New Insight Into Machinery of Immune Cells'
DURHAM, NC -- Researchers have identified new molecular
components of the machinery that regulates formation of the
tentacle-like filaments by which immune system T cells grasp
other cells. This embrace by such filaments is critical for the
T cell to establish communication with cells called "antigen
presenting cells" (APCs). Such communication enables the T cell
to program itself to target invading microbes for destruction.
Antigens are proteins in invading microbes that the immune
system detects to trigger a counterattack.
The researchers said their findings of the machinery of
formation of such "actin filaments" could offer targets for
drugs to induce the immune system to work more effectively to
fight infection; or to damp its stimulation in autoimmune
Led by Duke University Medical Center pharmacologist Ann
Marie Pendergast, the researchers published their findings in
the Jan. 10, 2006, issue of Current Biology. The research was
sponsored by the National Institutes of Health. Another paper
in the same issue -- by Daniel Billadeau and colleagues of the
Mayo Clinic College of Medicine -- confirmed the Duke
researchers' findings and also implicated the same machinery in
regulating calcium mobilization in T cells.
In their studies, Pendergast and her colleagues sought to
discover the signaling proteins in T cells responsible for
formation, or polymerization, of the protein actin into
filaments following activation of the T cells. Such actin
filament formation is crucial for the T cell to attach to APCs,
called B cells, which collect and display foreign proteins from
The ensuing "conversation" between T and B cells enables the
T cell to effectively identify and target such invaders for
destruction. The site of contact between the T and B cells has
been dubbed the "immunological synapse," because it is a
communication link between the cells just as synapses between
brain cells are the sites where one brain cell signals
"It was known activation of the enzyme Rac was a key
regulator of actin polymerization, but the downstream molecules
in this pathway have remained elusive," said Pendergast. "The
prevailing dogma was that this function is mediated by a
protein called WASp. However, mice lacking WASp can still form
immunological synapses, so we proposed that there was another
unidentified pathway that regulated the process."
In their experiments, the researchers concentrated on a
protein called "Abl interactor" (Abi), which had been
identified earlier in the Pendergast laboratory. The earlier
research had shown Abi to be an important adaptor protein in
the signaling pathway involving a key cell regulatory enzyme
called Abl. And Abl had already been shown to regulate
remodeling of the cell's structure -- called the cytoskeleton
-- that includes actin polymerization. Such remodeling is a key
process in cellular growth and adaptation.
The researchers sought to understand whether the interaction
between Abi and a complex of proteins that include the Wave
family of proteins, might regulate actin polymerization. The
Abi/Wave complex had already been shown to be involved in actin
polymerization in other cells, said Pendergast.
Using tracer molecules to tag the proteins in T cells, the
researchers found that both Abi and Wave homed in on sites of
actin filament formation. What's more, when the scientists
activated T cells with a "superantigen," they detected Abi at
the contacts between T cells and B cells. Their experiments
also revealed that the binding of Abi to Wave was required for
Abi to reach the contact point.
The researchers' experiments also revealed that the Abi/Wave
complex was naturally present in T cells. And when they knocked
down the levels of either protein, the T cells lost the ability
to polymerize actin at the immunological synapse.
Their experiments also revealed that, when T cells are
stimulated, Abi recruits the Wave complex to the site of
contact between T and B cells. They also found that mice
lacking Abi proteins have a significant impairment in the
immune system's production of a key immune cell trigger, called
IL-2, in response to T cell activation. Additionally, mice
deficient in Abi proteins have decreased T cell proliferation
in response to activating stimuli, found the researchers.
"These findings add important new players in the regulatory
pathway downstream of Rac," said Pendergast. "And since immune
system activation depends critically on the formation of the
synapse, these new players give us more targets for drugs to
treat both immune deficiency and the hyperstimulation of the
immune system in autoimmune disease."
Besides Pendergast, other co-authors were Scott Witherow,
Jing Gu, Elizabeth Chislock and Colleen Ring -- in the
Pendergast laboratory in the Department of Pharmacology and
Cancer Biology. Co-author Patricia Zipfel is in the Duke
Medical Center Department of Surgery. Stephen Bunnell of the
Tufts University School of Medicine also was a co-author.