Progress in serodiagnosis of Mycobacterium tuberculosis infection

Biomarkers for clinical and incipient tuberculosis: performance in a TB-endemic country

Background

Simple biomarkers are required to identify TB in both HIV⁻TB⁺ and HIV⁺TB⁺ patients. Earlier studies have identified the M. tuberculosis Malate Synthase (MS) and MPT51 as immunodominant antigens in TB patients. One goal of these investigations was to evaluate the sensitivity and specificity of anti-MS and –MPT51 antibodies as biomarkers for TB in HIV⁻TB⁺ and HIV⁺TB⁺ patients from a TB-endemic setting. Earlier studies also demonstrated the presence of these biomarkers during incipient subclinical TB. If these biomarkers correlate with incipient TB, their prevalence should be higher in asymptomatic HIV⁺ subjects who are at a high-risk for TB. The second goal was to compare the prevalence of these biomarkers in asymptomatic, CD4⁺ T cell-matched HIV⁺TB⁻ subjects from India who are at high-risk for TB with similar subjects from US who are at low-risk for TB.

Methods and Results

Anti-MS and -MPT51 antibodies were assessed in sera from 480 subjects including PPD⁺ or PPD⁻ healthy subjects, healthy community members, and HIV⁻TB⁺ and HIV⁺TB⁺ patients from India. Results demonstrate high sensitivity (∼80%) of detection of smear-positive HIV⁻TB⁺ and HIV⁺TB⁺ patients, and high specificity (>97%) with PPD⁺ subjects and endemic controls. While ∼45% of the asymptomatic HIV⁺TB⁻ patients at high-risk for TB tested biomarker-positive, >97% of the HIV⁺TB⁻ subjects at low risk for TB tested negative. Although the current studies are hampered by lack of knowledge of the outcome, these results provide strong support for the potential of these biomarkers to detect incipient, subclinical TB in HIV⁺ subjects.

Conclusions

These biomarkers provide high sensitivity and specificity for TB diagnosis in a TB endemic setting. Their performance is not compromised by concurrent HIV infection, site of TB and absence of pulmonary manifestations in HIV⁺TB⁺ patients. Results also demonstrate the potential of these biomarkers for identifying incipient subclinical TB in HIV⁺TB⁻ subjects at high-risk for TB.

Mycobacterium bovis BCG vaccine strains lack narK2 and narX induction and exhibit altered phenotypes during dormancy

Mycobacterium tuberculosis is the causative agent of tuberculosis, a disease that affects one-third of the world's population. The sole extant vaccine for tuberculosis is the live attenuated Mycobacterium bovis bacillus Calmette-Guerin (BCG). We examined 13 representative BCG strains from around the world to ascertain their ability to express DosR-regulated dormancy antigens. These are known to be recognized by T cells of M. tuberculosis-infected individuals, especially those harboring latent infections. Differences in the expression of these antigens could be valuable for use as diagnostic markers to distinguish BCG vaccination from latent tuberculosis. We determined that all BCG strains were defective for the induction of two dormancy genes: narK2 (Rv1737c) and narX (Rv1736c). NarK2 is known to be necessary for nitrate respiration during anaerobic dormancy. Analysis of the narK2/X promoter region revealed a base substitution mutation in all tested BCG strains and M. bovis in comparison to the M. tuberculosis sequence. We also show that nitrate reduction by BCG strains during dormancy was greatly reduced compared to M. tuberculosis and varied between tested strains. Several dormancy regulon transcriptional differences were also identified among the strains, as well as variation in their growth and survival. These findings demonstrate defects in DosR regulon expression during dormancy and phenotypic variation between commonly used BCG vaccine strains.

Identification of Rv0222 from RD4 as a novel serodiagnostic target for tuberculosis

Forty-seven Mycobacterium tuberculosis genes from the 'regions of difference' RD2-7, RD9-13 and RD15 were cloned and expressed, and the purified recombinant proteins were screened for their serodiagnostic potential. Evaluation of six selected proteins in serum samples from Danish resident tuberculosis patients and healthy controls led to identification of Rv0222 as the most promising serodiagnostic antigen. Recognition of the Rv0222 was compared with the 38 kDa protein and a fusion protein of the RD1 proteins ESAT-6 and CFP10 in a serum panel from pulmonary tuberculosis (TB) patients from Uganda. The highest overall sensitivity was observed for Rv0222 compared to BCG-vaccinated non-endemic healthy controls as well as symptomatic endemic controls. Importantly, Rv0222 identified human immuno deficiency (HIV) virus-positive patients and HIV-negative patients with the same overall sensitivity. The results emphasize the importance of cut-off values in TB endemic regions based on endemic control individuals to diagnose active TB, and identify Rv0222 as a promising new antigen for serodiagnosis of TB in both HIV-negative and HIV-positive patients.

Humoral immune responses of tuberculosis patients in Brazil indicate recognition of Mycobacterium tuberculosis MPT-51 and GlcB

The humoral responses to recombinant MPT-51 and GlcB was determined by using an enzyme-linked immunosorbent assay. Levels of immunoglobulin M (IgM) against MPT-51 and IgG against GlcB were higher among tuberculosis (TB) patients than among control individuals. When the MPT-51 and GlcB assays were combined, 90.8% specificity and 75.5% sensitivity were observed. MPT-51 and GlcB were recognized in the humoral responses of Brazilian TB patients.