 |
|
|
| Research
Group |
|
| Curriculum
Vitae |
|
Werner Held
studied Microbiology at the University of Bern and did his Ph.D. thesis
in immunology in the laboratory of Profs. Ch. Muller and M. Hess in the
Dept. of Pathology of the University of Bern (1987-90). He received his
postdoctoral training in the laboratories of Drs. H. Acha-Orbea and H.R.
MacDonald at the Lausanne Branch of the Ludwig Institute (1990-93) and
in the lab of Prof. D.H. Raulet at the University of California at Berkeley
(1993-96). In 1996 he received a career development award (START fellowship)
from the Swiss National Science Foundation and became Assistant Member
at the Lausanne Branch of the Ludwig Institute. He was promoted to Associate
Member of the Ludwig Institute in 2002 and to Associate Professor (ad
personam) at the University of Lausanne in 2006.
| Research
Interests |
|
Both NK cells
and CD8 T cells play important protective roles against infected and cancerous
cells. We are interested in the identification and understanding of effector
cell intrinsic, target cell-specific and environmental factors that contribute
to or prevent the elimination of diseased cells by Natural Killer cells
as well as CD8 T cells.
| Introduction |
|
NK cells and
CD8 T have the ability to determine whether host cells are healthy or
diseased and to kill the latter. The recognition strategies used by NK
cells and CD8 T cells are fundamentally different. Whereas T cells assemble
almost indefinite numbers of T cell receptors (TCR) for almost indefinite
numbers of ligands, NK cells determine whether host cells express specific
self-ligands at a normal surface density. To do so NK cells use arrays
of two types of receptors, one that stimulates the NK cell, and another
one that inhibits. Upon the interaction with a host cell, it is the balance
between inhibitory and activation signals that determines whether the
NK cell will kill.
NK cells express activation receptors for ligands that are constitutively
expressed on healthy host cells. Cytolysis of such cells is prevented
by inhibitory NK cell receptors that are specific for Major Histocompatibility
Complex class I (MHC-I) and MHC-I-independent ligands. When host cells
lose the expression of inhibitory ligand, which may be a feature of infected
or transformed cells, they become susceptible to NK cell mediated attack.
In this case NK cells fail to receive inhibitory signals, shifting the
balance towards NK cell activation. This is termed "missing-self
recognition". An alternative recognition strategy is that diseased
host cells induce the expression of endogenous self-molecules that serve
as ligands for activating NK cell receptors such as NKG2D. This is termed
"induced-self recognition".
| Current Projects |
|
Lymphocyte
development
Canonical Wnt (wingless/int-1) signaling is required for the maintenance
of self-renewing tissues in the adult such as the skin or the gut. The
role of this pathway for lymphocyte development or haematopoiesis is less
clear. We have shown that the lack of Tcf-1 (Tcf7), one of the nuclear
effectors of the canonical Wnt signaling pathway, resulted in the premature
death of a critical developmental intermediate of intrathymic T cell development,
the CD4+8+ thymocytes. In addition, NK cell development was also impaired.
Using a genetic complementation approach we showed that the N-terminal
domain in Tcf-1 was essential to rescue thymocyte survival in Tcf-1 deficient
mice. This domain binds ? catenin, the central intracellular mediator
of canonical Wnt signaling, suggesting that ?-catenin binding to Tcf-1
was critical for thymocyte development (Ioannidis et al.)
However, T cell development was normal in the absence of ?-catenin and
we showed that the closest ?-catenin homologue, ?-catenin (plakoglobin),
was also not limiting. Further, we found that T cell development, NK cell
development and haematopoiesis in general was also normal in the combined
absence of ?- and ?-catenin (Jeannet et al.). These data indicate that
T and NK cell development depends on Tcf-1 but is independent of ?? and
?-catenin.
We are currently testing possible roles of the canonical Wnt signaling
pathway for the function of NK cells and CD8 T cells.
Fine-tuning
NK cell effector function
NK cell effector functions are under the control of inhibitory and activating
cell surface receptors that interact with ligands expressed on other cells.
Unexpectedly, we found that Ly49A, which is the prototype inhibitory mouse
NK cell receptor specific for MHC class I molecules, not only interacts
with MHC class I molecules expressed on opposing cells (in trans), but
also with those on the same cell (in cis). While trans interaction mediates
classical NK cell inhibition, there was no evidence that cis interaction
resulted in inhibitory signaling. Cis association and trans interaction
occurs via the same binding site and is thus mutually exclusive. Consequently,
cis association sequesters Ly49A and restricts the number of inhibitory
receptors available to bind MHC-I on target cells. This reduces NK cell
inhibition via Ly49A. By lowering the threshold at which NK cell activation
exceeds NK cell inhibition, cis interaction allows an optimal discrimination
of normal from diseased host cells (Doucey et al.,).
NK cell education
NK cells show enhanced functional competence when they express inhibitory
receptors that are specific for inherited MHC-I molecules. This is referred
to as “NK cell education”. Education improves the functionality
of activating NK cell receptors (licensing) via an unknown mechanism.
Current models imply that NK cell education is dependent on an interaction
of inhibitory receptors with MHC-I expressed on other cells. However,
since the inhibitory Ly49A receptor can also bind MHC-I ligand on the
NK cell itself (in cis) we tested whether cis interaction played a role
for NK cell education. To this end we designed a Ly49A variant, which
can only engage MHC-I expressed on other cells but not in cis. This variant
readily inhibited NK cell effector function, however it failed to educate
NK cells. These data show that cis interaction of Ly49A is necessary for
NK cell education (Chalifour et al.). These data dissociate a classical
inhibitory from an educating function of Ly49A.
Structural
basis of MHC-I recognition in cis versus trans
It is not known how cell surface receptors like Ly49 can bind MHC-I expressed
in cis and trans, and why the two types of interactions have distinct
functional outcomes.
Ly49s are homodimeric type II glycoproteins, with each chain composed
of a ligand-binding C-type lectin-like domain, termed the natural killer
receptor domain (NKD), connected by a stalk of approximately 70 residues
to the transmembrane and cytoplasmic domain. Crystal structures of Ly49–MHC-I
complexes have shown that Ly49s engage MHC-I at a broad cavity beneath
the peptide-binding platform formed by the ?1, ?2 and ?3 domains, and
?2-microglobulin (?2m). Because trans and cis interactions utilize the
same binding site the ligand-binding domains have to revert their orientation
relative to the NK cell surface in order to bind MHC-I in trans versus
cis. Based on this together with available Ly49/MHC-I cocrystal structures
we have proposed a model in which cis–trans interactions are mediated
by two distinct Ly49 conformations (Held and Mariuzza, 2008).
This model is currently being tested.
Adapted from Held
and Mariuzza. 2008. Nat. Rev. Immunol 8: 269.
Models for cis and trans interactions of Ly49 receptors with MHC-I. (left)
The ?1, ?2 and ?3 domains of the MHC-I heavy chain are cyan; ?2-microglobulin
(?2m) is blue; Ly49 is red. The Ly49 homodimer on the natural killer (NK)
cell (bottom) binds two MHC-I molecules on the target cell (top). The
stalks (green) are drawn arbitrarily. To bind in trans, the stalks may
adopt a back-folded conformation (right) Cis interaction of Ly49 with
MHC-I. The Ly49 homodimer binds one MHC-I molecule on the NK cell itself.
In this case, the stalks may assume an extended conformation.
NK cell inactivation
NKG2D is a multi-subunit activation receptor expressed by NK cells and
activated CD8 T cells. This receptor allows NK cells to detect and eliminate
stressed, infected and transformed host cells. However, chronic exposure
of NK cells to cell-bound NKG2D ligand impairs NKG2D function both in
vitro and in vivo. We have further found that sustained NKG2D engagement
not only impacted NKG2D function but that heterologous NK cell activation
pathways were also affected. We have termed this phenomenon “cross-tolerance
induction”.
Receptors that activate NK cells via the DAP12 (KARAP) and DAP10 signaling
adaptors, such as mouse NKG2D and Ly49D, preferentially cross-tolerize
NK cell activation pathways that function independent of DAP10/12, such
as antibody-dependent cell mediated cytotoxicity (ADCC) (mediated by CD16)
and missing self-recognition (mediated via ill-defined activation receptors)(see
Figure below). Removal of the activation stimulus was sufficient to restore
the proper function of the various NK cell activation pathways (Coudert
et al.).
These data idnetify a class of NK cell activation receptors that can reversibly
tolerize mature NK cells. The suppression of the NK cells’ cytolytic
function likely reduces the NK cells’ efficacy to control endogenous
and exogenous stress. However this may be needed to limit tissue damage.
| Selected
publications |
|
1.
Chalifour, A., Scarpellino, L., Back, J., Brodin, P., Devèvre,
E., Gros, F., Lévy, F., Leclercq, G., Höglund, P., Beermann,
F., Held, W. 2009. A role for Ly49A-MHC class I cis interaction for NK
cell education. Immunity. 30: 337-347.
2. Coudert, J.D.,
Scarpellino, L., Gros, F., Vivier, E. and Held, W. 2008. Sustained NKG2D
engagement induces cross-tolerance of multiple distinct NK cell activation
pathways. Blood. 111: 3571-3578.
3. Doucey, M.-A., Scarpellino, L., Zimmer, J., Guillaume, P., Luescher,
I.F., Bron, C., Held, W. 2004. Cis-association of Ly49A with MHC class
I restricts natural killer cell inhibition. Nature Immunology 5: 328-336
4. Held, W., Roland,
J. and Raulet, D.H. 1995. Allelic exclusion of Ly49 family genes encoding
class I MHC-specifc receptors on NK cells. Nature 376: 355-358.
5. Held,, W and Mariuzza,
R.A. 2008. Cis interactions of immunoreceptors with MHC and non-MHC ligands.
Nature Reviews Immunology. 8: 269-278
6. Ioannidis V., Beermann,
F., Clevers, H. and Held, W. 2001. The ??catenin / TCF-1 pathway ensures
the survival of CD4+CD8+ thymocytes. Nature Immunology. 2: 691-697.
7. Jeannet, G., Scheller,
M., Scarpellino, L, Duboux, S., Gardiol, N., Back, J, Kuttler, F., Malanchi,
I., Birchmeier, W., Leutz, A., Huelsken, J. and Held, W. 2008 Long-term,
multilineage hematopoiesis occurs in the combined absence of ?-catenin
and ?-catenin. Blood. 111: 142-149
|
|
