2013 AIChE Annual Meeting
(176c) Effects of Coupled Dose and Rhythm Manipulation of Plasma Cortisol Levels On Leukocyte Transcriptional Response to Endotoxin in Humans
Authors
Healthy function of the immune system is intrinsically tied
to the biological rhythms running in harmony. There is a bi-directional
information flow between the immune and neuroendocrine systems such that; while
a wide range of immune parameters go through daily fluctuations under the
control of autonomic and endocrine rhythms; immune effectors, such as
cytokines, can, in turn, influence the phase of these rhythms (1). In the case of acute stress, such as
trauma or infection, inflammatory response is mounted by the coordinated action
of immune and neuroendocrine systems. Hypothalamus-pituitary-adrenal (HPA) axis
is a crucial mediator of this action because of its control on circadian and
ultradian glucocorticoid (GC) secretion patterns (2). These fluctuating patterns are
hypothesized to provide the basis for a ?continuous dynamic equilibration?,
fundamental to establishment of homeostasis and allostasis (3). Cortisol is the major among circulating
human GCs which, through feedforward and feedback regulatory loops, maintains
this dynamic state essential for proper response to stressors. Although GCs are
mostly famous for their immunosuppressive and anti-inflammatory actions that
have been clinically utilized for the treatment of autoimmune and inflammatory
disorders for decades; due to their non-linear effects on transcription and
highly complex interactions with other inflammatory mediators; their downstream
reactions are highly diverse. Effect of GCs on the immune response can be so
disparate, such as to permit, suppress or stimulate the response, depending on their
secretion frequency, concentration and timing with respect to a stressor (4-7).
Since GCs are both major endogenous hormones regulating
stress response as well as clinically used pharmacological agents; building a
solid understanding on their effects on immune cells is of primary importance.
With this vision in mind, this in vivo study in healthy human subjects
seeks to assess the responses of whole blood leukocytes to coupled dose and
rhythm manipulation of cortisol preceding an immune challenge induced by endotoxin
(LPS) administration. Healthy human model of LPS administration has been used
as a reproducible experimental platform providing mechanistic insights into how
cells and organs respond to systemic inflammation. The model transiently alters
many physiologic and metabolic processes in a qualitatively similar manner to
those observed after acute injury and systemic inflammation (8); therefore served as a suitable platform
for investigating the changes in immune response due to hormonal manipulations.
Transcriptional responses of leukocytes as well as the change in total
leukocyte count in blood and plasma cortisol levels were monitored over 24 hours
both before and after LPS administration. Differential gene expression and
clustering analyses were done separately for these two periods to allow
focusing both on the response to cortisol infusion alone and on the effects of
this treatment to subsequent LPS exposure.
Through continuous infusion; plasma cortisol concentration
was increased to and kept constant at a level associated with major physical
stress and almost twice as much as the normal daily peak. In response to this
treatment, total leukocyte count in blood was increased both during cortisol
infusion period and within 2 hours after LPS exposure, after which it started
to decrease. Transcriptional programming of leukocytes was also significantly
altered where a priming response at the end of 24hr infusion period was
evident. Enhanced expression of a number of pattern recognition and cytokine
receptors, receptor regulatory and signal transduction elements; as well as
tuned down protein translation and mitochondrial function portrayed a phenotype
sensitized against potential infectious threats. Promoter analysis within
cortisol responsive genes revealed that only less than half of this
transcriptional response might have been as a direct consequence of
glucocorticoid receptor (GR)-DNA binding, therefore transcriptional regulation
mechanisms other than DNA binding, such as tethering, may play a significant
role. Furthermore, absence of any correlations between the direction of the
response and different modes of transcriptional regulation underscored the
complexity of mechanisms that govern the transcriptional programming through
GR.
Despite these significant changes in transcriptional
programming in leukocytes prior to LPS administration, response to LPS followed
very similar patterns in both cortisol and saline pre-treated groups except one
significantly different cluster including probesets associated with major
players known to regulate the inflammatory response such as cytokines and
chemokines, anti-inflammatory signal transduction elements and receptors
mediating recruitment of leukocytes to sites of inflammation. Observed much
more intense response in the cortisol group in this cluster indicated that although
continuous cortisol infusion did not directly affect the transcription of major
inflammatory players, it indirectly affected their regulation, possibly through
modulating their up-stream signaling cascades.
In sum, altered natural fluctuation coupled with increased
concentration of plasma cortisol levels induced a primed phenotype in the
circulating leukocytes associated with sensitization of surveillance function
and tuning down of energy-intensive processes which may be linked to increasing
their efficiency in responding to subsequent infectious threats.
References