IP-10 is an additional marker for tuberculosis (TB) detection in HIV-infected persons in a low-TB endemic country
Introduction
In 2010, there was an estimated 1.14 million new cases of tuberculosis (TB) among Human Immunodeficiency Virus (HIV)-infected persons and TB accounted for 31% of Acquired Immunodeficiency Virus Syndrome (AIDS)-related deaths. HIV-infected individuals with latent tuberculosis infection (LTBI) and severe immune suppression have a high risk of progression to active TB disease and would thus benefit from accurate diagnosis and prophylactic treatment of LTBI.1, 2
A recent breakthrough in the diagnosis of Mycobacterium tuberculosis (Mtb) infection has been the development of T-cell-based Interferon (IFN)-γ Release Assays (IGRAs) that use antigens belonging to an Mtb region of difference 1 (RD1). Two commercial IGRAs are now available. Evidence reviewed elsewhere3, 4 suggests that they are more specific, correlate better with Mtb exposure in low-incidence settings and are less affected by Bacillus Calmette–Guérin (BCG)-vaccination than the tuberculin skin test (TST). However, although better than the TST, their accuracy in persons with HIV is still limited, particularly in “mitogen-unresponsive”5, 6, 7, 8, 9, 10, 11, 12 individuals and in those with low CD4+ T-cell counts. Therefore, innovative diagnostic tools for TB, and new and validated markers of Mtb infection and disease are needed to reduce the burden of TB-HIV epidemic.13
It has been recently shown that the accuracy of IGRAs may be enhanced by the addition of other Mtb-specific antigens,14, 15, 16, 17 by modifying the incubation time,18, 19, 20, 21 using peptides selected from RD1,22, 23 evaluating the response at the site of TB disease24, 25 or by measuring biomarkers other than IFN-γ.25, 26, 27 In particular, it has been shown that IFN-γ-inducible protein 10 (IP-10) may be an additional biomarker for LTBI detection after RD1-specific stimulation in both adults6, 15, 28, 29, 30, 31 and children.32 IP-10 is involved in trafficking monocytes and activated-Th1 cells to inflamed foci.33 Serum, pleural fluid and urine IP-10 levels have been evaluated as biomarkers for diagnosis, prognosis, and monitoring of treatment efficacy in inflammatory and infectious diseases including TB.8, 34, 35 IP-10, as already reported for IFN-γ, is a marker that does not discriminate between LTBI and active TB disease.28, 29, 30, 31, 34, 35
We recently demonstrated in India that in HIV-infected patients, IP-10 response to QuantiFERON TB-Gold In tube (QFT-IT) is associated with TB.6, 31 Differently from IFN-γ-based tests, in patients with active TB, IP-10-based assays were shown to be independent of mitogen response and CD4+ T-cell counts. IP-10 response to QFT-IT showed low specificity for active TB diagnosis (13% specificity) whereas the specificity of QFT-IT was 68% making the results uncertain in terms of being able to accurately detect TB. However, the study was performed in a high TB burden country, therefore the response could have been associated with LTBI. IP-10, which has a higher detection ability than IFN-γ (the marker used by QFT-IT) could thus be considered a valuable marker for TB.
To rule out this uncertainty and in an attempt to find better correlates of Mtb infection during HIV disease, we evaluated the performance of this assay in HIV-infected subjects enrolled in Italy, a country with a low incidence of TB. As a control, we evaluated the performance of an IP-10-based assay in HIV-uninfected subjects. Because there is no gold diagnostic standard for evaluating the accuracy of the immune diagnostic tests for LTBI, we used active TB cases to assess sensitivity of the IP-10 based test and low-risk populations to assess specificity, as previously reported for the commercial IGRA.7, 11, 36
Section snippets
Characteristics of the enrolled individuals
This study was approved by the Ethical Committee of our Institution, the L. Spallanzani National Institute for Infectious Diseases (INMI), approval number 2/2007. Informed written consent was required to participate in the study. Recruitment began in January 2008 and involved enrolled HIV-infected subjects with or without active TB one day a week until January 2011. HIV-uninfected subjects were enrolled as a control.
HIV-infected individuals without active TB were distinguished as being at low-
Characteristics of the enrolled individuals
We enrolled 195 subjects. Of the 77 who were HIV-uninfected, 40 were classified as healthy donors and 37 with active TB. The remaining 118 were HIV-infected and classified as 21 with active TB and 97 without active disease and then further classified as 62 at low- and 35 at high-risk of TB (Fig. 1). Demographic characteristics, duration of hospitalization, severity of the lung lesions evaluated by the chest X-rays, immune, virological and microbiological parameters, BCG-vaccination status and
Discussion
For the first time to our knowledge, we present the results of a prospective study conducted in HIV-infected individuals in a low TB endemic country (Italy). The study was designed to investigate if factors other than IFN-γ, such as IP-10, may improve the detection of the response to QFT-IT for immunodiagnosis of TB.
The results demonstrate that in HIV-infected subjects, besides IFN-γ, the IP-10 response to QFT-IT is associated with active TB and with TB risk factors. Among those with active TB,
Acknowledgements
The authors are grateful to all the patients, nurses and physicians who took part in this study. We also thank Teresa Chiacchio, Federica Turchi, Barbara De Stefani, Assunta Navarra and Gabriella De Carli for their support. We are deeply grateful to Ms Andrea Baker (INMI Rome, Italy) for the editing. The study was supported by grants from the Italian Ministry of Health: “Ricerca Corrente”, RF-IMI-2009-1302952; a grant from the European Community: HEALTH-F3-2009-241642. The funders had no role
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