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Infection and Immunity, December 2003, p. 7173-7177, Vol. 71, No. 12
0019-9567/03/$08.00+0 DOI: 10.1128/IAI.71.12.7173-7177.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.
CD8+-T-Cell Responses of Mycobacterium-Infected Mice to a Newly Identified Major Histocompatibility Complex Class I-Restricted Epitope Shared by Proteins of the ESAT-6 Family
Laleh Majlessi,1* Marie-Jésus Rojas,1 Priscille Brodin,2 and Claude Leclerc1
Unité
de Biologie des Régulations Immunitaires, INSERM
E352,1
Unité de
Génétique Moléculaire
Bactérienne, Institut Pasteur, 75724 Paris Cedex
15, France2
Received 23 April 2003/
Returned for modification 10 June 2003/
Accepted 20 August 2003

ABSTRACT
Here
we describe the identification of a new CD8
+-T-cell
epitope,
the GYAGTLQSL nonamer, shared by the TB10.3 and TB10.4
proteins
of the
Mycobacterium tuberculosis ESAT-6 family.
Cytotoxic T
cells from mycobacterium-infected mice efficiently
recognized
this epitope. GYAGTLQSL-specific T-cell hybridomas, which
were
able to recognize
Mycobacterium bovis BCG-infected
macrophages,
were generated and now allow investigation of
mycobacterial-antigen
processing through the major histocompatibility
complex class
I
pathway.

TEXT
The contribution of CD8
+ T cells to the control of
infection
with
Mycobacterium tuberculosis is supported by a
marked emergence
of gamma interferon-producing CD8
+
T lymphocytes in the lungs
of infected mice
(
5,
9,
10) and by the increased
susceptibility
of mice deficient in genes involved in the major
histocompatibility
complex class I (MHC-I) presentation pathway to this
infection
(
13). However,
the specificity of antimycobacterial CD8
+ T cells
and
the mechanisms of mycobacterial-antigen presentation by MHC-I
molecules
are poorly understood. Only a few MHC-I-restricted epitopes,
i.e.,
38-kDa: 225-234
(
17), MPT64:190-198
(
5), and antigen 85A
(Ag85A):144-152
(
3) were
shown to be recognized by antimycobacterial CD8
+ T
cells.
The MHC-I presentation of mycobacterial antigens maythus be
relatively inefficient, and CD8
+-T-cell
responses to this infection
may be induced by substantial expansion of
clones with limited
specificities
(
5). Therefore, the
identification of MHC-I-restricted
mycobacterial epitopes and
generation of tools able to detect
their presentation in the MHC-I
context can be of considerable
interest for the investigation of
CD8
+-T-cell immunity during
M. tuberculosis
infection.
We applied the SYFPEITHI program
(http://syfpeithi.bmi-heidelberg.com/)
to 400 open reading frames (ORFs) overlapping the first 10%
(Rv0001 to Rv0400) of the M. tuberculosis H37Rv genome
(http://genolist.pasteur.fr/TubercuList/)
(2). The scan of the first
10% of the genome was performed to obtain a reasonable number of
peptides to be synthetized and tested by cytotoxic-T-lymphocyte (CTL)
assay. One hundred fifty nonamers which possess consensus
H-2Kd-binding motifs were selected. Based on the computed
scores of known H-2Kd-restricted peptides, out of these 150
nanomers, only 84 with SYFPEITHI scores of
23 were considered
potentially immunogenic. Moreover, since hydrophobic epitopes may reach
an alternative MHC-I pathway
(6), we selected only
hydrophilous H37Rv peptides (11 out of the 84) (Table
1). As a positive peptide control, we used a previously described
H-2Kd epitope, Rv3804c (Ag85A):144-152
(3).
The selected
H37Rv-derived peptides were synthesized by Pepscan
Systems (Lelystad,
The Netherlands) or Neosystem (Strasbourg,
France) and monitored for
their capacity to generate CTL responses.
BALB/c (H-2
d) mice
(Iffa Crédo, L'Arbresle, France) received
two subcutaneous
(s.c.) injections of 50 µg of individual
peptides emulsified in
incomplete Freund adjuvant at a 2-week
interval. Eight days
following the last injection, their lymph
node cells were stimulated in
vitro for 5 days with the homologous
peptides and their CTL activity
was measured as previously described
(
4).
The Rv0140:79-87
peptide generated only a weak CTL response,
while the
GYAGTLQSL epitope, shared by the Rv0288 (TB10.4)
and
Rv3019c (TB10.3) proteins (referred to as TB10.3/4:20-28) and
the
Rv3804c (Ag85A):144-152 positive-control peptide, elicited
CTLs able to
efficiently lyse peptide-loaded P815 targets (Fig.
1A).
TB10.3 and TB10.4 are low-molecular-weight proteins with 84.4%
sequence
identity (Fig.
2) and constitute with TB12.9 (Rv3017c)
(
12)
a subfamily of the
highly immunogenic ESAT-6 protein family
(
1,
2,
15).
TB12.9 does not
share the GYAGTLQSL epitope. It is noteworthy
that the
actual knowledge of the
M. tuberculosis genome does
not allow
us to precisely anticipate which mycobacterial proteins
are secreted.
However, among the first 400 ORFs, 6 encode proteins
identified in the
culture supernatant of
M. tuberculosis and
6 others are
putatively secreted. Among the ORFs selected here,
only TB10.4 and
TB10.3 are secreted proteins.
We then screened these peptides for
their recognition by CTLs
from mice infected with
Mycobacterium
bovis BCG or with
M. tuberculosis.
The effector cells of
these mice specifically and efficiently
lysed P815 cells pulsed with
TB10.3/4:20-28 peptide or Rv3804c
(Ag85A):144-152 positive-control
peptide (Fig.
1B and C).
The
other H37Rv-derived peptides were unable to induce the lysis
of
target cells by such effectors. Splenocytes of controls injected
with
phosphate-buffered saline showed no TB10.3/4:20-28-specific
CTL
activity (data not shown), indicating that immunization
with
mycobacteria, rather than in vitro priming with TB10.3/4:20-28
peptide,
induced this CTL response. The TB10.3/4:20-28 epitope
is thus naturally
processed and presented by MHC-I molecules
in BCG- or
M.
tuberculosis-infected mice.
In this study, the frequency of
immunogenic CTL epitopes among peptides predicted by SYFPEITHI was
relatively low (1 out of 11). In a previous study using a similar
bioinformatic approach on mycobacterial immunogens, this frequency was
varied (4 out of 7 for H-2Kb and 1 out of 11 for
H-2Dd) (16).
However, recent studies show that a combination of the prediction of
MHC-I ligand with information on proteasomal cleavages improves the
efficiency of the identification of CD8+-T-cell
epitopes
(8).
Heat-killed (20
min, 86°C) BCG lost its capacity to induce a
TB10.3/4:20-28-specific CTL response (Fig.
3A). This result is in agreement with the hypothesis that the most obvious
way for mycobacteria to gain access to the host cell cytoplasm and its
MHC-I pathway would be membrane permeabilization of the primary
phagosomes inducible only by metabolically active mycobacteria
(7,
14). The
TB10.3/4:20-28-specific CTL activity of BCG-immunized mice was
inhibited when effectors were depleted of CD8+, but
not CD4+, T cells (Fig.
3B). Furthermore,
TB10.3/4:20-28-loaded L fibroblasts transfected with H-2Kd,
but not with H-2Dd or H-2Ld, were lysed by the
CTLs of BCG-immunized mice (data not shown). Thus, immunization of mice
with live BCG generates H-2Kd-restricted
TB10.3/4:20-28-specific CD8+ CTLs.
Using
splenocytes of BCG-immunized BALB/c mice, we generated
a panel of 10
CD8
+-T-cell hybridomas specific for the
TB10.3/4:20-28
epitope. As shown in Fig.
4A, four of these hybridomas, namely,
XH3, YB8, WC7, and KC6, produce
interleukin-2 (IL-2) when they
are stimulated with TB10.3/4:20-28
peptide in the presence of
H-2
d bone marrow-derived
dendritic cells. Unrelated H-2K
d-restricted
Rv3804c
(Ag85A):144-152 or LCMV-NP:118-126 peptides did not
stimulate these
hybridomas (data not shown). Assays of TB10.3/4:20-28
presentation by L
fibroblasts transfected with H-2K
d, H-2D
d,
or
H-2L
d demonstrated that XH3 and YB8 were restricted by
H-2K
d but that WC7 and KC6 were restricted by both
H-2K
d and H-2L
d (data not shown). Three out of
these 10 TB10.3/4:20-28-specific
T-cell hybridomas efficiently
recognized BCG-infected Raw 264.7
(H-2
d) macrophages (Fig.
4B), indicating that the
processing
of mycobacteria leads to MHC-I presentation of the
TB10.3/4:20-28
epitope. These hybridomas are the first-described stable
CD8
+-T-cell
clones specific for an MHC-I-restricted
mycobacterial antigen
and represent powerful tools which now allow
investigation of
cellular and molecular mechanisms of
mycobacterial-antigen presentation
via the MHC-I pathway.
Based
on the low quantities of ESAT-6 family proteins within
in vitro
mycobacterial cultures despite the substantial host
immune responses to
these proteins, it has been suggested that
their expression may be
upregulated during intracellular growth
in an infected host
(
11). Considering (i)
this hypothesis, (ii)
the fact that TB10.4 is strongly recognized by
peripheral blood
mononuclear cells of BCG-vaccinated or tuberculosis
patients
(
11), and (iii)
our data, it might be of particular interest
to investigate the
potential vaccinal interest of TB10.3 or
TB10.4 proteins against
infection with
M. tuberculosis.

ACKNOWLEDGMENTS
We gratefully acknowledge Gilles Marchal for
providing the BCG
preparations, Richard Lo-Man for his advice on the
generation
of T-cell hybridomas, and Melinda Pryor for her help in
bioinformatic
sequence analyses of secreted or membrane mycobacterial
proteins.
This work was supported by grants from the Institut
Pasteur (Programme Transversal de Recherche no. 110) and INSERM
E352.

FOOTNOTES
* Corresponding
author. Mailing address: Unité de Biologie des Régulations
Immunitaires, Institut Pasteur, 25, rue du Docteur Roux, 75724 Paris
Cedex 15, France. Phone: (33-1) 45.68.85.42. Fax: (33-1) 45.68.85.40.
E-mail:
lmajless{at}pasteur.fr.

Editor:
S. H. E. Kaufmann

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Infection and Immunity, December 2003, p. 7173-7177, Vol. 71, No. 12
0019-9567/03/$08.00+0 DOI: 10.1128/IAI.71.12.7173-7177.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.
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