|
|
||||||||
Department of Biochemistry and Molecular Biology, Mayo Clinic/Foundation, Rochester, MN 55905
| Abstract |
|---|
|
|
|---|
| Introduction |
|---|
|
|
|---|
1 and
2 domains affect
the overall conformation of the Ag-presentation domain of the complex,
it is still unclear whether such changes may have structural
consequences in the other regions of the molecule, or, conversely, the
effect that conformational changes in
3 or
ß2-microglobulin
(ß2m)2 have on
the conformation of the Ag-presentation domain. A recent report
describing the crystal structure of the complex between human CD8 and
HLA-A2 finds that the position of the
3 domain is different from
that in uncomplexed HLA-A2 (5). Taken together with the finding by
Garcia et al. that the presence of CD8 enhances the formation of stable
TCR/MHC complexes (6), these observations suggest that these shifts
might occur to accommodate a higher-affinity interaction. The present study describes features of the mouse class I molecule, Kb, which are important for the formation of the conformationally sensitive epitope of the mAb, AF6-88.5, produced by Loken and Stall (7). We find that even relatively subtle changes in any of the extracellular domains of the class I molecule, excluding the peptide-binding site, have an effect on the epitope recognized by AF6-88.5. This indicates that conformational perturbations that originate in one domain can radiate to affect the three dimensional structure of the other domains of the complex. Conformational plasticity provides a mechanism for transmitting signals along the class I molecule as a consequence of being bound by its ligands and receptors. While conventional views focus on the signals transmitted by the TCR and CD8 as a consequence of class I binding, we entertain the possibility that signals emanate from the class I molecule as a result of conformational changes induced by interactions with the TCR, CD8, or other ligands/receptors.
| Materials and Methods |
|---|
|
|
|---|
The B10.M/Sn, B10.S/SgMcdJ, BALB/cByJ, and B10.Q/SgJ mice, as well as the ß2m knockout and human ß2m (hß2m) transgenic mice used in this study were obtained from the colony of Chella David (Mayo Clinic, Rochester, MN); the C.B10-H2b (BALB.B) were obtained from Peter Wettstein (Mayo Clinic, Rochester, MN); the C57BL/6J and C57BL/6J-H-2bm3/Eg mice were obtained from The Jackson Laboratories (Bar Harbor, ME); the Kbm3/Dd transgenic mice as well as the Kb/Ld transgenic mice were produced in this laboratory in collaboration with the David Laboratory for unrelated studies; the TAP knockout mice (129/OLA TAP1) were obtained from Dr. Anton Berns (The Netherlands Cancer Institute, Amsterdam, Netherlands). The mice were maintained in the animal facility at the Mayo Clinic.
Cell lines
The R8 cells and its variants, which were a gift from Dr. Stanley Nathanson (Albert Einstein College of Medicine, Bronx, NY), have been described previously (8). The cell lines, which harbor the chimeric class I molecules, were generated by Ken Arakawa in this laboratory or described previously (9). Briefly, the mouse fibroblast L cells were transfected by CaPO4 precipitation of the construct, along with a thymidine kinase gene, onto the cells. The cells were cultured in RPMI 1640 complete medium, and transfectants were selected for viability with hypoxanthine/aminopterin/thymidine.
Abs and flow cytometry
The origin of the mAbs AF6-88.5, B8-24-3, K10.56, 28-13-3, and 64-3-7 have been listed previously (7, 10) and were used at binding-site-saturating concentrations. Detection of Abs was done using an FITC-conjugated goat anti-mouse IgG or IgM (Biosource International, Camarillo, CA). Flow cytometry was performed on a FACScan (Becton Dickinson, Mountain View, CA), and mean channel fluorescence values were obtained and converted to a linear scale.
Binding calculations
AF6-88.5 reactivity of variant and chimeric class I molecules
was first determined relative to an unaffected epitope, as indicated in
the tables and figures, and that proportion was compared with the
proportion found on the wild-type Kb molecule expressed in
comparable cell types (i.e., freshly isolated spleen cells, transfected
L cells, or the R8 lymphoma line). The values reported is the ratio of
the percent of AF6-88.5 binding to the variant molecule relative to the
percent of AF6-88.5 binding to the wild-type molecule. Each binding
experiment was repeated at least three times, and all data reported
were reproducible. Representative fluorescence values are shown in
Table II
.
|
| Results |
|---|
|
|
|---|
AF6-88.5 is a mAb with high specificity for the class I allele
Kb (7). The Ab was generated by immunizing a BALB/c mouse
with splenocytes from a C57BL/6 mouse. During tests to confirm the
reactivity of the Ab, we found that a mouse L cell line transfected
with a chimeric class I molecule composed of the
1 and
2 domains
of Kb, and the
3, transmembrane, and cytoplasmic domains
of Ld failed to bind AF6-88.5 in a FACS assay. Reactivity
against Kb was confirmed by showing that L cells
transfected with genomic clones of Kb were positive, while
transfectants of Ld were negative (Fig. 1
and Table I
). Furthermore, splenocytes from a B6
mouse reacted positively with the Ab, while splenocytes from a B10.M
transgenic mouse expressing the chimeric Kb/Ld
molecule did not (data not shown).
|
|
3 domain was unanticipated,
as there is no allelic cross-reactivity reported for this Ab by the
manufacturer or in the literature, and polymorphism in the
3 region
is limited. Additionally, the epitope recognized by this Ab had been
previously reported to reside in the
2 region of Kb
(11), although no mapping data is presented. As there are nine amino
acid residue differences between Kb and Ld in
their
3 domains (Ref. 12, Fig. 3
3 domain of Kb. One
prediction of this hypothesis is that a class I molecule with an
identical
3 sequence would be able to bind AF6-88.5. Kq
is the allele most closely related to Kb in the
3 domain
by sequence, with differences at position 225 (Ile instead of Thr) and
at position 268 (Glu instead of Lys) (12). Splenocytes from a B10.Q
mouse failed to bind AF6-88.5 in a FACS assay. B10.S (H-2s) splenocytes
expressing class I molecules that differ from the
3 sequence of
Kb at additional positions than Kq also did not
bind AF6-88.5 (data not shown). Examination of the three-dimensional
structure of Kb shows that one of the nine amino acids
different between Kb and Ld, position 264, lies
in relatively close proximity to the
2 domain. We performed
site-directed mutagenesis on the Ld
3 exon to change
residue 264 to glutamic acid (the amino acid present at that position
in Kb) to see if it participated in the formation of the
AF6-88.5 epitope. This change in the chimera was found not to be
sufficient to restore the AF6-88.5 reactivity (data not shown). To test
the hypothesis that AF6-88.5 recognizes a structure determined by the
3 domain of Kb more directly, a chimeric L cell
transfectant expressing a class I molecule bearing the
1 and
2
domains of Ld, and the
3, transmembrane, and cytoplasmic
domains of Kb were evaluated and found not to bind the Ab
(Table I
2 epitope with our
findings, a third chimeric class molecules consisting of the
1
domain of Ld and the
2,
3, transmembrane, and
cytoplasmic domains of Kb was investigated. This molecule
was found to be unstable, but could be monitored with Ld
1 and Kb
2-associated Abs (64-3-7 and K10.56,
respectively) and thermal stabilization. Using the Kb
1-specific Ab, B8-24-3, as a negative control, it is shown that this
chimera also failed to be bound by AF6-88.5 (Table I
|
|
1 and
3 regions. We next analyzed a panel of
Kb mutants derived by in vitro selection of R8 cells with
Abs specific for the
1 and
2 regions (8). This series of mutants
contain single amino acid substitutions that modify surface-exposed
residues on the peptide-binding domain. Each of the variants bound
AF6-88.5 in a manner equivalent to the parent with the exception of the
variants G162D, which has a slight disruptive effect, and N174K, which
enhances binding (Table II
2 helix, are spatially removed from the
1 and
3 regions of the Kb molecule, it is evident
that the AF6-88.5-defined epitope is sensitive to structural
perturbations in all three of the extracellular heavy chain domains.
|
To determine the effects of previously described mutations that
influence the structure of the peptide-binding site formed by the
1
and
2 domains of the Kb molecule, the ability of the
AF6-88.5 Ab to bind splenocytes from the Kbm3,
Kbm5, and Kbm8 mutant mice or L cells
expressing transfected Kbm1 and Kbm10 genes
were tested. Each of these mutant Kb molecules displays
altered peptide-binding properties and TCR recognition. For most cases,
that changes can be visualized with
1/
2-specific mAbs (9, 13, and
our unpublished observations). None of the mutations had any
significant influence on the expression of the AF6-88.5-defined epitope
(Table III
). Consistent with our previous finding that the
3 region
influences AF6-88.5-recognition, cells transfected with a chimeric
molecule that is comprised of the
1 and
2 domains of
Kbm3 and the
3, transmembrane, and cytoplasmic tail of
Dd were negative, as were splenocytes from a mouse
expressing the chimeric class I molecule from a transgene (Table III
).
To assess the role of peptides within the binding site, spleen cells
from mice genetically deficient for the TAP complex were analyzed.
Cells from these mice have reduced surface expression of
Kb, as the class I molecules are not assembled with peptide
within the endoplasmic reticulum (14). Reduced levels of Kb
were detected relative to wild-type cells, and the proportion of
AF6-88.5 reactivity relative to K10.56 reactivity was roughly half that
proportion on wild-type cells (Table IV
).
Therefore, we concluded that the absence of high-affinity peptide had
some effect on the AF6-88.5-defined epitope. A study has been
previously reported that found no influence of 40 different OVA-derived
peptide analogues on AF6-88.5 binding (11). This indicates that, while
the presence of high-affinity peptide has an effect on the AF6-88.5
epitope, the nature of peptides that are bound did not.
|
We wished to assess whether ß2m participated in the
formation of the AF6-88.5 epitope. We determined that Kb
complexed to the "b" allele of ß2m had no
effect on the AF6-88.5 epitope by analyzing BALB.B splenocytes (Table IV
). To determine whether the AF6-88.5-defined epitope could be
detected on ß2m deficient cells, spleen cells from a
ß2m-deficient mouse expressing Kb were
examined. These cells were found to be negative, which was not
surprising as these cells do not express any known class I epitopes on
their cell surface in appreciable quantities (15). Next, AF6-88.5
recognition was assessed on cells after restoring class I expression
with a human ß2m transgene. Despite the presence of
Kb on the surface of spleen cells from this mouse as
detected by several mAbs, the AF6-88.5-defined epitope was not detected
(Table IV
). From this, we conclude that mAb AF6-88.5 recognizes an
epitope that is dependent on all four extracellular domains of the
Kb molecule.
| Discussion |
|---|
|
|
|---|
3 domain of Ld. We
found it surprising that the nine amino acid differences in the
3
domain, most of which are far away from the Ag-binding domain in the
three-dimensional structure (Fig. 3
2 region. Furthermore, site-directed mutagenesis showed that the
eight distal-most residues from the peptide-binding domain in
3 that
differ between Kb and Ld were sufficient to
disrupt AF6-88.5 binding. That Kq, as well as all the other
chimeras between Kb and Ld, all failed to bind
the Ab suggested to us that there must be some kind of interaction
among the
1,
2, and
3 domains that determine the structure of
the epitope. The epitope forms only when Kb sequences are
present in each of the domains and is disrupted by amino acid
substitutions in each of the external domains of the protein.
The R8 cell line mutants, as well as the "bm" mutant series, were
originally isolated for changes in mAb and TCR-binding properties (8, 16). By definition, this set of mutants has structural divergence from
Kb mapping in the
1 and
2 regions. With the exception
of G162P and N174K, none had an impact on the AF6-88.5-defined
determinant, indicating that the conformational changes they produce
are not overlapping with the AF6-88.5-epitope. It should be pointed out
that the substitution at position 162 results in a proline at a
position that links two helices, and the substitution at 174 results in
two adjacent lysine residues. The former might not be predicted to have
an effect although it is destabilizing to AF6-88.5, and the latter
might be predicted to have a disruptive effect on the structure
although it results in stabilization of the structurally sensitive
epitope.
Others have found that ß2m has a role in stabilizing the
heavy chain, and there have been other mAb described that are dependent
on ß2m (17). Indeed, some have found that xenogeneic
ß2m has higher affinity for mouse heavy chains and
provided for greater stability of the complex than mouse
ß2m (18). Recent studies indicate that ß2m
plays an essential role in the endoplasmic reticulum as it interacts
with the heavy chain and facilitates its proper folding (19). It
is also worth noting that numerous murine class I molecules, when
complexed with human or bovine ß2m, acquire some affinity
for the monomorphic anti-HLA class I mAb, W6/32. This binding
appears to associate with the murine
2 domain (20, 21, 22), which would
demonstrate an influence of ß2m on murine heavy chain
structure. To assess what role ß2m might have in
formation of the epitope, we used a ß2m-deficient mouse
as well as a mouse ß2m-deficient cell line reconstituted
with hß2m. We found that the AF6-88.5-defined epitope is
dependent on both Kb and mouse ß2m (Table IV
).
Thus, all of the major elements of the class I complex influence the AF6-88.5-defined determinant. A report from Chen et al. cites the use of two antigenic OVA peptides and their reciprocally substituted analogues bound to Kb and found that the AF6-88.5 epitope formed regardless of which peptide was used. We were able to recapitulate this finding using RMA-S cells and the dominant OVA peptide (data not shown).
Examination of the three-dimensional structure of Kb
reveals a point where all three of the domains of the heavy chain as
well as ß2m come together (Fig. 3
). We have not been able
to define the exact determinant for AF6-88.5 and cannot formally
exclude that it is represented by this area. However, we find it
unlikely that it represents the center of the Ab-binding site given the
apparent inaccessibility of this point. Even if this were the area
where this Ab was binding, the positions of the residues that influence
binding still force us to conclude that structural perturbations can
radiate throughout all the domains of the molecule.
The majority of the efforts made to assess changes in class I conformation have focused on using different peptides bound in the binding cleft or mutants of the heavy chain that directly alter the physical properties of the peptide-binding site (23, 24, 25, 26). These studies fit current paradigms well, as all the structural data reported over the past decade has shown that this is the region where immune recognition takes place. It has therefore been of the most potential value to study the peptide binding region of class I in an attempt to understand the molecular details surrounding class I-TCR interactions.
Here we have shown that the mAb, AF6-88.5, recognizes an epitope on the mouse class I molecule, Kb, which is sensitive to modifications of residues in all parts of the heavy chain as well as to the nature of the ß2m, but is insensitive to the nature of bound peptide. This implies the existence of an additional conformationally sensitive site(s) on class I molecules. The question of whether such changes have biological relevance remains to be answered.
There is increasing evidence that molecular interactions involving
regions of the class I molecule outside of the peptide-presenting
domain not only play important functions during T cell/APC
interactions, but also have a role in the function of class I molecules
on intracellular signaling pathways (27, 28, 29). An example of a large
change in the positions of
3 and ß2m relative to
1
2 in the receptor-bound structure was observed and comes from
the recent description of the high-resolution crystal structure of the
entire complex of Kb bound by TCR (30). Another recent
example of structural plasticity comes from the analysis of HLA-A2
complexed with CD8 (5). The observation of structural shifts in the
3 domain that accompany binding of CD8 to the class I molecule
suggests a potential role for these structural changes in immune
recognition. The finding that CD8 enhances the affinity of the class I
TCR interaction (6) is supportive of this view. Another demonstration
of large-scale structural changes that occur as a consequence of
receptor-ligand interaction comes from Reich et al., where they
described the oligomerization of TCR/peptide-MHC complexes while such
oligomerization did not occur with either molecule by themselves or
with a nonantigenic peptide (31).
While the static structures described above point to a functional role
for regions of the class I molecule distinct from the TCR interaction
domain, an example showing significance for such regions on molecules
in a live membrane comes form the observation that Abs specific for the
3 domain have been shown to inhibit signals transduced through the
TCR (32). That the cytoplasmic domain of the class I molecule is not
required for this function implies that intermolecular interactions
occurring between other membrane molecules and the extracellular
domains of class I may be important (33). Whether activation of these
negative signals by Abs is the result of conformational changes or
cross-linking is not known.
There are now a large number of crystal structures of class I molecules
that have been determined, and
3 and ß2m are the areas
which show the greatest deviation from structure to structure. While
this fact makes it difficult to conclude that such structural
alterations are a consequence of receptor or ligand binding, it also
indicates that these areas are likely to be capable of such
fluctuations at the cell surface and that makes them attractive
candidates for mediating other intermolecular interactions. Our finding
that the AF6-88.5 mAb recognizes a conformationally sensitive epitope
outside of the peptide-binding site of class I not only indicates that
class I is capable of considerable structural fluidity, but that AF6
could serve as a valuable tool in exploring situations that may bring
about a conformational shift in Kb.
| Acknowledgments |
|---|
| Footnotes |
|---|
2 Abbreviations used in this paper: ß2m, mouse ß2-microglobulin; hß2m, human ß2-microglobulin. ![]()
Received for publication November 12, 1997. Accepted for publication August 28, 1998.
| References |
|---|
|
|
|---|
ß T cell receptor structure at 2.5 Å and its orientation in the TCR-MHC complex. Science 274:209.
helices and ß sheets of the class I peptide binding site. J. Immunol. 175:821.
1 phosphorylation. J. Immunol. 154:1167.[Abstract]
3 domain of human MHC class I molecules. J. Immunol. 153:1054.[Abstract]
This article has been cited by other articles:
![]() |
C. F. Towne, I. A. York, J. Neijssen, M. L. Karow, A. J. Murphy, D. M. Valenzuela, G. D. Yancopoulos, J. J. Neefjes, and K. L. Rock Puromycin-Sensitive Aminopeptidase Limits MHC Class I Presentation in Dendritic Cells but Does Not Affect CD8 T Cell Responses during Viral Infections J. Immunol., February 1, 2008; 180(3): 1704 - 1712. [Abstract] [Full Text] [PDF] |
||||
![]() |
L. A. Benoit and R. Tan Xenogeneic beta2-Microglobulin Substitution Alters NK Cell Function J. Immunol., August 1, 2007; 179(3): 1466 - 1474. [Abstract] [Full Text] [PDF] |
||||
![]() |
C. F. Towne, I. A. York, L. B. Watkin, J. S. Lazo, and K. L. Rock Analysis of the Role of Bleomycin Hydrolase in Antigen Presentation and the Generation of CD8 T Cell Responses J. Immunol., June 1, 2007; 178(11): 6923 - 6930. [Abstract] [Full Text] [PDF] |
||||
![]() |
C. F. Towne, I. A. York, J. Neijssen, M. L. Karow, A. J. Murphy, D. M. Valenzuela, G. D. Yancopoulos, J. J. Neefjes, and K. L. Rock Leucine Aminopeptidase Is Not Essential for Trimming Peptides in the Cytosol or Generating Epitopes for MHC Class I Antigen Presentation J. Immunol., November 15, 2005; 175(10): 6605 - 6614. [Abstract] [Full Text] [PDF] |
||||
![]() |
L. A. Benoit, J. Shannon, J. W. Chamberlain, and R. G. Miller Influence of Xenogeneic {beta}2-Microglobulin on Functional Recognition of H-2Kb by the NK Cell Inhibitory Receptor Ly49C J. Immunol., September 15, 2005; 175(6): 3542 - 3553. [Abstract] [Full Text] [PDF] |
||||
![]() |
J. Michaelsson, A. Achour, A. Rolle, and K. Karre MHC Class I Recognition by NK Receptors in the Ly49 Family Is Strongly Influenced by the {{beta}}2-Microglobulin Subunit J. Immunol., June 15, 2001; 166(12): 7327 - 7334. [Abstract] [Full Text] [PDF] |
||||
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |