A postgenomic approach to identification of Mycobacterium leprae-specific peptides as T-cell reagents

To identify Mycobacterium leprae-specific human T-cell epitopes, which coul d be used to distinguish exposure to M. leprae from exposure to Mycobacteri um tuberculosis or to environmental mycobacteria or from immune responses f ollowing Mycobacterium bovis BCG vaccination, 15-mer synthetic peptides wer e synthesized based on data from the M. leprae genome, each peptide contain ing three or more predicted HLA-DR binding motifs. Eighty-one peptides from 33 genes were tested for their ability to induce T-cell responses, using p eripheral blood mononuclear cells (PBMC) from tuberculoid leprosy patients (n = 59) and healthy leprosy contacts (n = 53) from Brazil, Ethiopia, Nepal , and Pakistan and 20 United Kingdom blood bank donors. Gamma interferon (I FN-gamma) secretion proved more sensitive for detection of PBMC responses t o peptides than did lymphocyte proliferation. Many of the peptides giving t he strongest responses in leprosy donors compared to subjects from the Unit ed Kingdom, where leprosy is not endemic, have identical, or almost identic al, sequences in Mi. leprae and M. tuberculosis and would not be suitable a s diagnostic tools. Most of the peptides recognized by United Kingdom donor s showed promiscuous recognition by subjects expressing differing HLA-DR ty pes. The majority of the novel T-cell epitopes identified came from protein s not previously recognized as immune targets, many of which are cytosolic enzymes. Fifteen of the tested peptides had greater than or equal to 5 of 1 5 amino acid mismatches between the equivalent M. leprae and M. tuberculosi s sequences; of these, eight gave specificities of greater than or equal to 90% (percentage of United Kingdom donors who were nonresponders for IFN-ga mma secretion), with sensitivities (percentage of responders) ranging from 19 to 47% for tuberculoid leprosy patients and 21 to 64% for healthy lepros y contacts. A pool of such peptides, formulated as a skin test reagent, cou ld be used to monitor exposure to leprosy or as an aid to early diagnosis.