Interferon-gamma responses after isoniazid chemotherapy for latent tuberculosis

Chemotherapy for Mycobacterium tuberculosis infection may decrease interferon (IFN)-gamma responses to early secretory antigenic target 6 and culture filtrate protein (CFP)-10; a reaction that could be useful to monitor the success of treatment. We investigated IFN-gamma responses in subjects with latent TB infection before and after isoniazid (INH) chemotherapy.

A total of 48 patients who had contact with a TB patient in a psychiatric hospital were suspected to have latent TB infection on the basis of a positive QuantiFERON-TB Gold (QFT-G) test and were offered INH treatment for 6 months. After INH chemotherapy, IFN-gamma responses were again quantified and compared with initial measurements.

Thirty-four patients completed 6 months of therapy and 28 were retested. Seven (25%) had a negative test and the other patients showed an overall decline. Geometric mean for early secretory antigenic target 6 decreased from 1.398 to 0.362 (P < 0.001), and that for CFP-10 from 0.312 to 0.120 (P < 0.001). A subsequent QFT-G test carried out 18 months after chemotherapy showed no further decline of IFN-gamma responses.

If the success of chemotherapy is defined as negative conversion in the QFT-G test, these results suggest that although the waning of QFT-G responses as a result of chemotherapy is an important characteristic of IFN-gamma responses, the extent of waning would not be sufficient to allow effective monitoring of the success of chemotherapy because a majority of contacts still showed positive responses in the QFT-G test even after chemotherapy.