The Journal of Immunology, 1998, 161: 4489-4492.
Copyright © 1998 by The American Association of Immunologists
Cutting Edge: Thymic Positive Selection and Peripheral Activation of Islet Antigen-Specific T Cells: Separation of Two Diabetogenic Steps by an I-Ag7 Class II MHC ß-Chain Mutant1
Osami Kanagawa2,
Barbara A. Vaupel,
Guan Xu,
Emil R. Unanue and
Jonathan D. Katz
Department of Pathology, Center for Immunology, Washington University School of Medicine, St. Louis, MO 63110
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Abstract
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The diabetes-susceptible class II MHC genes (in human and mouse)
share unique nonaspartic acid residues at position 57 of the class II
ß-chain. Transgenic expression of a mutant
I-Ag7, substituting histidine and serine at
position 56 and 57 of ß-chain with proline and aspartic acid
(I-Ag7PD), respectively, inhibits diabetes
development in the nonobese diabetic mouse model. Here, we demonstrate
that immature thymocytes expressing a diabetogenic islet Ag-specific
transgenic TCR are positively selected by I-Ag7PD class II
MHC to give rise to mature CD4+ T cells. However, splenic
APCs expressing the same I-Ag7PD fail to present pancreatic
islet Ag to mature T cells bearing this diabetogenic TCR. These results
indicate that nonaspartic acid residues at position 57 of class II MHC
ß-chain is important for diabetogenic CD4+ T cell
activation in the periphery but is not essential for the formation of a
diabetogenic T cell repertoire in the thymus.
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Introduction
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Insulin-dependent
diabetes mellitus (IDDM)3 is
a T cell-mediated autoimmune disease under the complex regulation of
both genetic and environmental factors (1, 2). In both human IDDM and
its mouse model, the NOD mouse, specific allelic forms of the class II
MHC genes are the most important genetic elements predisposing to
diabetes development (3, 4) The IDDM-susceptible class II MHC genes,
DQ2 and DQ8 in the human and I-Ag7 in the NOD mouse,
share a unique nonaspartic acid residue at position 57 of their
ß-chains (5, 6) Transgenic expression of a modified I-Ag7
(I-Ag7PD), in which histidine and serine residues at
position 56 and 57 of I-Ag7 ß-chain were replaced by
proline and aspartic acid, respectively, protected NOD mice from
developing diabetes (7, 8). How these specific residues of class II MHC
molecules modulate the development of diabetes is not well understood.
Here, we show that I-Ag7PD class II MHC molecules are
capable of positively selecting diabetogenic CD4 T cells in the thymus,
but fail to present pancreatic ß cell Ag to activate the same T cells
in the periphery.
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Materials and Methods
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Mice
cDNA encoding I-Ag7 ß-chain with proline and
aspartic acid at positions 56 and 57 (9) was introduced into a
transgenic expression vector under the control of an I-E class II
MHC-specific promoter (10). The transgenic DNA construct was injected
into fertilized BALB/c eggs, and mice were screened for surface
expression of transgene on peripheral blood lymphocytes using the
I-Ag7-specific mAb, 10.3.6.2. Transgenic mice carrying
islet Ag-specific TCR, BDC2.5, were described previously (11).
BDC2.5-transgenic mice were mated to CB17.SCID mice to produce BDC2.5
H-2d SCID. BALB/c and NOD mice were purchased from The
Jackson Laboratory (Bar Harbor, ME). (NOD x BALB/c)F1
mice were bred in our colony. All mice were kept under specific
pathogen-free conditions in the Washington University School of
Medicine animal facility.
Surface immunofluorescence analysis
Spleen cells (1 x 106/sample) were
incubated with biotinylated anti-I-Ad (MKD6),
anti-I-Ag7 (10.3.6.2) mAbs, or without Ab at 4°C for
25 min, then washed and reincubated with streptavidin-coupled FITC
(Caltag, South San Francisco, CA) for an additional 25 min. Samples
were analyzed with FACScan using CellQuest software (Becton
Dickinson, Mountain View, CA).
T cell proliferation assay
T cells (2 x 104) were cultured with 2.5
x 105 irradiated (2000 rad) spleen cells in the presence
of the indicated dose of Ag in a final volume of 200 µl 5% FCS in
DMEM in flat-bottom microtiter plate. For stimulation of spleen cells
with islet cells, spleen cells (1 x 105) from bone
marrow chimeric mice were stimulated with irradiated NOD spleen cells,
as described above. Cultures were harvested after a 72-h incubation
with a 6-h pulse with [3H]thymidine.
Bone marrow chimera
Bone marrow cells from BDC2.5 H-2d SCID mice
were treated with anti-Thy1 mAb (At83 1/10 v/v) and complement for
45 min at 37°C. T cell-depleted bone marrow cells (1 x
107) were injected into irradiated (850 rad) recipients.
Chimeric mice were analyzed 8 to 10 wk after reconstitution.
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Results and Discussion
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BALB/c transgenic mice (named BALBg7PD) expressing the
introduced I-Ag7PD ß-chain with the same tissue
distribution as the endogenous I-Ad class II molecules
(data not shown) were selected for the experiments. Spleen cells from
BALBg7PD, NOD, BALB/c, and (NOD x
BALB/c)F1 mice were stained with mAbs specific for
I-Ag7 and I-Ad ß-chains (10.3.6.2 and MKD6,
respectively). As shown in Figure 1
, the cells from
(NOD x BALB/c)F1 mice express equivalent
amounts of I-Ad and I-Ag7. The
I-Ag7 and I-Ad molecules differ only in their
ß-chains and share an I-A
d-chain (5). The equivalent
expression of the two ß-chains on the cell surface of (NOD x
BALB/c)F1 mice demonstrates that I-Ad and
I-Ag7 ß-chains have a similar affinity for the
I-Ad
-chain. However, in BALBg7PD mice, the
expression of the transgene-encoded Aß appears slightly lower than
that of endogenous I-Ad class II MHC molecules (Fig. 1
). We
found the same staining pattern in two independent founders. We know
that all of the mAb specific for the I-Ag7 ß-chain
interact with residues surrounding positions 56 and 57 (12). Moreover,
in transfected B cell lines expressing only I-Ag7PD
(I-A
d and ßg7PD), the cell surface
staining for the complex appears lower with the anti ß-chain Ab than
with the anti
-chain Ab (E. Carrasco-Marin, unpublished
observation). Therefore, it is likely that the lower staining intensity
of I-Ag7PD on transgenic spleen cells is due to the lower
affinity of the mAb to the modified I-Ag7 ß-chain.

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FIGURE 1. Expression of the transgenic I-Ag7PD molecule.
Spleen cells from BALB/c, NOD, (BALB/c x NOD)F1, and
I-Ag7 PD transgenic BALB/c (BALBg7PD) mice were
stained with no Ab (solid line), anti-I-A ßd Ab, MKD6
(bold line), and anti-I-Ag7 ß-chain Ab
10.3.6.2 (dotted line), followed by avidin-FITC.
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To assess the function of mutant I-Ag7PD on the APCs, we
measured their Ag-presenting capacity using three OVA-specific T cell
clones (Fig. 2
). The OVA-2 clone
recognizes OVA peptide (323339) presented by either I-Ad
or I-Ag7, as demonstrated in our previous report (13). This
cell line, as expected, responded to OVA Ag presented by splenic APC
from NOD, BALB/c, (NOD x BALB/c)F1, and
BALBg7PD mice. The OVA-3 clone responds to the same peptide
presented only by I-Ag7 (13) and, in contrast, failed to
respond to OVA Ag presented by BALBg7PD splenic APC. A
third clone, OVAPD, was established from BALBg7PD mice and
showed no reactivity to the OVA Ag presented by I-Ad class
II MHC. However, this clone did respond to OVA presented by NOD,
(NOD x BALB/c)F1, and BALBg7PD spleen APC
(Fig. 2
). Thus, the unique I-Ag7PD ß-chain associated
with endogenous I-A
d-chain was expressed on the
transgenic mouse APC and was capable of presenting protein Ag to T
cells.

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FIGURE 2. Ag presentation by spleen cells from I-Ag7PD transgenic
mice. OVA-2, OVA-3, and OVAPD T cells were stimulated with NOD,
BALBg7PD, BALB/c x NOD F1, and BALB/c
spleen cells in the presence (0.5 mg/ml) or absence of OVA as described
in Materials and Methods. Cultures were harvested after
a 72-h incubation with a 6-h pulse of
[3H]thymidine.
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The presentation of islet ß cell Ag to T cells by the
I-Ag7PD class II molecule was tested using the diabetogenic
CD4 T cell clone BDC2.5 (11, 14, 15). The BDC2.5 T cell clone,
established from diabetic NOD mice, responds to pancreatic ß cell Ag
presented by the I-Ag7 class II MHC molecules. This
diabetogenic T cell line responded to BALB/c-derived islet cells in the
presence of (NOD x BALB/c)F1 APC (Fig. 3
). In this assay, Ag from the islets are
transferred to the APC expressing class II MHC for presentation to the
BDC 2.5 T cell. Moreover, the presentation of islet Ag to BDC2.5 T cell
is I-Ag7 restricted, because non-NOD APC such as BALB/c
fail to present islet Ag (14). Importantly, no proliferative response
was observed to BALB/c islet cells in the presence of APC from the
BALBg7PD transgenic mouse (Fig. 3
). These results indicate
that the modification of I-Ag7 at positions 56 and 57 of
the ß-chain completely abolishes its capacity to present islet Ag to
the diabetogenic BDC2.5 T cells.

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FIGURE 3. Lack of islet Ag presentation by BALBg7PD spleen APC. Islet
Ag-specific CD4 T cell clones, BDC2.5, were stimulated with an
indicated number of irradiated (2000 rad) BALB/c-derived pancreatic
islet cells in the presence of either (BALB/c x
NOD)F1 (open circle) or BALBg7PD (open
triangle) spleen cells under the conditions described in
Materials and Methods. Cultures were harvested after a
72-h incubation with a 6-h pulse of [3H]thymidine.
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The BDC2.5 TCR transgene was introduced into the CB17.SCID strain
(H-2d) (16). The thymocytes from BDC H-2d SCID
mice were analyzed by surface immunofluorescence (Fig. 4
, Donor). A majority of thymocytes were
arrested at the immature CD4/CD8 double-positive stage and failed to
produce mature CD4 single-positive T cells. These results establish
that I-Ad is not a positively selecting MHC for BDC2.5
TCR-bearing thymocytes, while as previously established, the
I-Ag7 is the selecting I-A allele (15).

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FIGURE 4. Maturation of BDC2.5 transgenic TCR-bearing T cells in bone marrow
chimeras. Thymocytes from BDC2.5 TCR transgenic SCID mice with
H-2d background were stained with
phycoerythrin-anti-CD4 and FITC-anti CD8 Abs. Cells were analyzed
by FACScan using the CellQuest program. T cell-depleted bone marrow
cells from the same mice were injected into lethally irradiated
(NOD x BALB/c)F1, BALB/c, and BALBg7PD
transgenic mice. Eight weeks after bone marrow reconstitution,
thymocytes from bone marrow chimeras were analyzed for CD4 and CD8
expression as described in Materials and
Methods.
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Bone marrow cells from BDC H-2d SCID mice (Donor in Fig. 4
)
were transferred into lethally irradiated (850 rad) (NOD x
BALB/c)F1, BALB/c, and BALBg7PD transgenic
mice. Eight weeks after bone marrow reconstitution, thymocytes from the
bone marrow chimeras were analyzed (Fig. 4
). As expected, a large
number of CD4 single-positive T cells (32% of total thymocytes) were
generated in the (NOD x BALB/c)F1 thymus, whereas no
mature single-positive T cells were found in the thymus of BALB/c
recipients. In the BALBg7PD recipient thymus, the mature
CD4 single-positive T cells (26% of total thymocytes) were generated
as efficiently as in the (NOD x BALB/c)F1 recipient
thymus, indicating that BDC2.5 TCR-bearing T cells were positively
selected by I-Ag7PD class II MHC. It should be noted that a
majority of the thymocytes in all three different recipients expressed
Vß4 TCR (the TCR V ß-chain used by the BDC2.5 T cell) (11),
indicating that these thymocytes were derived from BDC TCR transgenic
bone marrow cells (data not shown). Furthermore, having used the BDC
H-2d SCID mice as bone marrow donors, we excluded the
development of T cells using endogenous TCR (16).
None of the chimeric mice developed diabetes over a 3-mo period. This
was not surprising given that all of the hemopoietic cells, including
all of the APC, in the chimeric mice are derived from BDC2.5
H-2d SCID bone marrow cells and are, by virtue of their
H-2d MHC expression, incapable of presenting diabetogenic
Ag to BDC.2.5 T cells. However, the peripheral T cells in the chimera
mice were functional because they responded to islet Ag in an
I-Ag7 MHC-restricted fashion as demonstrated in an in vitro
proliferation assay (Fig. 5
). We recently
found that the mice carrying both BDC2.5 TCR and I-Ag7PD
transgenes on the CB17 SCID background also showed no development of
diabetes (result not shown). In these mice, BDC2.5 TCR-positive
CD4-positive T cells developed efficiently, similar to the born marrow
chimera shown in this report.

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FIGURE 5. Islet Ag-specific response of T cells positively selected by
I-Ag7PD thymus. Spleen cells from (NOD x
BALB/c)F1 and BALBg7PD chimeric mice (from the
experiment shown in Fig. 4 ) were stimulated by the indicated number of
irradiated BALB/c-derived pancreatic islet cells in the presence of NOD
spleen cells under the conditions described in Materials and
Methods. Cultures were harvested after a 72-h incubation with a
6-h pulse of [3H]thymidine.
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In summary, the results presented here demonstrate that thymocytes
bearing the BDC2.5 TCR interact with the mutant I-Ag7
molecule (I-Ag7PD) in the thymus and that this interaction
transduces positive selecting signals. In contrast, no productive
interaction takes place between the mature BDC2.5 TCR-positive T cells
and the mutant I-Ag7 molecule and islet Ag(s). These
results help us to interpret previous studies in which an
I-Ag7 ß-chain with the PD changes was introduced as a
transgene into NOD mice (7, 8). These mice did not develop diabetes. We
would conclude that the protection was not caused by a negative effect
of mutant I-Ag7 on the selection of diabetogenic CD4 T
cells in the thymus. It should be noted that the I-Ag7
molecule as well as I-Ag7PD is a class II MHC molecule that
binds peptide poorly (9). This results in a poor negative selection
process (17) and could also promote peripheralization of a wide range
of T cells. We previously showed that the I-Ag7PD mutation
affected either recognition by T cells or peptide binding when testing
different T cell clones to different peptides (13). We could not
determine which circumstance applied in the case of islet Ags, since we
did not know their chemical identity. Thus, the lack of islet
Ag-specific activation of BDC2.5 T cell by I-Ag7PD APC
could be explained by 1) a lack of peptide binding or 2) poor or low
affinity interaction of the BDC2.5 TCR with I-Ag7PD-peptide
complex.
A similar dissociation between thymic selection and peripheral
activation of T cells has been reported in the class I MHC system by
Ohashi et al. (18). T cells bearing lymphocytic choriomeningitis virus
(LCMV)-specific, class I Db molecule-restricted TCR were
positively selected in a Dbm13 class I mutant mouse.
However, mature T cells in the Dbm13 mouse failed to
respond to LCMV antigenic peptide. Thus, both wild-type Db
and mutant Dbm13 were sufficient for positive selection of
the transgenic TCR bearing T cells in the thymus, but only the
wild-type MHC bound antigenic peptides for activation of the mature T
cells. Our findings and those of Ohashi et al. (18) indicate the great
plasticity of TCR/MHC/peptide interaction, as well as a lack of
absolute correlation between positively selecting MHC/peptide and
activating MHC/peptide complex for a single TCR. The biologic
significance of these findings in the establishment of T cell Ag
repertoire awaits further investigation.
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Acknowledgments
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We thank Michael White for production of the transgenic mice. We
thank Dr. Charles Kilo and the Kilo Diabetes and Vascular Research
Foundation for their generous support.
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Footnotes
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1 This work was supported by grants from the National Institutes of Health, the Juvenile Diabetes Foundation, and the Kilo Diabetes and Vascular Research Foundation. 
2 Address correspondence and reprint requests to Dr. Osami Kanagawa, Department of Pathology, Center for Immunology, Washington University School of Medicine, 660 South Euclid Avenue, St. Louis, MO 63110. E-mail address: 
3 Abbreviations used in this paper: IDDM, insulin-dependent diabetes mellitus; NOD, nonobese diabetic; I-Ag7PD, transgenic expression of a modified I-Ag7 in which histidine and serine residues at position 56 and 57 of I-Ag7 ß-chain are replaced by proline and aspartic acid, respectively. 
Received for publication August 7, 1998.
Accepted for publication August 27, 1998.
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