Peripherally Derived Macrophages Reduce Inflammation of CNS Lesion via Alterations of Local Microglial Functions

             

            Central Nervous System (CNS) lesions happen in an untold number of people every year, with numerous negative effects, including loss of senses, paralysis, and even death. They are often caused by physical injury, infections, or long-term diseases such as Multiple Sclerosis.(1) Most CNS lesions occur in the brain, and therefore can affect other parts of the body. Because of the broad range of symptomatology associated with them and the consequences they can cause, it is crucial that we increase our understanding of how the CNS repairs itself after lesion.

            Although it has been known that the immune system plays a role in recovery and repair of CNS lesions, it was previously unknown through exactly which mechanisms the immune system provided assistance. Because the CNS is protected by the Blood Brain Barrier (BBB), which is composed of astrocytic endfeet and acts as a physical barrier to only allow certain cells, substances and molecules into the CNS, scientists did not know exactly how peripheral monocyte-derived macrophages(2) (MDMs) regulated repair.

Previous studies had shown that the cellular response to CNS injuries and lesions was the rapid recruitment of tissue resident microglia (3) (4) and the later involvement of MDMs one to three days following injury (5). Although the arrival of MDMs decreased microglial proinflammatory functions, it was unclear exactly how this crosstalk occurred.

            A recent study from McGill (6) has examined the specific effects of MDMs on microglia in CNS lesions, explaining why their recruitment is a vital part of recovery. In the study, Greenhalgh et al. (2018) caused spinal cord injuries (SCIs) in mice, and used green fluorescent protein (GFP) to track both MDMs and resident microglia. They found that, in the lesions, the microglia often had beneficial effects initially in the first minutes-to-hours following injury by preventing lesion expansion; however, the microglia soon prove detrimental to recovery, and within a few days, caused a prolonged inflammatory response in which the microglia non-specifically phagocytized (ingested) cells within the lesion, promoted apoptosis (regulated death) of neighboring cells via release of inflammatory molecules, such as cytokines IL-1β, TNF, IL-6 and IL-10, and stimulated upregulation of microglial inflammatory phagocytosis and apoptosis-promoting genes.

         

  

However, the authors found that the recruitment of MDMs to sites of injury one-to-three days post-injury reverse many of these deleterious microglial functions. By co-culturing macrophages with adult microglia in vitro (outside the body), they were able to study the interactions between the two cell types, and found that macrophages suppressed microglial phagocytosis and inflammatory gene expression, via Prostaglandin E₂ binding to the EP2 receptor. Previous works found that Prostaglandin E₂ (PGE₂) signaling via the EP2 receptor, reduces IL-1β (a potent inflammatory cytokine) expression in microglia (7), and EP2 receptor activation inhibits phagocytosis (8).

By examining RNA transcript levels, the authors determined that the MDMs increased transcription of microsomal prostaglandin E synthase-1 (mPGES), which is required to make PGE₂, and resident microglia expressed high levels of the EP2 receptor. As well, the enzyme that degrades PGE₂ was found to have decreased expression in lesions, further implying that MDM PGE₂, via its binding to microglial EP2 receptors, inhibits microglial phagocytosis and proinflammatory gene expression.

To test their hypothesis in vitro, the authors treated microglia with an EP2 receptor antagonist, and examined microglial phagocytosis. They also performed an in vivo experiment (within a living animal) by genetically knocking out mPGES in macrophages, and inducing an SCI. Both experiments showed that when the PGE₂-EP2 pathway is disrupted, there is increased microglial phagocytosis; this supports their hypothesis that MDMs suppress microglial activity via this pathway.

        

    After determining the mechanisms for how MDMs decreased phagocytosis, the authors next examined what happened if infiltration of macrophages into the lesion was blocked. Because they knew that mice lacking CCR2 cannot recruit MDMs to CNS lesions (9), they studied CCR2 KO mice and found that these mice had, in addition to almost no MDMs, increased microglial phagocytic activity, and when they examined the genetic profiles of these microglia, they found high levels of MyD88, Cxd2 and NF-κB (which promote inflammation) and Trp53 and Bd2 (which are involved in stimulating apoptosis). As well, the CCR2 KO mice had greater microglial activation in lesions 28 days after SCI, which corresponded to increased myelin loss and worse locomotor recovery.

            These findings are important to both clinicians treating, and researchers studying, CNS lesions. Because many people suffer from these lesions and they have a variety of causes, it is imperative that we improve treatment of them. To do this, we need to better understand how CNS lesions heal. This article does that by examining the effects of the immune system on recovery, and how the MDMs are vital for healing and cessation of microglial inflammation. Although there is not much data available on CNS lesions because many can be asymptomatic and it is hard and expensive to identify a lesion even when a patient does present with symptoms, they undoubtedly take innumerable lives every year, can lead to serious diseases or symptoms, and cost us millions or billions every year. We must learn how to better treat them not only to save money, but, more importantly, prevent the pain, suffering and even death that can accompany them. Although this article was very helpful in explaining how MDMs are vital to healing CNS lesions, the authors did not explain if this would have any potential clinical relevance in the near future. One could hypothesize that if someone came into the Emergency Room with a CNS lesion, doctors could simply inject them with grafted MDMs to promote healing. However, this does not take into account the idea of immune rejection. If the body recognized foreign MDMs that were injected as “non-self”, it would create an immune response that would do much more harm than good. Therefore, I believe that future research should focus on how to increase MDM levels and circulation within the body. This would allow clinicians to promote healing without having to inject a patient with foreign MDMs, which brings with the risk of immune rejection.

Bibliography

1. WebMD Description of Brain/CNS Lesions: https://www.webmd.com/brain/brain-lesions-causes-symptoms-treatments#1

2. Description of MDMs: https://www.stemexpress.com/human-blood-products/peripheral-blood-monocyte derived-macrophages-frozen.html

3. Davalos D, Grutzendler J, Yang G, Kim J, Zuo Y, Jung S, et al. ATP mediates rapid microglial response to local brain injury in vivo. Nat Neurosci. 2005; 6(8):752–8

4. Sevenich L (2018) Brain-Resident Microglia and Blood-Borne Macrophages Orchestrate Central Nervous System Inflammation in Neurodegenerative Disorders and Brain Cancer. Front. Immunol. 9:697.

5. Schwab JM, Zhang Y, Kopp MA, Brommer B, Popovich PG. The paradox of chronic neuroinflammation, systemic immune suppression, autoimmunity after traumatic chronic spinal cord injury. Experimental Neurology. 2014; 258(0):121–9.

6. Greenhalgh AD, Zarruk JG, Healy LM, Baskar Jesudasan SJ, Jhelum P, Salmon CK, et al. (2018) Peripherally derived macrophages modulate microglial function to reduce inflammation after CNS injury. PLoS Biol 16(10): e2005264.

7. Caggiano AO, Kraig RP. Prostaglandin E Receptor Subtypes in Cultured Rat Microglia and Their Role in Reducing Lipopolysaccharide-Induced Interleukin-1 β Production. Journal of Neurochemistry. 1999; 72(2):565–75.

8. Aronoff DM, Canetti C, Peters-Golden M. Prostaglandin E2 Inhibits Alveolar Macrophage Phagocytosis through an E-Prostanoid 2 Receptor-Mediated Increase in Intracellular Cyclic AMP. The Journal of Immunology. 2004; 173(1):559–65.

9. Ma M, Wei T, Boring L, Charo IF, Ransohoff RM, Jakeman LB. Monocyte recruitment and myelin removal are delayed following spinal cord injury in mice with CCR2 chemokine receptor deletion. Journal of Neuroscience Research. 2002; 68(6):691–702.