Cynthia Bin-Eng Chee

Latent tuberculosis infection (LTBI) is characterised by the presence of immune responses to previously acquired Mycobacterium tuberculosis infection without clinical evidence of active tuberculosis (TB). Here we report evidence-based guidelines from the World Health Organization for a public health approach to the management of LTBI in high risk individuals in countries with high or middle upper income and TB incidence of <100 per 100 000 per year. The guidelines strongly recommend systematic testing and treatment of LTBI in people living with HIV, adult and child contacts of pulmonary TB cases, patients initiating anti-tumour necrosis factor treatment, patients receiving dialysis, patients preparing for organ or haematological transplantation, and patients with silicosis. In prisoners, healthcare workers, immigrants from high TB burden countries, homeless persons and illicit drug users, systematic testing and treatment of LTBI is conditionally recommended, according to TB epidemiology and resource availability. Either commercial interferon-gamma release assays or Mantoux tuberculin skin testing could be used to test for LTBI. Chest radiography should be performed before LTBI treatment to rule out active TB disease. Recommended treatment regimens for LTBI include: 6 or 9 month isoniazid; 12 week rifapentine plus isoniazid; 3-4 month isoniazid plus rifampicin; or 3-4 month rifampicin alone.

In clinical practice, latent infection with Mycobacterium tuberculosis is defined by the presence of an M. tuberculosis-specific immune response in the absence of active tuberculosis. Targeted testing of individuals from risk groups with the tuberculin skin test or an interferon-γ release assay is currently the best method to identify those with the highest risk for progression to tuberculosis. Positive predictive values of the immunodiagnostic tests are substantially influenced by the type of test, the age of the person who is tested, the prevalence of tuberculosis in the society and the risk group to which the person belongs. As a general rule, testing should only be offered when preventive chemotherapy will be accepted in the case of a positive test result. Preventive chemotherapy can effectively protect individuals at risk from the development of tuberculosis, although at least 3 months of combination therapy or up to 9 months of monotherapy are required, and overall acceptance rate is low. Improvements of the current generation of immunodiagnostic tests could substantially lower the number of individuals that need to be treated to prevent a case of tuberculosis. If shorter treatment regimens were equally effective than those currently recommended, acceptance of preventive chemotherapy could be much improved.

The tuberculin skin test (TST) and interferon gamma (IFN-γ) release assays (IGRAs) are used as adjunctive tests for the evaluation of suspected cases of active tuberculosis (TB). However, a positive test does not differentiate latent from active TB. We investigated whether flow cytometric measurement of novel combinations of intracellular cytokines and surface makers on CD4 T cells could differentiate between active and latent TB after stimulation with Mycobacterium tuberculosis-specific proteins. Blood samples from 60 patients referred to the Singapore Tuberculosis Control Unit for evaluation for active TB or as TB contacts were stimulated with purified protein derivative (PPD), ESAT-6 and CFP-10, or heparin-binding hemagglutinin (HBHA). The CD4 T cell cytokine response (IFN-γ, interleukin-2 [IL-2], interleukin-17A [IL-17A], interleukin-22 [IL-22], granulocyte-macrophage colony-stimulating factor [GM-CSF], and tumor necrosis factor alpha [TNF-α]) and surface marker expression (CD27, CXCR3, and CD154) were then measured. We found that the proportion of PPD-specific CD4 T cells, defined as CD154(+) TNF-α(+) cells that were negative for CD27 and positive for GM-CSF, gave the strongest discrimination between subjects with latent and those with active TB (area under the receiver operator characteristic [ROC] curve of 0.9277; P < 0.0001). Also, the proportions and absolute numbers of HBHA-specific CD4 T cells were significantly higher in those with latent TB infection, particularly CD154(+) TNF-α(+) IFN-γ(+) IL-2(+) and CD154(+) TNF-α(+) CXCR3(+). Finally, we found that the ratio of ESAT-6- and CFP-10-responding to HBHA-responding CD4 T cells was significantly different between the two study populations. In conclusion, we found novel markers of M. tuberculosis-specific CD4 cells which differentiate between active and latent TB.

BACKGROUND: Tuberculosis remains common in Singapore, increasing in incidence since 2008. We attempted to determine the molecular epidemiology of Mycobacterium tuberculosis complex (MTC) isolates locally, identifying major circulating genotypes and obtaining a glimpse of transmission dynamics. METHODOLOGY: Non-duplicate MTC isolates archived between 2006 and 2012 at the larger clinical tuberculosis laboratory in Singapore were sampled for spoligotyping and MIRU-VNTR typing, with case data obtained from the Singapore Tuberculosis Elimination Program registry database. Isolates between 2008 and 2012 were selected because of either multidrug-resistance or potential epidemiological linkage, whereas earlier isolates were randomly selected. Separate analyses were performed for the early (2006-2007) and later (2008-2012) study phases in view of potential selection bias. PRINCIPAL FINDINGS: A total of 1,612 MTC isolates were typed, constituting 13.1% of all culture-positive tuberculosis cases during this period. Multidrug-resistance was present in 91 (5.6%) isolates - higher than the national prevalence in view of selection bias. The majority of isolates belonged to the Beijing (45.8%) and EAI (22.8%) lineages. There were 347 (30.7%) and 133 (27.5%) cases clustered by combined spoligotyping and MIRU-VNTR typing from the earlier and later phases respectively. Patients within these clusters tended to be of Chinese ethnicity, Singapore resident, and have isolates belonging to the Beijing lineage. A review of prior contact investigation results for all patients with clustered isolates failed to reveal epidemiological links for the majority, suggesting either unknown transmission networks or inadequate specificity of the molecular typing methods in a country with a moderate incidence of tuberculosis. CONCLUSION: Our work demonstrates that Singapore has a large and heterogeneous distribution of MTC strains, and with possible cross-transmission over the past few years based on our molecular typing results. A universal MTC typing program coupled with enhanced contact investigations may be useful in further understanding the transmission dynamics of tuberculosis locally.

The total number of new tuberculosis (TB) cases notified in Singapore among citizens, permanent residents and foreigners rose by 46% from 2004 to 2010. During this period, the proportion of foreigners increased from 29% to 47% of the total case burden. In 2008, the TB incidence rate among Singapore citizens and permanent residents increased for the first time in ten years, despite the on-going efforts of the Singapore TB Elimination Programme. Additional measures and resources are clearly needed to curb this rising trend. Pivotal to this is to address TB among foreigners. The political will to battle TB in Singapore must result in action to remove barriers to diagnosis, to enable all TB patients to undergo treatment under directly observed therapy (DOT), and to ensure that all healthcare providers who manage TB patients are responsible and accountable to the public health system.