Antigenic profile of ICRC bacilli with special reference to isolation of immunogenic subunit.

A vaccine containing ICRC bacilli induces persistent immune conversion in lepromatous leprosy (LL) patients and lepromin-negative healthy subjects, in association with upgrading of the tissue responses in the former. With an idea to isolate the immunogenic 'subunit(s)', antigenicity of ICRC sonicate and its fractions were tested, with reference to both B- and T-cell responses. A very high molecular weight glycolipoprotein, named PP-I with an apparent molecular weight of 1,000,000, has been isolated using gel permeation high performance liquid chromatography (HPLC). PP-I, which focussed as a single band at pH 5 in an LKB isoelectric focussing column, quantitatively interacted with 80% of the circulating antibodies in pooled LL sera, and also induced a late (3 weeks) Mitsuda-type skin response which shows excellent correlation with host immunity against Mycobacterium leprae. These observations suggest that PP-I is a complex bifunctional antigen containing epitopes for both B and T cells. The PP-I fraction of ICRC and a similar high molecular weight HPLC fraction of M. leprae produced a line of identity against rabbit anti-ICRC serum in Ouchterlony gel diffusion and gave comparable skin responses in healthy volunteers in leprosy endemic areas. The data indicate that the PP-I fractions from the two organisms are antigenically closely related. Preliminary studies in human volunteers showed that administration of PP-I of ICRC resulted in immune conversion in lepromin-negative healthy subjects. PP-I thus appears to be the appropriate immunogen that could be used in preparation of a 'subunit' antileprosy vaccine.