Every
day, billions of red blood cells pass through the spleen, an organ that
is responsible for filtering out old or damaged blood cells. This task
is made more difficult when the blood cells are misshapen, as they are
in patients with sickle cell disease, which affects millions of people
throughout the world. Sickled blood cells can clog the spleen’s filters,
leading to a potentially life-threatening situation.
Researchers
have now designed a microfluidic device, or “spleen-on-a-chip,” that
can model how this phenomenon, known as acute splenic sequestration,
arises.
The
researchers found that low oxygen levels make it more likely that the
spleen’s filters will become clogged. They also showed that boosting
oxygen levels can unclog the filters, which may help to explain how
blood transfusions help patients suffering from this condition.
“If
we increase the oxygen levels, it will reverse the blockage,” says one
of the senior authors of the study. “This mimics what's done when
there's a splenic sequestration crisis. The first thing doctors do is
transfusion, and in most cases, that gives some relief to the patient.”
The research appears in Proceedings of the National Academy of Sciences this week.
Most
red blood cells have a lifespan of about 120 days, so nearly 1 percent
of the supply has to be removed every day. Within the spleen, blood
flows through tissue known as red pulp, which contains narrow passages
called inter-endothelial slits.
These
slits, formed by the spaces between the endothelial cells lining the
spleen’s blood vessels, have maximum opening dimensions significantly
smaller than those of a red blood cell. Any red blood cells that can’t
pass through these tiny openings, because they’re damaged, stiffened or
misshapen, become trapped and are destroyed by immune cells called
macrophages.
To
model the spleen’s filtration function, the researchers created a
microfluidic device with two modules — the S chip, which mimics the
interendothelial slits, and the M chip, which mimics the macrophages.
The device also includes a gas channel that can be used to control the
oxygen concentration of each chip to simulate conditions in the body.
Using
this device, the researchers sought to better understand acute splenic
sequestration, which occurs in about 5 percent of patients with sickle
cell disease, usually in children. When this happens, the spleen becomes
enlarged, and the patient becomes severely anemic. Doctors usually
treat it with blood transfusions, but if that doesn’t help, the spleen
may need to be surgically removed.
Working
with healthy red blood cells and sickled red cells from sickle cell
disease patients, the researchers allowed the cells to flow through
their device under controlled oxygen levels.
Under
normal oxygen conditions (20 percent oxygen) sickled cells created some
blockage at the slits, but there was still space for other blood cells
to pass through. However, when the oxygen level decreased to 2 percent,
the slits quickly became fully blocked.
When
the researchers increased the oxygen level again, the blockage cleared
up. This may partly explain why blood transfusions, which bring
oxygenated blood cells into the spleen, can help patients who are
experiencing acute splenic sequestration, the author says.
“Our
findings provide a general scientific framework to guide and
rationalize what doctors observe. They also help to elucidate how the
spleen provides a critical function to help filter blood cells,” another
senior author says.
The
researchers found that mildly deoxygenated conditions (5 percent
oxygen) cause some clogging but not enough to produce a splenic
sequestration crisis, which may explain why such crises occur rarely,
the author says.
The
researchers then used the other device module, the M chip, to model
what happens as red blood cells encounter macrophages under different
conditions. They found that when oxygen levels were low, sickled red
blood cells were much more likely to be trapped by macrophages and
ingested by them. In fact, so many blood cells were caught that
macrophages became overwhelmed and couldn’t destroy them fast enough,
contributing to the clogging of the slits.
The
researchers also found that stiff sickled cells retained their sickled
shape even after being ingested, which made it harder for macrophages to
break them down. “About half of these cells stay sickled for a very
long time and slow down the whole digestion process,” the author says.
When
oxygen levels were increased, the blood cells regained their normal
shape, even the cells that had been ingested. This allowed macrophages
to more easily digest them and clear up the clogged filters.
The
researchers are now using the spleen-on-a-chip to study how drugs used
to treat sickle cell disease, such as voxelotor and hydroxyurea, affect
the cell behavior that they observed in this study. They also hope that
the device could one day be used to help doctors analyze individual
patients’ blood cells and monitor how their disease is progressing.
https://www.pnas.org/doi/10.1073/pnas.2217607120
http://sciencemission.com/site/index.php?page=news&type=view&id=publications%2Fmicrofluidic-study-of&filter=22