Monocyte-derived cells but not Microglia cause Oxidative Tissue Damage in Neuroinflammation

Keywords

• Monocytes

• Mitochondrial Reactive Oxygen Species

• Multiple Sclerosis

Main Findings

In  this preprint, the authors investigate the source of reactive oxygen  species (ROS) that determine the severity of multiple sclerosis (MS) in  mice. Tissue damage due to ROS production is a key feature in MS.  However, the major contributing cells responsible for this damage remain  poorly classified. Using multiple human and murine high resolution  sequencing data sets, the authors generate an integrated map of brain  macrophages and identify an enrichment of a ROS gene expression  signature. Confirming these results in the murine data sets the authors  reveal and confirm that Monocyte-derived Cells (MdC) rather than  Microglia express highest levels of ROS. Using a combination of  inducible Cre lines, the investigators provide data indicating that ROS  production in EAE, a murine experimental model for MS, is independent of  the NADPH Oxidase 2-associated protein gp91phox. Identifying that MdC  show elevated expression of the mitochondrial complex I (a central part  of the electron transport chain) inspired the authors to explore the  role of reverse electron transport in generating mitochondrial ROS  (mtROS). Using a transgene encoding allowing the conditional  overexpression of mitochondrial-tagged Catalase (mCAT), the authors next  demonstrate that CCR2-expressing MdC, overexpressing cells mCAT show  significantly lower levels of mtROS productions. In line with these  observations, the authors report a decrease in infiltrating immune cells  (monocytes and T cells), a decrease in demyelination and a significant  amelioration of EAE pathology. The authors conclude that MdC-derived  mtROS rather then microglia contribute to ROS-mediated tissue damage in  EAE/MS.

Limitations

• While the source and pathway contributing to ROS are nicely dissected,  the fate of MdC or monocytes under the describe circumstances (mCAT  overexpression and brain inflammation) remain unclear. One wonders if  mCAT overexpression impacts inflammatory cytokine production and the  availability of DAMPs in the brain, or whether mCAT overexpression  alters myeloid cell migration, or survival.

• A more detailed spatial characterization of myeloid cells in active  lesions and the lesion edges would be a fantastic addition to the  preprint’s last figure. Why are there fewer MdC/inflammatory cells?  Pairing such an analysis with an assessment of neuronal cell death, the  characterization of T cells and their regional distribution would be  very valuable information.

• An  additional aspect that could improve the authors’ report is an answer  to the question of how inflammatory cytokines contribute to mtROS  production in EAE. Integrating the role of IFNg or GM-CSF into the  activity of complex I would be of great value particularly in light of  recently published data https://doi.org/10.1038/s41586-024-07167-9 . A role for RORgt+ T cells could be considered here as well.

• Another point that arises from the authors data is the question on what  pathway or mechanism facilitates RET in MdC but not in microglia?

• While the use of the reported Cre lines is a widely-accepted and elegant  strategy, it remains unclear to what extent, or if at all, infiltrating  monocytes contribute to the pool of “microglia-like cells” under  conditions of EAE. A clearer definition of MdCs’ contribution to the  population of “microglia/microglia-like” cells would strengthen the  authors’ claim and help to define whether a transition of MdC to cells  with features and phenotypes of microglia could be considered as path  towards ROS production by microglia/microglia-like cells. As a  re-occurring issue in the field, the authors’ report and experimental  set up has the potential to provide clarifying answers to this issue.

Significance/Novelty

The  authors elegantly demonstrate that RET and mtROS produced by  CCR2-expressing cells is a key contributor to tissue damage in EAE. The  use of the mCAT transgene makes this work particularly interesting. The  authors experimental set up strongly suggests that Monocyte-derived  Cells are the major producers of tissue-damaging ROS through the use of  mitochondrial RET. This pathway and its contribution to the disease are  novel and a great advancement to the field that now allows the  integration of reports on mitochondrial dysfunction and MS into a  testable, experimental setting.

This  work nicely demonstrates the use of an elegant in vivo system that  facilitates the exploration of an underappreciated pathway implicated in  inflammation-driven tissue damage. The provided results inspire the use  of such tools for many additional inflammatory conditions to dissect  the contribution of monocyte-derived cells and tissue-resident  macrophages to the severity of chronic and acutely inflammatory  diseases.

Credit

Reviewed by Arthur Mortha (University of Toronto, Department of Immunology) as  part of a cross-institutional journal club between the Icahn School of  Medicine at Mount Sinai, the University of Oxford, the Karolinska  Institute and the University of Toronto.

The author declares no conflict of interest in relation to their involvement in the review.