Case 2 - HPA-cytokine

Question 1

anti-inflammatory GC effects

Answer 1

• Inhibit synthesis of several cytokines including interleukin (IL)-1, IL-2, IL-3, IL-4, IL-5, IL-6, IL-8, IL-12, IL-18, granulocyte macrophage-colony stimulating factor (GM-CSF), tumor necrosis factor (TNF)-α, and interferon (IFN)-γ.
• These effects are transmitted intracellularly through the binding of GCs to GRs, resulting in suppression of pro-inflammatory gene expression or induction of suppressive factors:
  o The suppression of pro-inflammatory gene expression by the intranuclear GC-GR complex can be achieved via several ways (see figure 1):
   GR binds directly to transcription factors such as NF-κB and AP-1 to modulate their activity; by blocking the DNA binding of NF-κB and AP-1, GR represses the excessive production of inflammatory cytokines (direct protein-protein interaction)
   Direct binding to GC response elements (+GREs) or negative GREs (nGREs)
   Composite binding to DNA (GREs) and protein substrates
• specific anti-inflammatory effects of GCs (see figure 2):
  o Lymphocyte apoptosis
  o Decreased neutrophil infiltration into inflamed tissue
  o Decreased vasodilation
  o Decreased vascular permeability
  o Suppression of NK and Th1 cells (IL-12 mediated reduction of IFN-γ)
  o Inhibit Th cell differentiation (GC-GR complex binds transcription factors)

Question 2

pro-inflammatory effects of GCs

Answer 2

• Regulation of the T Cell Response and B Cell Activation:
  o GCs induce T cell accumulation in lymphoid organs at night, enhancing T cell responses against bacterial infections.
  o Diurnal changes of the HPA-axis affect immune responses against foreign antigens, impacting effector CD8 T cells, Tfh cells, germinal center B cells, and class-switched B cells
  o GCs promote immunoglobulin production via T–B cell interactions, supporting germinal center formation and immunoglobulin class switching.
• Control of T Helper Cells:
  o GCs suppress Th1 and NK cell responses but promote Th2 and Th17 cell differentiation, enhancing IL-4 production.

Question 3

acute vs chronic stress effect on immune function

Answer 3

acute:
- predominately anti-inflammatory (NFkB and AP-1 transrepression)
- pro-inflammatory (T cell SLO migration, Th2/17 activation)
–> net anti-inflammatory effect

chronic:
- increased pro-inflamm
- GC loose anti-inflamm effect –> GR resistance
–> net shift to pro-inflammatory

Question 4

GR resistance - general

Answer 4

Under healthy conditions, GCs induce a feedback inhibition on multiple levels of the HPA axis.
- Binding of GCs to the GRs modulates gene expression and signaling pathways involved e.g. in the release of CRH or ACTH, resulting in a decreased release of GCs.
- In contrast, GC-mediated negative feedback in MDD of the HPA axis is impaired, due to so-called “GC Resistance”, caused by a decreased function of the GR.
–> GC resistance results in immune cells being resistant to anti-inflammatory actions mediated by cortisol, such as repression of inflammatory cytokines like IL-1β, IFN-γ, TNFα, COX-2 and inducible NO-synthase (iNOS).
–> Therefore, this decreased sensitivity towards GCs ultimately results in increased inflammation in stressed patients. involved in MDD pathology

Question 4

GR resistance - mechs

Answer 4

Causes for the decreased function of GRs may include reduced GR sensitivity for GCs, reduced GR expression and lack of co-repressor activity.
Additional mechanisms that may lead to GC resistance:
* Dysregulated expression or activity of 11β-HSD2 → conversion of cortisol to cortisone (see figure 1)
* Enhanced expression of glucocorticoid-efflux transporters (P-glycoprotein (P-gp))
Cytokines can contribute to GC resistance:
* Cytokines can inhibit several of the GR functions, e.g. by changes in GR translocation and induction of GR isoforms which have reduced capacity to bind ligands. Moreover, cytokine-activated MAPK signaling might lead to GR phosphorylation, modulating its turnover and transcriptional activity. Moreover, NRLP3 inflammasome upregulation and the resulting caspase 1 mediated cleavage of the GR further contributes to GC resistance.
* Via induction of kynurenine synthesis, cytokines may also contribute to GC resistance –> inflammation and direct interaction

Question 5

cytokines and HPA modulation

Answer 5

Cytokines can directly stimulate HPA axis activity. Under healthy conditions, cytokines act on all three levels of the HPA axis and thereby increase the release of GCs, resulting in the suppression of the production of further pro-inflammatory cytokines in a negative feedback loop.

Question 6

cytokines in brain

Answer 6

Cytokines may act on the HPA axis via several different mechanisms. They can
(a) passively cross the BBB at “leaky” regions and directly activate neurons e.g. projecting to the hypothalamus,
(b) bind to endothelial cells or glial cells inducing the synthesis of central cytokines or second messengers, resulting in the activation of neurons of the HPA axis,
(c) cross the BBB by active transport
(d) stimulate the vagus nerve which projects to the nucleus tractus solitarius (NTS) in the brainstem, which stimulates the release of norepinephrine in the paraventricular nucleus (PVN).
(e) infiltrating immune cells

Question 7

cytokine effects in brain/HPA axis

Answer 7

PVN –> increased CRH –> increased AVP –> increased ACTH
(also binds to hippo –> inhibits the PVN)

anterior pituitary –> ACTH increased

adrenal cortex –> GC release

–> chronic cytokine release –> hyperactivity HPA –> GC resistance –> immune activation