ArticlesEnhanced contact tracing and spatial tracking of Mycobacterium tuberculosis infection by enumeration of antigen-specific T cells
Introduction
Control of the global tuberculosis epidemic could be enhanced by identification and treatment of symptom-free people who are latently infected with Mycobacterium tuberculosis, as well as those with active disease. Even in countries with a low prevalence of tuberculosis, 30–40% of new cases are probably caused by recent transmission of M tuberculosis from infectious cases.1 Immunocompetent individuals with M tuberculosis infection have a 10% risk of developing active disease in their lifetime; half this risk is in the first 1-2 years after exposure.2 Chemoprophylaxis of recently infected individuals prevents the development of active tuberculosis,3 and isoniazid preventive therapy is highly cost effective. It is therefore important to identify recently infected contacts and infected individuals at greatest risk of progression—eg, intravenous drug users, HIV-1 infected individuals, and children.4, 5
The tuberculin skin test (TST) for diagnosis of latent M tuberculosis infection has remained almost unchanged for a century. Intradermal inoculation of purified protein derivative (PPD), which is a crude precipitate of more than 200 M tuberculosis antigens that are common to M bovis BCG and environmental mycobacteria, elicits a local cutaneous delayed-type-hypersensitivity response in sensitised individuals. The broad antigenic cross-reactivity of PPD causes the poor specificity of TST; a positive reaction can be a response not only to M tuberculosis infection, but also to BCG vaccination or exposure to environmental mycobacteria.6, 7, 8 Because a third of the world's population is thought to be infected with M tuberculosis,9 and most people have been vaccinated with BCG, accurate identification of those infected with M tuberculosis for targeted chemoprophylaxis is difficult.5, 6, 8
M tuberculosis infection evokes a strong cell-mediated immune response, and detection of T cells that are specific to this bacterium might be a means to detect infection. We therefore selected ESAT-6, a secreted antigen that is expressed in M tuberculosis complex10 (M tuberculosis, M bovis, and M africanum), but is absent from all strains of M bovis BCG vaccine11, 12 and most environmental mycobacteria.13 ESAT-6 is highly immunogenic in animal models of tuberculosis,14, 15 tuberculosis patients,16, 17, 18, 19 and people exposed to tuberculosis bacteria (contacts).20, 21 Cellular immune responses to ESAT-6 have been detected by standard in-vitro assays in 60-80% of tuberculosis patients.22 We used a highly sensitive assay to detect ESAT-6-specific T cells, the ex-vivo ELISPOT assay for interferon gamma.23 The ELISPOT assay is based on the principle of a sandwich-capture ELISA. The assay detects interferon-gamma molecules in the immediate vicinity of the T cell from which they were secreted, while still at a high concentration. Thus, each spot represents the footprint of an antigen-specific T cell that secretes interferon gamma (spot-forming cell). ELISPOT assay can detect antigen-specific T cells from blood without an in-vitro stimulation step,23 hence incubation periods are short. We reported that ESAT-6-specific T cells were an accurate marker of M tuberculosis infection in 45 (96%) of 47 patients with culture-confirmed active disease.21 We also noted that 22 (85%) of 26 TST-positive household contacts of patients with sputum smear-positive pulmonary tuberculosis had circulating ESAT-6-specific T cells.21 These results suggested a new approach for the detection of recent tuberculosis infection.24
Our aim was to assess whether this approach could be used to identify individuals in routine clinical practice, who were at high risk of recent infection and to compare the results with those for TST. Unfortunately, no test can reliably confirm latent M tuberculosis infection in symptom-free individuals—ie, there is no gold standard for comparison—hence measurement of accuracy of a new assay is difficult. However, airborne transmission of M tuberculosis is promoted by close and prolonged contact with an infectious person,25, 26, 27 and a key factor is the amount of time a contact spends sharing room air with an infectious individual.28 We postulated that the ESAT-6 based ex-vivo ELISPOT assay would correlate better with intensity of exposure to M tuberculosis than TST, but would be independent of BCG-vaccination status. Therefore, we investigated people attending a contact-tracing clinic who had well defined but differing amounts of exposure to M tuberculosis.
Section snippets
Participants
We prospectively recruited healthy adult contacts from the Contact Tracing Clinic, Northwick Park Hospital, North West London Hospitals NHS Trust, London, UK. This clinic is held once a week and is attended by individuals thought to have been in contact with a newly identified case of tuberculosis. There were no exclusion criteria. We consecutively recruited 50 contacts of index cases who had sputum-smear-positive pulmonary tuberculosis. We identify cases in this report by number (HC 1–HC 54).
Results
20 participants were contacts of an index case (IC). Figure 1 shows the proximity of these contacts to IC, and results of ESAT-6 ELISPOT and TST. IC shared a house with two cohabitants, including a work colleague, and worked consecutively in two separate offices within a large university building. He had a productive cough throughout the last 3 months that he worked in office A (figure 1) and during the next 2 months when he worked in office B (figure 1), before his admission to hospital.
The
Discussion
We have shown that the ESAT-6 ELISPOT assay identifies individuals at high risk of M tuberculosis infection more accurately than the TST. The strong positive relation between ESAT-6 ELISPOT results and M tuberculosis exposure, and the lack of relation with BCG vaccination status, allow symptomless M tuberculosis infection to be distinguished from BCG vaccination, thereby avoiding unnecessary chemoprophylaxis in uninfected individuals.
Although the numbers of individuals in some exposure
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