ReviewMycobacterium tuberculosis-specific cytokine biomarkers to differentiate active TB and LTBI: A systematic review
Introduction
It is estimated that one-quarter of the world's population is infected with Mycobacterium tuberculosis (MTB).1 Despite effective treatment being available for over 70 years, it continues to be the leading cause of death from a single infectious disease worldwide2. One of the main factors contributing to this toll is the limitations of current diagnostic tests. As a result, an estimated 3 million cases of tuberculosis (TB) each year are not diagnosed or notified.3, 4
The diagnosis of active TB is made by visualisation of acid fast bacilli by smear microscopy and growth of MTB on culture, which is time consuming and has low sensitivity in children who have paucibacillary TB disease.5, 6 Nucleic acid amplification methods, such as GeneXpert MTB/RIF, are expensive and impractical in many resource-poor settings.7, 8
Latent TB infection (LTBI) is diagnosed by either the tuberculin skin test (TST) or by an interferon (IFN)-γ release assay (IGRA), which are based on detecting an immune response to MTB antigens.9, 10 A TST result can be affected by prior BCG vaccination, potentially producing a false-positive result, as well as by other factors such as immunosuppression or malnutrition, which can produce a false-negative result.10 IGRAs measure a T-cell immune response, through IFN-γ production, to MTB-specific antigens (early secretory antigenic target (ESAT)−6, culture filtrate protein (CFP)−10 +/- TB7.7).9 They are not affected by prior BCG-vaccination, but have lower sensitivity in children, particularly those under five years of age.11, 12
Neither the TST nor IGRAs can differentiate between active TB and LTBI.13 This distinction is crucial to determine appropriate treatment. A patient presenting with symptoms of active TB (e.g. cough, fever, weight loss, fatigue) may have a positive TST or IGRA result because they have active TB, or because they have LTBI and an intercurrent infection with a non-tuberculous respiratory illness. This is particularly a problem in countries where TB is endemic.
The World Health Organization (WHO) ‘End TB Strategy’ has called for the development of new diagnostic tests to address the shortcomings of existing tests to enable more accurate diagnosis of both LTBI and active TB.14 A growing number of studies have investigated the performance of novel MTB-specific cytokine responses, either alone or in combination, to improve the sensitivity and specificity of immunodiagnostic assays. These are envisaged as potential candidates for inclusion in novel immunodiagnostic tests for TB infection, and a number of studies have looked at their potential role as a stage-specific marker (i.e. whether a patient has active TB or LTBI).
This review summarises the results of all studies in humans that have investigated MTB-specific cytokine responses as diagnostic tools to differentiate between LTBI and active TB.
This review was done in accordance with the ‘preferred reporting items for systematic reviews and meta-analyses’ (PRISMA) statement.15 A systematic search of MEDLINE and EMBASE was done from 1946 to November 2019 using the OVID interface with the following search terms: (exp Mycobacterium tuberculosis/an, ch, im, ph OR exp Tuberculosis/bl, di, im, mi, ph, pp) AND (exp “intercellular signalling peptides and proteins” OR exp cytokines OR exp chemokines OR exp interferons OR exp interleukin 1 receptor antagonist protein OR exp interleukins OR exp lymphokines OR exp monokines OR exp tumour necrosis factors). Reference lists of identified relevant publications were also hand-searched.
Studies were included if they: 1) assessed the use of one or more cytokines (including chemokines) or soluble markers to discriminate between active TB and LTBI in humans, and 2) measured cytokines, chemokines or soluble markers in blood after stimulations with the MTB-specific peptides ESAT-6 and/or CFP-10. Studies were excluded if: 1) more than 10% of included patients were known to be immunosuppressed (e.g. HIV infection), as the immune response to TB infection may be altered in this group, and 2) diagnostic groups were not clearly defined, particularly where the control group included healthy household contacts (a group that may comprise TB-infected and TB-uninfected individuals). Meeting abstracts, studies without original data, studies not in English, and studies that evaluated IFN-γ alone were also excluded, as were studies that used flow cytometry due to the expensive and labour-intensive nature of this diagnostic modality.16
The search identified a total of 9670 studies. After removal of duplicates, 8512 studies were screened using titles and abstracts, of which 56 fulfilled the inclusion criteria16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71 (Fig. 1). These 56 studies reported the results from 1848 patients with active TB and 1633 patients with LTBI. The results of these studies are summarised in Supplementary Tables 1A-C.
Section snippets
Participants
Thirty of the 56 studies were done in countries with a high incidence of TB (≥40 cases per 100,000 population per year).72 Thirty-two studies were done in adults, 14 in children, and nine in a population comprising children and adults. One study did not specify the age of participants.71 Most (45 of 56) studies included a group of healthy controls, and 11 studies additionally included a group of ‘sick controls’ who were unwell with conditions other than active TB (to determine specificity).17,21
Discussion
This systematic review identified 56 studies investigating 100 cytokines for their potential to distinguish between active TB and LTBI. Although several cytokines have shown promise as stage-specific markers of TB infection, there is little consistency between studies and much heterogeneity in the changes reported for different cytokines.
The immunological mechanisms involved in containment of MTB after infection, resulting in LTBI or progression to active TB disease, are still largely unknown.68
Funding
National Health and Medical Research Council (NHMRC) grant number 1142043. ES is the recipient of a Murdoch Children's Research Institute (MCRI) Postgraduate Health Research Scholarship.
Declarations of Competing Interest
None.
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