Joined at the Bench
By Jason Socrates
Bardi
Last summer, when Professor Hugh Rosen came to The Scripps
Research Institute (TSRI) from Merck & Co., where he was the
executive director of Immunology and Rheumatology, he found
himself looking over rows of empty shelves and cabinets in
his laboratory.
Coming from industry, he says, he did not have chemicals
and reagents to bring with him. He had no cell lines and no
major laboratory equipment. Most importantly, he had no students
or postdocs with whom to work. His lab was something of a
blank slate—he didn't even have his own office table
or chair (fortunately, he was able to borrow these).
"It was an interesting moment," he reflects.
At that time, he was writing grants full-time to get his
research up and running. To clear his head, he would take
frequent breaks. It was during one of these breaks that he
struck up a conversation with his colleague Associate Professor
Michael McHeyzer-Williams, who was also a then-new recruit
to TSRI's Department of Immunology. Rosen and McHeyzer-Williams
share one contiguous laboratory space in a TSRI building that
houses the Institute for Childhood and Neglected Diseases.
McHeyzer-Williams was also setting up his laboratory, having
arrived near the end of 2001 with a core group of two postdocs
and one technician. They were just finishing the calibration
of his new, dedicated flow cytometer machine, getting it running
at peak performance.
After several conversations, McHeyzer-Williams recalls,
he suggested to Rosen that they do an experiment together.
They designed a "pilot" experiment looking at the late stages
of T cell selection in the thymus that was intended to combine
Rosen's expertise in chemical biology with McHeyzer-Williams's
expertise in flow cytometry.
But after they completed their pilot experiment and analyzed
the results, they saw that they had something completely new.
Out of the Thymus
The thymus is the organ that supplies the body with helper
and killer T immune cells, crucial mediators of the adaptive
immune response. T cells circulate through the blood to secondary
lymphoid organs—lymph nodes, Peyer's patch, the spleen.
If they encounter antigen, usually viruses or bacteria, in
the secondary lymphoid organ, they begin to proliferate and
to undergo clonal expansion, producing various effector cells.
These effector cells remove what they recognize as non-self.
A single T cell, one of two key players in the adaptive
immune response, can proliferate into a million cells in a
matter of days once it has been activated, helping to clear
the body of invading bacterial or viral threats.
Crucial to this effort is the maintenance of the T cells
in the periphery by the continual development and release
of mature of T cells from the thymus.
The thymus is constantly generating new T cells, but each
day only a small percentage survive. Well over 95 percent
of the T cells that are made in the thymus are destroyed there.
The small number of survivor T cells are selected to replenish
the peripheral T cell pool.
The maturation of T cells in the thymus occurs through a
highly sophisticated mechanism whereby the thymus sorts out
those cells that are potentially useful in the periphery from
those that are not. This is achieved by screening the cells
for their binding affinity for major histocompatibility complex
(MHC) molecules, the receptors that are present on antigen-presenting
cells recognized by the T cells' own receptors.
For mature T cells in the bloodstream, antigen-presenting
cells display pieces of pathogenic invaders (antigens) in
their MHC receptors, and this leads to the activation of T
cells that have the right receptor—one that binds antigen-loaded
MHC tightly. In the thymus, MHC molecules also play a crucial
role, and they must be recognized by the T cells. This positive
selection takes place in one part of the thymus, called the
cortex,, and the organ selects out all the T cells that are
unable to recognize MHC molecules.
Of the remaining cells, those that have been positively
selected for their ability to recognize self antigen, a further
selection takes place. Those that have been positively selected
in the cortex pass into the "medulla" portion of the thymus
where a negative selection awaits them.
While much is known about the positive thymic selection,
less is known of the mechanisms regulating the negative selection
that occurs in the medulla as the T cells undergo their final
maturation before they are released into the blood.
In the medulla, T cells that are highly reactive are selected
to die. This negative selection is an important complement
to the positive selection. If these cells were allowed to
get out of the thymus, some would attack our own tissue. We
would suffer from autoimmunity and might even reject our own
organs.
Now the paper by Rosen, McHeyzer-Williams, and their TSRI
colleagues Associate Professor Charles Surh and Research Associate
Christopher Alfonso may shed some light on this mechanism.
In their paper, they describe a new mechanism in which the
thymus may sense peripheral inflammation and modulate the
T cell response.
The S1P Receptors
The TSRI team focused on a family of receptors for a fatty
lipid molecule called sphingosine 1-phosphate (S1P) that is
produced by platelets—those flat, circulating, molecule-filled
protoplasmic disks in the blood that are necessary for clotting—and
by a variety of tissue cells.
Sphingosine 1-phosphate acts on a family of receptors called
the S1P receptors or the "edg receptors," originally defined
as endothelial differentiation genes. S1P is produced by endothelial
cells and other cells at sites in the body where there is
inflammation—and where there are inflammatory cytokines
like tumor necrosis factor-alpha, for instance. S1P lipids
activate the S1P receptors and regulate a range of physiological
functions that include cardiovascular function and blood pressure.
Rosen and his colleagues previously showed that S1P receptors
can also control the recirculation of lymphocytes, a mechanism
which was never understood before. Furthermore, they found
that either S1P lipids or synthetic chemical agonists of the
S1P receptors are able to alter the trafficking of lymphocytes
in a reversible way. These synthetic chemicals, they found,
are very potent and bind to S1P receptors in the low nanomolar
and sub-nanomolar range.
They have now shown that when S1P agonists interact with
thymus, they cause the T cells to lose a receptor on their
surface called CD69, promoting their maturation in the medulla.
In addition, a biological switch is activated that shuts off
emigration of mature T cells from thymus, preventing T cells
from reaching the periphery.
These small molecules also interrupt antigen responses by
misdirecting peripheral T cells to the wrong lymph nodes by
interrupting recirculation. Regulating the release of new,
mature T cells from the thymus while inhibiting the expansion
of the T cells that are already in the periphery synergistically
combine to create a strong immunosuppressive effect.
This immunosuppressive effect could potentially be used
to prevent the rejection of organ transplants or the effects
of autoimmune disease.
"What we have," says Rosen, "is a biological toggle switch—an
on-off switch—that is regulated as you activate these
receptors. You essentially shut off a switch and, as you activate
these receptors, the lymphocytes disappear in a reversible
way from peripheral blood and you can protect an animal or
a person from transplant rejection and from autoimmune-mediated
tissue damage."
Now, Rosen adds, his laboratory is focused on the study
of this mechanism under normal and autoimmune conditions.
"We're very excited about [the results]," says McHeyzer-Williams.
"Both of us have started a whole new direction of research
that we wouldn't have had. We have uncovered a new phase in
late thymiuc development using these powerful chemical tools
focused on complex biological processes. Our laboratory has
a wealth of experience in the analysis of T cells, their antigen
receptors and cell fate in vivo. We can now directly interrogate
the process of negative selection with increased cellular
and molecular resolution to provide insight into this fundamental
process."
To read the article, "Rapid induction of medullary thymocyte
phenotypic maturation and egress inhibition by nanomolar sphingosine
1-phosphate receptor agonist" by Hugh Rosen, Christopher Alfonso,
Charles D. Surh, and Michael G. McHeyzer-Williams, which was
published online by the Proceedings of the National Academy
of Sciences on September 3, 2003, please see: http://www.pnas.org/cgi/content/abstract/1832725100v1.
|
