Humoral response to defined epitopes of tubercle bacilli in adult pulmonary and child tuberculosis

Defining Discriminatory Antibody Fingerprints in Active and Latent Tuberculosis

Tuberculosis (TB) is among the leading causes of death worldwide from a single infectious agent, second only to COVID-19 in 2020. TB is caused by infection with Mycobacterium tuberculosis (Mtb), that results either in a latent or active form of disease, the latter associated with Mtb spread. In the absence of an effective vaccine, epidemiologic modeling suggests that aggressive treatment of individuals with active TB (ATB) may curb spread. Yet, clinical discrimination between latent (LTB) and ATB remains a challenge. While antibodies are widely used to diagnose many infections, the utility of antibody-based tests to diagnose ATB has only regained significant traction recently. Specifically, recent interest in the humoral immune response to TB has pointed to potential differences in both targeted antigens and antibody features that can discriminate latent and active TB. Here we aimed to integrate these observations and broadly profile the humoral immune response across individuals with LTB or ATB, with and without HIV co-infection, to define the most discriminatory humoral properties and diagnose TB disease more easily. Using 209 Mtb antigens, striking differences in antigen-recognition were observed across latently and actively infected individuals that was modulated by HIV serostatus. However, ATB and LTB could be discriminated, irrespective of HIV-status, based on a combination of both antibody levels and Fc receptor-binding characteristics targeting both well characterized (like lipoarabinomannan, 38 kDa or antigen 85) but also novel Mtb antigens (including Rv1792, Rv1528, Rv2435C or Rv1508). These data reveal new Mtb-specific immunologic markers that can improve the classification of ATB versus LTB.

Male Sex Bias in Immune Biomarkers for Tuberculosis

Males have a bias toward developing sputum smear-positive pulmonary tuberculosis, whereas other forms of the disease have an equal sex ratio. Immune responses are known to be affected by estrogen and testosterone. Biomarkers may therefore be affected by these hormones, especially between 16 and 45 years of age when the differences are most marked. Using large data sets, we examined whether the male bias was significant in terms of diagnosis or predictive ability for the development of disease in those exposed to tuberculosis. Despite the large numbers, the need to specify homogeneous population groups for analysis affected the statistical power to discount a useful biomarker. In general, males showed higher interferon-gamma responses to TB antigens ESAT-6 and CFP-10, whilst females had stronger tuberculin responses in those with sputum smear- and culture-positive tuberculosis, but smaller responses in those who were screened for tuberculosis and who did not develop disease. Importantly, in contacts of sputum smear-positive pulmonary tuberculosis, more males who did not develop tuberculosis had tuberculin skin tests in the range between 10 and 14 mm, suggesting that sex-specific cut-offs might be better than general cut-off values for determining who should receive preventive treatment. Immunocytochemistry of the tuberculin responses correlated with cell numbers only in females. Total and anti-lipoarabinomannan IgM antibody levels were lower in males, whereas total and anti-BCG IgE antibody levels were higher. Evaluation of biomarkers should take account of the spectrum of tuberculosis and male sex bias for sputum smear-positive pulmonary tuberculosis. These findings improve our understanding of how immune responses contribute to the pathogenesis of infectious tuberculosis as well as suggesting clinical applications of the differences between the sexes.

Immunological Aspects of Diagnosis and Management of Childhood Tuberculosis

The diagnosis of tuberculosis (TB) in children is difficult because of the low sensitivity and specificity of traditional microbiology techniques in this age group. Whereas in adults the culture of Mycobacterium tuberculosis (M. tuberculosis), the gold standard test, detects 80% of positive cases, it only detects around 30-40% of cases in children. The new methods based on the immune response to M. tuberculosis infection could be affected by many factors. It is necessary to evaluate the medical record, clinical features, presence of drug-resistant M. tuberculosis strains, comorbidities, and BCG vaccination history for the diagnosis in children. There is no ideal biomarker for all TB cases in children. A new strategy based on personalized diagnosis could be used to evaluate specific molecules produced by the host immune response and make therapeutic decisions in each child, thereby changing standard immunological signatures to personalized signatures in TB. In this way, immune diagnosis, prognosis, and the use of potential immunomodulators as adjunct TB treatments will meet personalized treatment.

B-Cells and Antibodies as Contributors to Effector Immune Responses in Tuberculosis

Tuberculosis (TB), caused by Mycobacterium tuberculosis (Mtb), is still a major threat to mankind, urgently requiring improved vaccination and therapeutic strategies to reduce TB-disease burden. Most present vaccination strategies mainly aim to induce cell-mediated immunity (CMI), yet a series of independent studies has shown that B-cells and antibodies (Abs) may contribute significantly to reduce the mycobacterial burden. Although early studies using B-cell knock out animals did not support a major role for B-cells, more recent studies have provided new evidence that B-cells and Abs can contribute significantly to host defense against Mtb. B-cells and Abs exist in many different functional subsets, each equipped with unique functional properties. In this review, we will summarize current evidence on the contribution of B-cells and Abs to immunity toward Mtb, their potential utility as biomarkers, and their functional contribution to Mtb control.

B lymphocytes in anti-mycobacterial immune responses: Pathogenesis or protection?

The role of B cells and antibodies in tuberculosis (TB) immunity, protection and pathogenesis remain contradictory. The presence of organized B cell follicles close to active TB lesions in the lung tissue raises the question about the role of these cells in local host-pathogen interactions. In this short review, we summarize the state of our knowledge concerning phenotypes of B cells populating tuberculous lungs, their secretory activity, interactions with other immune cells and possible involvement in protective vs. pathogenic TB immunity.

Antibodies and tuberculosis

Tuberculosis (TB) remains a major public health problem internationally, causing 9.6 million new cases and 1.5 million deaths worldwide in 2014. The Bacillus Calmette-Guérin vaccine is the only licensed vaccine against TB, but its protective effect does not extend to controlling the development of infectious pulmonary disease in adults. The development of a more effective vaccine against TB is therefore a pressing need for global health. Although it is established that cell-mediated immunity is necessary for the control of latent infection, the presupposition that such immunity is sufficient for vaccine-induced protection has recently been challenged. A greater understanding of protective immunity against TB is required to guide future vaccine strategies against TB.

In contrast to cell-mediated immunity, the human antibody response against M.tb is conventionally thought to exert little immune control over the course of infection. Humoral responses are prominent during active TB disease, and have even been postulated to contribute to immunopathology. However, there is evidence to suggest that specific antibodies may limit the dissemination of M.tb, and potentially also play a role in prevention of infection via mucosal immunity. Further, antibodies are now understood to confer protection against a range of intracellular pathogens by modulating immunity via Fc-receptor mediated phagocytosis. In this review, we will explore the evidence that antibody-mediated immunity could be reconsidered in the search for new vaccine strategies against TB.

Epitope-specific antibody levels in tuberculosis: biomarkers of protection, disease, and response to treatment

Monoclonal antibodies restricted to Mycobacterium tuberculosis can measure epitope-specific antibody levels in a competition assay. Immunodominant epitopes were defined from clinical samples and related to the clinical spectrum of disease. Antibody to the immunodominant epitopes was associated with HLA-DR15. Occupational exposure showed a different response and was consistent with recognition of dormancy-related proteins and protection despite exposure to tuberculosis (TB). Studies in leprosy revealed the importance of immune deviation and the relationships between T and B cell epitopes. During treatment, antibody levels increased, epitope spreading occurred, but the affinity constants remained the same after further antigen exposure, suggesting constraints on the process of epitope selection. Epitope-specific antibody levels have a potential role as biomarkers for new vaccines which might prevent the progression of latent to active TB and as tools to measure treatment effects on subpopulations of tubercle bacilli.

Antibody responses to mycobacterial antigens in children with tuberculosis: challenges and potential diagnostic value

The identification of easily detectable biomarkers for active tuberculosis (TB) is a global health priority. Such biomarkers would be of particular value in childhood TB, which poses greater diagnostic challenges than adult TB. Serum antibodies can be detected by simple formats that provide extremely rapid results. However, attempts to develop accurate serodiagnostic tests for TB have been unsuccessful. Whereas antibody responses to mycobacterial antigens in adult TB have been studied extensively and reviewed, the same cannot be said for serologic data in pediatric populations. Here we appraise studies on serological responses in childhood TB and discuss findings and limitations in the context of the developing immune system, the age range, and the spectrum of TB manifestations. We found that the antibody responses to mycobacterial antigens in childhood TB can vary widely, with sensitivities and specificities ranging from 14% to 85% and from 86% to 100%, respectively. We conclude that the limitations in serodiagnostic studies of childhood TB are manifold, thereby restricting the interpretation of currently available data. Concerns about the methodology used in published studies suggest that conclusions about the eventual value of serodiagnosis cannot be made at this time. However, the available data suggest a potential adjunctive value for serology in the diagnosis of childhood TB. Despite the difficulties noted in this field, there is optimism that the application of novel antigens and the integration of those factors which contribute to the serological responses in childhood TB can lead to useful future diagnostics.

Humoral immune response against 38- and 16-kDa mycobacterial antigens in childhood tuberculosis

Several enzyme-linked immunosorbent assays (ELISAs) based on mycobacterial antigens have been tried for the rapid diagnosis of tuberculosis (TB). In this study, the value of the 16 and 38-kDa mycobacterial antigens in the diagnosis of TB was investigated in pediatric patients in Izmir, Turkey in whom they were found using clinical and/or bacteriological methods. A commercial ELISA kit was used for measuring IgG against 38 and 16-kDa recombinant antigens. The humoral immune response was analyzed in a group of 32 TB patients (24 pulmonary, 3 lymphadenitis and 2 pleuritis, 2 meningitis and a disseminated TB) and in control groups consisting of 20 healthy children and 20 pulmonary diseases other than TB cases. The sensitivity, specificity, positive predictive value, and the negative predictive value of the test were found to be 25%, 90%, 66.7%, and 60%, respectively, in the TB cases. The ELISA test shows very good specificity, but low level of sensitivity and negative predict value. It was thought that it might be used in combination with other methods to increase diagnostic accuracy, especially for culture-negative TB pediatric cases, which are difficult to diagnose.

Serodiagnosis of childhood tuberculosis by ELISA

OBJECTIVE

Diagnosis of childhood tuberculosis remains an enigma despite many recent technological developments. The present study has been taken up with the aim to assess the diagnostic potential of mycobacterium tuberculosis excretory-secretory ES-31 antigen and affinity purified anti ES-31 antibodies in the serodiagnosis of different spectrum of childhood tuberculosis.

METHODS

Mycobacterium tuberculosis H37Ra excretory-secretory antigen (ES-31) and affinity purified goat anti ES-31 antibodies were used in stick penicillinase ELISA for IgG antibody detection and stick Sandwich penicillinase ELISA for detection of circulating free and immune complexed antigen in the sera of 230 children.

RESULTS

Analysis of tubercular antibody, circulating free and immune complexed antigen (CIC-Ag) was done in both pulmonary and extrapulmonary form of childhood tuberculosis and overall sensitivity of 81.4% with a specificity of 93% was achieved for detection of antitubercular IgG antibodies. Of the five cases of pulmonary tuberculosis showing absence of IgG antibody, 3 showed the presence of CIC-Ag and one was found positive for both free and CIC-Ag. Similarly out of 8 cases of extrapulmonary childhood tuberculosis missed by IgG detection 5 were found to be positive for CIC-Ag and 1 showed the positive reaction for both free and immune complexed antigens.

CONCLUSION

IgG antibody to excretory-secretory antigen ES-31 is found to be having good specificity with acceptable sensitivity in detecting different forms of childhood tuberculosis. Further detection of circulating free and/or immunecomplexed antigen can be used as an adjunct tool in the diagnosis of childhood tuberculosis.

Rapid dot sputum and serum assay in pulmonary tuberculosis

A rapid direct sputum (Sp.) and/or antibody assay, based on immunoblotting and enzyme immunoassay is described. The test can detect mycobacterial antigens or antibodies in clinical specimens from pulmonary tuberculosis (TB) patients. In this study, 87 sputa, 87 sera and 40 paired sputa and sera were utilized from smear-positive and smear-negative, culture-positive patients; 59 sputa, 37 sera and 22 paired sputa and sera from nontuberculosis respiratory disease patients and 68 sera from healthy controls. The antigen detection in sputum by dot assay has 86.1% sensitivity on active tuberculosis patients, 92.9% specificity, 91.6% positive predictive value (PPV), 88.2% negative predictive value (NPV) and 10.3% error. The antibody assay has 83.6% sensitivity, 95.4% specificity, 94.4% positive predictive value, 85.6% negative predictive value and 11% error. The test performed on paired sputum and serum (Sr.) samples has a sensitivity of 93.3%, which rose to 96.1% on smear-positive and culture-positive patients, but the specificity decreased to 83% in sputum, whereas in serum it was 92%. The results of the assay, combined with clinical and radiological data, could form the basis for starting an earlier course of treatment for tuberculosis.

Serodiagnosis of tuberculosis: comparison of immunoglobulin A (IgA) response to sulfolipid I with IgG and IgM responses to 2,3-diacyltrehalose, 2,3,6-triacyltrehalose, and cord factor antigens

Nonpeptidic antigens from the Mycobacterium tuberculosis cell wall are the focus of extensive studies to determine their potential role as protective antigens or serological markers of tuberculous disease. Regarding this latter role and using an enzyme-linked immunosorbent assay, we have made a comparative study of the immunoglobulin G (IgG), IgM, and IgA antibody responses to four trehalose-containing glycolipids purified from M. tuberculosis: diacyltrehaloses, triacyltrehaloses, cord factor, and sulfolipid I (SL-I). Sera from 92 tuberculosis patients (taken before starting antituberculosis treatment) and a wide group of control individuals (84 sera from healthy donors, including purified protein derivative-negative, -positive, healed, and vaccinated individuals, and 52 sera from nontuberculous pneumonia patients), all from Spain, were studied. The results indicated a significantly elevated IgG and IgA antibody response in tuberculosis patients, compared with controls, with all the antigens used. SL-I was the best antigen studied, showing test sensitivities and specificities for IgG of 81 and 77.6%, respectively, and of 66 and 87.5% for IgA. Using this antigen and combining IgA and IgG antibody detection, high test specificity was achieved (93.7%) with a sensitivity of 67.5%. Currently, it is widely accepted that it is not possible to achieve sensitivities above 80% in tuberculosis serodiagnosis when using one antigen alone. Thus, we conclude that SL-I, in combination with other antigenic molecules, could be a useful antigen for tuberculosis serodiagnosis.

Use of multiepitope polyproteins in serodiagnosis of active tuberculosis

Screening of genomic expression libraries from Mycobacterium tuberculosis with sera from tuberculosis (TB) patients or rabbit antiserum to M. tuberculosis led to the identification of novel antigens capable of detecting specific antibodies to M. tuberculosis. Three antigens, Mtb11 (also known as CFP-10), Mtb8, and Mtb48, were tested together with the previously reported 38-kDa protein, in an enzyme-linked immunosorbent assay (ELISA) to detect antibodies in TB patients. These four proteins were also produced as a genetically fused polyprotein, which was tested with two additional antigens, DPEP (also known as MPT32) and Mtb81. Sera from individuals with pulmonary and extrapulmonary TB, human immunodeficiency virus (HIV)-TB coinfections, and purified protein derivative (PPD)-positive and PPD-negative status with no evidence of disease were tested. In samples from HIV-negative individuals, the ELISA detected antibodies in >80% of smear-positive individuals and >60% smear-negative individuals, with a specificity of approximately 98%. For this group, smears detected 81.6% but a combination of smear and ELISA had a sensitivity of approximately 93%. The antigen combination detected a significant number of HIV-TB coinfections as well as antibodies in patients with extrapulmonary infections. Improved reactivity in the HIV-TB group was observed by including the antigen Mtb81 that was identified by proteomics. The data indicate that the use of multiple antigens, some of which are in a single polyprotein, can be used to facilitate the development of a highly sensitive test for M. tuberculosis antibody detection.

Clinical utility of serodiagnosis of tuberculosis

The use of TB-ELISA tests as a diagnostic tool offer a lot of scope in early diagnosis of serious forms of childhood tuberculosis. The characteristics of these tests have improved with the availability of purified and recombinant antigens and competition assays using monoclonal antibodies. Lower antibody titres to M. tuberculosis specific antigens in children and the presence of 'natural exposure' antibodies make the interpretation of these tests difficult at times. Caution must be exercised in interpreting their results due to problems inherent on currently available methods of TB-immunodiagnosis. The selection of best combination of antigens for serology, prospective clinical trials comparing success rate of serology with the standard different diagnostic procedures are required.

Human humoral responses to antigens of Mycobacterium tuberculosis: immunodominance of high-molecular-mass antigens

The selection of antigens of Mycobacterium tuberculosis for most studies of humoral responses in tuberculosis patients has been restricted to molecules that were either immunodominant in immunized animals or amenable to biochemical purification rather than those that were reactive with the human immune system. Delineation of antigens that elicit humoral responses during the natural course of disease progression in humans has been hindered by the presence of cross-reactive antibodies to conserved regions on ubiquitous prokaryotic antigens in sera from healthy individuals and tuberculosis patients. The levels of cross-reactive antibodies in the sera were reduced by preadsorption with Escherichia coli lysates, prior to studying their reactivity against a large panel of M. tuberculosis antigens to which the human immune system may be exposed during natural infection and disease. Thus, reactivity against pools of secreted, cellular, and cell wall-associated antigens of M. tuberculosis was assessed by an enzyme-linked immunosorbent assay (ELISA). Initial results suggested that the secreted protein preparation contained antigens most frequently recognized by the humoral responses of pulmonary tuberculosis patients. The culture filtrate proteins were subsequently size fractionated by preparative polyacrylamide gel electrophoresis, characterized by reaction with murine monoclonal antibodies to known antigens of M. tuberculosis by an ELISA, and assessed for reactivity with tuberculous and nontuberculous sera. Results show that a secreted antigen of 88 kDa elicits a strong antibody response in a high percentage of patients with pulmonary tuberculosis. This and other antigens identified on the basis of their reactivity with patient sera may prove useful for developing serodiagnosis for tuberculosis.

Human T cell responses to peptide epitopes of the 16-kD antigen in tuberculosis

The 16-kD protein constituent of the Mycobacterium tuberculosis complex has been known mainly for its prominent serological immunogenicity and species specificity in tuberculous infection. In this study, we evaluated the T cell immune repertoire in 27 sensitized healthy subjects and 46 patients with active tuberculosis using 14 overlapping 20mer peptides spanning the entire sequence of this protein. Four of the tested peptides individually stimulated proliferation of blood mononuclear cells from more than 50% of healthy controls. Tuberculosis patients reacted to a narrower peptide range and with a 17-27% lower rate of responses to the four most immunogenic peptides, but these differences do not distinguish in any simple way between the T cell repertoire of patients and sensitized healthy subjects. The most immunogenic peptide (91-110) was recognized by 67% of healthy subjects and by 50% of tuberculosis patients. Importantly, several non-responders to this peptide were stimulated with the other three most permissive peptides with sequences of 111-130, 71-91 and 21-40, resulting in an overall response rate to at least one of these four peptides of 93% in healthy controls and 74% in tuberculosis patients. In view of this additive effect between the most immunogenic peptides, their combined use may achieve sufficient sensitivity in a test aimed at the specific discrimination between infected and non-infected healthy subjects. The major interest in testing with these peptides rests in their species specificity, which is not achieved using purified protein derivative (PPD).

Clinical evaluation of the diagnostic value of measuring IgG antibody to 3 mycobacterial antigen preparations in the capillary blood of children with tuberculosis and control subjects

SETTING

Department of Paediatrics in a University Hospital.

OBJECTIVE

To evaluate ABC-ELISA as serodiagnostic tests for tuberculosis in children.

DESIGN

An avidin-biotin-peroxidase conjugated anti-human immunoglobulin (ABC) enzyme-linked immunosorbent assay (ELISA) was established to measure serum immunoglobulin G (IgG) antibodies to polymerized old tuberculin (OT), tuberculin purified protein derivative (PPD) and the 30,000 dalton (30 kDa) native antigen of Mycobacterium tuberculosis, in 122 tuberculosis patients and 187 healthy children.

RESULTS

In those patients who had received less than 3 months of chemotherapy the sensitivities of the tests based on OT, PPD and the 30 kDa antigen were 40.3%, 50.0% and 36.1% respectively; the positive predictive values were 92.6%, 94.7% and 93.8% respectively and the negative predictive values were 58.6%, 62.9% and 57.2% respectively.

CONCLUSION

The ABC-ELISA may be useful for the diagnosis of pulmonary and extrapulmonary tuberculosis in groups of children with a high prior probability of tuberculosis. Antibodies to the 30 kDa antigen persisted longer than did those to OT and PPD.

Does antibody to mycobacterial antigens, including lipoarabinomannan, limit dissemination in childhood tuberculosis?

Serum immunoglobulin (Ig) G responses to a variety of mycobacterial antigens were measured in children from the UK, in children with tuberculosis from Hyderabad, India and Dhaka, Bangladesh, classified according to whether the disease was disseminated or localized, and in non-tuberculous controls. Anti-lipoarabinomannan (LAM) IgG responses in UK children showed a marked trough between 6 months and 3 years coincident with the reported peak incidence of disseminated tuberculosis. Geometric mean IgG responses to sonicates of slow-growing mycobacteria (rich in LAM) in 36 children with disseminated tuberculosis were markedly lower than in 99 children with localized tuberculous lesions (for Mycobacterium scrofulaceum P < 0.01, for M. tuberculosis P < 0.01, and for M. vaccae P < 0.01). Responses to purified LAM were also lower in the disseminated tuberculosis group (P < 0.05) but there was no difference between the groups in their response to mycobacterial 65 kDa protein. Multiple regression analysis showed that the reduced response to sonicated mycobacterial antigens and to LAM in children with disseminated disease was independent of age, nutritional status, skin test reactivity, duration of previous symptoms, and city of origin. There was no evidence for sequestration of antibody to immune complexes. These findings are compatible with the hypothesis that children with low levels of antibody to sonicated mycobacterial antigen and to LAM, or those who cannot mount an antibody response, are predisposed to dissemination. A role for antibody in preventing disseminated forms of tuberculosis in childhood has implications for the development of improved vaccines and for the optimum timing of vaccination with bacille Calmette-Guérin.

Serodiagnostic value of the 19 kilodalton antigen of Mycobacterium tuberculosis in Indian patients

Previous studies in UK subjects suggested that the 19 kDa protein antigen of Mycobacterium tuberculosis might be valuable in the serodiagnosis of paucibacillary tuberculosis. In this study, antibody titres for the 19 kDa antigen were higher in healthy controls in India than in the UK. Consequently, the diagnostic sensitivity of this antigen and its TB23 epitope was negligible in Indian patients with tuberculosis. However, a diagnostic sensitivity of 50% was achieved in patients with skin tuberculosis on the basis of a high ratio between antibody titres for the whole antigen and its TB23 epitope.

Detection of lipoarabinomannan as a diagnostic test for tuberculosis

A coagglutination technique was established for the detection of lipoarabinomannan of Mycobacterium tuberculosis in human serum samples and evaluated for its utility in the diagnosis of tuberculosis at the Instituto Nacional de Enfermedades Respiratorias in Mexico City. The test had a sensitivity of 88% in patients with sputum-smear-positive active pulmonary tuberculosis. The sensitivity in patients with active pulmonary tuberculosis negative for acid-fast bacilli in sputum was 67%. Less favorable results were obtained for patients with AIDS and tuberculosis, with a sensitivity of 57%. The specificity in control patients with lung diseases different from tuberculosis and in healthy subjects was 100%. The positive predictive value was 100%, and the negative predictive value for patients with sputum-positive active pulmonary tuberculosis was 97%. The results of this study suggest that the detection of lipoarabinomannan is an accurate test for the diagnosis of pulmonary tuberculosis.

Antibodies to Mycobacterium tuberculosis-specific epitopes in lepromatous leprosy

Sera from patients with leprosy or tuberculosis and healthy subjects have been analysed for the presence of antibodies to four species-specific mycobacterial epitopes, four different viruses and five autoantigens. Antibodies to the Mycobacterium leprae-specific 35-kD protein and phenolic glycolipid I epitopes were not present in patients with active pulmonary tuberculosis. In contrast, antibody levels to species-specific epitopes of the 38-kD and 14-kD antigens M. tuberculosis were significantly elevated in patients with lepromatous leprosy. Neither of the two antigens is cross-reactive with M. leprae at the B cell level. However, it was considered that cross-reactive helper T cells could recall the response of M. tuberculosis-specific memory B cells, which had been primed through prior self-healing tuberculous infection. As an alternative explanation, the possible role of polyclonal B cell stimulation was considered. This seemed unlikely, however, since: (i) antibody levels to autoantigens, except anti-smooth muscle, were not elevated, and (ii) antibody levels to four distinct viruses, unlike those to all mycobacterial epitopes, showed no correlation with titres, to M. tuberculosis-specific epitopes.

A rapid, simple enzyme immunoassay for detection of antibody to individual epitopes in the serodiagnosis of tuberculosis

The antibody response to individual epitopes has previously been analysed by competition assay using 125I- or enzyme labelled monoclonal antibodies. A modification of the test is described in which competition of human sera with unlabelled mouse monoclonal antibodies at the limiting dilution is revealed by peroxidase labelled antimouse IgG conjugate. Analysis of 54 sera from patients with pulmonary (36) and extra-pulmonary (18) tuberculosis and 31 controls indicated that the modified test compares favourably with the test based upon directly labelled antibodies. Diagnostic sensitivity for five monoclonal antibodies evaluated was 11.1% (TB78), 35.2% (TB23 and TB68), 37.0% (TB71) and 61.1% (TB72) at 97.5% specificity. For TB72, sensitivity was highest for pulmonary disease (69.7%). The modified assay is also easier to standardise for screening new monoclonal antibodies using a single enzyme-labelled conjugate.

Human tuberculosis sera show prominent antibody responses to particulate fractions of Mycobacterium tuberculosis

Sera from smear-positive pulmonary tuberculosis patients and normal control subjects in Madras were analyzed by Western blotting for their reactivity with soluble and particulate (membrane-rich and cell wall-rich) antigen fractions extracted by sonication from Mycobacterium tuberculosis H37Rv. Discrimination between patient and control sera was best with particulate antigen fractions: 60% of patient sera reacted with a 38-kD antigen band and 90% reacted with a 55-kD band. Reactions of control sera with the 38- and 55-kD bands were infrequent and faint. The results suggest that a serodiagnostic test might be based on quantitation of responses to these two antigens.

Potential value of serology for diagnosis of extrapulmonary tuberculosis

The possible benefits for improved diagnosis of extrapulmonary tuberculosis were evaluated by means of a modified serological competition assay. Antibodies to the 38 kD Mycobacterium tuberculosis antigen were detected in 73% of cases of extrapulmonary tuberculosis and 70% of smear-negative cases of pulmonary tuberculosis at a chosen specificity of 98%. Of 55 patients (45%) whose diagnosis of tuberculosis was not made until 7 days or longer after admission, 76% were positive for the antibody. In this group the serological test would have allowed chemotherapy to be started on average 12.5 days earlier and would have obviated the need for 80% of diagnostic biopsies.

Further evaluation of an indirect enzyme-linked immunosorbent assay for the diagnosis of bovine tuberculosis

The sensitivity and specificity of an ELISA for the detection of bovine IgG anti-Mycobacterium bovis antibodies were 73.6% and 94.1%, respectively, as determined in 53 bacteriologically confirmed tuberculous cattle and 101 healthy cattle from a tuberculosis-free area. In addition, the results of ELISA and tuberculin tests in 149 cattle were compared with those of subsequent necropsy studies. Both tests failed to detect 2 animals with tuberculous lesions and positive culture; 3/12 cattle with M. bovis isolation and no lesions, and 2/7 with atypical mycobacterial infection reacted to tuberculin, but none had antibodies; in 128 cattle with neither lesions nor mycobacterial isolation, 6 were tuberculin reactors and 7 others had antibodies. Negative results were obtained by ELISA in 21/22 paratuberculous cattle. Antibodies were not detected in 88.9% to 96.4% of 697 cattle from two tuberculin negative herds of an endemic area. In a herd with proved M. bovis infection, distribution of seropositive animals in tuberculin and non-tuberculin reactors was similar. Antibody responses to cutaneous tuberculin stimuli were observed in 4 experimentally infected cattle, but only in 2/10 healthy controls after repeated PPD stimuli. Nine controls which had either received a single tuberculin dose or none showed no increase in antibody levels. The low sensitivity of this ELISA limits its usefulness as a diagnostic tool for bovine tuberculosis eradication campaigns. However, it could be helpful in epidemiological surveillance if its efficiency to identify infected herds is demonstrated.

Levels of antibody to defined antigens of Mycobacterium tuberculosis in tuberculous meningitis

The aim of the study was to develop an antibody immunoassay for tuberculous meningitis with a sensitivity greater than that of the initial bacterial smear while maintaining a 100% specificity. Antibody titers to five purified antigens of Mycobacterium tuberculosis were measured in cerebrospinal fluid by using an enzyme-linked immunosorbent assay technique. Seventy-four patients with tuberculous meningitis (26 culture positive) were compared with 26 patients with purulent meningitis, 69 patients with suspected but excluded tuberculous meningitis, and 29 patients with other neurological diseases. Antibody responses to both the 14-kilodalton (kDa) antigen and lipoarabinomannan were immunodominant, accounting for 74% of those with any antibody titer to a soluble extract of M. tuberculosis. A sensitivity of 61% was achieved (77% for the culture-positive samples) by using immunoglobulin G titers to lipoarabinomannan and the 14-, 19-, and 38-kDa antigens. Thus, estimation of levels of antibody in cerebrospinal fluid to selected mycobacterial antigens would be valuable in the diagnosis of tuberculous meningitis. The possible reasons for the immunodominance of the 14-kDa antigen and lipoarabinomannan in the pathogenesis of tuberculous meningitis are discussed.