|
Groves is the principal co-author, along with
Michael Dustin, a cellular immunologist at New York University (NYU),
of a paper published in the November 18, 2005 issue of the journal
Science, entitled: "Altered TCR Signaling from Geometrically
Repatterned Immunological Synapses." The lead author is Kaspar Mossman,
a graduate student in Groves' research group, and the second co-author
is Gabriele Campi, a graduate student at NYU with Dustin.
"Scientists, including ourselves, have been posing
elaborate theories about how the strength and duration of signals that
activate T cells are controlled by immunological synapses, without
having been able to do direct experimentation of key factors," said
Groves. "Three years ago, we had this fantasy pipe dream about an
experiment to measure how alterations in the geometric shapes of the
synapses – what we call spatial mutations – would affect T cell
signaling. Then we realized, we have the tools to create nanoscale
patterns, we can do this."
The human immune system is a remarkable
collaboration of different types of cells, working together to protect
our bodies from bacterial, parasitic, fungal or viral infections, and
against the growth of tumors. The process starts when "antigens,"
special markers on the surface of a cell, identify another cell as "non-self,"
and signal the cellular warriors of the immune system to kill the
invader. Leading this attack will be the T cells, lymphocytes from the
thymus. It is well established that the key to T cell activation is
the molecular signal coming off antigen-presenting cell surfaces. This
signal must be enhanced and sustained long enough for the T cells to
commit to mounting an immune response, and then must be cut off in
time to avoid antigen-induced cell suicide or "apoptosis" of the T
cells.
It has also been established that the control
center for T cell signaling is at the junction or point of contact
between T cells and antigens, dubbed the "immunological synapse"
because it resembles the synapse between two communicating nerve cells.
At the immunological synapse, a central cluster of T cell receptors
surrounded by a ring of adhesion molecules form what co-author Dustin
has described as a sort of "bull's-eye." The center of this bull's eye
has been dubbed the "central supramolecular activation cluster," or
c-SMAC, because it was believed to be the source of T cell activation.
"The original idea behind the c-SMAC was that the
larger the T cell receptor cluster, the stronger the T cell activation
signal," said Groves. "This simple vision of strength in numbers had
begun to show cracks, and now we have demonstrated that just the
opposite is true, the coalescence of the c-SMAC cluster extinguishes
the T cell activation signal. The duration of the activation signal is
related to the spatial organization of the T cell receptors rather
than cluster size."
Groves and his colleagues constructed their
synthetic membranes out of lipids which they assembled onto a
substrate of solid silica so that the membranes were able to float
freely a few nanometers above the substrate. This enabled the
researchers to preserve the membranes in their naturally fluid state,
allowing lipids and T cell receptor proteins to diffuse and interact
freely over macroscopic distances.
"The fluidity of our membranes created artificial
antigen-presenting cell surfaces that enabled the formation of
functional immunological synapses with living T cells," said Groves.
Groves and his colleagues were able to spatially
mutate the geometric shapes of the immunological synapses by embedding
the silica substrate with chrome lines that were only 100 nanometers (about
one ten-millionth of an inch) wide. These ultra-narrow chrome lines
served as barriers that restricted the motion of membrane lipids and T
cell receptor proteins. Using electron-beam lithography, the
researchers were able to configure the chrome lines into several
distinct patterns, including simple parallel lines, grids, and a
series of concentric hexagons.
"By changing the shape of the immunological synapse,
we showed that the synapse signal starts out in an amplified mode, and
that the transport of the T cell receptors towards the center weakens
and eventually extinguishes the signal, irrespective of the degree of
clustering," Groves said. "This may help explain why diseases of the
autoimmune system are so difficult to treat. T cell receptor proteins
do not respond like a conventional target, where if you hit the bull's
eye you trigger a signal. The spatial position of the receptor
determines the type of signal it triggers."
If scientists can learn more about the impact that
spatial arrangement has on the immunological synapse and its signaling
strength, the information could benefit the future development of
drugs for treating autoimmune diseases. Such information should also
help scientists better understand the chemical language by which cells
communicate with one another.
Groves said this new technique for spatial mutation
studies should be applicable to many intercellular signaling systems.
Already, he and his colleagues have begun applying it to study
neuronal synapse formation, and cell signaling mechanisms in the
development of cancer. They are also using it to look at the dynamic
range of signaling over which T cell receptors can respond.
"Essentially, these experiments amount to using
inorganic nanotechnology to physically grab a protein in a living cell
and move it to another position in that cell – then watch how the cell
responds," said Groves. "We used it to study the T cell as a paradigm
system, but the theme here is much more general. Whereas the spatial
position of molecules is rarely thought to play an important role in
the outcome of a chemical reaction, with our experimental technique we
are seeing that, in living cells, this is not the case. The spatial
position encodes information which can be directly translated into
altered chemical outcomes."
The earliest indications that spatial positions
could influence T cell signaling and that the synaptic pattern might
actually help to extinguish the signal came from the work of Arup
Chakraborty, a chemical engineering professor who, at the time, held a
joint Berkeley Lab/UC Berkeley appointment and is now with MIT
University. Chakraborty is a pioneer in the use of computer
simulations, called "experiments in silico," for studying important
problems in cellular immunology. In 2003, his computational models
indicated that the immunological synapse is responsible for intense
but self-limited T cell signaling. |
