Browsing by Author "Mildner, Alexander"

Oligodendrocytes myelinate axons for rapid impulse conduction and contribute to normal axonal functions in the central nervous system. In multiple sclerosis, demyelination is caused by autoimmune attacks, but the role of oligodendroglial cells in disease progression and axon degeneration is unclear. Here we show that oligodendrocytes harbor peroxisomes whose function is essential for maintaining white matter tracts throughout adult life. By selectively inactivating the import factor PEX5 in myelinating glia, we generated mutant mice that developed normally, but within several months showed ataxia, tremor and premature death. Absence of functional peroxisomes from oligodendrocytes caused widespread axonal degeneration and progressive subcortical demyelination, but did not interfere with glial survival. Moreover, it caused a strong proinflammatory milieu and, unexpectedly, the infiltration of B and activated CD8(+) T cells into brain lesions. We conclude that peroxisomes provide oligodendrocytes with an essential neuroprotective function against axon degeneration and neuroinflammation, which is relevant for human demyelinating diseases.

The chemokine receptor CCR2 plays a vital role for the induction of autoimmunity in the central nervous system. However, it remains unclear how the pathogenic response is mediated by CCR2-bearing cells. By combining bone marrow chimerism with gene targeting we detected a mild disease-modulating role of CCR2 during experimental autoimmune encephalomyelitis, a model for central nervous system autoimmunity, on radio-resistant cells that was independent from targeted CCR2 expression on endothelia. Interestingly, absence of CCR2 on lymphocytes did not influence autoimmune demyelination. In contrast, engagement of CCR2 on accessory cells was required for experimental autoimmune encephalomyelitis induction. CCR2Ly-6C(hi) monocytes were rapidly recruited to the inflamed central nervous system and were crucial for the effector phase of disease. Selective depletion of this specific monocyte subpopulation through engagement of CCR2 strongly reduced central nervous system autoimmunity. Collectively, these data indicate a disease-promoting role of CCR2Ly-6C(hi) monocytes during autoimmune inflammation of the central nervous system.

Previous studies have demonstrated a potential role of brain endogenous microglia and meningeal macrophages in inflammation and brain injury during bacterial meningitis. However, the contribution of previously engrafted monocytes and microglia to this process is still unknown. We therefore used genetically labelled bone marrow-derived cells from transgenic mice expressing the green fluorescent protein (GFP) under the chicken beta-actin promoter to deliver fluorescently labelled monocytes to the diseased brain. Approximately 24 hours after Streptococcus pneumoniae infection, GFP-expressing parenchymal microglia changed their morphology to an activated phenotype and upregulated major histocompatibility complex class II molecules. Bacterial meningitis increased the engraftment of GFP(+) monocytes and their differentiation to microglia during the post-inflammatory period, but not during acute meningitis. Importantly, these newly recruited monocytes became an integral part of the pool of parenchymal microglia and contributed to the clearance of damaged tissue by increased lysosomal activity and close location to apoptotic cells. Thus, circulating cells entering the brain such as monocytes/macrophages might provide a potential cellular target for the treatment of the tissue damage following meningitis via peripheral cell therapy.

Mononuclear phagocytes are important modulators of Alzheimer's disease (AD), but the specific functions of resident microglia, bone marrow-derived mononuclear cells, and perivascular macrophages have not been resolved. To elucidate the spatiotemporal roles of mononuclear phagocytes during disease, we targeted myeloid cell subsets from different compartments and examined disease pathogenesis in three different mouse models of AD (APP(swe/PS1), APP(swe), and APP23 mice). We identified chemokine receptor 2 (CCR2)-expressing myeloid cells as the population that was preferentially recruited to beta-amyloid (A beta) deposits. Unexpectedly, AD brains with dysfunctional microglia and devoid of parenchymal bone marrow-derived phagocytes did not show overt changes in plaque pathology and A beta load. In contrast, restriction of CCR2 deficiency to perivascular myeloid cells drastically impaired beta-amyloid clearance and amplified vascular A beta deposition, while parenchymal plaque deposition remained unaffected. Together, our data advocate selective functions of CCR2-expressing myeloid subsets, which could be targeted specifically to modify disease burden in AD.

The action of type I interferons in the central nervous system (CNS) during autoimmunity is largely unknown. Here, we demonstrate elevated interferon beta concentrations in the CNS, but not blood, of mice with experimental autoimmune encephalomyelitis (EAE), a model for CNS autoimmunity. Furthermore, mice devoid of the broadly expressed type I IFN receptor (IFNAR) developed exacerbated clinical disease accompanied by a markedly higher inflammation, demyelination, and lethality without shifting the T helper 17 (Th17) or Th1 cell immune response. Whereas adoptive transfer of encephalitogenic T cells led to enhanced disease in Ifnar1(-/-) mice, newly created conditional mice with B or T lymphocyte-specific IFNAR ablation showed normal EAE. The engagement of IFNAR on neuroectodermal CNS cells had no protective effect. In contrast, absence of IFNAR on myeloid cells led to severe disease with an enhanced effector phase and increased lethality, indicating a distinct protective function of type I IFNs during autoimmune inflammation of the CNS.

The I kappa B kinase complex induces nuclear factor kappa B activation and has recently been recognized as a key player of autoimmunity in the central nervous system. Notably, I kappa B kinase/nuclear factor kappa B signalling regulates peripheral myelin formation by Schwann cells, however, its role in myelin formation in the central nervous system during health and disease is largely unknown. Surprisingly, we found that brain-specific I kappa B kinase 2 expression is dispensable for proper myelin assembly and repair in the central nervous system, but instead plays a fundamental role for the loss of myelin in the cuprizone model. During toxic demyelination, inhibition of nuclear factor kappa B activation by conditional ablation of I kappa B kinase 2 resulted in strong preservation of central nervous system myelin, reduced expression of proinflammatory mediators and a significantly attenuated glial response. Importantly, I kappa B kinase 2 depletion in astrocytes, but not in oligodendrocytes, was sufficient to protect mice from myelin loss. Our results reveal a crucial role of glial cell-specific I kappa B kinase 2/nuclear factor kappa B signalling for oligodendrocyte damage during toxic demyelination. Thus, therapies targeting I kappa B kinase 2 function in non-neuronal cells may represent a promising strategy for the treatment of distinct demyelinating central nervous system diseases.

Activation of transcription factor NF-kappa B in the central nervous system (CNS) has been linked to autoimmune demyelinating disease; however, it remains unclear whether its function is protective or pathogenic. Here we show that CNS-restricted ablation of 'upstream' NF-kappa B activators NEMO or IKK2 but not IKK1 ameliorated disease pathology in a mouse model of multiple sclerosis, suggesting that 'canonical' NF-kappa B activation in cells of the CNS has a mainly pathogenic function in autoimmune demyelinating disease. NF-kappa B inhibition prevented the expression of proinflammatory cytokines, chemokines and the adhesion molecule VCAM-1 from CNS-resident cells. Thus, NF-kappa B-dependent gene expression in non-microglial cells of the CNS provides a permissive proinflammatory milieu that is critical for CNS inflammation and tissue damage in autoimmune demyelinating disease.

Inflammatory diseases of the CNS, such as MS and its animal model EAE, are characterized by infiltration of activated lymphocytes and phagocytes into the CNS. Within the CNS, activation of resident cells initiates an inflammatory cascade, leading to tissue destruction, demyelination, and neurologic deficit. TLRs recognize microbes and are pivotal mediators of innate immunity. Within the CNS, augmented TLR expression during EAE is observed, even in the absence of any apparent microbial involvement. To determine the functional relevance of this phenomenon during sterile autoimmunity, we studied the role of different TLRs as well as their common signaling adaptor MyD88 in the development of EAE. We found that MyD88(-/-) mice were completely EAE resistant. Surprisingly, this protection is partly due to engagement of the CpG receptor TLR9. Restricting the MyD88 or TLR9 mutation to host radio-resistant cells. including the cells within the CNS, revealed that engagement of radio-resistant cells modulated the disease course and histopathological changes. Our data dearly demonstrate that both TLR9 and MyD88 are essential modulators of the autoimmune process during the effector phase of disease and suggest that endogenous "danger signals" modulate the disease pathogenesis.

Myeloid cell recruitment is a characteristic feature of bacterial meningitis. However, the cellular mechanisms important for the control of Streptococcus pneumoniae infection remain largely undefined. Previous pharmacological or genetic studies broadly depleted many myeloid cell types within the meninges, which did not allow defining the function of specific myeloid subsets. Herein we show that besides CD11b(+)Ly-6G(+)CCR2(-) granulocytes, also CD11b(+)Ly-6C(high)CCR2(+) but not Ly-6C(low)CCR2(-) monocytes were recruited in high numbers to the brain as early as 12 h after bacterial challenge. Surprisingly, CD11b(+)Ly-6C(high)CCR2(+) inflammatory monacytes modulated local CXCL2 and IL-1 beta production within the meninges but did not provide protection against bacterial infection. Consistent with these results, CCR2 deficiency strongly impaired monocyte recruitment to the infected brains but was redundant for disease pathogenesis. In contrast, specific depletion of polymorphonuclear granulocytes caused elevated local bacterial titer within the brains, led to an aggravated clinical course, and enhanced mortality. These findings demonstrate that Ly-6C(high)CCR2(+) inflammatory monocytes play a redundant role for the host defense during bacterial meningitis and that predominantly CD11b(+)Ly-6G(+)CCR2(-) myeloid cells are involved in the restriction of the extracellular bacteria.