Mitochondrial Function of Immune Cells in Severe Sepsis and Septic Shock - a Prospective Observational Cohort Study
Evidence suggests that sepsis and septic shock severely impair mitochondria and that the
resulting mitochondrial dysfunction is related to the severity and outcome of the resulting
organ dysfunction. In sepsis mitochondrial abnormalities - biochemical and ultrastructural -
have been recognized in multiple organs, including liver, kidney, skeletal and heart muscle
tissue and blood cells. A systematic review on mitochondrial function assessed as oxygen
consumption, state 3 and state 4 respiration, respiratory enzyme activity, or tissue ATP
levels and turnover rates showed decreased function especially in sepsis models lasting more
than 16h . Depleted levels of reduced glutathione, an important intra-mitochondrial
antioxidant, in combination with increased generation of reactive oxygen species (ROS) and
reactive nitrogen species (RNS) inhibit oxidative phosphorylation and ATP generation. This
acquired intrinsic derangement in cellular energy metabolism contributes to reduced
activities of mitochondrial electron transport chain enzyme complexes and impaired ATP
biosynthesis and contributes to the organ dysfunction in sepsis.
Circulating immune cells play an important role in the pathophysiology of sepsis.
Stimulation of the immune system alters the energy requirements of immune cells;
down-regulation of immune-cell activity has been associated with prolonged sepsis. Immune
cell activation mandates an increase in energy requirements, thus a reduced production of
ATP due to impaired mitochondrial function may be a factor in modulating the immune response
in sepsis. Alterations in mitochondrial function and energetic failure have been reported in
peripheral blood mononuclear cells and seem to be associated with the modulation of the
immune response to sepsis. Macrophages incubated with endotoxin/interferon-γ show a decrease
in oxygen consumption and inhibition of mitochondrial I complex. Different mechanisms for
alteration of mitochondrial function in these cells have been proposed, including increases
in NO production and nitration of mitochondrial proteins , elevation of IL-10  or
prostaglandin levels. In human neutrophils intact mitochondrial function plays an important
role in chemotaxis and phagocytosis, impairment of these mechanisms leads to a decreased
defence ability to microbial challenges.
HIF-1α is a transcription factor that acts as a key regulatory factor in the evolution of
oxygen homeostasis. Under normoxic conditions HIF-1α is continuously synthesized and
degraded after hydroxylation by dioxygenases that utilize oxygen, Fe and α-ketoglutarate as
substrates. During hypoxia, the low availability of oxygen limits the reaction; HIF-1α is no
longer degraded and rapidly accumulates and triggers the transcription of genes involved in
oxygen homeostasis such as glycolytic enzymes, glucose transporters, vascular endothelial
growth factor (VEGF) and erythropoietin. Under hypoxic conditions, HIF mediates also a
decrease in mRNA levels of the respiratory chain proteins, preparing the cell to produce ATP
mainly from glycolysis and not from oxidative phosphorylation, thereby optimizing cell
energetics and homeostasis for survival and function during hypoxia.
Recent published reports have linked inflammation and endotoxin stimulation to HIF-1α
activation. HIF-1α has been shown to be up-regulated and stabilized in LPS-treated
macrophages and monocytes under normoxic conditions. HIF-1α levels were shown to be
decreased in macrophages deficient in TLR4 after LPS stimulation, suggesting that LPS
stimulation of HIF-1α is mediated by TLR4.
Data on mitochondrial function of human immune cells in severe sepsis is limited and the
potential correlation of mitochondrial energy requirements and production and the severity
of the patient's condition and outcome are not well established. The immunologic reaction in
the context of severe sepsis and septic shock consists of an interdependent, highly complex
system that involves different types of immune cells and pro- and anti-inflammatory
cytokines involved in a time-dependent process. Simple in-vitro studies assessing
mitochondrial function of a single type of immune cells and single cytokines just a one
point in time during the course of the septic process might not be an appropriate model to
mirror the complex interactions of the immune system. Serial measurements of mitochondrial
function of different key-player cells and specific pro- and anti-inflammatory cytokines and
apoptosis markers parallel with other clinical and laboratory markers of sepsis may offer a
more in-depth evaluation and understanding of this deleterious disease pattern.
The aim of the project is to comprehensively investigate changes in mitochondrial function
and morphology of immune cells in patients with severe sepsis and septic shock. We plan to
assess changes in mitochondrial function of monocytes, B cells and CD4+ T cells in
correlation to levels of various cytokines and to classic clinical and laboratory parameters
of severity of sepsis and outcome.
A total of 30 patients admitted to the intensive care unit (ICU) of a tertiary care hospital
due to severe sepsis or septic shock will be included in the study after obtaining written
informed consent of the patient or the patient's next of kin. Patients with any type of
chronic infectious, inflammatory or autoimmune diseases, after transplantations or receiving
immunosuppressive agents are excluded.
Controls: 30 healthy volunteers Assessment of mitochondrial function of
monocytes/granulocytes will be performed by measurement of mitochondrial complex activity
using a standard titration protocol to measure activation of complex I to IV. To measure
serum levels of IL-1, IL-6, IL-10 and TNF standard commercial kits will be used.
Observational Model: Cohort, Time Perspective: Prospective
Mitochondrial function of immune cells
At the time of ICU admission
Department of Intensive Care Medicine, Bern University Hospital (Inselspital) and University of Bern