Samia J. Khoury, MD, Co-director, Partners MS Center; Professor of Neurology, Brigham and Women’s Hospital and Harvard Medical School, Boston, MA

Introduction
In multiple sclerosis (MS), both genes and the environment contribute to disease susceptibility. Environmental contributions likely come from exposure to viral, bacterial, or other antigens during childhood, and may “set the stage” for MS. If the antigen in the environment is structurally similar to a component of a person’s own central nervous system (CNS) myelin, then the immune responses set in motion would produce an anti-myelin attack on self-myelin ( autoimmune attack). A common feature of the lesions in all forms of MS is the presence of immune cells of several types: T lymphocytes, B lymphocytes, and activated cells belonging to the “myeloid” line. Myeloid cell are white blood cells that are NOT lymphocytes; macrophages and microglia are part of this broad category of myeloid cells. They are all found within areas of active demyelination (see Figure 2). Activated T cells will readily go into the CNS in search of their target antigen, but if these T cells are specific for antigens that are not found in the CNS, they will not remain within the CNS (Rock & Peterson, 2006). Thus, the presence of lymphocytes within active MS lesions, indicates that these T cells are being actively and specifically retained in the CNS.

What Are Microglia and Why Are They Important in MS?
The central nervous system (CNS) contains neurons, the cells that process and transmit information by electrochemical signals, and a variety of other cells called glia that provide support and nutrition, maintain equilibrium of the milieu, form myelin, and participate in signal transmission in the CNS.

Microglia are a type of glial cell that acts to defend the CNS from infection. Microglia constitute approximately 15% of the total glial cell population within the brain. Microglia are constantly moving and analyzing the CNS for damaged neurons, plaques, and infectious agents. They act as “housekeepers” cleaning up random cellular debris. Microglia accomplish this task by engulfing materials through a process called phagocytosis. Engulfed materials generally consist of cellular debris, lipids, and dead cells in the non-inflamed state, and invading viruses, bacteria, or other foreign materials in the inflamed state. In addition to being able to destroy infectious organisms through phagocytosis, microglia can also release a variety of substances that can be toxic to cells (cytotoxic), such as nitric oxide and hydrogen peroxide. Both of these chemicals can directly damage cells and lead to neuronal cell death. Microglia also produce inflammatory mediators called cytokines (or chemical messengers) like IL-1 and TNF that promote demyelination of neuronal axons. These cytotoxic secretions are aimed at destroying infected neurons, viruses, and bacteria, but can also cause large amounts of collateral neural damage. As a result, chronic inflammatory response can result in large scale neural damage as the microglia ravage the brain in an attempt to destroy the invading infection. Acute neuroinflammation is generally caused by some neuronal injuryafter which microglia migrate to the injured site engulfing dead cells and debris. The term neuroinflammation generally refers to more chronic, sustained injury when the responses of microglial cells contribute to and expand the neurodestructive effects, worsening the disease process, as compared to “acute” neuroinflammation.

When microglia are activated they take on an irregular shape with “octopus arm” like extensions, and they increase their gene expression. Increased gene expression leads to the production of numerous potentially neurotoxic substances, which are important in the normal functions of microglia––that is cleaning up neuronal debris as we described before. The production of these neurotoxic substances is usually reduced once their task is complete. In chronic neuroinflammation, however, microglia remain activated for an extended period during which the production of neurotoxic substances is sustained longer than usual. This longer-than-usual production plays an important role in the disease process of MS––especially in progressive disease. One of the interesting observations is that in chronic MS there is diffuse activation of microglia throughout the brain, and these serve as a source of inflammation from inside the CNS, in contrast to earlier phases of MS when inflammation is believed to be initiated from outside the CNS. The diffuse microglia activation is also seen in the chronic phase of the animal model of MS (Figure 1).

Microglia are thought to play key regulatory and effector roles in MS (Muzio, Martino, & Furlan, 2007; Rock & Peterson, 2006). To what extent, and by what mechanisms, microglia contribute to the onset and progression of MS is a subject of substantial debate. There are three opposing views have been presented in the literature: (1) activated microglia are harmful for functions of the CNS, in that they produce neurotoxic molecules and attract immune cell; (2) mcroglial responses may participate in CNS inflammation, but it is the inappropriate presence of blood derived macrophages/dendritic and T cells that infiltrate the CNS that are the prime villains driving MS pathology; and (3) activated microglia limit CNS inflammation by producing factors that protect neurons (neurotrophic) and may even drive antigen-specific neuroprotective T-cell responses.

There are difficulties in proving or disproving these various theories. One of the key problems is in distinguishing microglia (residents of the CNS) from macrophages that have migrated into the CNS through the blood stream. The reason for this difficulty is that macrophages from the blood stream can take on a shape similar to microglia, Likewise, activated microglia can take different shapes and express surface markers (protein fingerprints) that make them look like macrophages (Santambrogio et al., 2001). Studies in animal models of MS are trying to unravel the role of microglia in disease. The most commonly used animal model is experimental autoimmune encephalomyelitits (EAE), a disease that mimics certain aspects of human MS and is induced in rodents (mice, rats) by immunization. From animal studies, we have learned that microglia can activate T cells, but multiple factors present in the healthy CNS are capable of directly regulating microglial function. In MS, viruses, toxins, and other changes in the CNS microenvironment may alter the balance of how microglia (and macrophages) interact with immune cells, neurons, and other glial cells. Only slight changes in the balance between these components may be sufficient to generate a downward spiral into severe neurodegenerative disease, characterized by local inflammation (too much or too little), causing demyelination and axonal transection, which may in turn may induce further inflammation and chronic demyelination (Carson, 2002).

What Activates Microglia?
Microglia can be activated by several products of inflammation or cytokines, for example lipopolysaccharide (LPS-a part of the bacterial cell wall) has been widely used as a activator of microglia and induces the production of cytokines (TNF-?, IL-6) and nitric oxide (NO). NO is cytotoxic to microbes, parasites, and tumors, and in that way is helpful during infection, but can also be harmful to tissue. Other activators of microglia are virus and bacterial proteins, beta-amyloid (a protein found in plaques of Alzheimer’s disease), interferon (IFN)-gamma and other proinflammatory cytokines, and thrombin (Table 1). There are several products that inhibit microglia activation for example CD200R, glucocorticoids, vitamins A and E, and cytokines such as transforming growth factor-beta (TGF-beta). CD200R is a protein expressed on the surface of microglia, macrophages, and other cells that interacts with CD200 (expressed by neurons) and gives the myeloid cells an inhibitory signal and decreases their activation. Mice that carry a genetic mutation that increases the expression of CD200, have milder MS-like disease when immunized compared to normal mice (Chitnis et al., 2007). Glucocorticois (also known as steroids) are produced by the body in response to stress and can inhibit microglia activation. Steroids are frequently used to treat exacerbations of MS and act by killing activated T cells, decreasing swelling caused by inflammation, and inhibiting activation of macrophages and microglia. Minocycline, a tetracycline antibiotic, is a lipid soluble antibiotic frequently used in treatment of acne. Minocycline is known to inhibit macrophage and microglia activation and improves symptoms of EAE (Brundula, Rewcastle, Metz, Bernard, & Yong, 2002). Pilot trials of minocycline in MS showed promising results (Metz et al., 2004; Zhang et al., 2008). There is a current multicenter trial in Europe, studying minocycline in relapsing-remitting MS. Peroxisome proliferator-activated receptors (PPAR) are nuclear hormone receptors characterized by their ability to regulate adipocyte differentiation and gene transcription. Activators of these receptors (agonists) include troglitazone, pioglitazone (ACTOS), and rosiglitazone, and were initially designed as antidiabetic drugs because of their insulin-sensitizing effects, and several are in clinical use. PPAR agonists, however, also exert anti-inflammatory effects and were shown to inhibit EAE (Feinstein et al., 2002). A pilot trial of pioglitazone in MS was performed and shown it to be safe (Pershadsingh et al., 2004).

A list of other microglia inhibitors is shown in Table 1. Several drugs have been developed to suppress microglia activation in chronic pathologic conditions, but these drugs are not targeted specifically against the microglia and hence may result in various forms of undesirable side effects. But there is great promise that some of the drugs that inhibit microglia may develop into MS therapeutic agents.

There is evidence that microglia also interact with the stem cell pools in the brain. In the adult brain there are pools of stem cells (neural stem cells) that can be called upon to repair following injury. Animal studies of EAE have shown that neural stem cells get activated and start the process of repair. There are several reasons, however, why the repair is incomplete: 1) The presence of inflammation in the brain may prevent the brain stem cells from differentiating, 2) inflammation may actually damage the brain stem cell pools, and 3) it is also possible that repeated bouts of inflammation and the persistence of disease can exhaust the capacity of the stem cells. Chronically activated microglia may inhibit the repair potential of neural stem cells. Studies from our laboratory in the animal model of MS show that chronically activated microglia are present during the late phases of the disease and are associated with impaired neural function (Rasmussen et al., 2007). More recent data from our group demonstrate the presence of activated microglia in the stem cell niche and evidence that drugs that inhibit microglia activation (such as minocycline) lead to improved stem cell repair function.

Conclusion
In summary, microglia are active participants throughout the MS disease process. The challenge is whether the dynamic properties of microglia can be therapeutically manipulated to minimize tissue injury and to maximize repair. To date, approved therapies for MS are directed at the peripheral compartment of the immune system. Developing new drugs that can target microglia and CNS macrophages, may lead to better treatments for the chronic stages of MS.

References
Brundula, V., Rewcastle, N. B., Metz, L. M., Bernard, C. C., & Yong, V. W. (2002). Targeting leukocyte MMPs and transmigration: Minocycline as a potential therapy for multiple sclerosis. Brain, 125(Pt 6), 1297-1308.

Carson, M. J. (2002). Microglia as liaisons between the immune and central nervous systems: Functional implications for multiple sclerosis. Glia, 40(2), 218-231.

Chitnis, T., Imitola, J., Wang, Y., Elyaman, W., Chawla, P., Sharuk, M., et al. (2007). Elevated neuronal expression of CD200 protects Wlds mice from inflammation-mediated neurodegeneration. American Journal of Pathology, 170(5), 1695-1712.

Feinstein, D. L., Galea, E., Gavrilyuk, V., Brosnan, C. F., Whitacre, C. C., Dumitrescu-
Ozimek, L., et al. (2002). Peroxisome proliferator-activated receptor-gamma agonists prevent experimental autoimmune encephalomyelitis. Annals of Neurology, 51(6), 694-702.

Metz, L. M., Zhang, Y., Yeung, M., Patry, D. G., Bell, R. B., Stoian, C. A., et al. (2004). Minocycline reduces gadolinium-enhancing magnetic resonance imaging lesions in multiple sclerosis. Annals of Neurology, 55(5), 756.

Muzio, L., Martino, G., & Furlan, R. (2007). Multifaceted aspects of inflammation in multiple sclerosis: The role of microglia. Journal of Neuroimmunology, 191(1-2), 39-44.

Pershadsingh H. A., Heneka, M. T., Saini, R., Amin, N. M., Broeske, D. J., & Feinstein, D. L. (2004) Effect of pioglitazone treatment in a patient with secondary multiple sclerosis. Journal of Neuroinflammation, Apr 20; 1(1):3.

Rasmussen, S., Wang, Y., Kivisakk, P., Bronson, R. T., Meyer, M., Imitola, J., et al. (2007). Persistent activation of microglia is associated with neuronal dysfunction of callosal projecting pathways and multiple sclerosis-like lesions in relapsing-remitting experimental autoimmune encephalomyelitis. Brain, 130(Pt 11), 2816-2829.

Rock, R. B., & Peterson, P. K. (2006). Microglia as a pharmacological target in infectious and inflammatory diseases of the brain. Journal of Neuroimmune Pharmacology, 1(2), 117-126.

Santambrogio, L., Belyanskaya, S. L., Fischer, F. R., Cipriani, B., Brosnan, C. F., Ricciardi-Castagnoli, P., et al. (2001). Developmental plasticity of CNS microglia. Proceedings of National Academy of Sciences U S A, 98(11), 6295-6300.

Zhang, Y., Metz, L. M., Yong, V. W., Bell, R. B., Yeung, M., Patry, D. G., et al. (2008). Pilot study of minocycline in relapsing-remitting multiple sclerosis. Canadian Journal of Neurological Sciences, 35(2), 185-191.

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