A 19-year follow-up of tuberculin reactors: Assessment of skin test reactivity and in vitro lymphocyte responses

Tuberculin skin test reactivity decreases with time such that repeat PPD skin testing may result in reactions of less than 10 mm. This reactivity may be boosted in some individuals with a second tuberculin skin test. The immunologic basis of these observations remains unclear. We studied the relationship between skin-test reactivity and in vitro blastogenic response to PPD in a cohort of 22 individuals (aged 28 to 81 years) known to be tuberculin reactors (induration ≥10 mm) in 1970. In 1989, 18 subjects remained reactive to PPD on the first skin test and responded to PPD in vitro (mean incorporation of ³H-thymidine by peripheral blood mononuclear cells, 22,650 cpm). Three subjects reverted (induration in response to PPD 3H-thymidine, 2,205 cpm). One subject boosted (increase of induration of at least 6 mm to ≥15 mm) on the second skin test and showed a concomitant in vitro boost in the blastogenic response to PPD (from 1,008 cpm to 47,837 cpm). In this cohort, interpretation of the two-step tuberculin skin test correlated closely with in vitro proliferative responses. Over a 19-year period, the majority of individuals maintained skin test reactivity and strong in vitro responses to PPD despite a lack of ongoing exogenous exposure to Mycobacterium tuberculosis. The immunologic basis for reversion appears to depend in part on a loss of lymphocyte blastogenic capacity. In the one individual who exhibited the booster phenomenon, repeat antigen stimulation resulted in a dramatic increase in the in vitro blastogenic responses.