Brain innate immunity in the regulation of neuroinflammation: therapeutic strategies by modulating CD200-CD200R interaction involve the cannabinoid system

Abstract

The central nervous system (CNS) innate immune response includes an arsenal of molecules and receptors expressed by professional phagocytes, glial cells and neurons that is involved in host defence and clearance of toxic and dangerous cell debris. However, any uncontrolled innate immune responses within the CNS are widely recognized as playing a major role in the development of autoimmune disorders and neurodegeneration, with multiple sclerosis (MS) Alzheimer's disease (AD) being primary examples. Hence, it is important to identify the key regulatory mechanisms involved in the control of CNS innate immunity and which could be harnessed to explore novel therapeutic avenues. Neuroimmune regulatory proteins (NIReg) such as CD95L, CD200, CD47, sialic acid, complement regulatory proteins (CD55, CD46, fH, C3a), HMGB1, may control the adverse immune responses in health and diseases. In the absence of these regulators, when neurons die by apoptosis, become infected or damaged, microglia and infiltrating immune cells are free to cause injury as well as an adverse inflammatory response in acute and chronic settings. We will herein provide new emphasis on the role of the pair CD200-CD200R in MS and its experimental models: experimental autoimmune encephalomyelitis (EAE) and Theiler's virus induced demyelinating disease (TMEV-IDD). The interest of the cannabinoid system as inhibitor of inflammation prompt us to introduce our findings about the role of endocannabinoids (eCBs) in promoting CD200-CD200 receptor (CD200R) interaction and the benefits caused in TMEV-IDD. Finally, we also review the current data on CD200-CD200R interaction in AD, as well as, in the aging brain.

Fig. (1)

Roles of “eat me” and “don’t eat me” signals in the CNS innate immune responses. Scheme showing how microglial cells differenciate “self” from “non-self” cues.

Fig. (2)

Binding of CD200-CD200R in the CNS. Scheme showing the binding of CD200 expressed in neurons, oligodendrocytes astrocytes and brain endothelial cells with its receptor CD200R expressed mainly in microglial cells. Both molecules are type-1 membrane glycoprotein that belong to the IgSF and contain two IgSF domains. CD200R has a short intra-cytoplasmic domain allowing the inhibitory signal to microglial cells. CD200 is expressed in a variety of lymphoid and non-lymphoid cells while CD200R is primarily expressed by myeloid cells (macrophages, monocytes and microglia).

Fig. (3)

Neuron-microglia crosstalk. A) In control conditions the crosstalk between neuron-microglia via the CD200-CD200R interaction is working correctly. The inhibitory signal is contributing to the maintenance of brain homeostasis and immune privilege. B) In inflammatory situations the crosstalk between neuron-microglia through the CD200-CD200R interaction is altered. The reduced inhibitory input from CD200 causes a disturbed equilibrium which results in activation of microglia and neuronal damage in MS, in the aging brain and AD.

Fig. (4)

Microglia-neurons co-cultures: The anti-inflammatory cytokines, IL-4 and IL-10, reduce neuronal death- induced by inflammatory stimuli while the impairment of CD200-CD200R interaction enhanced neuronal death. A) Representative time-lapse experiment of BV2 microglial cells/neurons subjected to LPS (10 µg/ml)/IFNγ (100U) stimulation for 24 h. The addition of IL-4 (10 ng/ml) or IL-10 (10 µg/ml) reduced neuronal death while the impairment of CD200-CD200R interaction using a blocker of CD200, CD200 blocking antibody (5mg/mL), augmented neuronal death as reflected in quantification experiments B, and C respectively. Quantification data show the mean ± SEM from four independent experiments. Statistics: B, *p<0.05 vs control; #p<0.05 vs LPS/IFNγ; ##p<0.01 vs LPS/IFNγ; C, *p<0.05 vs LPS/IFNγ; ##p<0.01 vs LPS/IFNγ.

Fig. (5)

Models showing AEA and 2-AG production and inactivation. AEA is produced by phospholipase D hydrolysing NAPE which is produced by N-acyltransferase. AEA is released toward the extracellular space and activates CB1, CB2 and TRPV1 receptors; AEA is hydrolysed by fatty acid amide hydrolase (FAAH) into arachidonic acid plus ethanolamine. 2-AG is produced by diacylglycerol lipase (DGL) hydrolysing diacylglycerol (DAG) which is produced by phosphatidylinositol-specific phospholipase C. 2-AG is released toward the extracellular space and activates CB1 and CB2 receptors, hydrolysed by monoacylglycerol lipase (MGL) into arachidonic acid plus glycerol.

Fig. (6)

Current signalling relationship between glia and neurons related to cannabinoids. A sustained elevation in intracellular calcium leads to increased eCBs production. At postsynaptic neurons eCBs production increases when metabotropic glutamate receptors and voltage-sensitive calcium channels are activated. In microglia and astrocytes, eCB production increases when purinergic P2X7 receptors are activated or when LPS or proinflammatory cytokines are acting in these glial cells. CB1 receptors are abundantly expressed on pre-synaptic terminals and inhibit neurotransmitter release. Astrocytes also express CB1 receptors at lower levels than neurons and their activation regulate energy metabolism and cell survival. Activated microglial cells express mainly CB2 receptors that inhibit cytokines and inflammatory mediators.

Fig. (7)

The inhibitor of AEA uptake, UCM-707, induces a recovery in the expression of CD200 mRNA in the spinal cord of TMEV-IDD mice. TMEV-infected mice were subjected at 60 days pi (during established disease) to the administration of the selective inhibitor of AEA uptake, UCM-707 (5 mg/kg) for 12 consecutive days. A) Level of expression of CD200 mRNA, evaluated by real time-PCR, in Sham mice and TMEV-infected mice subjected to UCM-707 or vehicle administration. TMEV-infected mice show downregulation of CD200 mRNA expression (p<0.05) compared to Sham animals. UCM-707 treatment significantly increased the expression of CD200 in the spinal cord of TMEV-infected mice (p<0.05). B) Levels of expression of CD200R mRNA, evaluated by real time-PCR, in Sham mice and TMEV-infected mice subjected to UCM-707 or vehicle administration. No significant changes in the level of CD200R expression were found among the different groups. UCM-707 treatment did not modify the level of CD200R expression. C) Expression of IL-1β mRNA, evaluated by real time-PCR, in Sham mice and in TMEV-infected mice subjected to UCM-707 treatment or vehicle administration. TMEV-infected mice show elevated levels of IL-1β (p<0.01) vs Sham mice in the spinal cord. UCM-707 treatment significantly down-regulates IL-1β expression (p<0.05). D) Expression of IL-10 mRNA, evaluated by real time-PCR, in Sham mice and TMEV-infected mice subjected to UCM-707 or vehicle administration. TMEV-infected mice show decreased level of expression of IL-10 in the spinal cord (P<0.05) vs Sham mice. UCM-707 treatment significantly up-regulates IL-10 expression in the spinal cord of TMEV-infected mice (P<0.05). E) Representative images of coronal spinal cord sections derived from Sham or TMEV-infected mice subjected or not to UCM-707 treatment stained with SMI32 for labelling axonal damage. The quantification of the images analysis shows that TMEV-infected mice present axonal degeneration (p<0.01) and UCM-707 treatment reduces axonal damage (p<0.05). Scale bars: 100 µm. F) Activity cage performance: horizontal activity (left panel), vertical activity (right panel). TMEV-infected mice display decreased horizontal (P<0.001) and vertical activity (P<0.05) vs Sham mice. UCM-707 treatment significantly improves motor activity of TMEV-infected mice (p< 0.001 for horizontal activity and p<0.05 for vertical activity). Mice were assessed one day after the end of the 12-day treatment protocol. All values in A, B, C, E and F represent mean ± SEM from 7-8 mice per group.