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Infection and Immunity, February 2002, p. 1002-1005, Vol. 70, No. 2
0019-9567/01/$04.00+0 DOI: 70.2.1002-1005.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.
Delivery of a MalE CD4+-T-Cell Epitope into the Major Histocompatibility Complex Class II Antigen Presentation Pathway by Bordetella pertussis Adenylate Cyclase
Ji
ina Loucká,1 Géraldine Schlecht,2 Jana Vodolánová,1 Claude Leclerc,2 Peter
ebo,1*
Laboratory of Molecular Biology of Bacterial Pathogens, Institute of Microbiology of the Academy of Sciences of the Czech Republic, CZ-142 20 Prague 4, Czech Republic,1
Unité de Biologie des Régulations Immunitaires, Institut Pasteur, 75724 Paris, France2
Received 20 June 2001/
Returned for modification 7 August 2001/
Accepted 6 November 2001

ABSTRACT
Recombinant adenylate cyclase toxoids are shown to deliver inserted
foreign CD4
+-T-cell epitopes into the major histocompatibility
complex class II presentation pathway, inducing a specific CD4
+-T-cell
response in vivo and yielding in vitro stimulation of specific
CD4
+ T cells at a 100-times-higher molar efficiency than the
free peptide containing the epitope.

INTRODUCTION
The
Bordetella pertussis adenylate cyclase (AC) toxin (ACT or
CyaA) can penetrate a variety of cells, and upon reaching their
cytosol, it perturbs cellular physiology by unregulated conversion
of ATP to cyclic AMP (
10). Genetic ablation of the AC activity,
however, does not affect cell invasiveness of ACT (
12). AC toxoids
can hence be used as a potent new tool for delivery of vaccinal
CD8
+-T-cell epitopes into the cytosolic major histocompatibility
complex (MHC) class I antigen presentation pathway (
5). Moreover,
it was recently found that ACT specifically targets the ß
2 integrin molecule CD11b, which is present on cells of the myeloid
lineage and in particular on professional antigen-presenting
cells (APCs), such as dendritic cells (
6). This opens appealing
possibilities of using the ACT toxoid as a nonreplicative vector
for antigen delivery aiming at stimulation of specific cellular
immune responses. We have, indeed, recently used recombinant
ACT for delivery of foreign epitopes and for induction of protective
antiviral, as well as therapeutic antitumoral CD8
+ cytotoxic
T-cell responses in mice (
1,
2,
4,
7,
12,
13,
15,
16).
In certain cases, such as particularly for antitumor immunity, activation of CD4+ T-helper cells in parallel to induction of CD8+ T cytotoxic cells appears, however, necessary for optimal and long-lasting immune responses (9, 14, 17). The CD4+ T-helper cells are generally activated by exogenous antigens that have been taken up by APCs via phagocytosis or endocytosis and processed through the MHC class II antigen presentation pathway. After processing, antigenic peptides bound to MHC class II molecules are presented at the cell surface and are recognized by T-cell receptors of CD4+ T cells. Therefore, we tested whether ACT can deliver CD4+-T-cell epitopes for efficient endosomal processing and MHC-class II-restricted presentation, yielding specific stimulation of CD4+-T-cell responses.
We used the previously defined permissive insertion sites along the ACT molecule (12) and generated a set of 11 ACT constructs, which carried an H-2b/H-2d-restricted CD4+-T-cell epitope NGKLIAYPIAVEALS from the Escherichia coli maltose binding protein MalE (11). The recombinant ACT molecules were expressed in E. coli and purified, and their cell-invasive capacity was characterized, using sheep erythrocytes as the surrogate target cell model (Table 1).
As expected, 9 of the 11 ACT/MalE constructs exhibited over 60% of membrane insertion (binding) and hemolytic capacity, and seven of these constructs conserved over 90% of the cell-invasive capacity of wild-type ACT. The capacity to bind and penetrate erythrocytes was nil for the ACT926/MalE construct, whereas the ACT594/MalE and ACT607/MalE proteins exhibited reduced hemolytic and cell-invasive activities. The ACT336/MalE construct did not exhibit any measurable AC activity, and its cell-binding and invasive activity could not be quantified. The full hemolytic activity, however, indicated that its cell-targeting capacity was intact.
To avoid interference of the toxin activity of ACT (conversion
of intracellular ATP to cyclic AMP) with the physiology of the
cells used for in vitro presentation assays, the AC activity
of the individual ACT/MalE constructs was genetically ablated
by disruption of their respective ATP binding sites (
4). The
obtained toxoids were purified close to homogeneity, as documented
in Fig.
1,
and their capacity to deliver the inserted MalE CD4
+-T-cell
epitope into the MHC class II antigen processing and presentation
pathway was first determined in vitro. This was measured as
the potency of AC toxoids, incubated with mouse splenocytes
as APCs, for stimulating interleukin 2 (IL-2) release from CRMC3
or FBCD1 T-cell hybridomas (
11), which specifically recognize
the NGKLIAYPIAVEALS peptide complexed at the surface of APCs
with the MHC class II
H-2b or
H-2d molecules, respectively.
As shown in Fig.
2,
APCs incubated with up to 55 nM mock AC
toxoid did not stimulate any IL-2 release from the two hybridoma
T cells. In contrast, the six ACT/MalE proteins, carrying the
epitope within the first 600 residues, mediated an efficient
delivery and subsequent presentation of the MalE peptide to
the
H-2b-restricted CRMC3 T cells already at 0.06 nM toxoid
concentrations (Fig.
2A). It should be noted that approximately
hundred-times-higher concentrations of the free synthetic peptide
were necessary to induce similar IL-2 production by the same
hybridoma (Fig.
2B). The two toxoids bearing the CD4
+-T-cell
epitope at positions 607 and 1648 induced intermediate IL-2
production. Finally, the three toxoids with the MalE peptide
at positions 751, 926, and 1334 induced very low responses of
the CRMC3 T-cell hybridoma, even at the highest concentration
tested (0.56 nM). A similar response pattern, albeit at higher
AC toxoid concentrations, was observed also for the FBCD1 T-cell
hybridoma, which recognizes the same epitope complexed with
H-2d molecules (Fig.
2C and D). It can, therefore, be concluded
that the constructs with the MalE epitope inserted within the
first third of the toxoid molecule were particularly efficient
in delivering the epitope for endosomal proteolytic processing
and subsequent MHC-class II-dependent antigenic presentation
resulting in productive recognition by T-cell hybridoma. These
results suggest that the carrier ACT moiety was accounting for
the delivery efficiency, by targeting the epitope into APCs.
Interestingly, there was no influence of the residues flanking
the epitope on MHC class II presentation. All three insertion
contexts that were used (Table
1) allowed a comparable efficiency
of T-cell hybridoma stimulation by inserts located within the
first 600 residues of ACT (compare Table
1 and Fig.
2). Moreover,
there was no clear-cut correlation between the capacities of
ACT constructs to penetrate erythrocyte membrane and their capacities
to deliver the MalE epitope for presentation on MHC class II
molecules of APCs. Erythrocytes lack the ACT receptor (CD11b)
and endocytosis mechanisms (
6). The erythrocyte invasion assay
allows, hence, only characterization of the membrane penetration
activity of ACT constructs, which cannot fully reflect the more
complex interactions of ACT with APCs. Nevertheless, the nil
erythrocyte-binding and penetration activity may explain the
very poor, if any, epitope delivery capacity of the ACT926/MalE
toxoid. The ACT with the MalE insert at positions 751 and 1334
exhibited, however, full erythrocyte penetration capacity, while
performing very poorly in the T-cell stimulation assay. It is
possible that upon interaction with cells, only certain portions
of the ACT polypeptide, such as its first 600 residues and the
very C-terminal portion comprising an insert at position 1648,
may effectively reach the endosomal entry of the MHC class II
pathway of APCs. Presentation of the inserts at positions 751
and 1334 may also be inhibited by structural constraints preventing
epitope excision within endosomes or by translocation of those
ACT portions to the cytosolic side of the endosomal membrane.
The present results, however, demonstrate that 6 out of 11 constructed AC toxoids delivered a model CD4+-T-cell epitope for presentation in vitro with high efficiency. Two of the best-performing constructs were, therefore, tested in vivo for the capacity to induce specific CD4+-T-cell responses. As shown in Fig. 3,
1 week after immunization the splenocytes of mice that intravenously received a single dose of 50 µg of the ACT108/MalE and ACT336/MalE toxoids exhibited a strong proliferative response to the cognate MalE peptide, as compared to splenocytes isolated from mice that received the same dose of mock AC toxoid. It can, hence be concluded that the ACT/MalE toxoids induced a CD4+-T-cell response specific for the MalE epitope.
The mechanism by which the MalE epitope inserted into ACT reaches
the MHC-class II-dependent antigen presentation pathway remains
to be elucidated. It is, however, plausible to assume that ACT
endocytosis may be promoted by interaction with its ß
2 integrin (CD11b) receptor (
6) and that this may direct the inserted
MalE epitope to the endosomal pathway. Upon proteolytic processing,
the excised epitope could then associate with MHC class II molecules,
leading to presentation at the cell surface. Recently, ACT was
also shown to efficiently deliver several CD8
+-T-cell epitopes
inserted within its AC moiety (
3). These results suggest that
ACT could be used for simultaneous delivery of CD4
+- and CD8
+-T-cell
epitopes to professional APCs and for parallel induction of
both T-helper and T cytotoxic responses. ACT appears, therefore,
to be a promising candidate for immunotherapy protocols targeting
cancer or chronic viral infections, where coactivation of specific
CD4
+- and CD8
+-T-cell responses is required.

ACKNOWLEDGMENTS
We thank Martina H

ibová for help with AC assays and Gilles Dadaglio for critical reading of the manuscript.
This work was supported by ANRS and by grants no. 310/01/0934, A502907, and ME167 of the Grant Agency, the Academy of Science, and the Ministry of Education, Youth, and Sports of the Czech Republic, respectively, and QLK2-CT-1999-00556 from the 5th Framework Program of the European Union.

FOOTNOTES
* Corresponding author. Mailing address: Institute of Microbiology CAS, Víde

ská 1083, CZ-142 20 Prague 4, Czech Republic. Phone: (4202) 475 2762. Fax: (4202) 475 2152. E-mail:
sebo{at}0040biomed.cas.cz.

Editor: J. D. Clements

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Infection and Immunity, February 2002, p. 1002-1005, Vol. 70, No. 2
0019-9567/01/$04.00+0 DOI: 70.2.1002-1005.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.
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