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From the authors

Divisions of ¹ Clinical Infectious Diseases, and ² Immune Cell-Analytics, Research Centre Borstel, Borstel, Germany.

We would like to thank S. Barry and colleagues for their comments on our recent article describing the local immunodiagnosis of active smear negative tuberculosis (TB) by enzyme-linked immunospot (ELISPOT) 1. In contrast to a previous article 2, we demonstrated that relating numbers of spot forming cells by ELISPOT to lymphocytes, rather than to all mononuclear cells, improves the analysis of local recruitment of antigen specific cells to the compartment of infection for the detection of smear-negative pulmonary TB (pTB). Although the ELISPOT is unable to simultaneously assign the quantity and phenotype of a cell, it is known from the use of flow-cytometry that responding cells represent CD4+ T-lymphocytes 3, 4. Based on this knowledge, the combination of ELISPOT results with lymphocyte counts seems a reasonable strategy to increase diagnostic accuracy, and may be of particular interest for resource-poor settings where ELISPOT assays are easier to perform than flow-cytometric approaches.

When analysing mycobacterial-specific immune responses, purified-protein derivate (PPD) naturally generates more intense immune responses in mononuclear cells from the human lung than region of difference-1 (RD) restricted antigens. In a previous publication 1 PPD, early-secretory antigenic target (ESAT)-6 and culture-filtrate protein (CFP)-10 specific immune responses were compared in blood and bronchoalveolar lavage (BAL) by ELISPOT of patients with smear-negative pTB and in patients with nontuberculous pulmonary diseases. Approximately twice the number of PPD specific interferon (IFN)- releasing BAL mononuclear cells (BALMCs) than RD-1 antigen-specific BALMCs were reported in patients with pTB 2. In contrast to the BALMC responses to RD-1-restricted proteins, PPD-specific BALMC responses were less specific as they did not discriminate well between patients with active pTB and non-TB control patients. In the non-TB control group, BALMCs from seven (28%) of a total 25 patients who were latently infected with Mycobacterium tuberculosis, yielded positive results by ELISPOT when stimulated with PPD 2. While bacille Calmette–Guérin vaccination does not seem to influence PPD-specific immune responses in the human lung (which might be related to its limited protection against TB disease), BAL cells are preferentially enriched for PPD-specific lymphocytes in individuals with latent M. tuberculosis infection 5. Recently, it was also demonstrated that the frequency of PPD-reactive interferon- producing CD4+ T-cells in the sputum of patients with active TB did not differ from individuals with presumptive latent TB infection 6.

While we constantly observe PPD-specific responses of BALMCs in ELISPOT of patients with a past history of pTB, these BALMC responses to RD-1-restricted antigens are usually absent in these patients if they do not have reactivation of TB.

A higher sensitivity of an assay that analyses PPD-specific immune responses versus RD-1-restricted immune responses in BALMCs as observed by Breen et al. 7 might, therefore, result in a lower test specificity and could result in a higher number of indeterminate test results. Interim results of an ongoing multicentre study of the TB-NET, comparing the performance of the BAL ELISPOT with RD-1-restricted antigens and Mycobacterium tuberculosis-specific nucleic acid amplification technique, still demonstrate a sensitivity of the BAL ELISPOT with ESAT-6 and CFP-10 for the detection of smear-negative pTB of >90% (data not shown) 8.

In a small case series of five individuals with infections due to nontuberculous mycobacteria (NTM) at our institution, BALMCs of four patients had positive PPD-specific ELISPOT responses. However, BALMC responses to ESAT-6 and CFP-10 were negative in four individuals in this group. The one individual with an NTM infection and a positive BALMC immune response to the RD-1-selected antigens had an infection with M. kansasii, which is known to encode RD-1 proteins.

While the frequency of region of difference-1-specific T-cells for the detection of smear negative tuberculosis in the sputum may be too low 9, 10, assaying the ex vivo reactivity of bronchoalveolar lavage T-cells towards Mycobacterium tuberculosis-specific antigens by flow cytometry may further improve the diagnostic accuracy of on-site immune based assays for the diagnosis of smear-negative tuberculosis.

Statement of interest

A statement of interest for C. Lange can be found at www.erj.ersjournals.com/misc/statements.shtml

REFERENCES

1. Jafari C, Ernst M, Strassburg A, et al. Local immunodiagnosis of pulmonary tuberculosis by enzyme-linked immunospot. Eur Respir J 2008;31:261–265.[Abstract/Free Full Text]
2. Jafari C, Ernst M, Kalsdorf B, et al. Rapid diagnosis of smear-negative tuberculosis by bronchoalveolar lavage enzyme-linked immunospot. Am J Respir Crit Care Med 2006;174:1048–1054.[Abstract/Free Full Text]
3. Barry SM, Lipman MC, Bannister B, Johnson MA, Janossy G. Purified protein derivative-activated type 1 cytokine-producing CD4+ T lymphocytes in the lung: a characteristic feature of active pulmonary and nonpulmonary tuberculosis. J Infect Dis 2003;187:243–250.[CrossRef][Web of Science][Medline] [Order article via Infotrieve]
4. Sester M, Sester U, Clauer P, et al. Tuberculin skin testing underestimates a high prevalence of latent tuberculosis infection in hemodialysis patients. Kidney Int 2004;65:1826–1834.[CrossRef][Web of Science][Medline] [Order article via Infotrieve]
5. Walrath J, Zukowski L, Krywiak A, Silver RF. Resident Th1-like effector memory cells in pulmonary recall responses to Mycobacterium tuberculosis. Am J Respir Cell Mol Biol 2005;33:48–55.[Abstract/Free Full Text]
6. Breen RA, Hardy GA, Perrin FM, et al. Rapid diagnosis of smear-negative tuberculosis using immunology and microbiology with induced sputum in HIV-infected and uninfected individuals. PLoS ONE 2007;2:e1335[CrossRef]
7. Breen RA, Barry SM, Smith CJ, et al. Clinical application of a rapid lung-orientated immunoassay in individuals with possible tuberculosis. Thorax 2008;63:67–71.[Abstract/Free Full Text]
8. Jafari C, Bossink A, Goletti D, et al. Comparison of bronchoalveolar-lavage enzyme-linked immunospot and PCR for the diagnosis of smear negative tuberculosis: interim analysis of a TBNET study. Pneumologie 2008;62:S130
9. Lange C, Ernst M, Strassburg A, Sarrazin H, Jafari C. Diagnosis of smear-negative tuberculosis in low prevalence countries: keeping the scope on the bronchoscope. www.plosone.org/annotation/listThread.action?inReplyTo = info:doi/10.1371/annotation/57f8f244-15cc-4bd2-8847-33b2c319ed3e&root = info:doi/10.1371/annotation/57f8f244-15cc-4bd2-8847-33b2c319ed3e Date last updated: May 8:2008
10. Brightling CE. Clinical applications of induced sputum. Chest 2006;129:1344–1348.[CrossRef][Web of Science][Medline] [Order article via Infotrieve]

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