The
tuberculin skin test (TST) using purified
protein derivative (
PPD) of Mycobacterium tuberculosis is traditionally used to diagnose
latent tuberculosis (TB)
infection (LTBI). However, LTBI diagnosis by peripheral blood mononuclear cell (PBMC)
interferon (IFN)-gamma responses to M.
tuberculosis-specific
antigens, early secreted antigenic target 6 kDa (ESAT-6) and culture filtrate
protein (CFP)-10 has greater specificity. We investigated the difference in antimycobacterium cellular immunity in TB contacts who were strong TST reactors but nonresponsive to the ESAT-6/CFP-10 assay compared with those with concordant results. Healthy TB contacts were tested using the above two assays and mycobacterium survival was measured after co-culture of infected macrophages with their PBMCs. Whether
PPD reactivity was tested by TST or by PBMC-specific IFN-gamma responses, strongly
PPD-reactive TB contacts without ESAT-6/CFP-10 responsiveness showed significantly better mycobacterium inhibition activity than ESAT-6/CFP-10-responsive TB contacts with the same
PPD reactivity. In the former group, stronger
PPD reactivity was associated with improved mycobacterium killing, whereas ESAT-6/CFP-10 responders showed the opposite result.
PPD-reactive ESAT-6/CFP-10-nonresponsive TB contacts in our population may have had protective immunity related to prior mycobacterium exposure. ESAT-6/CFP10-responsive TB contacts are more likely to have LTBI and, in this group, strong
PPD reactivity may paradoxically be associated with poor mycobactericidal activity.