title: Mobilisation of the splenic monocyte reservoir and peripheral CX₃CR1 deficiency adversely affects recovery from spinal cord injury.
abstract: Macrophages in the injured spinal cord originate from resident microglia and blood monocytes. Whether this diversity in origins contributes to their seemingly dual role in immunopathology and repair processes has remained poorly understood. Here we took advantage of Cx₃cr1(gfp) mice to visualise monocyte-derived macrophages in the injured spinal cord via adoptive cell transfer and bone marrow (BM) chimera approaches. We show that the majority of infiltrating monocytes at 7 days post-injury originate from the spleen and only to a lesser extent from the BM. Prevention of early monocyte infiltration via splenectomy was associated with improved recovery at 42 days post-SCI. In addition, an increased early presence of infiltrating monocytes/macrophages, as a result of CX₃CR1 deficiency within the peripheral immune compartment, correlated with worsened injury outcomes. Adoptive transfer of identified Cx₃cr1(gfp/+) monocytes confirmed peak infiltration at 7 days post-injury, with inflammatory (Ly6C(high)) monocytes being most efficiently recruited. Focal SCI also changed the composition of the two major monocyte subsets in the blood, with more Ly6C(high) cells present during peak recruitment. Adoptive transfer experiments further suggested high turnover of inflammatory monocytes in the spinal cord at 7 days post-injury. Consistent with this, only a small proportion of infiltrating cells unequivocally expressed polarisation markers for pro-inflammatory (M1) or alternatively activated (M2) macrophages at this time point. Our findings offer new insights into the origins of monocyte-derived macrophages after SCI and their contribution to functional recovery, providing a basis for further scrutiny and selective targeting of Ly6C(high) monocytes to improve outcomes from neurotraumatic events.
authors.affiliation: The University of Queensland, School of Biomedical Sciences, QLD 4072, Australia.
authors.forename: Linda V
authors.initials: LV
authors.lastname: Blomster
authors.forename: Faith H
authors.initials: FH
authors.lastname: Brennan
authors.forename: Hong W
authors.initials: HW
authors.lastname: Lao
authors.forename: David W
authors.initials: DW
authors.lastname: Harle
authors.forename: Alan R
authors.initials: AR
authors.lastname: Harvey
authors.forename: Marc J
authors.initials: MJ
authors.lastname: Ruitenberg
mesh_terms: D019264:Adoptive Transfer; D000704:Analysis of Variance; D000818:Animals; D000950:Antigens, Ly / Q000378:metabolism; D000073883:CX3C Chemokine Receptor 1; D004195:Disease Models, Animal; D005106:Exploratory Behavior / Q000502:physiology; D005434:Flow Cytometry; D049452:Green Fluorescent Proteins / Q000235:genetics; D008124:Locomotion / Q000502:physiology; D008264:Macrophages / Q000276:immunology / Q000473:pathology; D051379:Mice; D008810:Mice, Inbred C57BL; D008822:Mice, Transgenic; D009000:Monocytes / Q000276:immunology* / Q000473:pathology*; D009186:Myelin Sheath / Q000378:metabolism / Q000473:pathology; D019009:Proto-Oncogene Proteins c-kit / Q000378:metabolism; D019707:Receptors, Chemokine / Q000172:deficiency* / Q000235:genetics / Q000378:metabolism; D020127:Recovery of Function / Q000235:genetics*; D013119:Spinal Cord Injuries / Q000503:physiopathology*; D013154:Spleen / Q000473:pathology; D013997:Time Factors; D051941:fms-Like Tyrosine Kinase 3 / Q000378:metabolism
chemical_list: D000950:Antigens, Ly; D000073883:CX3C Chemokine Receptor 1; C471885:Cx3cr1 protein, mouse; C494192:Ly-6C antigen, mouse; D019707:Receptors, Chemokine; D049452:Green Fluorescent Proteins; D019009:Proto-Oncogene Proteins c-kit; D051941:fms-Like Tyrosine Kinase 3
keywords: CNS injury; Degeneration; Dendritic cell; Inflammation; Regeneration; Secondary injury