ESAT-6/CFP10 skin test predicts disease in M. tuberculosis-infected guinea pigs

Abstract

Background: Targeted preventive chemotherapy of individuals with progressive subclinical (incipient) disease before it becomes contagious would break the chain of tuberculosis transmission in high endemic regions. We have studied the ability of a skin test response to ESAT-6 and CFP10 (E6/C10) to predict later development of tuberculosis disease in the guinea pig model.

Methods and findings: Guinea pigs, either vaccinated with BCG or unvaccinated, were infected with a low dose of Mycobacterium tuberculosis by the aerosol route and the development of delayed type hypersensitivity responses to E6/C10 and to purified protein derivative (PPD) were followed until the onset of clinical disease. We demonstrated a negative correlation between the size of the skin test response and the time to the onset of clinical disease; a large E6/C10 skin test response correlated to a shorter survival time post skin testing, while a small E6/C10 skin test reaction correlated with a longer survival time (r = -0.6 and P<0.0001). No correlation was found using PPD.

Conclusions: Our data suggest that it may be possible to develop a prognostic skin test based on E6/C10 that will allow the identification of individuals with incipient disease, who have the highest risk of developing active tuberculosis in the near future.

Conflict of interest statement

Competing Interests: KW and PA are employed by Statens Serum Institut that owns patents on ESAT6.

Figures

Figure 1. Survival curves and skintest responses in M. tuberculosis infected guinea pigs.

Survival curves of BCG vaccinated guinea pigs (triangles) and unvaccinated guinea pigs (open squares) after M. tuberculosis infection. The animals were sacrificed when they had lost 15% of the maximum weight or had clinical symptoms indicative of severe TB (Fig 1A). Mean skin test reaction (+/−SEM) for M. tuberculosis-infected unimmunized and BCG vaccinated guinea pigs (n = 7), measured as erythema after intradermal injection of 2 ug E6/C10 (Fig. 1B) or 10 T.U. PPD (Fig. 1C). Each animal received a maximum of 4 tests with more than 8 weeks interval and this schedule did not sensitize naïve animals. The experiment was repeated twice with the same overall result and data shown for a representative experiment.

Figure 2. Relationship between disease outcome and skin test response to E6/C10 and PPD.

Depicted is the size of the skin test reaction to E6/C10 (figure A) and PPD (figure B) against weeks of survival post skin testing for each animal. Each symbol represents a skin reaction from one animal tested either at week at 4, 8 or 12 weeks post infection (the first skin test for each animal). There is a negative correlation between E6/C10 skin test response and survival time post skin testing (P