Download Opinion signal delivered by agonist MHC–peptide complexes. 10-time reduction in

192
Opinion
TRENDS in Immunology Vol.22 No.4 April 2001
Environmental control
of immunological
synapse formation
and duration
Michael L. Dustin, Paul M. Allen and Andrey S. Shaw
The coordination of T-cell migration and antigen recognition is crucial for an
effective immune response. We have proposed that this coordination is achieved
by formation of an immunological synapse between the T cell and the antigenpresenting cell (APC). Our view contrasts with the serial encounter model also
proposed in this issue of Trends in Immunology, which is based on transient
T cell–APC interactions when surrounded by collagen. Here, we propose a model
that reconciles immunological synapse formation and serial encounters based
on environmental control of immunological synapse formation.
Michael L. Dustin*
Paul M. Allen
Andrey S. Shaw
Center for Immunology
and the Dept of Pathology,
Washington University
School of Medicine,
660 S. Euclid Ave, St
Louis, MO 63110, USA.
*Present address:
Program in Molecular
Pathogenesis and the
Dept of Pathology,
Skirball Institute of
Molecular Medicine, and
New York University
School of Medicine, 540
First Avenue, New York,
NY 10016, USA.
e-mail: dustin@
saturn.med.nyu.edu
Understanding the requirements for T-cell activation
is fundamental to the design of immune-based
therapies for vaccination or to treat disease. The T cell
and antigen-presenting cell (APC) interact via the
formation of an organized immunological synapse (IS)
that produces a stop signal for T-cell migration
that lasts for hours or even days1–5. However, it is
also becoming clear that T cell–APC interactions
could be profoundly regulated by their tissue
microenvironment and these effects might adjust the
threshold for synapse formation and set the duration
of interaction6,7. The duration of T-cell receptor (TCR)
engagement has profound effects on T-cell activation
and differentiation8. In this issue of Trends in
Immunology, Friedl and Gunzer9 propose a model for
T-cell activation based on serial encounters between
T cells and APCs within collagen gels that challenges
the role of IS formation. We suggest that Friedl and
Gunzer take too broad an interpretation of their data,
and that IS formation is still a crucial event in the
immune response. Friedl and Gunzer9 have identified
another aspect of T-cell behavior that is readily
incorporated into our model and is helpful in
explaining the structural organization of lymphoid
tissues. We propose that environmental factors, such
as collagen extracellular matrix and certain
chemokine gradients, regulate IS formation to restrict
the site of primary immune responses and to control
the duration of T-cell encounters with the APC.
We have proposed that T-cell activation requires
formation of an IS, a supramolecular structure at the
interface between the T cell and a single APC that is
stable for many hours4. The IS provides a molecular
machinery for integration of MHC–peptide quality
and quantity. The first step in IS formation is a stop
signal delivered by agonist MHC–peptide complexes.
The stop signal is defined as a >10-time reduction in
T-cell migration velocity as a result of contact with
antigen that is sustained over a period of an hour. We
have found that this step in IS formation is subject to
regulation by environmental signals including
dominant chemokines (e.g. SLC, ELC and IP-10)6.
Dominant chemokine gradients allow T cells to
migrate through the antigen stop signal, but do not
induce a state of non-responsiveness because the
T cells can go on to form IS and proliferate when
removed from the chemokine gradient. Dominant
chemokines allow for environmental control of
immunological synapse formation.
More recently, Friedl and colleagues found that
T cells that interact with dendritic cells in collagen
gels in vitro do not stop in response to agonist
MHC–peptide complexes. Instead, rapidly migrating
T cells transiently synapse with many antigenpositive dendritic cells7. This is similar to classical
models for cytotoxic T-cell killing of multiple
targets10. Although the molecular basis of this effect
of collagen is not known, the implication is that
signals from collagen or associated proteins and
dendritic cells synergize to prevent the antigen stop
signal, while retaining the ability to fully activate
T cells. Collagen is essential for this effect because
T cells and dendritic cells form agonist MHC–peptidedependent clusters in suspension cultures (cell–cell
‘...environmental factors regulate IS
formation to restrict the site of
primary immune responses and to
control the duration of T-cell
encounters with the APC.’
contacts with minimal cell–matrix contacts). These
clusters are consistent with the function of a stop
signal, although the internal dynamics of the clusters
were not directly evaluated. Friedl and Gunzer9
propose a serial encounter model to explain T-cell
activation without IS formation and to explain how
this mode of interaction might produce outcomes of
proliferation and anergy depending upon the
frequency of interactions. Our view is that collagen
provides a new axis for regulation of IS formation and
this might help to explain the unique organization of
collagen in secondary lymphoid tissues.
Regarding the molecular basis of signal integration
by migrating lymphocytes versus stopped
lymphocytes, the major difference is polarization and
not adhesive strength. Migrating cells engage the
same number of adhesion molecules as stationary
cells, but the pattern of engagement is different. In a
migrating cell, the adhesion molecules are
asymmetric, with molecules accumulating at the rear
as they are moved backwards by cytoskeletal tension
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Opinion
Fig. 1. Testing TCR
interaction by migration
versus immunological
synapse formation.
(a) shows a migrating
cell – the increasing red
color represents the
accumulation of adhesion
molecules. The arrow
represents cytoskeletal
tension. Green represents
the accumulation of
MHC–peptide before they
disengage from TCR.
(b) shows the
immunological synapse
formation. Symbols are
the same as in (a), but the
adhesion molecules
accumulate towards the
center of the synapse as a
result of the central
orientation of cytoskeletal
tension. MHC–peptide
accumulated by the
parallel engaged TCR in
the back.
TRENDS in Immunology Vol.22 No.4 April 2001
(a)
(b)
Migration:
MHC–peptide disengage
Immunological synapse:
MHC–peptide accumulate
TRENDS in Immunology
(Fig. 1). By contrast, stopped cells sweep molecules
toward the center of the synapse and thus accumulate
molecules in a radial bulls eye pattern. The migrating
T cell will detach from the APC within minutes and
does not form a stable complex of TCR and
MHC–peptide, whereas the stopped cells accumulates
MHC–peptide complexes in the center of the IS to
generate a zone of parallel TCR engagement.
Therefore, any unique signals produced by the
parallel engagement of TCR in the center of the IS
(cSMAC) must be replaced by other signals provided
by the collagen gel or dendritic cells under conditions
of the Friedl study. There are several mechanisms by
(a)
(b)
Naive T cell no collagen –
immunological synapse
(c)
Naive T cell in collagen gel –
transient interaction
(d)
Naive T cell collagen sequestered –
immunological synapse
Effector T cell in collagen gel –
α1β1 α2β1 anchors slow exit
TRENDS in Immunology
Fig. 2. Collagen regulates immunological synapse formation. (a) In the absence of collagen fibrils and
dominant chemokine gradients the T cell stops migrating on contact with antigen bearing antigenpresenting cell (APC) leading to an immunological synapse. (b) In the presence of collagen fibrils the
T cell does not stop on contact with antigen bearing APC leading to serial encounters. (c) The
hypothesis is that in the lymph node the collagen fibrils are enclosed in fibroblastic reticular cell
sheath so the T cell forms an immunological synapse with antigen bearing APC. (d) Expression of
high affinity collagen receptors VLA-1 and VLA-2 could help retain antigen specific effector cells in
collagen rich inflammatory sites.
http://immunology.trends.com
193
which extracellular matrix signaling could provide a
co-stimulatory signal that contributes to full T-cell
activation11,12. It is also possible that dendritic cells
might present MHC–peptide complexes and
co-stimulatory molecules in a pre-associated form that
could accelerate signaling compared with other APC
types13. It is notable that MHC–peptide complexes
that interact with TCR in a sub-optimal manner
actually signal, but fail to stop the T cell and fail to
form an immunological synapse. Instead, the T cells
exposed to these partial agonist ligands migrate while
signaling in a manner that leads to weak or absent
proliferation4. Thus, tolerance might result when
T cells signal on the move, but in the absence of
collagen. In the presence of collagen, a stray naive
T cell might simply remain ignorant of its antigen
encounters and exit harmlessly through lymphatics
before achieving a superthreshold stimulus.
The finding that collagen is a potent negative
regulator of IS formation provides a rationale for the
mysterious organization of collagen fibers in the
lymph nodes, the most well-characterized site for
efficient primary immune responses14. The T-cell
area of resting lymph nodes is a cellular environment
with little or no collagen in contact with T cells. The
abundant collagen fibers are sheathed in stromal
cells to form the reticular fiber network.
Lymphocytes pass through this collagen-rich space
during extravasation, but then enter the collagenfree parenchyma, which is most similar to a dense
suspension of T cells laced with reticular cells and
dendritic cells (Fig. 2). Upon entry into lymph nodes,
the key dominant chemokine receptor SLC is
probably desensitized (during initial arrest on the
high endothelial venule surface15) and the T cells will
thus be in an optimal environment for IS formation.
The suspension-like nature of the T-cell areas of
lymph nodes is consistent with the demonstration of
clear antigen-dependent T cell–dendritic cell clusters
in lymph nodes in vivo16, similar to those observed by
Friedl in suspension cultures without collagen7.
These data are not consistent with a general serial
encounter model and are more consistent with the
formation of IS in lymph nodes in the 24–48 h time
frame following injection of antigen-positive
dendritic cells in vivo. Direct observations of
T cell–APC dynamics in lymph nodes in vivo will
be required to pass the ‘in vivo veritas’ test of IS
formation versus the serial encounter model. This
test might not be too far in the future as technologies
of vital microscopy and deep penetrating two-photon
excitation microscopy17 are poised to reveal the true
nature of the potent kiss between the T cell and the
dendritic cell.
By contrast, T cells in the dermis and other solid
tissues would always be exposed to abundant
collagen and thus would interact in a serial encounter
mode that might be ideal for T-cell effector functions,
as highlighted by Friedl7. Thus, the environment of
the T cell could define the duration of interactions. In
194
Opinion
TRENDS in Immunology Vol.22 No.4 April 2001
this model IS formation is favored in secondary
lymphoid tissues, the primary site of naive T-cell
activation. Interestingly, the B-cell follicles, in which
T–B cell collaboration is focused, are even free of
reticular fibers and would also be ideal locations to
form the well-characterized T cell–B cell IS (Refs
1,18). However, effector cells in the periphery will
interact with many macrophages to promote
References
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phagocytosis or target cells for cytotoxicity. This
serial encounter process serves to amplify the
response, particularly for less-abundant CD4+ T cells.
The IS model can incorporate the new data on
dominant chemokines and collagen regulation of IS
formation to provide insights into the multiple
signals required to orchestrate T-cell-dependent
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assembly with
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Andres G. Grandea III and Luc Van Kaer
The stable assembly of MHC class I molecules with peptides in the
endoplasmic reticulum (ER) involves several accessory molecules. One of these
accessory molecules is tapasin, a transmembrane protein that tethers empty
class I molecules to the peptide transporter associated with antigen processing
(TAP). Here, evidence is presented that tapasin retains class I molecules in the
ER until they acquire high-affinity peptides.
MHC class I molecules present peptides, usually eight
or nine amino acids in length, to CD8-expressing
cytotoxic T lymphocytes (CTLs). Most peptides that
bind class I molecules are derived from newly
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synthesized proteins in the cytosol1. Such proteins are
subject to limited degradation by large proteolytic
complexes termed proteasomes (Fig. 1). A subset of
the resulting peptides are translocated across the
endoplasmic reticulum (ER) membrane by the peptide
transporter associated with antigen processing (TAP),
and are loaded onto peptide-receptive MHC class I
molecules in the ER. Peptide-loaded class I molecules
are then transported to the cell surface for recognition
by class I-restricted CTLs. This antigen presentation
pathway is critically important for immune
surveillance against viruses and tumors.
The loading of MHC class I molecules with
peptides in the ER is facilitated by several accessory
molecules, including proteins with general
housekeeping functions and the dedicated chaperone
tapasin1–4. Although tapasin appears to play an
important role in the quality control of MHC class I
assembly, its precise function remains controversial.
Here, it is proposed that tapasin retains class I
molecules in the ER until optimal ligand selection is
completed.
Role of general housekeeping proteins in class I
assembly
Shortly after their synthesis, MHC class I heavy
chains associate with the ER-resident chaperone
calnexin1 (Fig. 1). In human cells, binding of class I
heavy chains to β2-microglobulin (β2-m) results in the
exchange of calnexin for the chaperone calreticulin;
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