Correlation of mycobacterium tuberculosis specific and non-specific quantitative Th1 T-cell responses with bacillary load in a high burden setting

Abstract

Background: Measures of bacillary load in patients with tuberculosis (TB) may be useful for predicting and monitoring response to treatment. The relationship between quantitative T-cell responses and mycobacterial load remains unclear. We hypothesised that, in a HIV-prevalent high burden setting, the magnitude of mycobacterial antigen-specific and non-specific T-cell IFN-γ responses would correlate with (a) bacterial load and (b) culture conversion in patients undergoing treatment.

Methods: We compared baseline (n = 147), 2 (n = 35) and 6 month (n = 13) purified-protein-derivative (PPD) and RD1-specific (TSPOT.TB and QFT-GIT) blood RD1-specific (TSPOT.TB; QFT-GIT) responses with associates of sputum bacillary load in patients with culture-confirmed TB in Cape Town, South Africa.

Results: IFN-γ responses were not associated with liquid culture time-to-positivity, smear-grade, Xpert MTB/RIF-generated cycle threshold values or the presence of cavities on the chest radiograph in patients with culture-confirmed TB and irrespective of HIV-status. 2-month IGRA conversion rates (positive-to-negative) were negligible [<11% for TSPOT.TB (3/28) and QFT-GIT (1/29)] and lower compared to culture [60% (21/35); p<0.01].

Conclusions: In a high burden HIV-prevalent setting T-cell IFN-γ responses to M. tuberculosis-specific and non-specific antigens do not correlate with bacillary load, including Xpert MTB/RIF-generated C(T) values, and are therefore poorly suited for monitoring treatment and prognostication.

Figure 1. Patient flow diagram and diagnostic outcomes stratified by final diagnostic category.

*128 patients had no chest X-ray data. ^†To be eligible for the study, patients needed to provide at least 2 sputum samples (spot or morning), both of which were used for concentrated fluorescent smear microscopy and liquid culture. When available, a paired second spot sputum sample was collected and archived. This was later used for Xpert MTB/RIF testing (n = 389). ^‡The TSOT.TB, QFT.GIT and the PPD ELISPOT tests were not all concurrently available throughout the duration of study and thus not all patients necessarily received a combination of all 3 IGRAs. The number of valid results for TSPOT.TB, QFT-GIT and PPD respectively were 457, 457, and 462, respectively. 9 TSPOT.TB indeterminate results and 52 indeterminate QFT-GIT results occurred and are excluded. Only patients with culture-confirmed TB were included in the main analysis for the comparison of IGRA-response with bacterial load.

Figure 2. IFN-γ responses measured using the TSPOT.TB, QFT-GIT and PPD ELISPOT assays and correlated with liquid culture time-to-positivity, smear status and grade, and Xpert MTB/RIF-generated C_T values in individuals with culture-confirmed TB.

Culture, smear-microscopy and the Xpert MTB/RIF assay were performed on a paired sputum sample collected at diagnosis. P-values and Spearman’s rank correlation coefficient (r_S) are shown where appropriate. Abbreviations: SFC, spot-forming colonies; IFN-γ, interferon–γ; QFT-GIT, QuantiFERON TB Gold-in-tube; ELISPOT, enzyme-linked immunosorbent spot; PBMCs, peripheral blood mononuclear cells; IU, international units; TTP, time-to-positivity; C_T, cycle threshold.

Figure 3. Longitudinal IFN-γ responses are shown for patients undergoing anti-TB treatment at 2 months (n = 35) and 6 months (n = 13).

Capped horizontal bars represent the median IFN-γ response at each time-point.