Although many studies have compared in vitro TB diagnostic tests with the venerable tuberculin skin test (TST), there is little understanding of the quantitative relationship between critical measures of antimycobacterial immunity used to detect TB infection. We, therefore, decided to determine the degree of redundancy between quantitative read-outs of in vivo and in vitro assays of antimycobacterial immunity.
We enrolled 475 healthy HIV-negative children and young adults living in a hyperendemic area of TB. We measured in vivo TST responses, and a 1:10 diluted 3- or 7-day whole-blood assay was used to determine the in vitro antigen-specific interferon (IFN)-γ cytokine release. The frequency of antigen-specific IFN-γ+CD4+ and IFN-γ+CD8+ cells was tested using intracellular cytokine staining after 1 day incubation.
In vivo TST responses segregated into two well-separated groups with either no measurable response (TST induration < 5 mm; n = 164) or a normally distributed group with TST indurations ≥ 5 mm with peak at 15 mm (n = 260). In vitro assays provided a less pronounced separation of responders and nonresponders. Correlation analysis of responses among persons with TST ≥ 5 mm demonstrated that extent of TST response was poorly correlated with IFN-γ release (coefficients of correlation ρ = 0.17-0.22) and frequency of IFN-γ+CD4+/CD8+ cells (ρ = 0.05-0.17) across three stimulating antigens (Mycobacterium bovis bacillus Calmette-Guérin, purified protein derivative, early-secreted antigenic target-6).
We conclude that in vivo and in vitro assays are nonredundant, complementary measures of antimycobacterial immunity. Both TST and in vitro assays provided valuable information about antimycobacterial immunity and by inference latent TB in the studied high-incidence TB settings.