The stable incidence of new
leprosy cases suggests that transmission of
infection is continuing despite the worldwide implementation of multidrug
therapy programs. Highly specific tools are required to accurately diagnose asymptomatic and early stage Mycobacterium leprae
infections which are the likely sources of transmission and cannot be identified by using the detection of
antibodies against phenolic
glycolipid I. One of the hurdles hampering T-cell-based diagnostic tests is that M. leprae
antigens cross-react at the T-cell level with
antigens present in other mycobacteria, like M.
tuberculosis or M. bovis bacillus Calmette-Guerin (BCG). Using comparative genomics, we previously identified five candidate
proteins highly restricted to M. leprae which showed promising features with respect to application in
leprosy diagnostics. However, despite the lack of overall sequence homology, the use of
recombinant proteins includes the risk of detecting T-cell responses that are cross-reactive with other
antigens. To improve the diagnostic potential of these M. leprae sequences, we used 50 synthetic
peptides spanning the sequences of all five
proteins for the induction of T-cell responses (
gamma interferon) in
leprosy patients, healthy household contacts (HHC) of
leprosy patients, and healthy controls in Brazil, as well as in
tuberculosis patients, BCG vaccinees, and healthy subjects from an area of nonendemicity. Using the combined T-cell responses toward four of these
peptides, all paucibacillary patients and 13 out of 14 HHC were detected without compromising specificity. The
peptides contain HLA binding motifs for various HLA class I and II alleles, thereby meeting an important requirement for the applicability of diagnostic tools in genetically diverse populations. Thus, this study provides the first evidence for the possibility of immunodiagnostics for
leprosy based on mixtures of
peptides recognized in the context of different HLA alleles.