Effect of adjusted cut-offs of interferon-γ release assays on diagnosis of tuberculosis in patients with fever of unknown origin

Background

Tuberculosis (TB) is a leading cause of fever of unknown origin (FUO). In recent years, interferon-γ release assays (IGRAs) have been widely utilized and the cut-off values given by the manufacturers are set in countries where rates of TB are not as high.

Methods

A prospective cohort study was conducted in a Chinese general hospital to evaluate the diagnostic performance of T-SPOT.TB (T-SPOT) and QuantiFERON-TB Gold (QFT) in detecting active TB (ATB) in a high TB endemic area. Test results were compared with the culture and clinically confirmed diagnosis. Further, we explored an alternative method of interpreting IGRAs by increasing the cut-off values.

Results

The sensitivity and specificity of T-SPOT in detecting ATB were 85.3% (95% CI 81.6–94.0%) and 71.8% (95% CI 67.3–76.0%), respectively. The sensitivity and specificity of QFT were 72.3% (95% CI 62.8–80.1%) and 77.0% (95% CI 72.7–80.8%), respectively. Receiver operating characteristic analysis was used for evaluation of different cut-off values. When the cut-off values were adjusted as 125 spot-forming cells (SFCs)/ 2.5*10⁵ cells for T-SPOT and 4.0 IU/ml for QFT, the specificity could be improved to > 90.0% (90.3% and 94.1%, respectively), and the sensitivity were 43.1% and 41.6%, respectively. The new adjusted cut-off values were validated in another independent validation cohort.

Conclusion

The adjusted cut-off values of the two assays considerably improved the diagnostic value when applied to FUO patients in clinical settings.

Effects of the fusion design and immunization route on the immunogenicity of Ag85A-Mtb32 in adenoviral vectored tuberculosis vaccine

Vaccines containing multiple antigens may induce broader immune responses and provide better protection against Mycobacterium tuberculosis (Mtb) infection as compared to a single antigen. However, strategies for incorporating multiple antigens into a single vector and the immunization routes may affect their immunogenicity. In this study, we utilized recombinant adenovirus type 5 (rAd5) as a model vaccine vector, and Ag85A (Rv3804c) and Mtb32 (Rv0125) as model antigens, to comparatively evaluate the influence of codon usage optimization, signal sequence, fusion linkers, and immunization routes on the immunogenicity of tuberculosis (TB) vaccine containing multiple antigens in C57BL/6 mice. We showed that codon-optimized Ag85A and Mtb32 fused with a GSG linker induced the strongest systemic and pulmonary cell-mediated immune (CMI) responses. Strong CMI responses were characterized by the generation of a robust IFN-γ ELISPOT response as well as antigen-specific CD4(+) T and CD8(+) T cells, which secreted mono-, dual-, or multiple cytokines. We also found that subcutaneous (SC) and intranasal (IN)/oral immunization with this candidate vaccine exhibited the strongest boosting effects for Mycobacterium bovis bacille Calmette-Guérin (BCG)-primed systemic and pulmonary CMI responses, respectively. Our results supported that codon optimized Ag85A and Mtb32 fused with a proper linker and immunized through SC and IN/oral routes can generate the strongest systemic and pulmonary CMI responses in BCG-primed mice, which may be particularly important for the design of TB vaccines containing multiple antigens.