This Article Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Schluger, N. W. Search for Related Content PubMed PubMed Citation Articles by Schluger, N. W.

Assessing Tuberculosis Transmission and Virulence

The Vanishing Tuberculin Skin Test

Columbia University College of Physicians and Surgeons, New York, New York

With the introduction of IFN- release assays (IGRAs), a new era in testing for latent tuberculosis infection (LTBI) has begun (1, 2). These assays test for latent tuberculosis by mimicking in a test tube at least some of the phenomena associated with a type IV delayed-type hypersensitivity reaction. Peripheral blood mononuclear cells (PBMCs) contained in whole blood samples are exposed to mycobacterial proteins, and IFN- release, primarily by sensitized T cells, is measured. The recent refinement of these assays to stimulate PBMCs with proteins that are released by Mycobacterium tuberculosis, but not the vaccine strain of M. bovis, offers the advantage of a test that can distinguish between true infection with M. tuberculosis and prior vaccination with bacille Calmette-Guérin (BCG) (3). To guide physicians in the United States, the Centers for Disease Control and Prevention (CDC) recently issued guidelines for the use of the one IGRA currently licensed in this country (4). These guidelines state that IGRAs can be used in place of tuberculin skin testing (TST) in all circumstances, although caveats are offered that point out the dearth of data regarding the use of these tests in certain populations, such as those with HIV infection and persons at extremes of age. The CDC guidelines reflect evidence that suggests that, in comparison with TST, currently available IGRAs are more specific (primarily because of fewer false-positive results in persons who have received BCG) and of roughly equal or perhaps slightly lower sensitivity (5, 6). The guidelines also point out the pressing need for further research to evaluate unresolved questions, particularly concerning the performance characteristics of IGRAs in various populations.

In this context, how should the study by Anderson and coworkers in this issue of the AJRCCM (pp. 1038–1042) be viewed (7)? These investigators conducted a contact investigation of schoolchildren exposed to an index case with advanced cavitary tuberculosis. They found that only 2 of 75 exposed children (2.7%) had positive results on skin testing (using the Heaf test), whereas 16 of 75 (22%) were positive when evaluated with an IGRA of the authors' own development.

There are several limitations in Anderson and colleagues' study. The sample size was relatively small, and only 62.5% of potential contacts agreed to participate. The cut-off for determining a positive IGRA was derived from a relatively small group of control subjects, and results in the control subjects demonstrated considerable splay in the results. The authors could detect no relationship between duration and proximity of contact to the index case and IGRA results, although they did not rigorously assess these factors. Skin testing in this study was done using the Heaf test, which is believed to be a less reliable method of assessing LTBI than Mantoux testing (8). For evaluating contacts, if Mantoux testing had been used (for which the American Thoracic Society and CDC recommend a cut-off of 5 mm of induration as defining a positive result in close contacts), it is likely that substantially more children, perhaps as many as 11 of 75 (15%), would have been classified as having LTBI, according to the data presented in Table 1. Such findings would have reduced the discrepancy between TST and IGRA results substantially.

The current report points out the difficulty of evaluating any new test in the absence of a true gold standard for LTBI. A natural history study of untreated exposed persons who are tested both with the standard TST and a new assay and then monitored to see which method most accurately predicts the development of active tuberculosis would efficiently answer the question of greatest interest, but it would not be ethical. Instead, we will need to rely on large and meticulously performed studies in which new tests are compared with TST in cohorts of patients whose epidemiologic risk for latent tuberculosis is characterized as carefully as possible.

The experiments in which the isolate (NPH4216) recovered from the index case was used to stimulate cytokine production from monocytes are quite interesting. The authors report that levels of tumor necrosis factor (TNF)- and interleukin (IL)-12p40 secreted in response to exposure to NPH4216 were lower than those produced when monocytes were exposed to a virulent laboratory reference strain, H37Rv, or a clinical isolate, CDC1551, which has been associated with a high rate of transmission in the community (9), although this strain ultimately does not, in fact, appear to be highly virulent (10, 11). These results are more intriguing than they are definitive. TNF- has been strongly implicated as an important factor in granuloma formation and in preventing progression from the latent state of infection to active disease, as evidenced by the high rate of illness that develops in patients receiving potent anti-TNF therapies (12, 13). IL-12 is a proinflammatory cytokine that is upstream of IFN-, the prototypic Th1-type cytokine and a critical component of the host immune response in tuberculosis (14–16). The finding that NPH4216 impairs secretion of these cytokines may explain, in part, the dramatic clinical presentation of the index case. The present study provides further evidence that the pathogen–host interaction is both a dynamic and complex one. Characterization of more clinical isolates in this manner, and careful correlation of the results with patient outcomes, should provide important insights into the transmission and pathogenesis of tuberculosis, and should aid in vaccine development.

FOOTNOTES

Conflict of Interest Statement: N.W.S. has received a research grant of $35,000 from Oxford Immunotec to evaluate the use of an IFN- release assay.

REFERENCES

1. Brodie D, Schluger NW. The diagnosis of tuberculosis. Clin Chest Med 2005;26:247–271.[CrossRef][Medline]
2. Pai M, Riley LW, Colford JM Jr. Interferon-gamma assays in the immunodiagnosis of tuberculosis: a systematic review. Lancet Infect Dis 2004;4:761–776.[CrossRef][Medline]
3. Harboe M, Oettinger T, Wiker HG, Rosenkrands I, Andersen P. Evidence for occurrence of the ESAT-6 protein in Mycobacterium tuberculosis and virulent Mycobacterium bovis and for its absence in Mycobacterium bovis BCG. Infect Immun 1996;64:16–22.[Abstract]
4. Mazurek GH, Jereb J, Lobue P, Iademarco MF, Metchock B, Vernon A. Guidelines for using the QuantiFERON-TB Gold test for detecting Mycobacterium tuberculosis infection, United States. MMWR Morb Mortal Wkly Rep 2005;54(RR-15):49–55.
5. Kang YA, Lee HW, Yoon HI, Cho B, Han SK, Shim YS, Yim JJ. Discrepancy between the tuberculin skin test and the whole-blood interferon gamma assay for the diagnosis of latent tuberculosis infection in an intermediate tuberculosis-burden country. JAMA 2005;293:2756–2761.[Abstract/Free Full Text]
6. Shams H, Weis SE, Klucar P, Lalvani A, Moonan PK, Pogoda JM, Ewer K, Barnes PF. Enzyme-linked immunospot and tuberculin skin testing to detect latent tuberculosis infection. Am J Respir Crit Care Med 2005;172:1161–1168.[Abstract/Free Full Text]
7. Anderson ST, Williams AJ, Brown JR, Newton SM, Simsova M, Nicol MP, Sebo P, Levin M, Wilkinson RJ, Wilkinson KA. Transmission of Mycobacterium tuberculosis undetected by tuberculin skin testing. Am J Respir Crit Care Med 2006;173:1038–1042.[Abstract/Free Full Text]
8. Targeted tuberculin testing and treatment of latent tuberculosis infection. Am J Respir Crit Care Med 2000;161:S221–S247.[Free Full Text]
9. Valway SE, Sanchez MP, Shinnick TF, Orme I, Agerton T, Hoy D, Jones JS, Westmoreland H, Onorato IM. An outbreak involving extensive transmission of a virulent strain of Mycobacterium tuberculosis. N Engl J Med 1998;338:633–639.[Abstract/Free Full Text]
10. Manca C, Tsenova L, Barry CE, Bergtold A, Freeman S, Haslett PA, Musser JM, Freedman VH, Kaplan G. Mycobacterium tuberculosis CDC1551 induces a more vigorous host response in vivo and in vitro, but is not more virulent than other clinical isolates. J Immunol 1999;162:6740–6746.[Abstract/Free Full Text]
11. Bishai WR, Dannenberg AM, Parrish N, Ruiz R, Chen P, Zook BC, Johnson W, Boles JW, Pitt ML. Virulence of Mycobacterium tuberculosis CDC1551 and H37Rv in rabbits evaluated by Lurie's pulmonary tubercle count method. Infect Immun 1999;67:4931–4934.[Abstract/Free Full Text]
12. Flynn JL, Goldstein MM, Chan J, Triebold KJ, Pfeffer K, Lowenstein CJ, Schreiber R, Mak TW, Bloom BR. Tumor necrosis factor-alpha is required in the protective immune response against Mycobacterium tuberculosis in mice. Immunity 1995;2:561–572.[CrossRef][Medline]
13. Keane J, Gershon S, Wise RP, Mirabile-Levens E, Kasznica J, Schwieterman WD, Siegel JN, Braun MM. Tuberculosis associated with infliximab, a tumor necrosis factor alpha-neutralizing agent. N Engl J Med 2001;345:1098–1104.[Abstract/Free Full Text]
14. Ladel CH, Szalay G, Riedel D, Kaufmann SH. Interleukin-12 secretion by Mycobacterium tuberculosis-infected macrophages. Infect Immun 1997;65:1936–1938.[Abstract]
15. Taha RA, Kotsimbos TC, Song YL, Menzies D, Hamid Q. IFN-gamma and IL-12 are increased in active compared with inactive tuberculosis. Am J Respir Crit Care Med 1997;155:1135–1139.[Abstract]
16. Stobie L, Gurunathan S, Prussin C, Sacks DL, Glaichenhaus N, Wu CY, Seder RA. The role of antigen and IL-12 in sustaining Th1 memory cells in vivo: IL-12 is required to maintain memory/effector Th1 cells sufficient to mediate protection to an infectious parasite challenge. Proc Natl Acad Sci USA 2000;97:8427–8432.[Abstract/Free Full Text]

This article has been cited by other articles:

				S. J. Tsiouris, D. Coetzee, P. L. Toro, J. Austin, Z. Stein, and W. El-Sadr Sensitivity analysis and potential uses of a novel gamma interferon release assay for diagnosis of tuberculosis. J. Clin. Microbiol., August 1, 2006; 44(8): 2844 - 2850. [Abstract] [Full Text] [PDF]

This Article Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Schluger, N. W. Search for Related Content PubMed PubMed Citation Articles by Schluger, N. W.