Evidence for boosting Mycobacterium tuberculosis-specific IFN-γ responses at 6 weeks following tuberculin skin testing

To the Editors:

We read with great interest the paper by Leyten et al. 1 in the present issue of the European Respiratory Journal, which assessed whether the tuberculin skin test (TST) might affect the results of the QuantiFERON®-TB Gold (QFT-G), a blood-based Mycobacterium tuberculosis-specific interferon-γ release assay (IFNGRA).

This is an important topic in the UK with respect to the recently published National Institute for Clinical Excellence (NICE) guidelines 2 for the diagnosis and treatment of tuberculosis (TB). The guidelines state that if a suspected case of TB yields a negative initial TST then the subject should be re-tested at 6 weeks, at which point an IFNGRA may be used in conjunction with the TST. Experts within the field have previously been worried about the potential boosting effect of the TST on IFNGRA responses. However, to date, no study has been published in the UK.

As part of a project within our undergraduate medical degree course at Imperial College (London, UK), we conducted a similar project to that of Leyten et al. 1 using a smaller cohort of subjects. In addition, we looked at the effects of the TST administration not only on the QFT-G (Cellestis, Darmstadt, Germany), but also on responses in the other commercially available IFNGRAs, including T-SPOT.TB (T-SPOT; Oxford Immunotec, Abingdon, UK). Furthermore, we re-assessed responses at 6 weeks post-TST, since this time-point is in line with the NICE recommendations.

In total, we studied 10 subjects with no history of TB contact or disease. One subject was eventually discovered to have had previous exposure to TB. This subject’s results were included in our study as a point of interest. Our results are summarised in table 1⇓.

Overall our study concurs with that of Leyten et al. 1 for the early time-point. Out of the 10 subjects, nine had negative results at baseline and at 48 h in both assays used.

We found no evidence for boosting of QFT-G or T-SPOT responses from negative to positive following TST administration, if the IFNGRA blood sample was taken at the time of reading of the TST.

However, at the 6-week time-point we noticed discrepancies between initial and follow-up results for the QFT-G. Out of our 10 subjects, nine were QFT-G negative at the initial time-point and of these, three (33%) had become QFT-G positive at 6 weeks (table 1⇑). We interpret these findings as evidence for boosting interferon-γ responses through TST at 6 weeks.

These findings raise the question: why can a boosting effect be seen in the QFT-G over a 6-week-period and not in T-SPOT? We suggest that this may be because QFT-G uses the additional M. tuberculosis-specific antigen TB7.7, as well as the antigens early secreted antigenic target (ESAT)-6 and culture filtrate protein (CFP)-10, which are found in T-SPOT. The additional antigen may evoke a more persistent immunological response, which outlives the response of the common antigens and, hence, yields a positive QFT-G result at a later time-point.

Our results regarding the T-SPOT assay are in keeping with a study by Richeldi et al. 3, who found that repeated TSTs over a period of 21 months did not induce a T-cell response to ESAT-6 or CFP-10 and, therefore, does not lead to false-positive results in the T-SPOT assay thus maintaining high specificity.

Our results regarding a boosting effect on QFT-G after 6 weeks are in keeping with a study by Cauthem et al. 4, which showed that 30.9% of TST nonreactors showed a response at either 7 or 90 days after initial administration indicating a long-lived boosting effect. Thomson et al. 5 also found that the boosting effect rarely occurs within 1 week, suggesting a reason as to why QFT-G was not positive at the 48-h time-point.

We would like to encourage a larger study of the 6 week Mycobacterium tuberculosis-specific interferon-γ release assay responses, to be conducted in subjects with and without prior tuberculosis exposure to confirm our findings. The potential boosting effect in the QuantiFERON®-TB Gold assay should be taken into account within the current National Institute of Clinical Excellence guidelines.