Granzyme B as a diagnostic marker of tuberculosis in patients with and without HIV coinfection.

Immunodiagnostic tests for tuberculosis (TB) are based on the estimation of interferon γ (IFN-γ) or IFN-γ-secreting CD4(+) T cells following ex vivo stimulation with ESAT6 and CFP-10. Sensitivity of these tests is likely to be compromised in CD4(+) T-cell-depleted situations, like HIV-TB coinfection. CD4(+) and CD8(+) T cells, isolated from 3 groups, viz., HIV-negative patients with active TB, HIV-TB coinfected patients, and healthy household contacts (HHCs) were cocultivated with autologous dendritic cells, and the cytokine response to rESAT6 stimulation was compared between groups in supernatants. While CD4(+) T-cell stimulation yielded significantly elevated levels of IFN-γ and interleukin 4 in HIV-negative TB patients, compared to HHCs, the levels of both these cytokines were nondiscriminatory between HIV-positive TB patients and HHCs. However, CD8(+) T-cell stimulation yielded significantly elevated granzyme B titers in both groups of patients, irrespective of HIV coinfection status. Hence, contrary to IFN-γ, granzyme B might be a useful diagnostic marker for Mycobacterium tuberculosis infection particularly in HIV coinfected patients.

Application of lipoarabinomannan antigen in tuberculosis diagnostics: current evidence.

Tests based on the detection of mycobacterial lipoarabinomannan (LAM) antigen in urine have emerged as potential point-of-care tests for tuberculosis (TB). We aimed to assimilate the current evidence regarding the diagnostic performance of LAM assays and to ascertain their clinical indication in settings with high and low prevalence of HIV-TB co-infection. Owing to suboptimal sensitivity, the urinary LAM assays are unsuitable as general screening tests for TB. However, unlike traditional diagnostic methods, they demonstrate improved sensitivity in HIV-TB co-infection which further increases with low CD4 counts. Accordingly, these assays are indicated as rule-in tests for TB in patients with advanced HIV-induced immunosuppression, and facilitate the early initiation of antituberculous treatment in them. They also offer incremental sensitivity and specificity when used as adjunct tests to smear microscopy and chest radiography in HIV-TB co-infection. They obviate the biohazards associated with sputum samples and provide an alternative diagnostic tool in sputum-scarce patients. Notwithstanding these advantages, the specificity of these assays is variable, which is mostly attributable to misclassification bias and cross-reactivity with non-tuberculous mycobacteria or other commensal flora. Furthermore, the inability to detect low titres of antigen in HIV-uninfected patients makes these assays unsuitable for use in settings with a low HIV prevalence. Future research targeted towards inclusion of specific monoclonal antibodies and more sensitive immunoassay platforms might help to improve the diagnostic performance of these assays and extend their applicability to the general population of patients with TB.

Ethnicity-tailored novel set of ESAT-6 peptides for differentiating active and latent tuberculosis.

Differentiation between active and latent TB is a diagnostic challenge in TB-endemic regions. The commercially available IFN-γ-release assays are unsuitable for achieving this discrimination. We, therefore, screened ESAT-6 and CFP-10 proteins through population coverage analysis to identify minimal sets of peptides that can discriminate between these two forms of TB in a North Indian population. Comparing the diagnostic performance of a set of 2 ESAT-6 peptides (positions: 16-36; 59-79) to that of the QuantiFERON(®)-TB Gold IT (QFTGIT) assay, we observed significant difference in IFN-γ and TNF-α levels between patients (n = 15) and their age- and sex-matched healthy household contacts (n = 15). While the mean (±SD) IFNγ titer was 241.8 (±219.24) IU/ml for patients, the same in controls was 564.2 (±334.82) IU/ml (p = 0.039). Similarly the TNFα response was significantly higher in patients, compared to controls (796.47 ± 175.21 IU/ml vs. 481.81 ± 378.72 IU/ml; p = 0.047). IL-4 response to these peptides was non- discriminatory between the two groups. The QFTGIT Assay, however, elicited no significant difference in IFN-γ, TNF-α or IL-4 levels. Hence we conclude that IFN-γ or TNF-α response to these ESAT-6 peptides has the potential to differentiate between active and latent TB in our population.