Igor Kobsar

Immune Cells Contribute to Myelin Degeneration and Axonopathic Changes in Mice Overexpressing Proteolipid Protein in Oligodendrocytes

Overexpression of the major myelin protein of the CNS, proteolipid protein (PLP), leads to late-onset degeneration of myelin and pathological changes in axons. Based on the observation that in white matter tracts of these mutants both CD8+ T-lymphocytes and CD11b+ macrophage-like cells are numerically elevated, we tested the hypothesis that these cells are pathologically involved in the primarily genetically caused neuropathy. Using flow cytometry of mutant brains, CD8+ cells could be identified as activated effector cells, and confocal microscopy revealed a close association of the T-cells with MHC-I+ (major histocompatibility complex class I positive) oligodendrocytes. Crossbreeding the myelin mutants with mice deficient in the recombination activating gene-1 (RAG-1) lacking mature T- and B-lymphocytes led to a reduction of the number of CD11b+ cells and to a substantial alleviation of pathological changes. In accordance with these findings, magnetic resonance imaging revealed less ventricular enlargement in the double mutants, partially because of more preserved corpora callosa. To investigate the role of CD8+ versus CD4+ T-lymphocytes, we reconstituted the myelin-RAG-1 double mutants with bone marrow from either CD8-negative (CD4+) or CD4-negative (CD8+) mice. The severe ventricular enlargement was only found when the double mutants were reconstituted with bone marrow from CD8+ mice, suggesting that the CD8+ lymphocytes play a critical role in the immune-related component of myelin degeneration in the mutants. These findings provide strong evidence that a primary glial damage can cause secondary immune reactions of pathological significance as it has been suggested for some forms of multiple sclerosis and other leukodystrophies.

Role of immune cells in animal models for inherited neuropathies: facts and visions

Mice heterozygously deficient in the peripheral myelin adhesion molecule P0 (P0+/− mice) are models for some forms of Charcot–Marie–Tooth (CMT) neuropathies. In addition to the characteristic hallmarks of demyelination, elevated numbers of CD8‐positive T‐lymphocytes and F4/80‐positive macrophages are striking features in the nerves of these mice. These immune cells increase in number with age and progress of demyelination, suggesting that they might be functionally related to myelin damage. In order to investigate the pathogenetic role of lymphocytes, the myelin mutants were cross‐bred with recombination activating gene 1 (RAG‐1)‐deficient mice, which lack mature T‐ and B‐lymphocytes. The immunodeficient myelin mutants showed a less severe myelin degeneration. The beneficial effect of lymphocyte‐deficiency was reversible, since demyelination worsened in immunodeficient myelin‐mutants when reconstituted with bone marrow from wild‐type mice. Ultrastructural analysis revealed macrophages in close apposition to myelin and demyelinated axons. We therefore cross‐bred the P0+/− mice with spontaneous osteopetrotic (op) mutants deficient in the macrophage colony‐stimulating factor (M‐CSF), hence displaying impaired macrophage activation. In the corresponding double mutants the numbers of macrophages were not elevated in the peripheral nerves, and the demyelinating phenotype was less severe than in the genuine P0+/− mice, demonstrating that macrophages are also functionally involved in the pathogenesis of genetically mediated demyelination. We also examined other models for inherited neuropathies for a possible involvement of immune cells. We chose mice deficient in the gap junction component connexin 32, a model for the X‐linked form of CMT. Similar to P0‐deficient mice, T‐lymphocytes and macrophages were elevated and macrophages showed a close apposition to degenerating myelin. We conclude that the involvement of T‐lymphocytes and macrophages is a common pathogenetic feature in various forms of slowly progressive inherited neuropathies.

Evidence for macrophage‐mediated myelin disruption in an animal model for Charcot‐Marie‐Tooth neuropathy type 1A

Charcot‐Marie‐Tooth neuropathy type 1A (CMT1A) is the most common inherited neuropathy in humans and is mostly caused by a 1.5‐Mb tandem duplication of chromosome 17 comprising the gene for the peripheral myelin protein 22‐kDa (PMP22). Although there are numerous studies on the functional role of PMP22, the mechanisms of myelin degeneration under PMP22‐overexpression conditions have not yet been fully understood. We have shown previously that in mouse mutants hetero‐ or homozygously deficient for two other myelin components, P0 and Cx32, respectively, immune cells contribute to the demyelinating neuropathy. To test this possibility for PMP22 overexpression, we investigated a putative mouse model for CMT1A, i.e., the mouse strain C61 mildly overexpressing human PMP22 in peripheral nerves. Electron microscopic and electrophysiologic investigations revealed that this mouse strain develops pathologic features similar to those found in CMT1A patients. A novel finding, however, was the upregulation of CD8‐ and F4/80‐positive lymphocytes and macrophages, respectively, in peripheral nerves. The observation that macrophages enter endoneurial tubes of the mutants and obviously phagocytose morphologically normal myelin strongly suggests that the myelin degeneration is mediated at least partially by these phagocytic cells. By gene array technology and quantitative RT‐PCR of peripheral nerve homogenates from PMP22 mutants, monocyte chemoattractant protein‐1 (MCP‐1; ccl2) could be identified as a putative factor to attract or activate macrophages that attack myelin sheaths in this model of CMT1A.

Macrophage-related demyelination in peripheral nerves of mice deficient in the gap junction protein connexin 32.

Mice deficient in the gap junction protein connexin 32 (Cx32) develop a slowly progressing demyelinating neuropathy, with enlarged periaxonal collars, abnormal non-compacted myelin domains and axonal sprouts. These mice serve as a model for the X-linked form of inherited demyelinating neuropathies in humans. Based on our previous findings that macrophages are involved in demyelination in other myelin mutants (i.e. mice heterozygously deficient in P0), we considered the possibility that macrophages might be also mediators of demyelination in Cx32-deficient mice. Indeed, we detected an age-related increase in the number of macrophages in demyelinating nerves of Cx32-deficient mice. In addition, immunoelectron microscopy revealed macrophages in an apposition to degenerating myelin reminiscent of a macrophage-mediated demyelinating neuropathy. We conclude that involvement of macrophages might be a widespread phenomenon in genetically-determined demyelination.

Attenuated demyelination in the absence of the macrophage-restricted adhesion molecule sialoadhesin (Siglec-1) in mice heterozygously deficient in P0.

Mouse mutants heterozygously deficient for the myelin component P0 mimic some forms of inherited neuropathies in humans. We have previously shown that both T lymphocytes and macrophages contribute to the demyelinating neuropathy. Both cell types appear to influence each other mutually, i.e., impaired T lymphocyte development in RAG-1-deficient P0 mutants leads to decreased macrophage numbers and retarded macrophage activation causes reduced T lymphocyte numbers in the peripheral nerves of P0(+/-) mice. In the present study, we investigated the possible role of the macrophage-restricted sialic acid-binding Ig-like lectin sialoadhesin (Sn, Siglec-1) in the pathogenesis of inherited demyelination in P0(+/-) mice. We found that most peripheral nerve macrophages express Sn in the mutants. Myelin mutants devoid of Sn show reduced numbers of CD8+ T lymphocytes and macrophages in peripheral nerves and less severe demyelination, resulting in improved nerve conduction properties. Our findings are potentially important in the development of future treatment strategies for inherited demyelinating neuropathies.

Neuroprotective effect of the immune system in a mouse model of severe dysmyelinating hereditary neuropathy: enhanced axonal degeneration following disruption of the RAG-1 gene

In mouse models of later onset forms of human hereditary demyelinating neuropathies, the immune system plays a crucial pathogenic role. Here, we investigated the influence of immune cells on early onset dysmyelination in mice homozygously deficient of the myelin component P0. In peripheral nerves of P0(-/-) mice, CD8+ T-lymphocytes increased with age. Macrophages peaked at 3 months followed by a substantial decline. They were mainly of hematogenous origin. To evaluate the functional role of immune cells, we cross-bred P0(-/-) mutants with RAG-1-deficient mice. At 3 months, the number of endoneurial macrophages did not differ from the macrophage number of immunocompetent myelin mutants, but the later decline of macrophages was not observed. Quantitative electron microscopy revealed that in plantar nerves of 6-month-old double mutants, significantly more axons had degenerated than in immunocompetent littermates. These data suggest a neuroprotective net effect of T-lymphocytes on axon survival in inherited, early onset dysmyelination.

Role of immune cells in animal models for inherited peripheral neuropathies

Mice expressing half of the normal dose of protein zero (P0+/− mice) or completely deficient gap-junction protein connexin 32−/− micemimic demyelinating forms of inherited neuropathies, such as Charcot-Marie-Tooth (CMT) neuropathies type 1B and CMT type 1X, respectively. In both models, an almost normal myelin formation is observed during the first months of life, followed by a slowly progressing demyelinating neuropathy. In both models, there is a substantial increase of CD8+ T-lymphocytes and macrophages within the demyelinating nerves. Recently, this has also been observed in mice mildly overexpressing human peripheral myelin protein 22 kD mimicking the most common form of CMT, CMT type 1A. In all demyelinating models, the macrophages show close contacts with intact myelin sheaths or demyelinated axons, suggesting an active role of these cells in myelin degeneration. Additionally, fibroblast-like cells contact macrophages, suggesting a functional role of fibroblast-like cells in macrophage activation. By cross-breeding P0+/− and gap-junction protein connexin 32−/− mice with immunodeficient recombination activating gene-1-deficient mutants, a substantial alleviation of the demyelinating phenotype was observed. Similarly, cross-breeding of P0+/− mice with mutants with a defect in macrophage activation led to an alleviated phenotype as well. These findings demonstrate that the immune system is involved in the pathogenesis of demyelinating neuropathies. In contrast, in P0−/− mice, which display a compromised myelin compaction and axonal loss from onset, immune cells appear to have a neuroprotective effect because cross-breeding with recombination activating gene-1 mutants leads to an aggravation of axonopathic changes. In the present review, we discuss the influence of the immune system on inherited de- and dysmyelination regarding disease mechanisms and possible clinical implications.

Origin of pathogenic macrophages and endoneurial fibroblast-like cells in an animal model of inherited neuropathy.

Macrophages have recently been shown to be critically involved in the pathogenesis of genetically determined demyelination in mice heterozygously deficient for P0 (P0(+-)). Since little is known about the origin of these cells, we created chimeric P0(+-) mice by transplanting bone marrow from green fluorescent protein (GFP)-transgenic mice into irradiated P0(+-) mice. When analyzing chimeric P0(+-) mice, we could determine two populations (GFP(+) and GFP(-)) of endoneurial macrophages that became phagocytic for myelin and increased in number. We found that both GFP(-) resident macrophages and GFP(+) macrophages proliferated in peripheral nerves of P0(+-) mice but not in nerves of chimeric or nonchimeric P0(++) mice. These findings demonstrate a so far poorly recognized role of resident endoneurial macrophages in demyelinating neuropathies. Surprisingly, we also found GFP(+) cells that unequivocally showed the morphological characteristics of fibroblasts. These blood-borne fibroblast-like cells express the common hematopoetic stem cell marker CD34 and might comprise another cell type of potential importance for immune regulation in hereditary demyelinating neuropathies.

Pathology of a mouse mutation in peripheral myelin protein P0 is characteristic of a severe and early onset form of human Charcot-Marie-Tooth type 1B disorder

Mutations in the gene of the peripheral myelin protein zero (P0) give rise to the peripheral neuropathies Charcot-Marie-Tooth type 1B disease (CMT1B), Déjérine-Sottas syndrome, and congenital hypomyelinating neuropathy. To investigate the pathomechanisms of a specific point mutation in the P0 gene, we generated two independent transgenic mouse lines expressing the pathogenic CMT1B missense mutation Ile106Leu (P0sub) under the control of the P0 promoter on a wild-type background. Both P0sub-transgenic mouse lines showed shivering and ultrastructural abnormalities including retarded myelination, onion bulb formation, and dysmyelination seen as aberrantly folded myelin sheaths and tomacula in all nerve fibers. Functionally, the mutation leads to dispersed compound muscle action potentials and severely reduced conduction velocities. Our observations support the view that the Ile106Leu mutation acts by a dominant-negative gain of function and that the P0sub-transgenic mouse represents an animal model for a severe, tomaculous form of CMT1B.

Monocyte chemoattractant protein-1 is a pathogenic component in a model for a hereditary peripheral neuropathy

Macrophages are critically involved in the pathogenesis of genetically caused demyelination, as it occurs in models for inherited demyelinating neuropathies. It is presently unknown which factors link the Schwann cell-based myelin mutation to the activation of endoneurial macrophages. Here we identified the chemokine monocyte chemoattractant protein-1 (MCP-1) as a first and crucial factor upregulated in Schwann cells of mice heterozygously deficient for the myelin protein zero. The chemokine could be identified as an important mediator of macrophage immigration into peripheral nerves. Furthermore, a 50% reduction of chemokine expression by crossbreeding with MCP-1-deficient mice reduced the increase in macrophage numbers in the mutant nerves and lead to a robust amelioration of pathology. Surprisingly, the complete absence of MCP-1 aggravated the disease. Our findings show that reducing but not eliminating chemokine expression can rescue genetically caused demyelination that may be an interesting target in treating demyelinating diseases of the peripheral nervous system.