Dr. Ken West
When undergraduates are taught about antigen presenting cells
they are typically told about macrophages and B lymphocytes.
While these antigen presenting cells (APC) are involved in
the processing and presentation of antigen to T lymphocytes
the most important APC is the dendritic cell(DC).
Dendritic cell function is critically dependent on their
location and state of maturation. Immature DC reside in all
peripheral tissues (areas of high antigen encounter) and are
equipped to capture and process antigen. The Langerhans cell
of the skin is one of the most studied of these immature DC.
In response to activation signals, such as LPS and TNF-alpha,
these immature DC undergo maturation. During maturation the
DC down-regulate their ability to process antigen and up-regulate
MHC class II as well as costimulatory and adhesion molecules
developing into potent T cell stimulators. At the same time
these DC migrate into the lymph node. In the secondary lymphoid
organs, DC express high levels of chemokines which preferentially
attract naive (CD45RA+) T cells. In addition, DC
maturation is associated with morphological changes resulting
in the expression of numerous delicate dendrites.
Mature DC are 100 fold more potent than other APC in stimulating
resting T cells and are probably the only APC capable of activating
a primary immune response. They possess several features which
contribute to their superiority in stimulating resting T cells
and activating primary immune responses. DC express high levels
of costimulatory molecules such as B7-2 and the expression
of MHC-peptide complexes is 10-100 fold higher on DC than on
other APC such as B cells and monocytes. In addition, DC express
high levels of IL-12 upon maturation which further augments
innate and acquired immunity.
DC are unique among APC in that they can cluster naive T
cells and can interact with many T cells at one time. The majority
of T cell activation occurs within these DC-T cell clusters.
Clustering facilitates binding of the T cell receptor to MHC
Class II and the interaction of various costimulatory receptor
ligand pairs resulting in T cell activation. DC unlike other
APC are also able to cluster naïve T cells in both an
antigen dependant and independent fashion. The antigen independent
clustering is presumably important because it allows T cells
to "sample" the MHC-peptide complexes on the surface of the
DC in order to find specific MHC-peptide complexes that it
The unique function of DC in the immune response has placed
them at the forefront of immunotherapy for many clinical diseases.
DC have been demonstrated to be important for activating immune
responses against viruses and organ transplants. DC are currently
being evaluated in clinical trials in activating immune responses
against various different types of cancer. DC can also be modulated
to induce tolerance (or unresponsiveness) to our own tissues
and therefore are likely to be critical for our understanding
and treatment of autoimmune diseases.
DC and the immunological synapse with T cells
Monther Al-Alwan , Julie Bunker
Collaboration on signaling with David Byers of the Atlantic Research Centre The
junction between an APC and a T cell has been nicknamed "the immunological
synapse". This specialized contact between the T cell and the APC results
from reorganization of the T cell actin cytoskeleton and is characterized
by the accumulation of F-actin and other cytoskeletal proteins in the T cell
at the contact point with the APC. These active changes in the T cell cytoskeleton
result in the dynamic clustering of T cell surface receptors and signaling
molecules into a supramolecular activation cluster (SMAC) at the interface
with the APC and provides an optimal environment for signaling molecules
downstream of the TCR.
It has been widely believed that the APC cytoskeleton is passive
in the formation of the immunological synapse; however, previous studies
in this area have used activated T cells with B cells as APC. In contrast,
my lab has recently demonstrated that the dendritic cell (DC) actin
cytoskeleton actively participates in the development of the immunological
synapse with resting CD4+T cells.
We have demonstrated, using confocal microscopy, that during interaction
with a resting antigen specific T cell the DC actin
cytoskeleton focally polarizes towards the T cell. This polarization
consists of accumulation of filamentous actin (F-actin) and fascin,
an actin bundling protein, at the interface with the T cell. We have
shown using actin cytoskeletal inhibitors that DC cytoskeletal rearrangement
was critical for both the clustering and the activation of resting
CD4+ T cells. This novel observation demonstrates that the establishment
of the immunological synapse between APC and resting T cells requires
participation of the cytoskeleton in both cell types.
While this establishes the involvement of the DC in formation of
the immunological synapse many questions remain unanswered. What are
the signaling pathways that result in DC cytoskeletal rearrangement?
What proteins are physically involved in the cytoskeletal rearrangement?
Does this cytoskeletal rearrangment result in the reorganization and
activation of adhesion and costimulatory molecules? Studies to answer
these questions are being actively pursued in the lab.
The actin bundling protein, fascin, and its role
in dendritic cell function
Monther Al-Alwan, Pat Colp
The name dendritic cell is derived from their morphology. These cells possess
many dendrites which are projections from the surface of the cell. Dendrite
expression becomes more pronounced with maturation of the dendritic cells.
We have recently investigated the expression of fascin, a DC actin bundling
protein, during the maturation of DC. Interestingly, fascin expresion is
confined to dendritic cells, neurons and tumors and of course the first two
both have dendrites. We have demonstrated that fascin is expressed only in
mature dendritic cells in vivo and in vitro and its expression correlates
highly with the presence of dendrites and the ability of DC to activate T
cells. We inhibited the expression of fascin using antisense oligonucleotides
and found that it had a marked effect on dendrite formation with 85% of DC
losing their dendrites. Importantly, inhibition of fascin expression significantly
reduced the ability of DC to activate allogeneic T cells. This study was
the first to demonstrate that the dendrites have a role in the ability of
the dendritic cell to activate the T cell.
Work from other groups has suggested that fascin expression in other
cell types might be involved in motility. We have also shown that fascin
is only expressed in vivo in DC that have started to migrate from peripheral
tissues to the lymph node indicating that it might also be involved in
DC migration. We are currently using a variety of techniques to study the
involvement of fascin in migration of DC in vitro and in vivo. We are also
undertaking more general experiments on DC migration.
The role of fascin in tumor metastasis
(in collaboration with Shawn Murray of Pathology and Richard Langley of Dermatology)
In addition to its role in the function and migration of DC there is increasing
evidence to implicate the actin bundling protein, fascin, in the motility
of other cells. The most striking work came from Yamashiro and colleagues
who transfected a non motile epithelial cell line with a cDNA construct for
fascin. The cell line became markedly dendritic and was able to migrate across
a tissue culture plate. In addition, fascin expression has been documented
in a number of tumor cell lines and a recent report suggested that it was
also expressed in aggressive forms of breast cancer. We are currently studying
the role of fascin in the metastasis of cancer of the skin, breast and genitourinary
tract. Fascin is expressed at the leading edge of high grade tumors implicating
it as a prognostic factor. Future studies will evaluate the direct role of
fascin expression in melanoma and other tumors using transplantable tumor
models and antisense fascin inhibition.
Immunosuppressive agents and their effects on
Dendritic cell function
Immunosuppressive drugs are routinely used in the treatment of clinical disorders
such as arthritis and to prevent transplant rejection. Our current understanding
of the mechanisms of these agents is based on studies evaluating their effects
on T lymphocytes. However, DC function and maturation plays a critical role
in the activation of immune responses and the effects of immunosuppressive
drugs on DC have not been evaluated. We have demonstrated that immunosuppressive
drugs currently in clinical use have pronounced effects on DC maturation
and survival influencing their ability to activate immune responses. Ongoing
studies are evaluating the basic mechanisms of action of these drugs on gene
regulation in DC. We are also evaluating novel immunosuppressive drugs which
may affect appropriate DC migration and homing.
Other projects in the lab deal with clinical and pathological correlations
in transplantation and in disease of the skin.