Long-term Stability of Resistance to Latent Mycobacterium tuberculosis Infection in Highly Exposed Tuberculosis Household Contacts in Kampala, Uganda

Abstract

Background: Resistance to latent Mycobacterium tuberculosis (M.tb) infection, identified by persistently negative tuberculin skin tests (TST) and interferon-gamma release assays (IGRA), after close contact with pulmonary tuberculosis (TB) patients has not been extensively characterized. Stability of this "resistance" beyond 2 years from exposure is unknown.

Methods: 407 of 657 eligible human immunodeficiency virus (HIV)-negative adults from a TB household contact study with persistently negative TST (PTST-) or with stable latent M.tb infection (LTBI) were retraced 9.5 years (standard deviation = 3.2) later. Asymptomatic retraced contacts underwent 3 IGRAs and follow-up TST, and their M.tb infection status classified as definite/possible/probable.

Results: Among PTST- with a definite classification, 82.7% were concordantly TST-/ quantiferon-TB Gold- (QFT-), and 16.3% converted to TST+/QFT+ LTBI. Among original LTBI contacts, 83.6% remained LTBI, and 3.9% reverted their TST and were QFT-. Although TST and QFT concordance was high (κ = 0.78), 1.0% of PTST and 12.5% of original LTBI contacts could not be classified due to discordant TST and QFT results. Epidemiological variables did not differ between retraced PTST- and LTBI contacts.

Conclusion: Resistance to LTBI, defined by repeatedly negative TST and IGRA, in adults who have had close contact with pulmonary TB patients living in TB-endemic areas, is a stable outcome of M.tb exposure. Repeated longitudinal measurements with 2 different immune assays and extended follow-up provide enhanced discriminatory power to identify this resister phenotype and avoid misclassification. Resisters may use immune mechanisms to control aerosolized M.tb that differ from those used by persons who develop "classic" LTBI.

Keywords: TB exposure; TB outcomes; case-contact study; resistance to infection.

Figure 1.

Retracing study design and classification of retraced TB household contacts. A, Study design: TB household contacts with either persistently negative tuberculin skin tests (PTST−) or latent M.tb infection (LTBI) were identified in Kampala, Uganda between 2002 and 2012 (phase 1* [12]). PTST− and LTBI contacts from phase 1 were identified for retracing between 2014 and 2017, if they were > 15 years old in 2014, and matched by age bracket and household. Retraced contacts underwent 3 QFT tests and 1 TST (phase 2) over 1–2 years of additional follow-up. B, Classification of retraced contacts: PTST−, LTBI, or TST− contacts with less than 12 months of follow-up (TST incomplete [Inc]) in phase 1 were retraced and classified, based on 3 QFT from phase 2 and 2 TST from phases 1 and 2, into resister (RSTR; all tests negative), LTBI (all tests positive), converter (TST− in phase 1, QFT+/TST+ in phase 2), reverter (TST+ Inc phase 1, QFT−/TST−Inc phase 2) or TST/QFT discordant (TSTs consistent, QFTs consistent but in opposing directions). Among 250 subjects not consented, 6 had developed active TB between phases 1 and 2, 32 declined consent, 39 were unavailable, and 173 were untraceable. Contacts who developed TB were not eligible for retracing. The 34 HIV+ subjects excluded from further analysis included 27 who were HIV+ in phase 1 and 7 HIV conversions between phases 1 and 2. Classification as definite was based on availability and consistency of all 5 measures (2 TST and 3 QFT). Probable or possible classification was used when 1 assay was missing or equivocal. Unclassifiable subjects either didn’t complete phase 2 or had very inconsistent TST and QFT results (see Suppl. Methods). Abbreviations: F/U, follow up; HIV, human immunodeficiency virus; QFT, quantiferon-TB Gold; RSTR, resister phenotype; TB, tuberculosis; TST, tuberculin skin test.

Figure 2.

TST (mm) and outcomes after retracing PTST− and TST+ TB contacts. A, Boxplots illustrating 1st quartile, median, and 3rd quartile of TST in mm for definite RSTR, reverters, converters, and LTBI. B, Outcomes after retracing PTST− TB contacts. Definite RSTR: 5 of 5 tests (3 QFT, initial and retracing TST) negative. Definite converter: 3 of 3 QFT and follow-up TST were positive. TST/QFT discordant: 3 of 3 QFT positive but initial and retracing TST were negative. C, Outcomes after retracing of TST+ TB contacts. Definite LTBI: 5 of 5 tests (3 QFT, initial and retracing TST) positive. Definite reverter: 3 of 3 QFT and follow-up TST were negative. TST/QFT discordant: 3 of 3 QFT negative but initial and retracing TST were positive. Abbreviations: LTBI, latent M.tb infection; QFT, quantiferon-TB Gold; RSTR, resister phenotype; TB, tuberculosis; TST, tuberculin skin test.

Figure 3.

Plasma cytokines in RSTR and LTBI. Cytokines and chemokines were measured in plasma from 31 Definite RSTR and 31 LTBIs with a Luminex platform. Results for BAFF, interferon (IFN)-alpha, interleukin (IL)-1 alpha, MCSF, MIP1 alpha, and TNF-Rp55 are shown as mean pg/ml +/−SD. No statistically significant differences were detected between the two groups. Results for other 10 cyto- and chemokines are shown in Supplementary Figure 1. Abbreviations: BAFF, B-cell activating factor; IFN, interferon; IL1, interleukin-1; LTBI, latent M.tb infection; MCSF, macrophage colony stimulating factor 1; MIP1, macrophage inflammatory protein 1-alpha; RSTR, resister phenotype; SD, standard deviation; TNFR, tumor necrosis factor receptor superfamily member 3.