Multiple sclerosis (MS) is an autoimmune disease in which leukocytes pass through the blood-brain barrier and attack and destroy the myelin sheaths that encase the axons of nerve cells in the central nervous system (CNS). Normally, leukocyte access to the CNS is restricted by an intricate network of molecular signaling pathways that are not well understood. Researchers from the Department of Internal Medicine, Washington University School of Medicine, St. Louis, Missouri, led by Dr. Robyn Klein, have identified several of the key pathways (Cruz-Orengo et al. 2011). Their findings may lead to novel MS therapies.
MS is the most common cause of acquired disability in adults. Although it typically affects people between the ages of 20 and 50, it can strike at any age. MS affects two to three times as many women as men and is more common among people with northern European ancestry, but there is no evidence that it is inherited. Approximately 400,000 Americans have MS, and it may affect as many as 2.5 million people worldwide (National Multiple Sclerosis Society).
The mouse model of MS
Laboratory mice are not actually susceptible to MS. However, mouse models of MS can be produced by inducing mice with experimental autoimmune encephalomyelitis (EAE) via injections of myelin peptides, such as myelin basic protein (MBP), proteolipid protein (PLP), or myelin oligodendrocyte glycoprotein (MOG), in complete Freund's adjuvant. EAE susceptibility and severity vary among mouse strains, probably because it (MS susceptibility in humans) is due to varying chemokine expression patterns (Friese et al. 2006; Gold et al. 2006). Among the several EAE-susceptible mouse strains is C57BL/6J (B6J, 000664), which develops chronic MOG-induced EAE.
Clues that CXCL12, CXCR7, and cytokines are involved
In previous studies, Dr. Klein and her colleagues observed that the chemokine CXCL12 is conspicuously absent from the blood-brain barrier microvasculature of people with MS. This chemokine was known to restrict the access of CXCR4-expressing leukocytes to the CNS. Studies of EAE in mice suggested that CXCL12's disappearance from the blood-brain microvasculature is mediated by several T cell cytokines, including IL1B, TNF, IFNG, and IL17. One of CXCL12's receptors, CXCR7, had been recently identified and found to be extensively expressed along the microvasculature. Studies indicated that its primary function is to sequester CXCL12, but none of those studies had been conducted in living mammals.
Klein and her colleagues wondered if CXCR7 might be responsible for CXCL12's disappearance at the blood-brain barrier microvasculature, and, if so, whether its activity is cytokine-regulated.
CXCR7 is an EAE-modifying molecule
To determine if CXCR7's expression pattern and role in EAE, Dr. Klein and her colleagues constructed a "reporter" mouse (CXCR7GFP/+) that expresses enhanced green fluorescent protein (EGFP) wherever CXCR7 is expressed. Using this mouse, the researchers were the first to show that CXCR7 is expressed throughout the meninges and CNS microvasculature. Immunizing this mouse with MOG-specific T cells from EAE-induced B6J mice markedly increases CXCR7 expression in the blood-brain barrier microvasculature. Klein and her team treated groups of EAE-induced B6J mice – both at the time of EAE induction and after EAE onset – with CCX771, a highly specific and CNS-penetrating CXCR7 inhibitor. They found that, whether administered at EAE induction or after EAE onset, CCX771 dose-dependently ameliorates EAE. Mice receiving the highest CCX771 doses develop the fewest and least severe meningeal and parenchymal inflammatory lesions, with leukocytes infiltrating shallower and less extensive CNS regions and TNF being expressed primarily in the meninges. Additionally, CCX771 dose-dependently reduces parenchymal expression of the vascular cell adhesion molecule 1 (VCAM1 – a protein that mediates leukocyte-endothelial cell signal transduction and the adhesion of leukocytes to vascular endothelium), restricting leukocytes to the meninges and preventing microvasculature inflammation.
Cytokines modulate lysosomal internalization of CXCL12 by CXCR7
In mammals, CXCR7 was known to mediate CXCL12 transport to lysosomes for degradation, and, as mentioned above, T cell-derived cytokines were known to mediate blood-brain barrier loss of CXCL12 and the subsequent entry of leukocytes into the CNS. Klein and her team wanted to know if the effects of those cytokines and CXCR7 were related. They cultured primary brain microvessel endothelial cells and found that IL1B and IL17 increase CXCR7 expression and the internalization of CXCL12 into lysosomes. They also found that CCX771 dose-dependently inhibits CXCL12 internalization, indicating that CXCL12 internalization is mediated by CXCR7.
In summary, Klein and her colleagues demonstrated that, in an EAE mouse model of MS, IL17 and IL1B up-regulate the expression of CXCR7 at the sites of inflammation. As a result, CXCR7-mediated CXCL12 internalization into lysosomes increases, allowing leukocytes to pass through the microvasculature blood-brain barriers and enter CNS parenchyma. In contrast, CCX771 inhibits CXCR7's activities, reduces parenchymal VCAM1 expression and increases CXCL12 levels in the blood brain barrier microvasculature, reducing leukocyte infiltration into the CNS parenchyma. These findings suggest that blocking CXCR7 may ameliorate MS.
Cruz-Orengo L, Holman DW, Dorsey D, Zhou L, Zhang P, Wright M, McCandless EE, Patel JR, Luker GD, Littman DR, Russell JH, Klein RS. 2011. CXCR7 influences leukocyte entry into the CNS parenchyma by controlling abluminal CXCL12 abundance during autoimmunity. J Exp Med 208:327-39.
Friese MA, Montalban X, Willcox N, Bell JI, Martin R, Fugger L. 2006. The value of animal models for drug development in multiple sclerosis. Brain 129(Pt 8):1940-52.
Gold R, Linington C, Lassmann H. 2006. Understanding pathogenesis and therapy of multiple sclerosis via animal models: 70 years of merits and culprits in experimental autoimmune encephalomyelitis research. Brain 129(Pt 8):1953-71.