Clinical usefulness of serological measurements obtained by antigen 60 in mycobacterial infections: development of a new concept

Improved sensitivity for detection of tuberculosis cases by a modified Anda-TB ELISA test

Tuberculosis is still a major health problem in most developing countries and its incidence is rising in many industrial countries. The diagnosis of tuberculosis depends primarily on identification of mycobacteria and on clinicoradiological evidence of the disease. Compared to other diagnostic methods, serological tests are faster and do not necessarily require samples that contain the tubercle bacilli. We have evaluated a modified version of a commercially available enzyme immunoassay test to detect the presence of circulating anti-mycobacterial IgG and IgM antibodies in tuberculosis patients. The sensitivity and the specificity of the test reaches 87% and 95% respectively. In conclusion, the modified Anda-TB enzyme immunoassay test offers a good and reliable test for diagnosis of tuberculosis in suspected cases of active pulmonary tuberculosis.

Tuberculosis I: a conceptual frame for the immunopathology of the disease

An analysis of the cellular and humoral immune responses after bacille Calmette-Guerin (BCG) vaccination and during tuberculosis treatment favors the hypothesis of an immune defence developed in four overlapping successive stages. The initial immune response is innate. The following two intermingle innate and specific responses against low molecular weight oligopeptidic and nonpeptidic antigens, as muramyldipeptide and trehalose dimycolate, and large molecular weight nonpeptidic antigens such as lipoarabinomannan. The ultimate specific response is directed against protein antigens as Antigen 60. BCG and primary tuberculosis (TB) infections induce cellular and humoral immune responses essentially against oligopeptidic and small and large molecular weight nonpeptidic antigens. Immune responses against non-peptidic substances contribute to the immunoprotection of the infected person who develops a primary infection. Some infected people allow the expression of the immunosuppressive activity of the pathogen. This results in the synthesis of interleukin-10 (IL-10), which suppresses the formation of interferon-gamma (INF-gamma) and IL-2, and of IL-6, which suppresses T-cell responses. These patients have a skewed immune response against non-peptidic antigens and present with symptoms. They will not recover unless responses directed against proteinic antigens occur, which restore INF-gamma and IL-2 production. The formation of immumoglobulin-G (IgG)-type antibodies and of a cellular immunity against mycobacterial peptidic antigens is essential for a good protection against a post-primary infection.

Anti-Kp 90 IgA antibodies in the diagnosis of active tuberculosis

BACKGROUND

Accurate diagnosis of active tuberculosis (TB) has been difficult historically, yet a great demand persists for a rapid and reliable diagnostic method. Detection of Mycobacterium tuberculosis anti-Kp 90 IgA antibodies is one of the more novel techniques.

STUDY OBJECTIVES

To evaluate the diagnostic value of a recently developed enzyme-linked immunosorbent assay (ELISA) test, which detects IgA antibodies against M tuberculosis Kp 90 antigen, and to compare the results with conventional diagnosis and the polymerase chain reaction (PCR) method.

PARTICIPANTS

Serum, ethylenediaminetetraacetic acid (EDTA)-blood, and body fluid samples were obtained from 51 patients with active TB and 71 control subjects. The clinical diagnosis of TB was supported by a positive culture (n = 6), detection of acid-fast bacilli on smear (n = 35), or both (n = 10).

MEASUREMENTS AND RESULTS

IgA antibodies were detected in sera and/or body fluid samples from 82% of patients with TB and 10% of controls. M tuberculosis DNA was detected in body fluid sample of 96% and blood sample of 49% of patients with TB by PCR. None of the blood and 5.6% of the body fluid specimens from controls were PCR-positive.

CONCLUSIONS

Anti-Kp 90 IgA antibodies were detected using ELISA in 78% of serum and 69% of body fluids from patients with TB, therefore, this test is promising for the diagnosis of active TB and appears to be more reliable, particularly for body fluid samples.

Serological diagnosis of childhood tuberculosis by estimation of mycobacterial antigen 60-specific immunoglobulins in the serum

SETTING

An ELISA assay based on mycobacterial antigen 60 (A60) for the estimation of specific immunoglobulins in the serum has been used successfully for the rapid diagnosis of tuberculosis in studies done predominantly in Western countries. In a recent Indian study, encouraging results were reported in adult tuberculosis.

OBJECTIVE

To evaluate the utility of this ELISA test for rapid diagnosis of different clinical forms of tuberculosis in Indian children.

DESIGN

ELISA test based on mycobacterial A60 was used to estimate specific IgM, IgA and IgG antibodies in the sera obtained from 452 cases of tuberculosis and 161 controls in the paediatric population of Delhi, India.

RESULTS

Of the 161 controls, only 7.4% were positive for IgM, 4.3% for IgG, 3.7% for IgA and 8% when a combination of IgM and IgA was considered. Of 58 cases of definite pulmonary tuberculosis, 55.2% were positive for IgM, 32.7% for IgG, 36.2% for IgA and a high positivity of 72.4% was seen when IgA and IgM estimations were combined. The corresponding figures in 150 cases of definite extrapulmonary tuberculosis were 57.3%, 36.6%, 38% and 76.6%. A relatively weak serology was observed in 244 cases of probable tuberculosis. A very high positivity (95%) was seen in acid-fast bacilli-positive cases of tuberculosis.

CONCLUSIONS

Our findings point to a very good specificity (92%) and a reasonably good sensitivity (75.5%) of the test when combined IgM and IgA antibody titres are considered in the diagnosis of childhood tuberculosis.

Mycobacterial infections: are the observed enigmas and paradoxes explained by immunosuppression and immunodeficiency?

The enigmas and paradoxes observed in tuberculous patients, in Bacille Calmette-Guérin-vaccinated people and in Bacille Calmette-Guérin-treated cancer patients have been examined, in an attempt to explain them through the mechanisms of immunodeficiency and immunosuppression. A dual effect is postulated: an immunosuppression induced by the infecting mycobacteria that adds to a pre-existing or emerging state of immunodeficiency of the infected individual. The immunological cellular and humoral anergies observed at the beginning of a tuberculous therapy are usually lifted after the first two weeks of treatment. This restoration of immune responsiveness may be attributed to the destruction or to the growth inhibition of immunosuppressive mycobacteria. The observation that drugs cytocidal in vitro do not always sterilize the patients under treatment whereas bacteriostatic drugs do, may find an explanation in the dual immunosuppression induced by cytocidal drugs and mycobacteria. The fact that Bacille Calmette-Guérin applied as an immunotherapy to residual cancer has either a favorable or an unfavorable action may be due to the immunosuppressive activity attached to some Bacille Calmette-Guérin strains and to some cancers. The variable protective activity of Bacille Calmette-Guérin vaccines may be due to the immunological status of the vaccinated people and the compositional differences between strains. The protective activity of subunit vaccines in experimental models can be attributed to the elimination of immunosuppressive factors present in whole killed mycobacteria.

Diagnostic utility of the estimation of mycobacterial Antigen A60 specific immunoglobulins IgM, IgA and IgG in the sera of cases of adult human tuberculosis

SETTING

An ELISA assay based on mycobacterial Antigen A60 for the estimation of Mycobacterium-specific immunoglobulins in the serum has been used successfully for the rapid diagnosis of tuberculosis in studies done in the Western countries. There are hardly any similar large scale studies in India.

OBJECTIVE

To evaluate the utility of this ELISA test for rapid diagnosis of different clinical forms of tuberculosis in an adult Indian population.

DESIGN

ELISA test based on mycobacterial antigen A60 (Anda Biologicals, France) was used to estimate specific IgM, IgA and IgG antibodies in the sera obtained from 337 cases of tuberculosis and 131 controls in the population of Delhi (India).

RESULTS

Of the 131 controls, only 9.9% were positive for IgM, 7.6% for IgG, 6.1% for IgA and 9.9% when an IgA and IgG combination was considered. Of 122 cases of active pulmonary tuberculosis, 41% were positive for IgM, 86.8% for IgA, 88.5% for IgG and a very high positivity (98.3%) was seen when IgA and IgG estimations were combined. A relatively low seropositivity was observed in 25 cases of pleural tuberculosis. The corresponding figures in 130 cases of extrapulmonary tuberculosis were 22.3%, 68.4%, 73.8% and 86.15%. When 60 cases of pulmonary tuberculosis who had been successfully treated with antituberculosis drugs were analyzed the rates of seropositivity fell to 11.6%, 46.6%, 58.3% and 66.6% respectively.

CONCLUSION

Our findings point to a very good sensitivity (91.6%) and specificity (90.0%) of the test when combined IgA and IgG antibody titres are considered, to detect cases of adult tuberculosis. The role of IgM estimation can be restricted to the detection of cases of reactivation of tuberculosis.

Humoral immune response against antigen 60 in BCG-vaccinated infants

An ELISA assay based on the A-60 antigen complex from Mycobacterium bovis BCG cytoplasm was used to detect anti-mycobacterial antibodies of different classes in the sera of 63 BCG-vaccinated infants during the 6-month post-vaccination period. The mean IgM and IgA levels increased, whereas the mean IgG level decreased after BCG vaccination. However, in a minority of cases only Ig levels were above the cut-off line: this was true for IgM in 11/63 (17%) cases and for IgA in 14/63 (22%) of cases but none of the tested infants was anti-A60 IgG ELISA positive. Fifty-two infants (83%) were tuberculin-positive eight weeks after vaccination, and no significant difference in mean antibody levels of tuberculin-positive and negative cases was observed, except for IgG (p < 0.05).

Serological analysis by A60 antigen ELISA and BCG immunoblotting in domestic carnivores experimentally vaccinated with mycobacterium bovis BCG

The serological response to Mycobacterium bovis BCG vaccination was studied in 3 cats, 3 dogs and 3 puppies. The animals received two doses of 0.1 mg of BCG and were studied over a period of 8 months simultaneously by ELISA with antigen A60 (Anda Biologicals, Strasbourg, France) and immunoblotting with BCG. The two methods detected an increase of antibodies at 3 or 5 weeks. In the cats, specific antibody titer remained high and stable for more than one year, in the dogs they diminished quickly within 23 weeks. Carnivores elicit a serological response against specific protein antigens in the bands corresponding to 18 and 25kDa, 30 and 45kDa, and especially 35 and 42kDa bands, and to minor bands at higher molecular weight. Non-specific bands at 50-55 kDa and 70 kDa, assigned to heat shock proteins, were enhanced by BCG vaccination. Cat immunosera recognized on A60 immunoblots the specific homologous bands at 20-25 kDa and 32 kDa, but also two other dominant bands: one was partially specific (65 kDa) and the other was absent from the M. bovis and M. paratuberculosis profiles. The suitability of A60 ELISA for detection of mycobacterial infection in carnivores may be highlighted by immunoblot analysis.

Serological response of tuberculosis patients to antigen 60 of BCG

Two ELISA tests (IgG and IgM) for the serodiagnosis of tuberculosis, both based on antigen 60 (A60) of M. bovis BCG, were applied to 1,644 controls and patients to analyse the immune response in different forms of this infectious disease. Out of 200 healthy individuals, 148 being tuberculin--positive BCG-vaccinated adults, only 10 contacts--nurses of the pneumology department and laboratory technicians of the mycobacterial laboratory--were found positive for anti-A60 IgG. One quarter of hospitalized patients affected by non-tuberculous pneumopathies (194 in total) were found weakly positive for anti-A60 IgG. We suppose that these positive cases have suffered from inapperant infections and are in a "persistent state". Out of 344 cases of primary pulmonary tuberculosis, 88% were positive for anti-A60 IgG and 75% for the corresponding IgM. Among 97 cases of primary extra-pulmonary tuberculosis, 94% were found IgG positive and 33% IgM positive. The difference between active and inactive post-primary (chronic) tuberculosis was striking: about 100% of both pulmonary and extra-pulmonary cases (367 altogether) had high titers of anti-A60 IgG but IgM positivity was observed in only 15% of the cases, whereas in inactive and quiescent noncavitary tuberculosis (442 cases), 57% of the patients were weakly positive for anti-A60 IgG and none were positive for IgM. Kinetics of synthesis of anti-A60 IgG and IgM were analysed in primary and post-primary (chronic) active tuberculosis. The IgM tracing immune response to A60 was shorter and lower during primary tuberculosis as compared to post-primary tuberculosis. Our findings point to the high prognostic value of the A60- ELISA test for tuberculosis. Anti-A60 IgM mark initial stages of the disease or reactivation processes whereas anti-A60 IgG last longer than IgM and provide an evaluation of the intensity of the infectious process. Repeated serological tests allow monitoring of the course of the infection and the efficacy of therapy. The test is negative in healthy BCG-vaccinated persons (tuberculin-positive) and healed tuberculous infection cases. The combined use of both IgG and IgM tests helps in the correct diagnosis of "false positive" cases.