Antigen-specific suppressor cells in subclinical leprosy infection.

A two-stage in-vitro culture system was used to assay cells which suppress the lympho-proliferative response to Mycobacterium leprae (ML). Responses to ML, purified protein derivative of tuberculin, and streptokinase-streptodornase were preferentially suppressed by mitomycin-treated cells which had been primed with the same antigen in a 7-day primary culture. Healthy subjects exposed to leprosy for more than 3 years showed strong suppression of the response to ML antigens (11 of 12 showed more than 40% suppression), whereas those exposed for 3 months to 3 years showed much less suppression (12 of 15 showed less than 40% suppression). The in-vitro generation of strong ML-specific suppression may reflect the maturation of a well-regulated and protective immune response. However, premature induction and in-vivo activation of these suppressor cells could predispose to disseminated (lepromatous) forms of leprosy. With this assay it would be possible to assess the ability of proposed leprosy vaccines to engage strongly the regulatory network controlling the immune response to ML in the same way as long-term exposure to the natural infection.