Download MHC tailored for diabetes cell therapy

Gene Therapy (2005) 12, 553–554
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NEWS AND COMMENTARY
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Type 1 diabetes
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MHC tailored for diabetes cell therapy
M Trucco and N Giannoukakis
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Gene Therapy (2005) 12, 553–554. doi:10.1038/sj.gt.3302431
Published online 13 January 2005
Type 1 diabetes (T1D) is an autoimmune disease whose clinical onset
most frequently presents in adolescents who are genetically predisposed. The Journal of Clinical
Investigation has recently published
exciting results from Tian and coworkers, who have demonstrated
that the disease can be prevented
in nonobese diabetic (NOD) mice.
Using gene supplantation, the
authors substituted the ‘diabetessusceptible’ beta chain of the major
histocompatibility complex (MHC)
class II molecule in hematopoietic
stem cells, with a ‘diabetes-resistant’
allelic transgene, with positive results.1
In the late 1980s, McDevitt and coworkers fine-mapped and identified
the most influential hereditary factor
in T1D: a single amino acid of the
beta chain of the HLA-DQ histocompatibility molecule.2 Although
T1D is recognized to be a multigenic disease, in humans the principal genetic susceptibility component
was proposed to be any allelic form
of the HLA-DQ molecule that lacks a
charged amino acid at position 57 of
its beta chain. Conversely, resistance
to disease is associated with the
inheritance of HLA-DQ alleles containing a charged amino acid such as
aspartic acid, at the same position
(Asp-57).
Physical explanation of the unusual importance of this particular
single amino-acid location for the
development of the autoimmune
characteristics of T1D, came with
the elucidation of the crystal structure of the HLA-DQ8 molecule, a
non-Asp-57 molecule, which confers
the highest susceptibility to the disease.3 The most important feature of
the susceptibility HLA-DQ8 molecule relevant to diabetes immunology is that its crystal structure is
identical to the homologous I-Ag7
molecule present in the NOD
mouse.4 This strain of mouse spontaneously exhibits T1D with etio-
pathogenetic characteristics very
similar to the disease in humans.
The susceptibility status, in immunological terms, can be correlated
with impaired peptide lodging,
impaired peptide presentation to T
cells with consequent reduction in
positive selection of regulatory T
cells or by the impaired negative
selection of self-reactive T cells.
Indirect evidence of these hypotheses derives from transgenic NOD
mice that express class II genes other
than I-Ag,7 which do not develop
diabetes,5 and from the fact that
transplantation of allogeneic bone
marrow from strains that do not
spontaneously develop diabetes, also
prevents the occurrence of diabetes
in NOD mice.6
On this basis, the strategy used by
Tian et al1 is attractive in its simplicity: instead of approaching the
problem using an alloreactive bone
marrow transplant, with all its inherent severe contraindications (eg
graft versus host disease), the authors
transfected ex vivo the gene encoding
a resistant, Asp-57+ beta chain into
the bone marrow cells isolated from
the diabetes-prone NOD mouse. The
expression of the newly formed
diabetes-resistant molecule in the
reinfused hematopoietic cells was
sufficient to prevent T1D onset in
the NOD recipient, even in the
presence of the native, diabetogenic,
non-Asp-57+ molecule (Figure 1).
Mechanistically, the data suggest a
model in which a subset of the
engineered bone marrow cells migrate, populate the thymus and
become antigen-presenting cells involved in the negative selection of
thymocytes that would otherwise
mature into autoreactive T cells. In
fact, diabetes-free NOD mice exhibited no emergence into the blood
stream of T cells capable of responding to putative autoantigens, nor
insulitis, the presence of beta cellreactive T cells in the pancreatic islets
themselves. The mice remained dia-
betes-free even after cyclophosphamide treatment, a maneuver that
tests the robustness of a prophylactic
antidiabetic therapy.7
Considering that in USA, over one
million individuals are affected by
T1D with B30 000 new cases just
diagnosed last year, alternatives that
may enrich the armamentarium of
the diabetes therapist can only be
welcome. Despite the promises offered by this study, however, prophylactic modalities are not ready
for clinical translation, chiefly because of how susceptibility is defined, but more importantly because
the current generation of susceptibility markers do not identify all those
who will become diabetic. Indeed,
the converse is even more sobering
in that not all individuals identified
as genetically ‘high-risk’ will succumb to the disease.8 Under this
scenario, tinkering with the immune
system introduces a variable of instability that could precipitate autoimmunity
in
someone
who
otherwise would not have ever been
affected. Equally germane questions
about the clinical applicability of this
and other similar approaches include
safety and tolerability of conditioning regimen(s) that may be required
prior to reconstitution. Tian et al1
preconditioned the recipient with
lethal doses of irradiation to ablate
the recipient bone marrow, a step not
easily translatable into possible clinical trials. Level of thymic engraftment of the engineered cells is
another area that should be investigated: the relatively few stem cells
present in the bone marrow should
be successfully targeted. Also, the
persistence of transgene expression
should be guaranteed. The need for
long-lasting expression suggested
the use of a retrovirus for the
transgene carrier, a choice that might
bring along known, negative, clinical
consequences.9
Nevertheless, given that solutions
like pancreatic islet allotransplantation are plagued by the paucity of
pancreas donors and the toxicity of
the immunosuppressive drugs that
precludes its implementation in
young recipients, this gene therapybased approach may be extremely
useful in facilitating new types of
therapy. In the diabetic NOD mouse,
the abrogation of autoimmunity per
se seems to be sufficient to promote
regeneration or rescue of the insulinproducing beta cells in the host
News and commentary
M Trucco and N Giannoukakis
554
Epithelial Thymus Cells
A
B
C
Positive and Negative Thymic Selection
D
T1D
Figure 1 In an healthy individual, the maturation of the T cells, coming from precursors present in
the bone marrow, is taking place in the thymus, where they undergo a positive and a negative
selection. In the thymus, peptides (in red) from antigens of self-tissues are presented to the various
immature double positive (CD8, in gray, and CD4, lighter gray) T cells (A–C) via the MHC molecule.
MHC class II molecules are heterodimers composed of an alpha (in orange) and a beta (in yellow)
chain that form their antigen combining site. When, like for the A cell, the T-cell receptor (TCR: in
blue, alpha chain; in azure, beta chain) has, for the MHC molecule/self-peptide complex, a very low
affinity (in the cartoon, contours of the MHC molecule/self-peptide complex do not fit with the
contours of the TCR molecule) the developing T cell does not receive the necessary positive signal to
survive and exit the thymus for release into the periphery. However, if the affinity between the MHC
molecule/self-peptide complex and the TCR is too high, like for the B cell (in the cartoon, the contours
of the MHC molecule/self-peptide complex fit precisely into the contours of the TCR molecule), the T
cell undergoes negative selection and dies inside the thymus. The T cell shown in (C) receives instead,
a positive survival signal because of the high-affinity interactions between its TCR with the MHC
molecule; an affinity, however, that is not further enhanced by the presence of a self-peptide in its
groove, so that the negative selection does not take place. This T cell matures and goes in the
circulation to protect the body from foreign (non-self) invaders, with which it is able to efficiently
interact. The immunological basis of type 1 diabetes is schematically described in T1D. Here the D cell
binds to an MHC molecule (orange and green chains) conferring susceptibility to diabetes (like the
HLA-DQ8 in humans and the I-Ag7 in the mouse), because does not present the self-peptide properly.
The T cell, then, even if potentially autoreactive (D has the same TCR as B), is not subjected to
negative selection and is free to leave the thymus to circulate in the blood. T cells that are potentially
reactive to self-antigens but failed to be deleted inside the thymus, are able to attack tissues of the body
expressing these same antigens generating autoimmunity. The approach taken by Tian and coworkers1 can be illustrated imagining that the diabetogenic I-Ag7 molecule, carrying a non-Asp-57
beta chain (in green like in D), was supplemented, in the hematopoietic cells of the NOD mouse, with
a nondiabetogenic MHC molecule, like the one interacting with (A, B, or C). The ex vivo transfection
of a gene encoding an Asp-57+ beta chain (in yellow), into the bone marrow stem cells, allowed the
reconstruction of an efficient MHC molecule (orange and yellow chains) that, once the cells were
returned into the donor, allowed the restoration of an efficient negative selection in the thymus (like for
B), sufficient per se to delete autoreactive T cells and consequently to prevent diabetes.
Gene Therapy
endocrine pancreas.10 Indeed, the
same hurdles will also manifest
should embryonic stem cell lines be
considered as a possible substitute
for the transplant donor source.
Thus, the report by Tian and coworkers1 further strengthens the
rationale for stitching genes into
autologous cells with the ultimate
objective of a gene-engineered cell
therapy for human autoimmune diseases. ’
M Trucco and N Giannoukakis are at the
Diabetes Institute of the University of Pittsburgh, Rangos Research Center, Children’s
Hospital of Pittsburgh, Pittsburgh, PA, USA.
E-mail: [email protected]
Published online 13 January 2005
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