Antibody markers of incident tuberculosis among HIV-infected adults in the USA: a historical prospective study.

OBJECTIVE: To assess whether serum levels of antibodies against Mycobacterium tuberculosis antigens increase before diagnosis of active tuberculosis (TB). DESIGN: Serial serum samples were obtained from 30 human immunodeficiency virus (HIV) co-infected individuals who developed active TB during a multicenter prospective study on pulmonary complications of HIV/AIDS conducted among >1300 subjects in the USA in the 1980s. Multiple serum samples from 47 matched control individuals who did not develop TB in the same study were also tested. Immunoglobulin G (IgG) antibodies to 10 M. tuberculosis proteins were detected by enzyme-linked immunosorbent assay (ELISA), and data were analyzed by descriptive and inferential statistical techniques to assess patterns, trends and differences in antibody levels relative to time from TB diagnosis. RESULTS: Antibodies to five antigens (ESAT-6, 38 kDa Ag, 16 kDa Ag, malate synthase and MTSA-10/CFP-10), but not to five other antigens (Rv2626c, ferredoxin A, glutamine synthetase, alanine dehydrogenase and Ag85) increased before diagnosis of TB relative to control levels. The earliest increase in the TB group was detected for MTSA-10/CFP-10 (24-30 months pre-diagnosis). CONCLUSIONS: Levels of serum antibodies to particular proteins of M. tuberculosis increase before microbiological and clinical symptoms of active TB. The use of antibody biomarkers for prognostic purposes should therefore be feasible.

Integration of microscopy and serodiagnostic tests to screen for active tuberculosis.

SETTING: University of California San Diego Medical Center, USA. OBJECTIVE: To create a simple screening strategy for tuberculosis (TB) that includes antibody detection assays to improve the accuracy of microscopic examination of sputum for acid-fast bacilli (AFB smear). METHODS: Serum samples were obtained from 190 patients suspected of having active TB. TB diagnosis was established by Mycobacterium tuberculosis culture. HIV status was determined by commercial serologic tests. IgG antibody levels were measured by ELISA using purified M. tuberculosis antigens. Data from 130 randomly selected patients were used to develop a screening strategy; data from the remaining 60 patients were used for validation. RESULTS: AFB smear had 70% sensitivity and 88% specificity. In algorithms integrating single or multi-antigen ELISA with AFB smear and HIV results, the sensitivity improved over each test alone. The algorithm that included a four-antigen ELISA (38 kDa antigen, lipoarabinomannan, MPT-64 and glutamine synthase) had a sensitivity of 93% and a specificity of 76%. Compared to AFB smear, the sensitivity of the algorithm was significantly higher, while the specificity was not statistically different. CONCLUSION: This study demonstrates that a screening strategy can be created by integrating multi-antigen ELISA with AFB smear and HIV testing.

Immunological characterization of antigens encoded by the RD1 region of the Mycobacterium tuberculosis genome.

Development of immunoassays specific for the diagnosis of tuberculosis requires antigens unique to Mycobacterium tuberculosis. In a search for such antigens we tested six proteins encoded by RD1, a region present in M. tuberculosis and virulent M. bovis genomes but missing from the DNA of all substrains of M. bovis Bacillus Calmette-Guerin (BCG). The six proteins (Rv3871, Rv3872, Rv3873, MTSA-10, ESAT-6 and Rv3878) were purified to near-homogeneity from recombinant Escherichia coli. When tested for the ability to elicit antibody responses and delayed type hypersensitivity in tuberculous guinea pigs, only two of six antigens, ESAT-6 and MTSA-10, elicited strong skin reactions, while vigorous antibody responses were observed to all six proteins. When antibody responses to RD1 antigens were evaluated in sera from patients having pulmonary tuberculosis and from control subjects (patients having mycobacterioses other than tuberculosis, and healthy persons), a sizeable proportion (25%) of tuberculosis patients but none of the control subjects, had antibodies against MTSA-10 and/or ESAT-6. We conclude that MTSA-10 and ESAT-6 are promising candidates for immunodiagnostic assays specific for tuberculosis.