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Infection and Immunity, February 2002, p. 1010-1013, Vol. 70, No. 2
0019-9567/01/$04.00+0 DOI: 70.2.1010-1013.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.
Antigenic Specificity of the Mycobacterium leprae Homologue of ESAT-6
John S. Spencer,1* Maria Angela M. Marques,1 Monica C. B. S. Lima,1,2,3 Ana Paula Junqueira-Kipnis,1 Bruce C. Gregory,1 Richard W. Truman,4 and Patrick J. Brennan1
Department of Microbiology, Colorado State University, Fort Collins, Colorado; Hansens Disease Laboratory, Oswaldo Cruz Institute, Oswaldo Cruz Foundation,,1
and Laboratory of Immunopathology, School of Medical Sciences, State University of Rio de Janeiro, Rio de Janeiro, Brazil;,2
National Hansen's Disease Center, Louisiana State University, Baton Rouge, Louisiana3
Received 9 August 2001/
Returned for modification 1 October 2001/
Accepted 31 October 2001

ABSTRACT
The sequence of the
Mycobacterium leprae homologue of ESAT-6
shows only 36% amino acid correspondence to that from
Mycobacterium tuberculosis. Anti-
M. leprae ESAT-6 polyclonal and monoclonal
antibodies and T-cell hybridomas reacted only with the homologous
protein and allowed identification of the B- and T-cell epitopes.
The protein is expressed in
M. leprae and appears in the cell
wall fraction. Thus,
M. leprae ESAT-6 shows promise as a specific
diagnostic agent for leprosy.

INTRODUCTION
The global implementation of effective chemotherapy for leprosy
has resulted in the diminution of cases from approximately 10
million in 1985 to about 800,000 today (
27). However, there
is no evidence as yet of any reduction in the number of new
cases (
28), and we know little about the transmission of leprosy
or the time elapsed between infection and disease. The single
greatest need from leprosy research is definitive diagnostic
tools to help understand transmission and allow early detection
of disease. The recent completion of the sequencing of the genomes
of
Mycobacterium tuberculosis (
4) and
Mycobacterium leprae (
5)
provides the opportunity to identify leprosy-specific antigens.
An analogous approach applied to
Mycobacterium bovis BCG allowed
the identification of deleted genes and the development of antigens
that can distinguish between
M. tuberculosis infection and vaccination
with BCG (
17). Among those antigens were two low-molecular-weight
M. tuberculosis culture filtrate proteins, ESAT-6 and CFP10
(
2,
10), both encoded by genes in the RD1 region, a genetic
segment that has been deleted from all strains of BCG. When
tested together in a gamma interferon assay of peripheral blood
mononuclear cells from
M. tuberculosis-infected and BCG-vaccinated
individuals, the sensitivity and specificity of the response
were 84 and 100%, respectively, with no responses in purified
protein derivative-negative individuals (
1).
Although previous studies have identified a number of M. leprae proteins (7, 11, 19) and peptides (6, 26) capable of inducing gamma interferon responses in leprosy patients, a comparative analysis of the M. tuberculosis and M. leprae genomes should reveal new specific antigens, potential diagnostic and epidemiological tools for leprosy. In this report, comparative analysis of the M. leprae and M. tuberculosis ESAT-6 homologues suggests that the M. leprae product holds promise in this respect.

Comparison of the sequences of ESAT-6 from M. leprae and M. tuberculosis.
Whereas
M. tuberculosis contains 14 members of the ESAT-6 family
(
23), the
M. leprae genome shows evidence of only 4 (
5,
8).
A comparison of the alignment of the sequences of the 95-amino-acid
(aa)-length ESAT-6 protein from
M. tuberculosis (
22) with its
counterpart from
M. leprae showed 36% homology overall (Fig.
1). Although there was identity between 9 out of 13 amino acids
(69% homology) in the region bounded by aa 34 and 46, this is
the only instance with more than 4 consecutive, identical amino
acids. The rest of the sequence shows only one or two identical
amino acids, interrupted by conserved and nonconserved stretches.

Cloning and production of recombinant M. leprae ESAT-6.
The DNA sequence encoding the full-length
M. leprae ESAT-6 protein
(designated ML0049) (
5,
8) was cloned from
M. leprae genomic
DNA using Vent
Pfu DNA polymerase (Promega, Madison, Wis.).
PCR amplification was carried out with the forward primer 5"-CATATGATACAGGCGTGGCAC-3"
and reverse primer 5"-AAGCTTCCCGGTGAACATACT-3" designed to introduce
NdeI and
HindIII sites to the 5" and 3" ends of the open reading
frame. The pBluescript vector (Stratagene, La Jolla, Calif.)
was digested with the restriction endonucleases
NdeI and
HindIII,
ligated to the PCR products, and transformed into competent
Escherichia coli TOP10 cells. The
esx gene was subcloned into
the expression vector PET 23b(+) (Novagen, Madison, Wis.) and
transformed into BL21(DE3) pLys S cells by the heat shock method.
Single colonies expressing ESAT-6 were grown in Luria-Bertani
medium with ampicillin and induced with isopropyl ß-
D-thiogalactopyranoside.
Recombinant ESAT-6 (rESAT-6) found in inclusion bodies was solubilized
in 8 M urea in 20 mM Tris-0.1 M NaH
2PO
4 (pH 8.0) buffer, loaded
onto a nickel-nitrilotriacetic acid resin column, and eluted
with imidazole (
20). The purity of the recombinant protein was
established by sodium dodecyl sulfate-polyacrylamide gel electrophoresis
(SDS-PAGE) (
16); lipopolysaccharide content was less than 0.2
ng/mg. Recombinant
M. tuberculosis ESAT-6 was prepared under
similar conditions.

Antibody binding specificity of M. leprae ESAT-6.
Comparison of the sequences of ESAT-6 from
M. leprae and
M. tuberculosis (Fig.
1) suggested few conserved regions capable
of eliciting cross-reactive antibodies. To prove the point,
BALB/c mice were immunized with an emulsion containing 50 µg
of either
M. leprae or
M. tuberculosis rESAT-6 in a 1:1 ratio
of phosphate-buffered saline and incomplete Freund's adjuvant.
Monoclonal antibodies (MAbs) were produced (
29) using the myeloma
B-cell line SP2/0 (
21). MAbs 1C7.2F1 (immunoglobulin G1 [IgG1]),
2F4.2C4 (IgG2a), 7B10.2B2 (IgG1), 7G7.2A5 (IgG1), and 8C9.2B5
(IgG1) were produced as cell culture supernatant or purified
over a protein G-Sepharose affinity column. The reactivity of
the antisera to the homologous and heterologous proteins confirmed
the lack of cross-reactivity (Fig.
2). Neither antiserum reacted
to any significant degree with the heterologous antigen by enzyme-linked
immunosorbent assay (ELISA) or Western blotting (Fig.
3). Overlapping
peptides covering the entire
M. leprae protein, and the two
immunodominant
M. tuberculosis ESAT-6 peptides (
3) were synthesized
(Table
1) and reacted with the polyclonal and monoclonal antibodies
(Table
2). Polyclonal antisera specific for the entire
M. leprae ESAT-6 did not react to the homologous N-terminal peptide (p1,
aa 1 to 20) or to the last two C-terminal peptides (p8, aa 71
to 90; p9, aa 81 to 95) or to either of the heterologous
M. tuberculosis immunodominant peptides (p1, aa 1 to 20; p6, aa
50 to 71). It reacted most intensely to p2 (aa 11 to 30), slightly
more weakly to p3 through p6 (p3, aa 21 to 40; p4, aa 31 to
50; p5, aa 41 to 60; p6, aa 51 to 70), and weakly to p7 (aa
61 to 80). These data are in accord with analysis of the
M. leprae ESAT-6 peptides using the Jameson-Wolf antigenic index
(
12) and the Kyte-Doolittle hydrophilicity plot (
15). The
M. tuberculosis ESAT-6-specific polyclonal sera showed no reactivity
to any of the
M. leprae ESAT-6 peptides but reacted well to
both of the homologous ESAT-6 peptides (p1 and p6) (Table
2).
Of the five MAbs produced, all of which reacted strongly to
the homologous ESAT-6 as determined by Western blotting, four
(1C7.2F1, 2F4.2C4, 7G7.2A5, and 8C9.2B5) recognized the single
(p2) peptide, suggesting that p2 contains the dominant B-cell
epitope.

Detection of native M. leprae ESAT-6 in subcellular fractions of M. leprae.
M. tuberculosis ESAT-6 is found mainly as a secreted protein
in culture filtrate (
22). However,
M. leprae is derived from
the spleens and livers of experimentally infected armadillos,
rendering it very difficult to detect the protein in the tissue
milieu. Instead, the subcellular fractions of
M. leprae (
18)
were solubilized, and the proteins were subjected to Western
blotting (Fig.
4). ESAT-6 was observed in the cell wall but
not in the cytosol or membrane fractions.

Identification of T-cell epitopes on M. leprae ESAT-6.
BALB/c mice (
H-2d) were immunized into the hind footpad with
40 µg of
M. leprae rESAT-6 in incomplete Freund's adjuvant,
and lymph node cells were restimulated in vitro with dendritic
cells pulsed with 0.5 µg of rESAT-6/ml, fused with the
T-cell fusion partner BW

-ß (
25), and plated. Clones
that grew in individual wells were screened using dendritic
cell antigen-presenting cells with 0.5 µg of rESAT-6/well.
Hybrids were tested for the production of interleukin 2 (IL-2)
(
13), and positive hybridomas were further tested for their
response to the nine overlapping peptides (p1 to p9) of
M. leprae ESAT-6. Clone 2A3 responded to peptide p4, and clones 3D8 and
6B7 responded to the C-terminal peptide, p9. None of the
M. leprae ESAT-6-specific-T-cell hybridomas responded to either
of the two
M. tuberculosis ESAT-6 peptides, p1 or p6. Using
B-cell lymphoma lines bearing either
I-Ad (M12.B5),
I-Ed (M12.A2)
(
9), or both (A20) (
14), all three of the T-cell hybridomas
were shown to recognize their respective peptides using the
I-Ad restriction element (Table
3). In contrast, with
M. tuberculosis ESAT-6, only one peptide was shown to stimulate T cells from
the
H-2d haplotype (BALB/c strain) mice, the N-terminal peptide
(aa 1 to 20), whereas mice of
H-2k and
H-2a haplotypes recognized
a separate internal peptide, aa 51 to 70 (
3).
View this table:
[in this window]
[in a new window]
|
TABLE 3 Presentation of peptides p4 and p9 by class II major histocompatibility complex I-Ad haplotype on APC to M. leprae ESAT-6-specific-T-cell hybridomas
|

Conclusion.
Comparative analysis of the complete genome sequences of
M. leprae and
M. tuberculosis established that gene deletion and
decay have resulted in the formation of 1,116 nonfunctional
pseudogenes, resulting in an elimination of many key metabolic
activities of
M. leprae (
5). Thus,
M. leprae barely maintains
its existence with a minimal gene set (
24,
30). There are an
estimated 135 functional coding sequences in the
M. leprae genome
that show no similarity to any known genes, and some of these,
if found to actually produce a functional immunogenic protein,
may be useful in the development of new epidemiological and
diagnostic tools. The
M. leprae version of ESAT-6, in light
of its exceptional specificity, may qualify, although its promise
was revealed by pursuing a different principle: proteins that
share sizable correspondence to the
M. tuberculosis counterpart
but prove to have immunological specificity.

ACKNOWLEDGMENTS
This work was supported by NIH NIAID contract NO1 AI-55262.
We thank John Belisle for the gift of M. tuberculosis ESAT-6 (generated through NIH NIAID contract NO1 AI-75320) and Alessandro Sette and Morten Harboe for key discussions. We thank Elisa French, Julia Granowski, and Rick Heimbichner for technical assistance and Marilyn Hein for preparation of the manuscript.

FOOTNOTES
* Corresponding author. Mailing address: Department of Microbiology, Colorado State University, Fort Collins, CO 80523-1677. Phone: (970) 491-2432. Fax: (970) 491-1815. E-mail:
john.spencer{at}colostate.edu.

Editor: J. M. Mansfield

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Infection and Immunity, February 2002, p. 1010-1013, Vol. 70, No. 2
0019-9567/01/$04.00+0 DOI: 70.2.1010-1013.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.
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