74 Mycobacterium bovis-culture positive animals and 72 uninfected animals, all of which tested positive in the caudal fold tuberculin skin test eight to 28 days before the blood test. The use of combinations of ESAT-6 and CFP10 antigens, either as recombinant proteins or peptides, detected similar percentages of M bovis-infected animals as the PPD-based IFN-gamma test, but produced significantly fewer false positive reactions. The PPD-based IFN-gamma test was very effective in differentiating animals vaccinated against Johne's disease that were skin-test positive from those with bovine tuberculosis, and the use of PPD or specific mycobacterial antigens minimised the number of false positive reactions in animals with skin tuberculosis.

Mycobacterium bovis AN5 antigens vary in their ability to induce nitric oxide production in blood monocytes of experimentally infected cattle

Reactive nitrogen intermediates (RNI) are the principal effector molecules of activated monocyte/macrophage populations, responsible for killing and inhibiting the growth of virulent mycobacteria. In vitro nitrite production by blood monocytes of cattle inoculated with live Mycobacterium bovis AN5 was assessed from 0 day through 45 weeks post inoculation (PI). High in vitro nitrite production was observed at the 8th and 12th weeks PI in sensitized cattle but reactivity had fallen by the 20th week PI. To assess the in vitro nitrite producing ability of monocytes induced by individual polypeptides within culture filtrate antigens (CFA) of M. bovis AN5, cellular blotting was performed using peripheral blood mononuclear cells (PBMC) at the 12th week PI. It was observed that polypeptides of MW 70, 65, 60, 25, 24 and 22 kDa of CFA induced high nitrite production by blood monocytes while many polypeptides had little or no effect.

Combined use of Paracoccidioides brasiliensis recombinant 27-kilodalton and purified 87-kilodalton antigens in an enzyme-linked immunosorbent assay for serodiagnosis of paracoccidioidomycosis

The diagnosis of paracoccidioidomycosis (PCM) has relied on the identification of the host's humoral response by using a variety of immunological methods, such as complement fixation and immunodiffusion. Although these approaches are useful, historically their sensitivity and specificity have often been compromised by the use of complex mixtures of undefined antigens. The use of combinations of purified, well-characterized antigens appears preferable and may yield optimum results. Accordingly an indirect enzyme-linked immunosorbent assay (ELISA) using combinations of the previously described 27-kDa recombinant antigen and the 87-kDa heat shock protein were used for diagnosis and follow-up of patients with PCM. A total of 37 patients classified according to their clinical presentations (7 with the acute or subacute form of the disease, 22 with the chronic form of the disease, and 8 with the chronic unifocal form) were studied. Eighteen of these patients were also evaluated at every follow-up appointment. Forty serum samples from patients with other diseases and 50 serum samples from healthy individuals were also studied. Detection of anti-27-kDa and anti-87-kDa antibodies in sera of patients with PCM by ELISA using a combination of the two purified proteins showed a sensitivity of 92% with a specificity of 88% in comparison with normal human sera and 90% in comparison with the heterologous sera. These results demonstrated a significant increase in sensitivity and specificity compared to results when the antigens were used separately. Thus, the use of combinations of well-defined antigens appears to offer clear advantages over the use of single antigens when diagnosing PCM.

Mycobacterium bovis-infected white-tailed deer (Odocoileus virginianus): detection of immunoglobulin specific to crude mycobacterial antigens by ELISA

White-tailed deer (Odocoileus virginianus) have recently emerged as a source of Mycobacterium bovis infection for cattle within North America. The objective of this study was to evaluate the antibody response of M. bovis-infected deer to crude mycobacterial antigens. Deer were experimentally inoculated with M. bovis strain 1315 either by intratonsilar instillation or by exposure to M. bovis-infected (i.e., in contact) deer. To determine the time course of the response, including the effects of antigen administration for comparative cervical skin testing, serum was collected periodically and evaluated by enzyme-linked immunosorbent assay (ELISA) for immunoglobulin (i.e., IgG heavy and light chains) reactivity to mycobacterial antigens. The reactivity to M. bovis purified protein derivative (PPDb) exceeded (P < 0.05) the reactivity to M. avium PPD (PPDa) only after in vivo administration of PPDa and PPDb for comparative cervical testing of the infected deer. The mean immunoglobulin response, as measured by ELISA, of intratonsilar-inoculated deer to a proteinase K-digested whole-cell sonicate (WCS-PK) of M. bovis strain 1315 exceeded (P < 0.05) the mean of the prechallenge responses to this antigen at approximately 1 month after inoculation and throughout the remainder of the study (i.e., approximately 11 months). This response also exceeded (P < 0.05) that of the uninfected deer. Although this is encouraging, further studies are necessary to validate the use of the proteinase K-digested M. bovis antigens in the antibody-based assays of tuberculosis.

Isolation of a 19-kDa mycobacterium, bovis-specific antigen, different from MPB70/80, by chromatofocusing

Two antigens, 19-kDa each, were purified from Mycobacterium bovis culture filtrate protein extract by chromatofocusing. Antigen I had a 4.5 pI, and its amino terminal (DPVDAVINTTCNYGQVVAALNATDP) showed a 100% homology with the hypothetical protein Rv1174c. Antigen II had a pI of 6.0 pI and its amino terminal (GDLVGPGCAEYAAANPTGPASVQGM) showed a 100% homology with M. bovis MPB70/80. Antigen I is a hetero-dimer formed by a glycosylated, 10.5-kDa, monomer and a non-glycosylated 8-kDa monomer with identical amino terminal sequences. Both antigens were recognized by the sera of PPD+ animals, but antigen I did not crossreact with sera of human PPD+ individuals. Antigen I was a weak inducer of lymphocyte proliferation and IFN-gamma production. Our results show that M. bovis expresses a 19 kDa glycoprotein, homologue to the product of M. tuberculosis gen Rv1174c, which may prove useful for bovine TB diagnostic assays.

Use of recombinant proteins in antibody tests for bovine tuberculosis

Tuberculosis (TB) in cattle remains a major zoonotic and economic problem in many countries. Since the standard diagnostic assay, the intradermal test (IDT) with bovine PPD tuberculin, has less than optimal accuracy in all situations, other diagnostic methods such as serological assays have been investigated. Because of fundamental concerns for the low sensitivity and specificity of previous ELISA protocols, a profiling ELISA with nine purified, recombinant proteins of TB complex mycobacteria, was employed on samples from four groups of cattle: (a) naturally Mycobacterium avium-exposed and experimentally Mycobacterium bovis-infected, (b) officially-certified TB-free herds, (c) exposed to M. bovis in two field TB outbreaks and scored as bovine reactors in the gamma-IFN assay for bovine TB, (d) paratuberculosis (para TB)-infected. The described ELISA proved to be highly specific. In fact, the antibody (Ab) response could be consistently detected in 3 out of 3 endotracheally-infected calves and in 1 out of 3 contact-infected calves. There was also a very low prevalence of low-titered, non-specific Ab responses in paraTB-infected animals. As for the animals exposed to field TB outbreaks, 16 out of 28 gamma-IFN positive cattle were also Ab-positive; importantly, 7 out of 12 gamma-IFN positive, IDT-negative cattle showed Ab responses to TB proteins. In general, the profile of the Ab response varied among animals; the reaction to single recombinant antigens was sometimes transient and fluctuating, whereas the panel of antigens on the whole was indeed more effective in Ab detection.

An enzyme-linked immunosorbent assay using immonoaffinity-purified antigen in the diagnosis of caprine paratuberculosis and its comparison with conventional ELISAs

An enzyme-linked immunosorbent assay (APA-ELISA) using an immunoaffinity-purified antigen was developed and compared with the unabsorbed and absorbed ELISA procedures, using a crude antigenic preparation, for its efficacy in detecting antibodies in goat sera against Mycobacterium ovium paratuberculosis. Serum samples from 89 goats belonging to three different flocks, two with a history and evidence of paratuberculosis and one without it, were subjected to each ELISA, which had been standardized on known positive sera from goats experimentally infected with paratuberculosis. Faecal culture, faecal examination and histopathology were used as indicators of infection. The diagnostic sensitivities of the unabsorbed, absorbed and APA-ELISA were 81.8%, 77.3% and 77.3% and the specificities were 90.6%, 93.7% and 96.8%, respectively. The positive predictive values of APA-ELISA (94.4%) was the highest, followed by absorbed ELISA (80.9%) and unabsorbed ELISA (72.0%). The negative predictive values for APA-ELISA, absorbed ELISA and unabsorbed ELISA were 93.0%, 92.7% and 93.8%, respectively. The results indicated the value of APA-ELISA in avoiding the need to absorb individual test sera with Mycobacterium phlei and giving more consistent results than the absorbed ELISA. The APA-ELISA was also better than the other two procedures in terms of specificity and positive predictive values.

Towards more accurate diagnosis of bovine tuberculosis using defined antigens

Diagnostic accuracy is of paramount importance in test-and-slaughter programmes for the eradication of bovine tuberculosis (TB). Currently applied methods, such as in vivo skin testing and in vitro interferon-gamma (IFN- gamma) testing, utilize purified protein derivatives (PPDs), which are poorly-defined mixtures containing many individual antigenic components. It is known that false-positive responses to these reagents can occur in cattle which are not infected with TB, largely because of that antigenic complexity. This paper reviews recent approaches to the characterization of more precisely defined diagnostic tools which can be used to develop tests with greater specificity. For example, the low mass secreted protein ESAT-6 has been shown to be capable of differentiating TB-infected cattle from those which develop responsiveness to PPD through contact with environmental mycobacteria or vaccination with BCG. The information which has accumulated in recent years has shown that the increased specificity is associated with some decrease in test sensitivity, but the overall advantages of being able to make precise diagnostic decisions will have significant advantages in many situations.

Novel monoclonal antibodies against major antigens of Mycobacterium bovis

MPB70 and MPB83 are among the most characteristically exported proteins defining a strongly expressed phenotype of Mycobacterium bovis. These proteins are known to be homologous to osteoblast-specific factor 2. By in vitro culture of mycobacteria they appear to have a limited species distribution and to be relatively specific for M. bovis. Virtually identical genes are however, present in Mycobacterium tuberculosis. In order to facilitate further research into the immunobiology of these proteins and their potential application for differential diagnosis of tuberculosis as a result of M. bovis, we describe the reactivities of 20 monoclonal antibodies (MoAbs) to these proteins. Immunizing with bovine PPD generated 10 MoAbs. These antibodies reacted preferentially with the soluble MPB70 antigen using reducing conditions in SDS-PAGE with western blotting. Ten MoAbs were generated by immunizing mice with fractions derived from a whole cell sonic extract of M. bovis. These antibodies reacted preferentially with the surface exposed MPB83 lipoglycoprotein.

Characterization of the early antibody response in bovine tuberculosis: MPB83 is an early target with diagnostic potential

A 26-kDa antigen has been shown to be a dominant antibody target in Mycobacterium bovis-infected cattle. In this study, that antigen was used as an immunogen to raise a panel of mouse monoclonal antibodies. The majority of those bound to native protein with a molecular mass of 26 kDa and to recombinant MPB83, strongly suggesting that MPB83 is an important B-cell antigenic target in bovine tuberculosis. In order to provide assessment of the potential of measuring antibody responses to the native protein, one monoclonal antibody, 1F11, was incorporated into an enzyme-linked immunosorbant assay format to trap antigen from a crude bacterial extract. Despite some disadvantages of this format, serum samples from cattle which had been infected experimentally with M. bovis, and from tuberculin skin-test-negative and -positive field cattle were tested for the presence of antibodies. Data from the skin-test-negative cattle allowed an arbitrary cut-off value to be established and, under these conditions, test sensitivity and specificity were estimated at 37.5 and 89%, respectively. These results indicate potential for MPB83 in the development of assays for serological diagnosis of bovine tuberculosis.

IgG isotype antibody responses to epitopes of the Mycobacterium bovis protein MPB70 in immunised and in tuberculin skin test-reactor cattle

Serological assays may have merit in identifying animals in advanced stages of bovine tuberculosis, but most tests have had sub-optimal sensitivities and specificities. The Mycobacterium bovis protein MPB70 has been identified as a B-cell target with diagnostic potential in measurement of pre- and post-skin-test antibody responses. One observation, which has potential practical application, has been that skin testing with tuberculin boosts IgG(1) anti-MPB70 antibody responses in cattle with tuberculous lesions. However, serological cross-reactivities with bacteria, such as Nocardia asteroides, have been described for this protein. With the aim of identifying candidate reagents for improved diagnostic tests, this study investigated IgG isotype antibody responses to MPB70 at the epitope level and, because of the previous findings, focused on IgG(1) responses following skin testing. Screening of a panel of overlapping synthetic peptides using sera from cattle immunised with MPB70 and cattle infected with M. bovis showed that two regions of the protein (residues 21-70 and 101-120) contain dominant B-cell epitopes. No individual epitope appeared to be selectively recognised by one isotype of IgG antibody. Investigation of IgG(1) responses showed that recognition of the epitope within residues 51-70 was boosted strongly by tuberculin injections in skin-test positive cattle and that this memory response was generally a feature of cattle which were found to have macroscopic, tuberculous lesions.

Toward the development of diagnostic assays to discriminate between Mycobacterium bovis infection and bacille Calmette-Guérin vaccination in cattle

A scientific review of the recent sharp increase in bovine tuberculosis in Great Britain has concluded that the development of a cattle vaccine holds the best prospect for long-term disease control. It is important to develop a diagnostic test that differentiates between vaccinated and Mycobacterium bovis-infected animals, to ensure that test-and-slaughter control strategies can continue alongside vaccination. The mycobacterial antigens ESAT-6, MPB64, and MPB83 are expressed at high levels in M. bovis but are expressed at low levels or not at all in bacille Calmette-Guérin (BCG) Pasteur. Promiscuous bovine T cell epitopes of these antigens were identified and formulated into a peptide cocktail. This cocktail and a cocktail composed of recombinant forms of the 3 antigens was able to distinguish cattle infected with virulent M. bovis from those vaccinated with BCG and from those sensitized to avian tuberculin in lymphocyte transformation and interferon-gamma assays.

Diversity of antigen recognition by serum antibodies in experimental bovine tuberculosis

Tuberculosis in cattle remains a major zoonotic and economic problem in many countries. The standard diagnostic assay for bovine tuberculosis, the intradermal tuberculin test, has low accuracy. Therefore, alternative immunodiagnostic methods, such as serological assays, are needed for detection of infected animals. Development of an accurate serodiagnostic test requires a detailed understanding of the humoral immune responses during bovine tuberculosis and, in particular, identification of the key antigens of Mycobacterium bovis involved in antibody production. In this study, we characterized antibody responses in cattle experimentally infected with M. bovis. Sequential serum samples were collected every 3 to 4 weeks for up to 27 months postinfection. Circulating immunoglobulin G antibody levels were measured by an enzyme-linked immunosorbent assay using 12 highly purified recombinant proteins of M. bovis. Six proteins, ESAT-6, 14-kDa protein, MPT63, MPT70, MPT51, and MPT32, were identified as major seroreactive antigens in bovine tuberculosis. A remarkable animal-to-animal variation of antigen recognition by serum antibodies was observed. Kinetic analyses of the antibody production to individual antigens during infection revealed that the heterogeneous antigen recognition profile changed markedly in a given infected animal as disease progressed.

T-cell recognition of mycobacterial proteins MPB70 and MPB64 in cattle immunized with antigen and infected with Mycobacterium bovis

Defined antigenic reagents and knowledge of T-cell responses are required for the design of improved diagnostic tests for bovine tuberculosis. The limited species distribution of Mycobacterium bovis antigens MPB70 and MPB64 has indicated their potential for inclusion in future tests. The strategy adopted in this study was to define bovine T-cell responses to these antigens at the epitope level, using cattle immunized with recombinant forms of the antigens, and to compare these responses with cattle which had been experimentally infected with M. bovis. Panels of synthetic peptides (20-mers with 10-residue overlaps) were used and five epitopes were identified and found to be powerful stimulators of T-cell responses in both types of animal (residues 81-100 and 174-190 for MPB70, and residues 1-20, 41-60 and 181-200 for MPB64). Further investigation in larger numbers of cattle (n = 14) of mixed breeds from tuberculosis-infected herds confirmed that each peptide produced response in several of the cattle, but no single peptide was recognized by all animals. However, the limited numbers of animals in this study suggest that peptide reagents may identify as many positive animals as the intact antigenic protein and could form components of a future diagnostic test. The use of cattle immunized with the proteins of interest has proved to be an interesting model for studying the nature of bovine T-cell responses to defined mycobacterial proteins.

Immunochemical characterization of the MPB70/80 and MPB83 proteins of Mycobacterium bovis

MPB70 and MPB80 (MPB70/80) and MPB83 are closely related antigens which are highly expressed in Mycobacterium bovis. MPB70/80 are soluble secreted antigens, while MPB83 is an exported lipoprotein associated with the bacterial surface. In the present study, these antigens had different mobilities in sodium dodecyl sulfate-polyacrylamide gel electrophoresis under reducing and nonreducing conditions. These differences may be explained by the fact that MPB70 and MPB83 both have two internal cysteine residues which would create ring structures by disulfide bonding. We analyzed the structures of MPB70/80 and MPB83 by using monoclonal antibodies (MAbs) raised against bovine purified protein derivative or whole M. bovis cells. MAb 1-5C reacted specifically with MPB70 and MPB80, and MAb MBS43 reacted specifically with MPB83, while the other antibodies, including several previously described MAbs, bound all three antigens. MAbs and polyclonal antibodies reacted strongly with reduced protein and less well with nonreduced protein, indicating involvement of linear epitopes. Epitopes of MAbs Bov-1, 2-6B, 1-5C, and 1-1D were mapped by using synthetic peptides of MPB70. Sequence comparison showed the peptide with the 1-5C-reactive epitope to have three residues different from those in the homologous region of MPB83. Exchanges of A for S in position 112 or Q for E in position 116 abolished the reactivity of MAb 1-5C. Polyclonal rabbit antibodies to native purified MPB70 reacted strongly with peptides 6, 7, and 8 of the N-terminal half of mature MPB70. Cattle sera of experimentally M. bovis-infected animals recognized a broader spectrum of peptides. These findings indicate that there is diagnostic potential for these proteins and that there is also a possible role for antibodies in elucidation of the host-mycobacterium relationship involving a surface-bound and exposed lipoprotein, MPB83, and its highly homologous soluble secreted MPB70/80 counterparts.

Mycobacterial antigen-specific antibody responses in bovine tuberculosis: an ELISA with potential to confirm disease status

Serological assays may help to identify animals in advanced stages of bovine tuberculosis, but most of the tests available have suboptimal sensitivities and specificities. This study was designed to determine whether the antibody responses to defined antigens (rMPB70, rMPB64 and rMPB59) of Mycobacterium bovis at the immunoglobulin subclass level could be used to develop improved serological tests. In experimentally infected cattle it was found that the predominant serum antibody response was to rMPB70, and that an IgG1 response to this antigen was boosted strongly by skin testing. Studies in naturally infected cattle suggested that this memory IgG1 anti-rMPB70 response may be able to differentiate between skin test-reactor animals with and without lesions by comparing the ratio of the antibody response before and after skin testing. The study has provided a clearer understanding of the kinetics of antibody responses to defined mycobacterial antigens at the subclass level in bovine tuberculosis and has made it possible to develop a novel ELISA system which may be useful in disease diagnosis.

Characterisation of a lipoprotein in Mycobacterium bovis (BCG) with sequence similarity to the secreted protein MPB70

The monoclonal antibody, mAb3C4, raised against sonicated Mycobacterium bovis (Mb) BCG (Tokyo strain 172) cells recognises a 23-kDa protein in the cell wall. The gene encoding this protein was cloned and sequenced and found to be 100% homologous to mpb83 and mpt83 and the putative protein to have a 76% sequence similarity to the secreted, Mb-specific protein, MPB70. MPB83 contains the amino acid (aa) sequence LAGC, which corresponds to the consensus sequence for bacterial lipoprotein modification and processing. MPB83 associated with the detergent phase when separated with Triton X-114 confirming that it is a lipoprotein. When the putative site of acylation, the Cys in the sequence LAGC, was substituted with Ser, the mutated MPB83 associated with the aqueous phase. The cloned gene was used to determine the distribution of mpb83 in various Mycobacterium species. The gene was present in the M. tuberculosis (Mt) complex organisms, as well as in M. kansasii. In addition, Southern blot analysis of Mb and Mt DNA indicated that the mpb83 and mpb70 genes are located close to each other on the genome. Western blot analysis of cell lysates of various Mycobacterium species indicated that only Mt H37Rv and H37Ra produced proteins which reacted with mAb3C4. Furthermore, only two out of six of the Mb field isolates produced detectable antigen, indicating that expression of the mpb83 gene is variable within the Mt complex organisms.

Purification of Mycobacterium bovis BCG Tokyo antigens by chromatofocusing, lectin-affinity chromatography, and hydrophobic interaction chromatography

A combination of chromatofocusing, lectin-affinity chromatography, and hydrophobic interaction chromatography resulted in a simple purification of protein antigens of Mycobacterium bovis BCG Tokyo culture filtrate. Identification was established on the basis of chromatographic separation, sodium dodecyl sulfate-polyacrylamide gel electrophoresis determination of molecular weights, and N-terminal amino acid determination. Chromatofocusing on PBE 94 accomplished the separation of BCG85B from other BCG85 complex antigens and partial separation of MPB64 and MPB70 antigens. Subsequently, MPB64 and MPB70 were completely separated on a high-performance liquid chromatography TSK Phenyl 5PW hydrophobic interaction chromatography column. This column also separated BCG85B from a 17-kDa protein with an N-terminal amino acid sequence of A-V-P-I-T-G-K-L-G-S-E-L-T-M-T-D-( )-V-G-Q, which is similar to the sequence of MPT63. Concanavalin A-Sepharose-affinity chromatography separated MPB64 from a 43- and 47-kDa doublet with an amino acid sequence of D-P-E-P-A-P-P-V-P-P-V-P-A-( )-A-A-S-P, which is similar to the sequence of MPT32 and which appears to be glycosylated.

Modification of the Mycobacterium bovis extracellular protein MPB70 with fluorescein for rapid detection of specific serum antibodies by fluorescence polarization

The principle of fluorescence polarization described by Perrin (F. Perrin, J. Phys. Radium 7:390-401, 1926) was applied to the development of a novel assay that used fluorescein-labeled Mycobacterium bovis secretory protein MPB70 for rapid detection of anti-MPB70 antibodies in selected sera from three M. bovis-infected species (elk, Ilama, and bison). Labeling of purified MPB70 with fluorescein isothiocyanate resulted in the incorporation of 0.96 +/- 0.08 (mean +/- standard deviation; n = 3) fluorescein group per MPB70 molecule. The labeled protein fluoresced strongly with an emission maximum at 518 nm when excited with light of a wavelength near 493 nm, and its immunoreactivity with anti-MPB70 monoclonal antibody 4C3/17 was not altered by modification with fluorescein. The fluorescence polarization assay protocol was optimized for analysis of serum samples by incorporating into the assay buffer 0.05% lithium dodecyl sulfate, which prevents the occurrence of some nonspecific interactions. Sera from M. bovis-infected animals, selected on the basis of exhibiting the presence of anti-MPB70 antibodies, as detected by enzyme-linked immunosorbent assay (ELISA), reacted with fluorescein-labeled MPB70, resulting in an increase in polarization of up to 330 milli-polarization units, in contrast to the values for noninfected sera (167 to 178 mP), which were close to that obtained in the absence of specific antibodies (164.7 +/- 3.3 mP; n = 6). These results demonstrated the feasibility of using fluorescein-labeled MPB70 to detect anti-MPB70 antibodies by fluorescence polarization and suggested that the assay described here can be an alternative to ELISA or other antibody assay systems. The advantages of this original methodology and its general applicability to the diagnosis of infectious diseases are discussed.

Secretion of the mycobacterial 19-kilodalton protein by Escherichia coli, a novel method for the purification of recombinant mycobacterial antigens

We have reported previously (R.G. Hewinson and W.P. Russell, J. Gen. Microbiol. 139:1253-1259, 1993) the secretion by Escherichia coli of a recombinant form of the immunogenic protein MPB70 of Mycobacterium bovis when the protein is translated from its native initiation codon. N-terminal sequence analysis of the purified protein revealed that the signal peptide of MPB70 was cleaved by an endopeptidase of E. coli at the same cleavage site as reported for the protein in M. bovis. Since both the B- and T-cell antigenicities of the purified recombinant protein were similar to that of the native protein, the 19-kDa antigen of M. bovis was used as a model to test whether the signal peptide of MPB70 could direct the secretion of heterologous proteins in E. coli and whether antigen produced in this way retained antigenicity superior to that of recombinant protein produced as a fusion to glutathione-S-transferase. A chimeric protein was produced in which the signal peptide of MPB70 was fused to the 19-kDa antigen of M. bovis at amino acid residue 23. This chimeric protein was found to be secreted into the periplasm and culture medium of recombinant E. coli, and the signal peptide was cleaved by an endopeptidase of E. coli during secretion. Secretion of the 19-kDa antigen facilitated purification of the antigen by two-stage preparative electrophoresis which gave yields of 2.5 mg of purified, soluble 19-kDa antigen from 2.5 g (wet weight) of E. coli. Antigen purified in this way retained both B- and T-cell antigenicities. Moreover, the nonspecific mitogenic activity of the purified 19-kDa antigen was low, while the magnitude of the T-cell response induced by the purified antigen was considerably higher than that observed with purified antigen produced as a fusion protein with glutathione-S-transferase.

Homology between the MPB70 and MPB83 proteins of Mycobacterium bovis BCG

Isolation of MPB83 from Mycobacterium bovis BCG Tokyo culture fluid is described. MPB70 and MPB83 have similar molecular mass as judged by SDS-PAGE but differ in isoelectric points. Peptides isolated after CNBr cleavage of MPB83 revealed extensive homology as well as distinct differences from corresponding parts of the amino acid sequence deduced from the mpb70 gene cloned by Terasaka et al. Antibodies produced by immunization with MPB70 and MPB83 had distinctly different fine specificity revealing cross-reactivity between the proteins. These findings indicate that two distinct, homologous genes code for these proteins. Sensitization with live BCG Tokyo also induced T cell responses to MPB83 with development of delayed type hypersensitivity in guinea pigs.

Evaluation of three serological assays for the diagnosis ofMycobacterium bovisinfection in brushtail possums

Three serological tests for the diagnosis of Mycobacterium bovis infection were evaluated on 29 possums (Trichosurus vulpecula) with tuberculosis and on 100 possums from a tuberculosis-free area. An indirect ELISA using M. bovis culture filtrate as the antigen had a sensitivity of 45% and a specificity of 96%, while an indirect ELISA using a M. bovis specific antigen (MPB70) had a sensitivity of 21% and a specificity of 98%. A blocking ELISA which utilised a monoclonal antibody against MPB70 had a sensitivity of 28% and a specificity of 99%. Combination of the test results of the three ELISAs resulted in an increase in sensitivity to 51% and a decrease in specificity to 93%. A previous study has shown that possums experimentally infected with M. bovis produced cellular responses to M. bovis antigens relatively early in the infection, but these responses decreased in the terminal stages of the disease. In contrast, analysis of serological responses in the current study from sequentially collected sera of possums experimentally and naturally infected with M. bovis showed that antibody was first detected late in the disease.

A western blot characterization of Mycobacterium bovis antigens recognized by cattle sera

The immune response to Mycobacterium bovis in cattle was assessed by Western blot. The antibody recognition pattern to M. bovis whole cell extracts and culture supernatant antigens was studied by using sera from M. bovis-infected (n = 62) and healthy (n = 38) cattle. Although the recognition patterns were highly variable, some proteins were regularly detected, mainly those with molecular masses of 17, 23, 28, 42, 66, 71 and 80 kDa in cellular extracts, and with molecular masses of 23 and 33 kDa in supernatants. Whole cell extract antigens were more frequently recognized than culture supernatant antigens. Healthy controls produced only a weak antibody response. The antibody response was variable, depending on tuberculosis stage. In early stages very few antibodies were detected. A response against the 66-kDa stress protein was mounted in intermediate tuberculosis and remained stable in more advanced disease. In late diseases, the preferentially recognized antigens were a 28-kDa cellular protein and supernatant antigens. The 28-kDa protein was studied in some detail. As determined by using monoclonal antibodies, the 28-kDa protein is different from superoxide dismutase. This protein aggregated in stored cell extracts and was not totally transferred to nitrocellulose. The principal conclusions of this work are: (i) whole cell extract proteins are more frequently recognized than the secreted proteins and (ii) a 28-kDa protein is a major antigen in late disease.

Soluble Mycobacterium bovis protein antigens: studies on their purification and immunological evaluation

The eradication of bovine tuberculosis is an ultimate aim of the beef industry and the development of accurate diagnostic tests will expedite eradication. Characterization of Mycobacterium bovis antigens, and a detailed understanding of their immune reactivity will aid in the development of more specific and sensitive diagnostic tests. With this aim, studies were conducted which have resulted in the purification and immunological characterization of the major soluble M. bovis antigens. The purified antigens were found to contain cross-reactive epitopes and immunological responses to these proteins varied among individual animals. Thus if more specific diagnostic tests are to be formulated, they will have to be at the epitope level, using only species-specific epitopes and not whole proteins. Due to the genetic diversity of the response of infected cattle to individual epitopes, a large cocktail of such epitopes will be necessary for the development of a sensitive test.

In vitro immunodiagnostic assays for bovine tuberculosis

The immune response to mycobacterial infections in cattle is predominantly cellular in nature and current diagnostic tests for M. bovis are based on the measurement of T cell responses. The low sensitivity of serological assays for tuberculosis is therefore not surprising and serological tests will at best be used to complement rather than replace cellular assays. The recently developed bovine interferon gamma (IFN-gamma) assay is a rapid (24 hour) and simple whole blood in vitro assay, which in Australian field trials was found to be significantly more sensitive than the intradermal tuberculin test for the diagnosis of bovine tuberculosis. The problem of false-positive reactions, due to the cross-reactive nature of the antigen preparations used, can largely be overcome by using a comparative assay in which an animal's IFN-gamma response to bovine PPD and avian PPD are compared. Although reasonably M. bovis specific proteins have been identified and characterised, their use in either serological or cellular diagnostic assays is likely to be restricted due to the genetic diversity of the bovine immune response to M. bovis infection.

Diagnosis and epidemiology of bovine tuberculosis using molecular biological approaches

Conventional methods for diagnosis and epidemiological studies of bovine tuberculosis and other mycobacterial diseases are far from ideal. In the last twelve years, molecular biology has provided new approaches which have enabled detailed studies to be made of the molecular characteristics of Mycobacterium bovis, the causative organism of bovine tuberculosis. These characteristics have been investigated for their potential use in diagnosis and epidemiological studies. Restriction fragment analysis of genomic DNA from isolates of M. bovis has provided a highly discriminatory typing system which has been used extensively for epidemiological studies. DNA elements in M. bovis have been investigated for their potential use in diagnostic assays based on the polymerase chain reaction. Individual antigens of M. bovis and their reactive epitopes have been identified and investigated for their usefulness in both serological and T cell assays of animal infection. Serological assays have proven disappointing and the full potential of T cell assays has yet to be determined. Currently, T cell assays based on recombinant antigens have not been as sensitive as those based on natural antigens.

Cellular and humoral immune responses of cattle to purified Mycobacterium bovis antigens

Cellular responses to several purified antigens of Mycobacterium bovis were examined in experimentally infected cattle over a period of 36 months, using in vitro cellular proliferation and interferon-gamma assays. These antigens (12, 19, 22a, b, 24, 25, 30, 32, 39, 65 and 70 kDa) included the majority of M. bovis protein antigens described to date and are highly homologous to those purified from M. tuberculosis. Cellular responses were examined at 3-month time intervals during the 36-month course of infection. All purified antigens induced cellular immune responses in the infected animals. The onset and magnitude of response to individual antigens varied among the animals. At any specific time during the period of infection one or more antigens appeared to be immunodominant but the immunodominance profile changed as the infection progressed. Humoral immune responses were low or absent in the first half of the infection period, but increased substantially for some of the antigens during the second half. Variation was observed among the different animals as to which antigens they recognized.

Vaccination against tuberculosis: is BCG more sinned against than sinner?

While extensive experimental studies of tuberculosis (Tb) have provided the foundation data for the discovery of cell-mediated immunity, there remains much to be disclosed about the critical pathways of immunity involved in this infectious process and the factors necessary to produce protective immunity. Studies on the aetiology and pathology of this disease have failed to elucidate the mechanisms of protective immunity. Although Tb research has been neglected for the past 30 years, the re-emergence of Tb worldwide as a significant zoonotic disease has re-focused research in this area. Scientific solutions for the control of Tb in man or domestic animals have not been found using empirical methods. Composite studies involving animal models of experimental infection will be necessary to critically evaluate vaccine efficacy and eludiate the basic immunological mechanisms involved in both disease and immunity. Available data which suggest that disease-related hypersensitivity and immunity are dissociable highlight the prospect that immunity to infection may be induced without compromising the continued need for ongoing systems of immunodiagnosis to exclude disease. In populations with a high prevalence of disease it is likely that a combination of immunodiagnosis, chemotherapy and immunoprophylaxis will be required to eradicate the disease.