Underestimated Peripheral Effects Following Pharmacological and Conditional Genetic Microglial Depletion.

Jinming Han,Harald Lund,Keying Zhu,Yueshan Fan,Kai Zhou,Robert A Harris,Klas Blomgren,Xing-Mei Zhang

doi:10.3390/ijms21228603

Abstract

Microglia, predominant parenchymal resident macrophages in the central nervous system (CNS), are crucial players in neurodevelopment and CNS homeostasis. In disease conditions, pro-inflammatory microglia predominate over their regulatory counterparts, and are thus a potential immunotherapeutic target. It has been well documented that microglia can be effectively depleted using both conditional genetic Cx3cr1Cre-diphtheria toxin receptor (DTR)/diphtheria toxin subunit A (DTA) animal models and pharmacological colony-stimulating factor 1 receptor (CSF1R) inhibitors. Recent advances using these approaches have expanded our knowledge of the multitude of tasks conducted by microglia in both homeostasis and diseases. Importantly, experimental microglial depletion has been proven to exert neuroprotective effects in an increasing number of disease models, mostly explained by reduced neuroinflammation. However, the comprehensive effects of additional targets such as circulating monocytes and peripheral tissue macrophages during microglial depletion periods have not been investigated widely, and for those studies addressing the issue the conclusions are mixed. In this study, we demonstrate that experimental microglial depletion using both Cx3cr1CreER/+Rosa26DTA/+ mice and different doses of CSF1R inhibitor PLX3397 exert crucial influences on circulating monocytes and peripheral tissue macrophages. Our results suggest that effects on peripheral immunity should be considered both in interpretation of microglial depletion studies, and especially in the potential translation of microglial depletion and replacement therapies.

Highlights

Microglia are resident macrophages in the central nervous system (CNS), acting as key players in immune surveillance, neural circuits and synapse formation [1]
IntracistteerrnnaallininjejectcitoionnofoPfLPXL3X393739d7oedsoneostnexoetretxaesritgnaifsiicgannitfiecfafenctt eoffnescptleonnicsmplyeenliocidmcyelellso. id ce(lAls,.B)(AT,Bhe) Tnhuemnbuemr banerdapnedrcpeenrtcaegnetaogferoefdrepdulppumlpamcraocprhoapgheasg(e±s (S±EMSE) Min) itnhethsepslepelneenfoflloolwloiwnging initnrtarcaicsitsetrenrnaal lininjejeccttiioonn oof PLX33339977((PPLLXX33339977trteraetamtmenetngtrgoruopu,pp,inpkinbkarbsa; rcso;nctoronlt,rbolla,cbklabcakrsb, anr=s,4n, 3=, 4, 3,rreessppeectcitviveleyl)y.).(C(C,D,D) )TThehennuummbebreranadndpperecrecnetnatgaegeofofspsplelneincicLLyy6C6Chihmi monooncoyctyeste(s±(±SESMEM) f)oflloolwloiwnging initnrtarcaicsitsetrenrnaal lPPLLXX33339977ttrreeaattmment (PLX33339977 ttrreeaattmmeennttggrroouupp,ppininkkbabrasr;sc;ocnotnrotrlo, lb,lbaclakcbkabrsa,rns, =n4=, 34, 3, rersepsepcetcivtievleyl)y.).nnss: :nnoottssiiggnniifificcaant. We demonstrated that both conditional genetic and pharmacological microglial depletion approaches have significant effects on circulating monocytes and peripheral tissue macrophages, and that these effects can be abrogated by direct delivery of pharmacological inhibitors directly into the CNS
Our results demonstrated that numbers of circulating classical Ly6Chi monocytes and non-classical Ly6Clow monocytes significantly decreased in Cx3cr1CreER/+Rosa26DTA/+ mice following tamoxifen treatment, with non-classical Ly6Clow monocytes being most affected

Summary

Introduction

Microglia are resident macrophages in the central nervous system (CNS), acting as key players in immune surveillance, neural circuits and synapse formation [1]. Conditional genetic and pharmacological tools have been widely used in order to ablate microglia in research settings, as we have previously reviewed [3,4,5] These studies have expanded our understanding of microglial biology in both homeostasis and diseases. Cx3cr1Cre-diphtheria toxin receptor (DTR) mouse, with a tamoxifen-inducible Cre-recombinase expressed under control of the Cx3cr promoter, was bred in order to deplete microglia after the activation of tamoxifen and intraperitoneal administration of diphtheria toxin [7]. This conditional genetic approach can deplete approximately 80% of microglia in the mouse brain [7]. It is important to note that other peripheral immune cells including activated T cells and NK cells express CX3CR1 [11]

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Conclusion