Multiple sclerosis is a chronic inflammatory disease that affects the brain and spinal cord, or central nervous system (CNS), leading to sensory and motor impairments. MS is more common in women than in men and is typically diagnosed between the ages of 20 and 40. Approximately 400,000 individuals in the United States currently have MS and more than 2.1 million people worldwide live with the disease. MS is diagnosed as one of two forms, either relapsing-remitting (RRMS) or primary progressive (PPMS). The vast majority of patients are initially diagnosed with RRMS, which is characterized by periods of exacerbation, or flare-ups, followed by periods of remission. Patients with PPMS do not experience remissive phases. Most of the medications approved for MS treatment are aimed toward ameliorating the relapsing-remitting disease course. Currently, MS is commonly treated with an interferon beta (IFN beta) drug, which reduces disease activity, in combination with other medications that target the various symptoms experienced by patients (2).
The clinical presentation of MS varies from person to person; however, pathologically, the disease results from an autoimmune attack on myelin sheath (a protective covering around nerve fibers). Immune cells (lymphocytes) become self-reactive against certain proteins that make up myelin and subsequently destroy it. This process is called demyelination. When the myelin sheath is damaged, signaling between nerve cells becomes slowed or prevented altogether. As a result, simple tasks like walking become quite difficult, and patients may experience vision impairments, episodes of numbness and tingling, loss of balance and coordination, as well as other symptoms. Unfortunately, the mechanism through which an individual develops MS is not entirely understood, and therefore, the repertoire of treatment targets is limited. In order to gain a better understanding of the cause and progression of MS, researchers use various animal models of the disease. The most commonly used MS model is experimental autoimmune encephalomyelitis (EAE), which can be induced in a variety of animal species including certain rodents and non-human primates. In the upcoming November issue of the Journal of Neuropathology and Experimental Neurology, a study by Kap and colleagues investigates whether or not B cell depletion (an experimental treatment for MS) is a valid therapeutic target (1). The authors employed an EAE study in the common marmoset (monkey species). Marmosets were first utilized to examine the clinical and pathological features of MS in 1996, and have been found to exhibit a disease course more closely related to human MS than that observed in rodent models. The marmoset EAE model demonstrates widespread demyelination in both white matter and grey matter of the CNS, strongly resembling the conditions of MS in humans. Furthermore, marmosets have similar immune and nervous system genes to humans, establishing another advantage of using this model (3).
In the present study, EAE was induced through a single inoculation of recombinant human MOG (myelin oligodendrocyte glycoprotein), a myelin protein that, during MS, becomes incorrectly recognized as foreign antigen. Antigen is an entity that binds to specific receptors on lymphocytes causing lymphocyte activation and a subsequent immune response. Accordingly, when MOG is injected into an animal, the animal’s immune cells are essentially tricked into ‘thinking’ that even self-MOG is foreign, and an immune response is initiated against the myelin protein.
The marmosets in the present study were observed each day for 15 weeks after EAE induction and given a daily clinical score based on a scale ranging from O to 5, where 0 indicates no clinical signs and 5 indicates death due to EAE. At 21 days post-induction, half of the animals were treated with a CD20 antibody (HuMab 7D8), which is an antibody directed against a protein found on the surface of B cells. CD20 antibody treatment provokes a robust and long-lasting depletion of B cells from blood and lymph nodes. The rest of the animals received a placebo treatment. In addition to the data collected from behavioral observation and scoring, MRI was used to detect and analyze the presence of lesions (areas of demyelination) in white and gray matter of the brain. The monkeys were paired according to age and body weight for MRI observation. The first MRI for all marmosets was made at day 18 post-induction and at this point, none of the animals showed detectable brain lesions. A second MRI was taken when at least one marmoset in a pair had demonstrated evident neurological symptoms with a clinical score of 2 or greater (2 = ataxia, optic disease). Data collected from the second MRI demonstrated that B cell depletion effectively prevented white matter lesion development (no significant differences of gray matter lesions were observed at this point), as all marmosets treated with the CD20 antibody had no white matter lesion formation. MRI was additionally taken postmortem; these scans revealed that, when compared with the control animals, CD20 antibody-treated marmosets had significantly smaller white matter lesions, if any, characterized by less myelin damage, and furthermore never developed any gray matter lesions over the course of the study. The CNS of the animals was then examined microscopically (histological analysis) by staining tissue sections of brain, spinal cord, and optic nerve to observe the molecular and cellular characteristics of lesions. This analysis uncovered actively demyelinating white matter lesions in all of the control animals, while none of the CD20 antibody-treated marmosets showed signs of active demyelination. Additionally, the extent of demyelination in spinal cords was substantially lower in the CD20 antibody-treated marmosets than in controls. No demyelination occurred in the optic nerves of any CD20 antibody-treated marmosets. There were no B cells found in the spinal cords of the CD20 antibody-treated marmosets and there was a reduced population of T cells in these animals as well. The presence of macrophages (a type of immune cell involved in inflammation) was greatly reduced in the spinal cords of the CD20 antibody-treated marmosets but not in control animals. IgG antibodies (a subset of antibodies involved in targeting myelin proteins during MS) were not detected in the CNS of CD20 antibody-treated animals, suggesting that antibody-mediated immunity is hindered by B cell depletion. Together, the findings demonstrate that the depletion of B cells decreased demyelination and inflammation in CNS white matter. Histological analysis further revealed that gray matter demyelination in the control animals consisted of macrophages, B cells, and T cells, while no gray matter lesions could be detected in any of the CD20 antibody-treated marmosets.
To summarize, the abovementioned findings together demonstrate that depletion of CD20-postive B cells presents an effective therapy for clinically protecting against EAE, reducing the volume of white matter lesions and the extent of inflammation/demyelination, and most notably, completely preventing gray matter pathology. However, the means by which B cells contribute to the disease course of MS is not fully understood. B cells may play a role in demyelination through the production and secretion of antibodies directed against myelin proteins, or alternatively, by the increased release of cytotoxic cytokines (molecules that directly cause the death of cells), which is observed in MS patients (4). B cells might also act as antigen presenting cells (APCs), resulting in T cell activation. Therefore, with B cell depleting treatment, T cells that would normally attack the myelin sheath do not become activated. This is supported by previous observations where B cell depletion substantially reduced T cell proliferation and cytokine production in a marmoset EAE model (5).
The principal treatments for MS have generally targeted T cells, with the angle that autoreactive T cells propel the immune system into self-destruct mode in the CNS, and therefore must be inhibited somehow. These therapies are only partially effective and often have various adverse effects. In 2008, however, a clinical trial was completed and published in the New England Journal of Medicine that opened a new domain for MS treatment – the B cell (6). The study showed promising results for the treatment of RRMS patients with a CD20 antibody drug called rituximab (commonly under the brand name Rituxan). Rituximab is approved by the FDA for treating rheumatoid arthritis and certain lymphomas, yet its use in treating MS is still considered experimental. Stephen L. Hauser, MD, lead author of the 2008 Rituxan clinical trial explained that B cell targeted treatment “represents a paradigm shift that has profound implications for our understanding of MS” (7). The results from the study by Kap and colleagues offer clear support for the therapeutic benefits of CD20 antibody treatment in EAE, and accordingly strengthen the foundation for clinical use of rituximab in treating MS. Additional successful trials on rituximab will be necessary before it receives approval for use in patients with MS. Future research will hopefully continue to explore and expand upon this new and promising frontier for MS treatment.
1. 1. Kap, Y., J. Bauer, N. van Driel, W. Bleeker, P. Parren, E. Kooi, J. Geurts, J. Laman, J. Craigen, E. Blezer, and B. ’t Hart. 2011. B-cell depletion attenuates white and gray matter pathology in marmoset experimental autoimmune encephalomyelitis. J Neropathol Exp Neurol. 70: 992-1005.
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5. 5. Kap, Y., N. van Driel, E. Blezer, P. Parren, W. Bleeker, J. Laman, J. Craigen, and B. ‘t Hart. 2010. Late B cell depletion with a human anti-human CD20 IgG1kappa monoclonal antibody halts the development of experimental autoimmune encephalomyelitis in marmosets. J Immunol. 185: 3990—4003.
6. 6. Hauser, S., E. Waubant, D. Arnold, T. Vollmer, J. Antel, R. Fox, A. Bar-Or, M. Panzara, N. Sarkar, S. Agarwal, A. Langer-Gould, and C. Smith. 2008. B-cell depletion with rituximab in relapsing-remitting multiple sclerosis. N Engl J Med. 358: 676-688.
7. 7. Boyles, S. (2008, February 13). Rituxan shows promise for MS. WebMD Health News. http://www.webmd.com/multiple-sclerosis/news/20080213/rituximab-shows-promise-for-ms.