Acute vs. Chronic Peripheral Immune Stimulation: Differences in Effects on Microglia and Inflammation

The concept of “immune memory” has long been an important, albeit, little understood part of our study of the immune system. For centuries, we have unknowingly utilized immune memory against foreign pathogens (1) that the body has seen before. Whenever a new virus or infection appeared, people who had been exposed to it seemed to suffer fewer symptoms, which we now know is due to this immune memory. We have even “hijacked” this endogenous process and sculpted it to fit our own needs by creating vaccines, which provide immunological memory against particular pathogens to adapt our response, resulting in less damage if we encounter the specific pathogen again. However, we still do not know exactly how immune memory works.

            There are two main elements of the immune system: the innate component, and the adaptive component.(2) The innate component is usually the first line of defense, and along with providing physical barriers against external pathogens (such as skin or mucous), it also attacks any pathogen that enters the body. It relies on large numbers of cells that are not extremely accurate or specific.(3) However, the adaptive immune system is the exact opposite. It is part of a delayed response, and produces T and B cells that specifically target the pathogen that has entered the body. Although it takes longer to initiate, it is much more accurate and helpful in eradication of a particular pathogen.(4)

            Until the last few years, scientists thought that immunological memory existed only in cells of the adaptive immune system.(5) However, a new paper by Wendeln et al. (2018) studied the function of microglia, a component of the innate immune system and located in the Central Nervous System (CNS), and their function after peripheral immune stimulation. Because the Blood Brain Barrier physically separates the brain from the rest of the body and restricts entry of most cells or pathogens into the brain, the brain is classified as “immunoprivileged”, and microglia, in addition to maintaining homeostasis, act as the main immune cell in the brain. However, microglia are not specific to certain pathogens, and are therefore part of the innate immune system. Wendeln et al. found that intraperitoneal application (injection into the abdomen) of a peripheral inflammatory stimulus “trained” microglia to alter their function and either promoted or inhibited inflammation in the brain for up to six months, depending on whether the inflammatory stimulus was acute or chronic. When the scientists tested their hypothesis in a mouse model of Alzheimer’s pathology, they found that the acute peripheral stimulus caused the microglia to decrease their clearance of Amyloid-Beta, which is one of the two main proteins that build up in Alzheimer’s Dementia. However, when the peripheral stimulus was induced chronically (four times), the microglia became “tolerized”, and released lower levels of proinflammatory cytokines (signaling molecules that are released by immune cells and can increase or decrease inflammation), resulting in less inflammation and lower amyloid-β buildup, compared to “trained” microglia.

            The authors injected the mice with four doses of lipopolysaccharide, LPS (an immune system stimulator (6)), over the course of four days. After the first dose, there was an increase in proinflammatory cytokines in the blood, but not the brain. After the second injection, levels of proinflammatory IL-1β, TNF-ɑ, IL-6, IL-12 and IFN-γ were all increased in the brain, showing how the brain-resident microglia had been “trained” to induce a proinflammatory response after peripheral immune stimulation. The third dose of LPS provided similar results to the second, but interestingly, the fourth dose eliminated most, if not all, proinflammatory secretion of TNF-ɑ, IL-1β and IL-6, while increasing anti-inflammatory IL-10. This indicates that the microglia had become “tolerized”.

Note: Anti-Iba1 stains Microglia

           To ensure that it was microglia that were responsible for the inflammation within the brain, the authors used genetic knockout mice whose microglia lacked Tak1, HDAC1 and HDAC2; without these genes, the microglia were unable to modify transcription of genes as part of immunological memory. The microglia in these knockout mice were unable to become “trained” or “tolerized”, and therefore levels of TNF-ɑ, IL-1β and IL-6 did not increase after the second dose of LPS before decreasing after the fourth dose, as they did in the normal wild-type mice. However, what I found most interesting about this aspect of the experiment was that no LPS was found in the brain; although it produced a peripheral immune response, somehow it was the peripheral response and not the LPS itself that signaled to microglia within the brain to release proinflammatory cytokines.

            The authors next examined how “training” and “tolerizing” microglia affected long-term brain immune responses, and therefore disease progression. They used APP23 mice, which are an animal model for Alzheimer’s Disease, and produced large plaques of amyloid-β, which activated the microglia.(7) They injected these mice with four doses of LPS and found that, again, acute peripheral inflammation increased the levels of proinflammatory cytokines while chronic inflammation resulted in fewer levels of these cytokines. However, they also found that the “trained” microglia, in addition to releasing IL-1β, IL-6 and IL-12, also caused greater buildup of amyloid plaque, while the “tolerized” microglia were able to decrease plaque load and total amyloid-β levels. This could have important clinical consequences; it implies that chronic peripheral immune stimulation, which happens in many diseases or autoimmune disorders, can “train” microglia to promote inflammation and amyloid buildup in the brain, accelerating the course of Alzheimer’s Disease.

            To determine how acute and chronic peripheral immune stimulation affected microglia, the authors analyzed the genetics of the microglia and determined which genes had different expression levels during acute vs. chronic exposure. They found that acute exposure (one or two doses of LPS) caused increased expression of hypoxia inducible factor-1ɑ, which resulted in inflammation and the release of proinflammatory cytokines via lactate secretion.(8) On the other hand, chronic exposure increased expression of genes involved in the “Rap1 signaling pathway”, which promoted phagocytosis and does not cause increased secretion of proinflammatory cytokines, therefore not resulting in inflammation.(9) Similar results were found in the APP23 mice, which explained why lower levels of amyloid-β were found in the chronically exposed mice, as their “tolerized” microglia had increased phagocytic capabilities.

            Future experiments should determine how exactly peripheral immune stimulation is able to pass through the Blood Brain Barrier and induce changes in microglia. As the authors found no LPS within the brain, it cannot be that the stimulant itself crosses into the brain to activate microglia. If scientists are able to determine exactly what in the periphery causes the microglia to become “trained” or “tolerized”, it can radically affect our treatment of neurological diseases, like Alzheimer’s Disease. If the signal is able to be blocked, we could decrease microglial activation, which would prevent inflammation within the brain and protect neurons, which would help to slow the course, or even stop the progression of these severe and widespread neurological diseases.

Bibliography

1.     Definition of a Pathogen: https://www.sciencedaily.com/terms/pathogen.htm

2.     PubMed Description of the Innate and Adaptive Immune Systems: https://www.ncbi.nlm.nih.gov/books/NBK279396/

3.     Definition of the Innate Immune System and its Functions from Molecular Biology of the Cell, 4th Edition: https://www.ncbi.nlm.nih.gov/books/NBK26846/

4.     Explanation of and Illustrations for the Adaptive Immune System:

http://library.open.oregonstate.edu/aandp/chapter/21-3-the-adaptive-immune-response-t-lymphocytes-and-their-functional-types/

5.     Netea MG, Latz E, Mills KHG, O'Neill LAJ. Innate immune memory: a paradigm shift in understanding host defense. Nature Immunology. 2015; 16:675–679.

6.     Description of the Structure and Function of LPS: https://www.sigmaaldrich.com/technical-documents/articles/biology/glycobiology/lipopolysaccharides.html

7.     Sturchler-Pierrat C, Staufenbiel M. Pathogenic mechanisms of Alzheimer's disease analyzed in the APP23 transgenic mouse model. Annals of the New York Academy of Sciences. 2000; 920:134– 139.

8.     Haas R, Smith J, Rocher-Ros V, Nadkarni S, Montero-Melendez T, D’Acquisto F, et al. (2015) Lactate Regulates Metabolic and Pro-inflammatory Circuits in Control of T Cell Migration and Effector Functions. PLoS Biol 13(7): e1002202.

9.   Chung, J., Serezani, C. H., Huang, S. K., Stern, J. N., Keskin, D. B., Jagirdar, R., Brock, T. G., Aronoff, D. M., … Peters-Golden, M. (2008). Rap1 activation is required for Fc gamma receptor-dependent phagocytosis. Journal of immunology (Baltimore, Md. : 1950), 181(8), 5501-9.

Link to the Article Here: Innate immune memory in the brain shapes neurological disease hallmarks