Targeting Platelets in Multiple Organ Dysfunction Syndrome in Sepsis
Sepsis is a systemic inflammatory response to an infection. However, symptoms
are produced by the host defense system, as opposed to the invading pathogen.
Mortality rates remain high, despite advances in critical care medicine, and sepsis is the
tenth leading cause of death in the United States. The frequent precursor to mortality
from sepsis is multiple organ dysfunction syndrome (MODS), which is characterized by
apoptosis in organs such as the lung, intestine, vascular endothelium, and lymphoid
tissue. Notably, platelets have been shown to accumulate in the microvasculature of
these tissues. Previous work in the laboratory has shown that sepsis induces an
alteration in the megakaryocyte-platelet transcriptional axis, resulting in strongly
cytotoxic platelets that express the serine protease granzyme B and are capable of
inducing apoptosis in splenocytes ex vivo. Given these findings, we aimed to further
investigate the role of platelet cytotoxicity in sepsis and the mechanism by which they
induce cell death. We performed a mortality study comparing survival from sepsis in
wild type and granzyme B null mice. We found that granzyme B null mice survived
longer following induction of polymicrobial sepsis in comparison to wild type mice.
Additionally, lung and spleen sections from septic granzyme B null mice showed
decreased apoptosis, as measured by TUNEL staining, than sections from wild type mice.
Given the apparent importance of granzyme B in cell death in sepsis, we sought to
determine the mechanism by which platelet granzyme B enters target cells. Granzyme B
is commonly released with perforin, which enables entry into target cells, but it can also
enter cells independently of perforin. By co-incubating septic perforin null or wild type
platelets with healthy splenocytes ex vivo, we determined platelet granzyme B does not
PREVIErequire perforin to induce cell death, as splenocyte apoptosis was similar regardless of
the septic platelet genotype. However, septic platelets do require contact to induce
apoptosis in splenocytes ex vivo. Separation of septic platelets from splenocytes with a
semi-permeable membrane significantly decreased splenocyte apoptosis. This finding
suggested pharmacologically targeting platelet adhesion to other cell types could prevent
platelet-induced apoptosis. To determine if this was the case, we exposed septic platelets
to eptifibatide, a platelet GPIIb/IIIa platelet receptor inhibitor, before co-incubation
with healthy splenocytes ex vivo. Splenocytes incubated with eptifibatide-exposed
platelets had significantly less apoptosis in comparison to those without. To further test
the potential of platelet adhesion inhibition as a therapeutic target in sepsis, we
conducted an in vivo mortality study in which polymicrobial sepsis was induced in wild
type mice that were subsequently treated with three intravenous injections of
eptifibatide or PBS vehicle. Mice treated with eptifibatide had slower progression of
sepsis and survived longer following induction of polymicrobial sepsis. However,
apoptosis, as measured by TUNEL staining, was evident in lung and spleen sections from
septic mice regardless of treatment. In conjunction with our other studies, this
preclinical study emphasizes the importance of platelet cytotoxicity in sepsis. As such,
inhibiting platelet adhesion could be a therapeutic target for the treatment of sepsis andMODS and warrants further study
Targeting Platelets in Multiple Organ Dysfunction Syndrome in SepsisSepsis is a systemic inflammatory response to an infection. However, symptomsare produced by the host defense system, as opposed to the invading pathogen.Mortality rates remain high, despite advances in critical care medicine, and sepsis is thetenth leading cause of death in the United States. The frequent precursor to mortalityfrom sepsis is multiple organ dysfunction syndrome (MODS), which is characterized byapoptosis in organs such as the lung, intestine, vascular endothelium, and lymphoidtissue. Notably, platelets have been shown to accumulate in the microvasculature ofthese tissues. Previous work in the laboratory has shown that sepsis induces analteration in the megakaryocyte-platelet transcriptional axis, resulting in stronglycytotoxic platelets that express the serine protease granzyme B and are capable ofinducing apoptosis in splenocytes ex vivo. Given these findings, we aimed to furtherinvestigate the role of platelet cytotoxicity in sepsis and the mechanism by which theyinduce cell death. We performed a mortality study comparing survival from sepsis inwild type and granzyme B null mice. We found that granzyme B null mice survivedlonger following induction of polymicrobial sepsis in comparison to wild type mice.Additionally, lung and spleen sections from septic granzyme B null mice showeddecreased apoptosis, as measured by TUNEL staining, than sections from wild type mice.Given the apparent importance of granzyme B in cell death in sepsis, we sought todetermine the mechanism by which platelet granzyme B enters target cells. Granzyme Bis commonly released with perforin, which enables entry into target cells, but it can alsoenter cells independently of perforin. By co-incubating septic perforin null or wild typeplatelets with healthy splenocytes ex vivo, we determined platelet granzyme B does notPREVIErequire perforin to induce cell death, as splenocyte apoptosis was similar regardless ofthe septic platelet genotype. However, septic platelets do require contact to induceapoptosis in splenocytes ex vivo. Separation of septic platelets from splenocytes with asemi-permeable membrane significantly decreased splenocyte apoptosis. This findingsuggested pharmacologically targeting platelet adhesion to other cell types could preventplatelet-induced apoptosis. To determine if this was the case, we exposed septic plateletsto eptifibatide, a platelet GPIIb/IIIa platelet receptor inhibitor, before co-incubationwith healthy splenocytes ex vivo. Splenocytes incubated with eptifibatide-exposedplatelets had significantly less apoptosis in comparison to those without. To further testthe potential of platelet adhesion inhibition as a therapeutic target in sepsis, weconducted an in vivo mortality study in which polymicrobial sepsis was induced in wildtype mice that were subsequently treated with three intravenous injections ofeptifibatide or PBS vehicle. Mice treated with eptifibatide had slower progression ofsepsis and survived longer following induction of polymicrobial sepsis. However,apoptosis, as measured by TUNEL staining, was evident in lung and spleen sections fromseptic mice regardless of treatment. In conjunction with our other studies, thispreclinical study emphasizes the importance of platelet cytotoxicity in sepsis. As such,inhibiting platelet adhesion could be a therapeutic target for the treatment of sepsis andMODS and warrants further study
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