Hauke Schmidt

Microglia in the adult brain arise from Ly-6ChiCCR2+ monocytes only under defined host conditions

Microglia are crucially important myeloid cells in the CNS and constitute the first immunological barrier against pathogens and environmental insults. The factors controlling microglia recruitment from the blood remain elusive and the direct circulating microglia precursor has not yet been identified in vivo. Using a panel of bone marrow chimeric and adoptive transfer experiments, we found that circulating Ly-6ChiCCR2+ monocytes were preferentially recruited to the lesioned brain and differentiated into microglia. Notably, microglia engraftment in CNS pathologies, which are not associated with overt blood-brain barrier disruption, required previous conditioning of brain (for example, by direct tissue irradiation). Our results identify Ly-6ChiCCR2+ monocytes as direct precursors of microglia in the adult brain and establish the importance of local factors in the adult CNS for microglia engraftment.

Distinct and nonredundant in vivo functions of IFNAR on myeloid cells limit autoimmunity in the central nervous system

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.

CCR2+Ly-6Chi monocytes are crucial for the effector phase of autoimmunity in the central nervous system

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. CCR2+Ly-6Chi 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 CCR2+Ly-6Chi monocytes during autoimmune inflammation of the central nervous system.

Local Type I IFN Receptor Signaling Protects against Virus Spread within the Central Nervous System

Several neurotropic viruses such as vesicular stomatitis virus (VSV) induce peripheral neutralizing Ab responses and still can infect cells within the CNS. To address whether local type I IFN receptor (IFNAR) triggering plays a role in controlling virus replication within the brain, we generated mice with a cell type-specific IFNAR deletion in neuroectodermal cells of the CNS (NesCre+/−IFNARflox/flox). Intranasal VSV infection with 103 PFU was well tolerated by wild-type mice, whereas conventional IFNAR−/− mice died within 2–3 days. In contrast, brain-specific NesCre+/−IFNARflox/flox mice survived until day 5–6 and then became hemiplegic and died. Terminally ill NesCre+/−IFNARflox/flox mice showed 10- to 100-fold higher virus loads in the brain than IFNAR−/− mice, whereas little or no virus was found in other organs. In wild-type animals, virus could be reisolated only from the olfactory bulb until day 6 where also STAT1 activation as a measure of IFNAR triggering was detected. Virus infection was found exclusively in glomerular structures of the olfactory bulb, whereas surrounding cells that showed STAT1 phosphorylation as a measure of IFNAR trigging were free of virus. Our data indicate that upon intranasal VSV instillation, early and localized IFNAR triggering in the glomerular layer of the olfactory bulb is critically required to prevent viral spread over the entire CNS and thus confers survival.

Streptococcus pneumoniae Infection aggravates experimental autoimmune encephalomyelitis via Toll-like receptor 2.

ABSTRACT The course of autoimmune inflammatory diseases of the central nervous system (CNS) can be influenced by infections. Here we assessed the disease-modulating effects of the most frequent respiratory pathogen Streptococcus pneumonia on the course of experimental autoimmune encephalomyelitis (EAE). Mice were immunized with myelin oligodendrocyte glycoprotein 35-55 (MOG35-55) peptide, challenged intraperitoneally with live S. pneumoniae type 3, and then treated with ceftriaxone. EAE was monitored by a clinical score for 35 days after immunization. EAE was unaltered in mice infected with S. pneumoniae 2 days before and 21 days after the first MOG35-55 injection but was more severe in animals infected 7 days after the first MOG35-55 injection. The antigen-driven systemic T-cell response was unaltered, and the intraspinal Th1 cytokine mRNA concentrations at the peak of disease were unchanged. The composition of CNS-infiltrating cells and subsequent tissue destruction were only slightly increased after S. pneumoniae infection. In contrast, the serum levels of tumor necrosis factor alpha and interleukin-6 and spinal interleukin-6 levels were elevated, and the expression of major histocompatibility complex class II molecules, CD80, and CD86 on splenic dendritic cells were enhanced early after infection. Serum cytokine concentrations were not elevated, and EAE was not aggravated by S. pneumoniae infection in Toll-like receptor 2 (TLR2)-deficient mice. In conclusion, infection with S. pneumoniae worsens EAE probably by elevation of proinflammatory cytokines and activation of dendritic cells in the systemic circulation via TLR2 and cross talk through the blood-brain barrier.

Overexpression of Lymphotoxin in T Cells Induces Fulminant Thymic Involution

Activated lymphocytes and lymphoid-tissue inducer cells express lymphotoxins (LTs), which are essential for the organogenesis and maintenance of lymphoreticular microenvironments. Here we describe that T-cell-restricted overexpression of LT induces fulminant thymic involution. This phenotype was prevented by ablation of the LT receptors tumor necrosis factor receptor (TNFR) 1 or LT beta receptor (LTbetaR), representing two non-redundant pathways. Multiple lines of transgenic Ltalphabeta and Ltalpha mice show such a phenotype, which was not observed on overexpression of LTbeta alone. Reciprocal bone marrow transfers between LT-overexpressing and receptor-ablated mice show that involution was not due to a T cell-autonomous defect but was triggered by TNFR1 and LTbetaR signaling to radioresistant stromal cells. Thymic involution was partially prevented by the removal of one allele of LTbetaR but not of TNFR1, establishing a hierarchy in these signaling events. Infection with the lymphocytic choriomeningitis virus triggered a similar thymic pathology in wt, but not in Tnfr1(-/-) mice. These mice displayed elevated TNFalpha in both thymus and plasma, as well as increased LTs on both CD8(+) and CD4(-)CD8(-) thymocytes. These findings suggest that enhanced T cell-derived LT expression helps to control the physiological size of the thymic stroma and accelerates its involution via TNFR1/LTbetaR signaling in pathological conditions and possibly also in normal aging.

Type I interferon receptor signalling is induced during demyelination while its function for myelin damage and repair is redundant.

The type I interferons, interferon-beta and alpha (IFN-beta, IFN-alpha), are widely used for the treatment of autoimmune demyelination in the central nervous system (CNS). Their effects on de- and remyelination through the broadly expressed type I IFN receptor (IFNAR), however, are highly speculative. In order to elucidate the role of endogenous type I interferons for myelin damage and recovery we induced toxic demyelination in the absence of IFNAR1. We demonstrate that IFNAR signalling was induced during acute demyelination since the cytokine IFN-beta as well as the IFN-dependent genes IRF7, ISG15 and UBP43 were strongly upregulated. Myelin damage, astrocytic and microglia response, however, were not significantly reduced in the absence of IFNAR1. Furthermore, motor skills of IFNAR1-deficient animals during non-immune demyelination were unaltered. Finally, myelin recovery was found to be independent from endogenous IFNAR signalling, indicating a redundant role of this receptor for non-inflammatory myelin damage and repair.