Application of a whole blood mycobacterial growth inhibition assay to study immunity against Mycobacterium tuberculosis in a high tuberculosis burden population.

Richard Baguma,Michael J Brennan,Lynnett Stone,Marwou De Kock,Erica Smit,Willem Hanekom,E Jane Hughes,Jonathan Day,Adam Penn-Nicholson,Mzwandile Erasmus,Bernadette Pienaar,Mark Hatherill,Michele Van Rooyen,Lebohang Makhethe,Thomas J Scriba,Robert S Wallis,Pere-Joan Cardona

doi:10.1371/journal.pone.0184563

Abstract

The determinants of immunological protection against Mycobacterium tuberculosis (M.tb) infection in humans are not known. Mycobacterial growth inhibition assays have potential utility as in vitro surrogates of in vivo immunological control of M.tb. We evaluated a whole blood growth inhibition assay in a setting with high burden of TB and aimed to identify immune responses that correlate with control of mycobacterial growth. We hypothesized that individuals with underlying M.tb infection will exhibit greater M.tb growth inhibition than uninfected individuals and that children aged 4 to 12 years, an age during which TB incidence is curiously low, will also exhibit greater M.tb growth inhibition than adolescents or adults. Neither M.tb infection status, age of the study participants, nor M.tb strain was associated with differential control of mycobacterial growth. Abundance and function of innate or T cell responses were also not associated with mycobacterial growth. Our data suggest that this assay does not provide a useful measure of age-associated differential host control of M.tb infection in a high TB burden setting. We propose that universally high levels of mycobacterial sensitization (through environmental non-tuberculous mycobacteria and/or universal BCG vaccination) in persons from high TB burden settings may impart broad inhibition of mycobacterial growth, irrespective of M.tb infection status. This sensitization may mask the augmentative effects of mycobacterial sensitization on M.tb growth inhibition that is typical in low burden settings.

Highlights

Almost a quarter of the world’s population is estimated to be infected with Mycobacterium tuberculosis (M.tb) [1], driving the most deadly epidemic due to an infectious agent
We sought to assess the utility of the whole blood mycobacterial growth inhibition assays (MGIA) to measure functional inhibition of mycobacterial replication by blood leukocytes in healthy individuals with or without M.tb infection
Since individuals with M.tb infection typically have higher, more activated and/or differentiated mycobacteria-specific T cell responses [25], we hypothesized that QuantiFERON-TB Gold In-Tube Assay (QFT)+ individuals would control the growth of M.tb to a greater degree than uninfected individuals

Summary

Introduction

Almost a quarter of the world’s population is estimated to be infected with Mycobacterium tuberculosis (M.tb) [1], driving the most deadly epidemic due to an infectious agent. Several risk factors for developing TB disease following M.tb infection have been identified, including compromised immunity due to HIV co-infection, diabetes, gender and age [3]. Pre-adolescent children older than 4 years of age have much lower rates of progression to TB disease following infection than adolescents or adults [4,5,6,7]. In such children pulmonary TB typically manifests as a mild, pauci-bacillary disease, whereas adolescents and adults more commonly present with multi-bacillary disease, more pronounced lung infiltration with immunopathology and lung cavitation [6,7]. The low risk of TB in children within this “golden age of TB” presents an opportunity to study natural resistance and/or characteristics of successful immunity to M.tb in humans, which are not well understood

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