By
From the Microbiology and Tumor Biology Center, Karolinska Institute, S-17177 Stockholm, Sweden
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Abstract |
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Triggering of a T cell requires interaction between its specific receptor (TCR) and a peptide
antigen presented by a self-major histocompatibility complex (MHC) molecule. TCR recognition of self-MHC by itself falls below the threshold of detection in most systems due to low
affinity. To study this interaction, we have used a read-out system in which antigen-specific
effector T cells are confronted with targets expressing high levels of MHC compared with the
selecting and priming environment. More specifically, the system is based on CD8+ T cells
selected in an environment with subnormal levels of MHC class I in the absence of 2-microglobulin. We observe that the MHC restriction element can trigger viral peptide-specific T
cells independently of the peptide ligand, provided there is an increase in self-MHC density.
Peptide-independent triggering required at least four times the natural in vivo level of MHC
expression. Furthermore, recognition of the restriction element at expression levels below this
threshold was still enough to compensate for lack of affinity to peptides carrying alanine substitutions in major TCR contact residues. Thus, the specificity in TCR recognition and T cell
activation is fine tuned by the avidity for self-MHC, and TCR avidities for peptide and MHC
may substitute for each other. These results demonstrate a functional role for TCR avidity for
self-MHC in tuning of T cell specificity, and support a role for cross-reactivity on "self" during
T cell selection and activation.
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Introduction |
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Triggering of a CD8+ T cell requires TCR interaction
with a specific peptide bound to an MHC class I molecule. The requirement for the selecting MHC molecule in
recognition of antigen is known as MHC restriction (1, 2).
Although interactions between TCR and the restriction element have been detected only in the presence of specific
peptides, it is believed that thymic positive selection imposes a low self-MHC affinity onto the peripheral T cell
pool (3). Early studies of TCR-MHC interactions involved analyses of MHC mutants for their ability to disrupt
T cell reactivity (7). Mutations in the -helical regions
in the
1/
2 domains of the MHC molecule were found
to perturb recognition of specific antigen, indicating that
the TCR physically contacts the
-helices of MHC molecules. However, measurements of affinity between TCR and peptide-MHC complexes have revealed relatively low
affinities for MHC in complex with agonist peptides and
slightly lower affinities for MHC in complex with antagonist peptides, whereas no detectable affinities have been
found for MHC in complex with unrelated peptides (10-
13). Thus, the idea of TCR affinity to self-MHC in the absence of specific antigen has been hard to support biochemically.
Recently, crystallographic studies have shown a physical
interaction between TCR and MHC in the presence of
specific peptide (14, 15). Of the total surface area contacted
by the TCR in the MHC complex, approximately one
third is provided by the peptide while MHC 1/
2 domains contribute two thirds of the contact area. Based on
alanine scanning mutagenesis of the 2C TCR, it has been estimated that almost two thirds of the binding energy in
antigen-specific binding is attributable to contacts with the
-helices (16). Does the interaction between TCR and
MHC as observed in crystallographic and biochemical
studies also contribute to the avidity required for T cell
triggering, and can it be detected in the absence of the specific peptide? Further, how does this interaction influence T cell specificity for the antigenic peptide? The answers to
these questions are of major importance for the understanding of several aspects of physiological T cell function,
including MHC restriction (1, 2), thymic selection (17),
and maintenance of T cell memory (18). Interestingly, recent evidence indicates that T cells require interaction with
endogenous peptide-MHC complexes for long-term survival, suggesting continuous cross-recognition of self for
maintenance of immunological memory (19).
T cell avidity for self has been demonstrated in functional assays. CTLs triggered by allogeneic MHC can specifically kill bystander cells of self-MHC haplotype, indicating that self-MHC can mediate binding to CTLs that are
efficiently triggered by their antigen, although it cannot
trigger CTLs directly (22). Second, mice with low MHC
class I expression select T cells adapted to that environment, whose avidity to self-MHC is revealed when assessed
in the context of normal MHC class I levels. For example, CTLs generated against allogeneic cells in such mice specifically cross-react with cells expressing high levels of self-MHC class I (23). Similarly, T cell hybridomas derived
from CD3/
-deficient mice expressing very low levels of
cell surface TCR also react against cells expressing self-MHC class I molecules, once a normal level of TCR expression has been restored by CD3
/
transfection (26).
Thus, in systems where either the MHC or the TCR are
expressed at subnormal levels, it is possible to generate T
cells for the study of TCR affinity/avidity for self-MHC,
which are not present in normal mice with high MHC expression due to negative selection.
To study the role of self-MHC avidity in MHC-restricted
CTLs, we immunized 2-microglobulin-deficient (
2m
/
)1
mice, which have low expression of conformed MHC
class I (27, 28), with the H-2Db-restricted immunodominant lymphocytic choriomeningitis virus (LCMV) GP 33-41 (GP33) peptide. We obtained CD8+ CTLs which were
specific for the GP33 epitope when loaded onto cells with
low MHC class I expression (MHClow). However, unlike
GP33-specific C57Bl/6 (B6) CTLs, these CTLs also killed
cells expressing high levels of self-MHC class I (MHChigh)
in the absence of specific peptide. Further experiments revealed a dual specificity of these CTLs: peptide specificity
against MHClow targets, and peptide-independent specificity for the restriction element when tested against MHChigh
targets. By mAb blocking of the MHC restriction element,
we found that peptide-independent triggering of a CTL
clone with high avidity for self-MHC required at least four
times the in vivo expression of MHC. Most interestingly,
at lower MHC expression an increased recognition of self-MHC could compensate for lack of TCR interaction with
the specific peptide, as evaluated with peptides carrying alanine substitutions in major TCR contact residues. These
results indicate that TCR avidity for the MHC molecule
functionally contributes to T cell specificity, and that TCR
affinities for MHC and peptide are interchangeable. Our
results have important implications for the understanding of
T cell specificity and the role of self-MHC in peripheral T
cell function.
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Materials and Methods |
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Mice.
B6, transporter associated with antigen processing (TAP)1Cell Lines.
RMA is a subline of the Rauscher virus-induced B6 lymphoma RBL-5, and RMA-S is a TAP2-deficient variant of RMA (33). T2 is a hybrid between the two human cell lines 0.174 and CEM, and has an antigen-processing deficiency due to a deletion on the MHC class II region including the TAP1 and TAP2 genes. T2Db is an H-2Db transfectant of T2. All cell lines were maintained at 37°C and 5% CO2 in RPMI 1640 tissue culture medium supplemented with 5% FCS, 50 µg/ml streptomycin, 100 µg/ml penicillin, and 2 mM L-glutamine. Con A-activated blasts were generated by culturing erythrocyte-depleted splenocytes in 5 µg/ml of Con A for 2 d in tissue culture medium as described above with 10% FCS.Synthetic Peptides.
The following synthetic H-2Db-presented peptides were synthesized using solid phase F-moc chemistry: LCMV GP33 KAVYNFATM (34, 35); LCMV GP 33-4A (GP33-4A) KAVANFATM; LCMV GP 33-34A (GP33-34A) KAAANFATM; LCMV GP 33-348A (GP33-348A) KAAANFAAM; influenza PR8 NP 366-374 (NP366) ASNENMETM (36); and Yersinia enterocolitica Yop51 249-257 (Yop249) IQVGNTRTI (37).Generation of LCMV GP33-specific CTL Cultures and Clones.
GP33-specific CD8+ CTLs were elicited in B6 andmAbs and FACS® Analysis.
B22-249.1 is a mouse mAb which binds to a conformation-dependent epitope on theCTL Assay, Cold Target Competition, and CTL Blocking with mAb.
CTL activity was measured in a standard 51Cr-release assay. In brief, peptide-coated target cells were prepared by incubating cells with indicated concentrations of peptide for 1 h at 37°C. Coated cells were labeled with 10 µl 10 mCi/ml 51Cr for 1 h at 37°C. Titrated amounts of effector cells were incubated with 3 × 103 51Cr-labeled target cells for 4 h at 37°C, 5% CO2. After incubation, released radioactivity was measured and specific lysis was calculated according to the formula: % specific release = [(experimental release ![]() |
Results |
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We first compared the levels of properly
conformed H-2Db molecules on the cell surface of 2m
/
,
TAP1/
2m
/
, and B6 (TAP1/
2m+/+) control cells
stained with an mAb directed against H-2Db (B22-249.1)
or H-2Kb (Y3) conformation-sensitive epitopes. A substantial level of conformed cell surface H-2Db molecules was
found on
2m
/
cells, whereas conformed H-2Kb was detected at lower levels, in line with previously published results (27, 28; Fig. 1). This is compatible with H-2Db being
more independent of
2m during folding and transport of MHC class I free heavy chains (27, 28). On Con A blasts
deficient for both
2m and TAP1, the staining with B22-249.1 was virtually at background levels, which implies that
the pool of conformed H-2Db molecules on
2m
/
cells is
dependent on a functional TAP complex.
|
B6 and 2m
/
mice
were immunized with antigenic peptides restricted to either H-2Db or H-2Kb. All tested peptides primed responses
in B6 mice (data not shown), whereas only one, the H-2Db-
restricted LCMV GP33, primed a response in
2m
/
mice. This is in line with the low but significant levels of folded H-2Db molecules on the surface of
2m
/
cells
(27; Fig. 1). CTLs from both B6 and
2m
/
mice primed
with the GP33 peptide killed RMA-S cells pulsed with the
GP33 peptide used for priming but not a control influenza NP366 peptide (Table I). CTL responses were mediated by
CD8+ cells as determined by mAb- and complement-mediated depletion of effector populations in vitro (data not
shown). FACS® analysis of both polyclonal bulk populations and clones confirmed the TCR+CD8+ phenotype of
the responding CTLs in
2m
/
as well as B6 mice (Fig. 2).
Cell surface expression levels of both TCR and CD8 were
similar in CTLs from
2m
/
and B6 mice. However, a
marginal increase in CD8 expression levels was detected in
some
2m
/
CTLs compared with B6 CTLs. This increase
may be due to the low levels of H-2Db expressed during selection and priming in the absence of
2m. However, lysis
performed by
2m
/
CTLs did not show increased dependency on CD8 compared with B6 CTLs, as determined by
mAb blocking of CD8 (data not shown).
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|
No priming of CTLs was observed in thymectomized
2m
/
mice that had been irradiated and reconstituted
with fetal liver (Table I). Control mice (receiving irradiation and fetal liver reconstitution without thymectomy)
generated a normal antipeptide CTL response, showing
that the responding CD8+ T cell population in
2m
/
mice was dependent on the thymus for development. In
addition, a normal antipeptide response was also found in
mice that had been thymectomized but not irradiated,
showing that the thymus was not necessary during the
priming of the response (Table I). The latter control indicated that the generated CTLs were not positively selected
in the thymus in response to the peptide injected for
immunization. We conclude that
2m
/
mice have a
peripheral pool of CD8+ T cells which are thymus dependent, H-2Db restricted, and peptide specific despite expressing very low levels of H-2Db molecules, corresponding to a few percent of those expressed by B6 mice.
Alloreactive CTLs from mice with low
MHC class I expression have an increased avidity to self-MHC (23). By priming peptide-specific, self-MHC-
restricted CTLs in 2m
/
mice it is possible to study the
role of self-MHC avidity for T cell specificity in MHC-
restricted T cells. To compare the avidity for self-MHC class
I in B6 and
2m
/
CTLs specific for the GP33 peptide,
these CTLs were tested for their ability to kill RMA target
cells (MHChigh) in the absence of loaded peptides. B6 CTLs
failed to kill RMA cells, whereas
2m
/
CTLs efficiently
killed these target cells (Fig. 3, A and B, respectively). Cold
target competition experiments made with bulk CTLs
showed that the
2m
/
CTL killing of labeled RMA cells
was completely inhibited by RMA-S pulsed with GP33
but not by RMA-S without peptide (Fig. 3 C). This
showed that a majority of the clones in the
2m
/
CTL
population were specific for both self-MHC expressed at
high levels and the peptide antigen presented by MHC at
low levels.
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Three GP33-specific 2m
/
CTL clones were generated, all of which also killed RMA cells. However, the
clone C10 was more efficient in killing RMA-S loaded
with GP33 compared with RMA (Fig. 3 D), whereas clone
27/30 killed RMA slightly better than RMA-S plus GP33
(Fig. 3 E), and clone 3C5 killed both of these target cells to
a similar extent (Fig. 3 F). This pattern is compatible with a
clonal variation in TCR avidity for peptide versus MHC
within the
2m
/
CTL population. In line with the results obtained with bulk cultures, the GP33-specific B6
CTL clone 2C10 did not recognize RMA in the absence of
GP33 peptide (Fig. 3 G).
To test the reactivity of the CTLs against MHC class I in
the absence of peptides, we used RMA-S cells incubated at
cold temperature (26°C), which induce high levels of functionally "empty" MHC class I molecules (39), and T2Db
cells expressing Db molecules mostly devoid of peptides.
Clone 27/30 also killed RMA-S cells after cold temperature incubation (Fig. 4 A). Furthermore, T2Db was recognized irrespective of loaded peptide, whereas T2 control targets were not killed (Fig. 4 B). Thus, the 2m
/
CTL
clone 27/30 displays an avidity for the restriction element of the GP33 peptide, even in the absence of specific peptide. It should be noted that these CTLs were primed to respond only against GP33, indicating that the ability to recognize self-MHC had been selected for during thymic
development and priming. Although these results do not
exclude that
2m
/
CTLs also have an increased avidity
for self-peptides, we use the term "peptide independent" to
describe the specificity for the restriction element displayed
by the GP33-specific
2m
/
CTLs.
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We next compared the avidity of 2m
/
CTLs
for self-MHC with and without added GP33, by blocking
of the H-2Db ligands. We used an mAb specific for properly conformed H-2Db molecules (B22-249.1), the binding
of which is not affected by GP33 (data not shown). RMA
target cells were killed at similar levels regardless of the
presence or absence of specific peptide (Fig. 5). However,
in the absence of GP33 peptide the CTL killing activity
was efficiently blocked by 5.0 µg/ml of B22-249.1, whereas virtually no blocking was observed when the RMA target
cells were prepulsed with the GP33 peptide at 37°C (Fig.
5 A). FACS® analysis of B22-249.1 binding to RMA cells
did not allow accurate quantitation of the fraction of free
H-2Db ligands at half-maximal lysis, since half-maximal
blocking of CTL lysis was achieved at a concentration at
which staining was close to maximal (Fig. 5 B). However,
these experiments clearly demonstrate that triggering of
2m
/
CTLs in the absence of GP33 peptide requires a
considerably higher number of H-2Db ligands.
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We next made a similar series of experiments using
T2Db cells as targets. These cells express the B22-249.1
epitope at levels <10% those of RMA (Table II) but are
still killed by the 2m
/
GP33-specific CTL clone 27/30
in the absence of specific peptide (Fig. 4 B). Lysis of T2Db
cells was blocked already at a 1.0 µg/ml concentration of
B22-249.1 in the CTL assay (Fig. 5 C), indicating that a
large fraction of cell surface H-2Db molecules is required
for triggering of this CTL clone. Pulsing of T2Db cells with
GP33 peptide prohibited blocking at all mAb concentrations tested (Fig. 5 C). This effect was peptide specific,
since neither NP366 nor Yop249 had any effect on avidity
as determined by blocking with mAb. By titration of B22-249.1 in FACS® analysis of T2Db (Fig. 5 D), the fraction of
free H-2Db ligands on T2Db at half-maximal peptide-independent lysis was estimated at ~80% (comparing Fig. 5, C
and D). Thus, ~80% of the H-2Db molecules on T2Db
were necessary for peptide-independent recognition by the
high-avidity CTL clone 27/30, which corresponds to
about four times the level of folded H-2Db on
2m
/
cells
(Table II). Taken together, these data indicate that
2m
/
CTLs use peptide-independent recognition of a high number of H-2Db ligands to compensate for the absence of specific peptide.
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Finally, we analyzed the consequences of self-MHC avidity in GP33-specific B6 and
2m
/
CTLs in terms of peptide specificity, when MHC
was expressed at lower levels. Neither B6 nor
2m
/
CTLs kill RMA-S (MHClow) in the absence of specific
peptide, and RMA-S cells require a pulse of ~100 pM
GP33 peptide to become sensitive targets to both B6 and
2m
/
CTLs (Fig. 6). This shows that B6 and
2m
/
CTLs require roughly equal amounts of H-2Db/GP33
complexes to get a triggering signal. However, when using GP33 peptide variants alanine-substituted at one or several
TCR contact residues (reference 40, and data not shown),
the
2m
/
CTLs recognized RMA-S cells pulsed with up
to 1,000-fold less peptide than B6 CTLs (Fig. 6). Substitution at one position (GP33-4A) already drastically reduced
the efficiency of B6 CTLs against peptide-pulsed RMA-S
targets, whereas
2m
/
CTLs could still recognize peptides with alanine substituted at two positions. Thus, although
2m
/
CTLs were still peptide specific, they were
less dependent on the TCR contact residues in the peptide
when the peptide was loaded on RMA-S target cells
(which express about three times the levels of H-2Db found
on
2m
/
cells [Table II]). These data indicate that the
avidities for peptide and MHC both contribute functionally to the triggering of CTLs, and that they can be considered separately. Further, increased avidity for the restriction
element compensates for a reduced avidity for the peptide.
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Discussion |
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In this paper we have investigated
the influence of TCR interaction with self-MHC in recognition of MHC-bound peptides. To address this issue, we
used MHC class I-restricted CD8+ T cells selected and
primed in an environment with high (B6) or low (2m
/
)
MHC expression (25). We find that self-MHC can deliver
a triggering signal independently of specific peptide, provided there is an increase in self-MHC density. The low
surface density of H-2 expressed on RMA-S cells was not
sufficient to trigger either
2m
/
or B6 CTLs in the absence of the specific antigen, but increasing the ligand density
by cold temperature incubation could sensitize RMA-S to
the
2m
/
CTL clone 27/30. Furthermore, since T2Db
cells were also killed irrespective of loaded peptide, recognition of syngeneic class I was most probably peptide independent. These results indicate that the interaction between
TCR and self-MHC as observed in crystals not only provides the structural framework of specific T cell recognition, it also contributes a part of the avidity required for
CTL triggering.
Recognition of MHChigh targets by 2m
/
CTLs was
blocked by mAb against properly conformed Db molecules,
whereas the presence of specific peptide prohibited blocking.
These results indicated that the lack of GP33 was compensated by using an increased number of low-avidity interactions with self-MHC. The results further suggest that a minimum of four
times the level of MHC class I expression present during selection and priming in vivo were necessary for activation by self-MHC to occur in vitro. RMA-S expresses only about three
times the level of MHC found on
2m
/
cells (Table II),
which may explain why
2m
/
CTLs were peptide specific
when tested against RMA-S targets. In the presence of specific
GP33 peptide, no mAb-mediated blocking of MHChigh target
cell killing was observed. This supports the notion that MHC-restricted CTLs have a high avidity for the peptide antigen and
a low avidity for self-MHC.
Interestingly, we found that T cells with an increased
avidity for MHC class I molecules had a more relaxed peptide specificity, i.e., they were less dependent on the exact
peptide sequence. While both B6 and 2m
/
GP33-specific CTLs had similar sensitivity for GP33,
2m
/
CTLs
had superior sensitivity for peptides lacking one or several TCR contact residues of the GP33 peptide. Thus, increased recognition of the restriction element compensates
for lack of TCR peptide affinity in recognition of low-
affinity peptide ligands.
Our data argue that recognition of the two entities of the MHC complex, peptide and MHC heavy chain, can be considered separately (41). Increased recognition of MHC could substitute for lack of peptide recognition. The indication that TCR avidity for its ligand can be subdivided in this way opens the possibility of extending the differential avidity model of T cell selection and recognition (42).
Implications for the Avidity Threshold in T Cell Recognition.The present data can be interpreted within a model for T
cell recognition based on the total avidity contributed by
TCR affinity for MHC, affinity for peptide, and the number of MHC-peptide complexes. Increased avidity for
MHC can compensate for lack of avidity for peptide, suggesting that these binding forces are functionally interchangeable and can be considered separately (Fig. 7 A).
This would suggest a variation in the avidities for MHC
and presented antigens among mature CTLs (See also Fig.
3, D-G). Interestingly, mice deficient in terminal deoxynucleotidyl transferase (TdT), which lack N-region additions in the TCR, display increased promiscuity in T cell
recognition of peptide ligands (43). In this situation, MHC
was suggested to provide compensatory avidity in recognition of peptide antigen by TdT/
CTLs. Further, the data
in this paper support an inverse correlation between the
number of MHC-peptide ligands necessary for triggering and TCR avidity for the ligand (for data, see Fig. 5; for
model, see Fig. 7 B). Lower avidity for an antigen would
then be compensated by an increase in antigen density.
This feature becomes most important for CTLs with a relatively high MHC avidity, since self-MHC can be regarded
as a high-density and low-affinity ligand.
|
By combining Fig. 7, A and B, a hypothetical three- dimensional diagram can be generated in which the surface depicts how the TCR affinities for peptide and MHC, and the number of ligands necessary for triggering, are interchangeable (Fig. 7 C). The diagram proposes that when the TCR avidities for both peptide and MHC are low, the number of ligands necessary to achieve triggering will be high. We would like to suggest further that the same model would apply for thymic selection, where the threshold coordinates for positive selection would be considerably lower on all axes while coordinates for negative selection would be closer to the threshold for triggering of effector functions.
Aspects of CD8+ T Cell Selection in the Absence ofSelection of H-2Db-restricted T cells specific for the GP33
epitope in 2m
/
mice is thymus dependent, and results
in a peripheral repertoire biased towards recognition of the
restriction element expressed with and without self-peptides. However, peptide-independent triggering occurs
only when MHC is expressed at levels considerably higher than those encountered by the T cells in vivo. This indicates repertoire calibration during thymic selection to fit
the self-MHC ligand density in the periphery. Our results
fit very well with the notion that T cells being selected in
the thymus view self-MHC as a low-affinity ligand. Self-ligands capable of triggering induce deletion, and in the periphery self-MHC never reaches the ligand density to trigger CTLs in the absence of triggering antigens. It should be
noted that in addition to the increased recognition of
MHC, the
2m
/
CTLs may also display an increased
avidity for self-peptides. There may be a clonal variation in
recognition of MHC and peptide within the
2m
/
CTL
population, where the clone 27/30 represents the most
peptide-independent phenotype.
Considering the low expression of MHC present during
selection in the 2m
/
mice, it is possible that the CD8+
T cells use upregulation of (co)receptor expression to
achieve positive selection. Indeed, we have observed a
marginal increase in CD8 expression in some of the
2m
/
CTLs, which potentially could also contribute to the elevated recognition of self-MHC. However, in functional
experiments based on recognition of GP33 and alanine-substituted variants, we have observed that
2m
/
CTLs
are less susceptible to CD8 blocking with anti-CD8
mAbs compared with B6 CTLs. This result would suggest, rather,
a decreased dependency on CD8 in triggering of
2m
/
CTLs (data not shown).
Accumulating evidence suggests that the CD8+ T cells
present in the 2m
/
mice have retained important characteristics of the
2m+/+ wild-type concerning development
and recognition of MHC. First,
2m
/
mice can generate
in vivo CTL responses against MHC class I-restricted peptide (44; and this study) and viral (45) antigens, and reject skin grafts over a minor histocompatibility barrier (46).
Also,
2m
/
CTLs can recognize peptide antigens on
2m+/+ targets (this study). Second, development of
2m
/
CD8+ T cells is dependent on the thymus (this study), and
results in a CD8+ T cell repertoire biased towards recognition of syngeneic class I (23; and this study). Third, H-2Db
heavy chains expressed in the absence of
2m are recognized by conformation-dependent mAbs (27, 28, 47; and
this study), and expression is TAP dependent (32; and this
study). Note also that all detectable
2m
/
CTL reactivity
against self-MHC could be blocked by an mAb specific for
properly conformed H-2Db molecules, which reduces the
likelihood for a role of aberrantly conformed free Db heavy
chains in this system. Fourth, wild-type CTLs can recognize endogenously processed antigens (48) and alloantigens
(27, 48) on
2m
/
cells. Taken together, these data suggest that T cell selection follows the normal rules in
2m
/
mice, although the selection window is dramatically shifted
towards low ligand density. The confrontation of such T
cells with cells expressing normal MHC levels allows detection of the T cell avidity for self-MHC. However, the
functional avidity must be established already during selection and must exist during priming. Thus, we propose that
T cells in normal mice have a similar avidity for self-MHC
which would be detectable by exposing them to cells with
supraoptimal MHC levels.
We have demonstrated that avidity for self-MHC can trigger MHC-restricted T cells independently of specific peptide if ligand density of self-MHC is sufficiently increased. This interaction compensates for lack of TCR affinity for peptide, and it depends on a TCR-MHC interaction of relatively low affinity that requires high numbers of MHC ligands. The data and the avidity threshold model discussed contribute to our understanding of T cell specificity in the periphery and during thymic selection.
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Footnotes |
---|
Address correspondence to Rickard Glas at his present address, Department of Pathology, Harvard Medical School, 200 Longwood Ave., Boston, MA 02115. Phone: 617-432-4779; Fax: 617-432-4775; E-mail: glas{at}hms.harvard.edu
Received for publication 2 June 1998 and in revised form 4 December 1998.
We thank Hidde Ploegh, Adnane Achour, Benedict Chambers, and Hans-Gustaf Ljunggren for discussions and reading of the manuscript.
This work was supported by the Swedish Cancer Society, Karolinska Institute, and Robert Lundbergs minnesstiftelse. R. Glas is supported by a fellowship from The Swedish Foundation for International Cooperation in Research and Higher Education (STINT), Stockholm, Sweden, and by Alex och Eva Wallströms stiftelse.
Abbreviations used in this paper
2m,
2-microglobulin;
B6, C57Bl/6;
LCMV, lymphocytic choriomeningitis virus;
TAP, transporter associated
with antigen processing.
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