Identification and characterization of the ESAT-6 homologue of Mycobacterium leprae and T-cell cross-reactivity with Mycobacterium tuberculosis

Abstract

In this paper we describe identification and characterization of Mycobacterium leprae ESAT-6 (L-ESAT-6), the homologue of M. tuberculosis ESAT-6 (T-ESAT-6). T-ESAT-6 is expressed by all pathogenic strains belonging to the M. tuberculosis complex but is absent from virtually all other mycobacterial species, and it is a promising antigen for immunodiagnosis of tuberculosis (TB). Therefore, we analyzed whether L-ESAT-6 is a similarly powerful tool for the study of leprosy by examining T-cell responses against L-ESAT-6 in leprosy patients, TB patients, and exposed or nonexposed healthy controls from areas where leprosy and TB are endemic and areas where they are not endemic. L-ESAT-6 was recognized by T cells from leprosy patients, TB patients, individuals who had contact with TB patients, and healthy individuals from an area where TB and leprosy are endemic but not by T cells from individuals who were not exposed to M. tuberculosis and M. leprae. Moreover, leprosy patients who were not responsive to M. leprae failed to respond to L-ESAT-6. A very similar pattern was obtained with T-ESAT-6. These results show that L-ESAT-6 is a potent M. leprae antigen that stimulates T-cell-dependent gamma interferon production in a large proportion of individuals exposed to M. leprae. Moreover, our results suggest that there is significant cross-reactivity between T-ESAT-6 and L-ESAT-6, which has implications for the use of ESAT-6 as tool for diagnosis of leprosy and TB in areas where both diseases are endemic.

FIG. 1.

(A) Amino acid alignment of L-ESAT-6 and T-ESAT-6 protein sequences. Conserved amino acids are indicated by plus signs. The level of identity between these two ESAT-6 proteins is 36%, and the level of homology is 63%. (B) Genome region of M. leprae and M. tuberculosis in which the ESAT-6 homologue genes are situated.

FIG. 2.

IFN-γ production against L-ESAT-6 (A), T-ESAT-6 (B), M. leprae (C), and M. tuberculosis (D) by PBMC of 21 leprosy patients (•), 10 TB patients, including 4 patients who had no contact with M. leprae (TB no lep) (▾), 11 non-BCG-vaccinated, healthy controls (▪) who had not been exposed to M. leprae or M. tuberculosis, and 7 healthy Dutch individuals who had contact with TB patients (⧫). Median values are indicated by horizontal lines.

FIG. 3.

IFN-γ production by PBMC from Brazilian leprosy patients (•), TB patients (▾), and healthy controls (HC) (▪) in response to L-ESAT-6 (A), T-ESAT-6 (B), M. leprae (C), and M. tuberculosis (D).