216 Inhibition of brutons tyrosine kinase (BTK) prevents inflammatory macrophage differentiation: a potential role in SLE

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A novel microglial subset plays a key role in myelinogenesis in developing brain

Microglia are resident macrophages of the central nervous system that contribute to homeostasis and neuroinflammation. Although known to play an important role in brain development, their exact function has not been fully described. Here, we show that in contrast to healthy adult and inflammation-activated cells, neonatal microglia show a unique myelinogenic and neurogenic phenotype. A CD11c(+) microglial subset that predominates in primary myelinating areas of the developing brain expresses genes for neuronal and glial survival, migration, and differentiation. These cells are the major source of insulin-like growth factor 1, and its selective depletion from CD11c(+) microglia leads to impairment of primary myelination. CD11c-targeted toxin regimens induced a selective transcriptional response in neonates, distinct from adult microglia. CD11c(+) microglia are also found in clusters of repopulating microglia after experimental ablation and in neuroinflammation in adult mice, but despite some similarities, they do not recapitulate neonatal microglial characteristics. We therefore identify a unique phenotype of neonatal microglia that deliver signals necessary for myelination and neurogenesis

Local self-renewing of microglia after genetic ablation is dependent on Interleukin-1 signaling

The development of new fate mapping techniques helped to elucidate the ontogeny of microglia during embryonic development. Microglia precursors arise during the primitive hematopoiesis at E7.5 and develop into early progenitors found in the yolk sac at E9.5. From there they migrate through the circulation to the CNS parenchyma between E10.5-E13.5, where they persist throughout adulthood. However, the mechanisms by which these unique cells are maintained in the adult CNS still remain elusive. In this study, we describe the Cx3Cr1CreER:iDTR system, a genetic model that allows for specific, conditional ablation of microglia in adult mice. With the help of this new genetic model we were able to deplete microglia with at least 90% efficiency. Surprisingly, the depletion was followed by rapid repopulation of the microglia compartment. The repopulation relied on CNS-resident cells, independent of bone-marrow-derived precursors, whereas the existence of a certain pre-determined microglia progenitor within the CNS is not yet completely proven. During the repopulation process, microglia formed clusters of highly proliferative cells that migrated away until normal microglia numbers were reestablished and steady state distribution was achieved. The trigger for the rapid repopulation is not yet clear, but could be driven by local progenitors in response to IL-1R signaling, since proliferating microglia expressed high levels of the interleukin-1 receptor (IL-1R), and treatment with an IL-1R antagonist during the repopulation phase impaired microglia proliferation. Furthermore, according to other tissue resident macrophages, microglia also demonstrate their potential for efficient self-renewal without the contribution of peripheral myeloid cells.Die Entwicklung neuer Fate-mapping- Techniken hat dazu beigetragen die Ontogenese von Mikroglia während der embryonalen Entwicklung weiter aufzuklären. Mikrogliale Vorläuferzellen entstehen während der primitiven Hämatopoese am embryonalem Tag E7.5 und entwickeln sich weiter zu frühen Vorläuferzellen, die an Tag E9.5 im Dottersack zu finden sind. Von hier aus wandern sie zwischen Tag E10.5-E13.5 durch das Blut zum ZNS, wo sie bis zum Erwachsenenalter verharren. Die Mechanismen, durch die diese einzigartigen Zellen im erwachsenen ZNS aufrechterhalten bleiben sind allerdings noch ungeklärt. In dieser Studie beschreiben wir das Cx3Cr1CreER:iDTR System, ein genetisches Model zur spezifischen und konditionalen Depletion von Mikroglia in erwachsenen Mäusen. Die Depletionseffizienz der Mikroglia Population in diesem Model lag dabei bei mindestens 90%. Überraschenderweise hatte diese Depletion eine schnelle Repopulation des mikroglialen Kompartimentes zur Folge. Diese Repopulation geschah unabhängig von den myeloiden Vorläufern des Knochenmarks und basiert grundlegend auf ZNS-eigenen Zellen. Allerdings konnte die Existenz von bestimmten vorgeprägten Mikroglia-Vorläuferzellen im erwachsenen ZNS noch nicht vollständig bewiesen werden. Jedoch werden während des Repopulationsvorgangs zahlreiche Zellkonglomerate von sich teilenden Mikroglia gebildet. Von diesen Zellkonglomeraten ausgehend wandern die einzelnen Zellen in das Gewebe, bis die normale Zellzahl und Verteilung von Mikroglia wieder erreicht ist. Der Auslöser für diese schnelle Repopulation ist noch unklar, könnte jedoch durch lokale Vorläuferzellen in Antwort auf Interleukin-1 Rezeptor (IL-1R) Signalisierung erfolgen, da sich teilende Mikroglia große Mengen des IL-1R exprimieren. Zudem beeinträchtigt die lokale Behandlung mit einem IL-1R Antagonisten während der Repopulationsphase die Teilungsrate der Mikroglia. Analog zu anderen Gewebsmakrophagen konnten Mikroglia zudem ihr Potential für effiziente Selbsterneuerung demonstrieren, ohne von peripheren myeloiden Zellen abhängig zu sein

EBI2 is highly expressed in multiple sclerosis lesions and promotes early CNS migration of encephalitogenic CD4 T cells

Arrival of encephalitogenic T cells at inflammatory foci represents a critical step in development of experimental autoimmune encephalomyelitis (EAE), the animal model for multiple sclerosis. EBI2 and its ligand, 7α,25-OHC, direct immune cell localization in secondary lymphoid organs. CH25H and CYP7B1 hydroxylate cholesterol to 7α,25-OHC. During EAE, we found increased expression of CH25H by microglia and CYP7B1 by CNS-infiltrating immune cells elevating the ligand concentration in the CNS. Two critical pro-inflammatory cytokines, interleukin-23 (IL-23) and interleukin-1 beta (IL-1β), maintained expression of EBI2 in differentiating Th17 cells. In line with this, EBI2 enhanced early migration of encephalitogenic T cells into the CNS in a transfer EAE model. Nonetheless, EBI2 was dispensable in active EAE. Human Th17 cells do also express EBI2, and EBI2 expressing cells are abundant within multiple sclerosis (MS) white matter lesions. These findings implicate EBI2 as a mediator of CNS autoimmunity and describe mechanistically its contribution to the migration of autoreactive T cells into inflamed organs

Genetic Cell Ablation Reveals Clusters of Local Self-Renewing Microglia in the Mammalian Central Nervous System.

SummaryDuring early embryogenesis, microglia arise from yolk sac progenitors that populate the developing central nervous system (CNS), but how the tissue-resident macrophages are maintained throughout the organism’s lifespan still remains unclear. Here, we describe a system that allows specific, conditional ablation of microglia in adult mice. We found that the microglial compartment was reconstituted within 1 week of depletion. Microglia repopulation relied on CNS-resident cells, independent from bone-marrow-derived precursors. During repopulation, microglia formed clusters of highly proliferative cells that migrated apart once steady state was achieved. Proliferating microglia expressed high amounts of the interleukin-1 receptor (IL-1R), and treatment with an IL-1R antagonist during the repopulation phase impaired microglia proliferation. Hence, microglia have the potential for efficient self-renewal without the contribution of peripheral myeloid cells, and IL-1R signaling participates in this restorative proliferation process