Repopulation of a microglia-depleted central nervous system : molecular characterization during homeostasis and disease

Microglia are predominant tissue resident macrophages within the central nervous system (CNS), and contribute to both CNS development and homeostasis. During disease conditions microglia undergo transcriptional re-programming and their dysfunction is implicated in a multitude of disorders, such as multiple sclerosis (MS). How microglia could be therapeutically targeted is a current research focus. Recent experimental microglial depletion methods using conditional genetic targeting and pharmacological therapies have broadened our perspective of these multi-tasking microglia. Newly repopulated microglia following experimental microglial ablation hold great promise for reducing neuroinflammation and treating a variety of neurological disorders.

In Study 1 our results indicated that microglia could be ablated (approximately 95%) by systemic use of tamoxifen in Cx3cr1CreER/+Rosa26DTA/+ mice. Microglial repopulation ensued through both the proliferation of surviving microglia in the CNS, and from the infiltration of Ly6Chi monocytes. Under this condition infiltrating monocytes could be shaped into microglia-like cells by the CNS microenvironment. Furthermore, isolated newly repopulated resident microglia and infiltrating microglia-like cells following experimental depletion exhibited differential functionality in vitro, such as phagocytic capacity and cytokine production.

In Study 2 we used the microglial depletion and repopulation model mentioned above and demonstrated that the presence of infiltrating microglia-like cells following ablation could exacerbate experimental autoimmune encephalomyelitis (EAE) symptoms in Cx3cr1CreER/+Rosa26DTA/+ female mice. This was not evident in male mice, indicating a potential sex effect. Under this condition there was a higher expression of major histocompatibility complex class II and a greater secretion of proinflammatory cytokines during the acute period in the female mice.

In Study 3 we discovered a novel subpopulation of microglia that escape the genetic modification of Cx3cr1 in Cx3cr1CreER-EYFP/+Rosa26DTA/+ mice. Following microglial depletion using tamoxifen, newly repopulated Cx3cr1highEYFP– microglia had an advantage over Cx3cr1CreER-EYFP/+ and Cx3cr1lowEYFP+ microglia. We also found that microglial repopulation was tightly regulated by the CX3CL1-CX3CR1 signaling. The numbers of repopulated CNS-resident microglia were significantly decreased, while the numbers of infiltrating microglia-like cells were increased during repopulation in mice devoid of Cx3cr1.

In Study 4 we demonstrated that experimentally removing microglia using both Cx3cr1CreER/+Rosa26DTA/+ mice and PLX3397 treatment had crucial effects on circulating monocytes and splenic macrophages, a finding that had previously received little attention. We therefore proposed that clinical translation of preclinical studies using microglial depletion should take peripheral effects into consideration.

List of scientific papers

I. Competitive repopulation of an empty microglial niche yields functionally distinct subsets of microglia-like cells. Harald Lund, Melanie Pieber, Roham Parsa, Jinming Han, David Grommisch, Ewoud Ewing, Lara Kular, Maria Needhamsen, Alexander Espinosa, Emma Nilsson, Anna K. Överby, Oleg Butovsky, Maja Jagodic, Xing-Mei Zhang, Robert A. Harris. Nature Communications. 2018 Nov 19;9(1):4845.
https://doi.org/10.1038/s41467-018-07295-7

II. Sex-specific effects of microglia-like cell engraftment during experimental autoimmune encephalomyelitis. Jinming Han, Keying Zhu, Kai Zhou, Ramil Hakim, Sreenivasa Raghavan Sankavaram, Klas Blomgren, Harald Lund, Xing-Mei Zhang, Robert A. Harris. International Journal of Molecular Sciences. 2020 Sep 17;21(18):6824.
https://doi.org/10.3390/ijms21186824

III. Microglial niche repopulation competition following genetic depletion is regulated by CX3CL1-CX3CR1 signaling. Kai Zhou, Jinming Han, Harald Lund, Nageswara Rao Boggavarapu, Volker Lauschke, Shinobu Goto, Ahmed M Osman, Yuyu Wang, Asuka Tachi, Cuicui Xie, Ying Sun, Dong Liang, Wei Han, Keying Zhu, Kristina Gemzell-Danielsson, Christer Betsholtz, Xing-Mei Zhang, Changlian Zhu, Bertrand Joseph, Robert A. Harris, Klas Blomgren. [Manuscript]

IV. Underestimated peripheral effects following pharmacological and conditional genetic microglial depletion. Jinming Han, Yueshan, Fan, Kai Zhou, Keying Zhu, Klas Blomgren, Harald Lund, Xing-Mei Zhang, Robert A. Harris. International Journal of Molecular Sciences. 2020 Nov 15;21(22):8603.
https://doi.org/10.3390/ijms21228603