Using a costimulant assay, in vitro Con A responses of patients across the leprosy spectrum were found to be markedly suppressed by phenolic glycolipid-I (PGL-I), a unique antigen of M. leprae. The degree of inducible suppression as well as the number of leprosy patients showing suppression of mitogenic responses was higher with PGL-I as compared with integral M. leprae (p less than 0.05 to less than 0.01). Both untreated lepromatous (60%) as well as tuberculoid leprosy (67%) patients showed significant suppression ranging from 13 to 64% and 12 to 79%, respectively. Thus, PGL-I appears to have a universal suppressive effect on Con A responses and is unlikely to play a central role in determining the leprosy spectrum.

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