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•Antibody-mediated CSF1R inhibition is associated with neurological toxicity during acute and chronic CNS GVHD.•Targeting donor IFNGR signaling is a promising therapeutic strategy for ameliorating cGVHD of the CNS.

Graft-versus-host disease (GVHD) remains the leading cause of nonrelapse mortality after allogeneic stem cell transplantation for hematological malignancies. Manifestations of GVHD in the central nervous system (CNS) present as neurocognitive dysfunction in up to 60% of patients; however, the mechanisms driving chronic GVHD (cGVHD) in the CNS are yet to be elucidated. Our studies of murine cGVHD revealed behavioral deficits associated with broad neuroinflammation and persistent Ifng upregulation. By flow cytometry, we observed a proportional shift in the donor-derived T-cell population in the cGVHD brain from early CD8 dominance to later CD4 sequestration. RNA sequencing of the hippocampus identified perturbations to structural and functional synapse-related gene expression, together with the upregulation of genes associated with interferon-γ responses and antigen presentation. Neuroinflammation in the cortex of mice and humans during acute GVHD was recently shown to be mediated by resident microglia-derived tumor necrosis factor. In contrast, infiltration of proinflammatory major histocompatibility complex (MHC) class II+ donor bone marrow (BM)–derived macrophages (BMDMs) was identified as a distinguishing feature of CNS cGVHD. Donor BMDMs, which composed up to 50% of the CNS myeloid population, exhibited a transcriptional signature distinct from resident microglia. Recipients of MHC class II knockout BM grafts exhibited attenuated neuroinflammation and behavior comparable to controls, suggestive of a critical role of donor BMDM MHC class II expression in CNS cGVHD. Our identification of disease mediators distinct from those in the acute phase indicates the necessity to pursue alternative therapeutic targets for late-stage neurological manifestations.

Graft-versus-host disease (GVHD) remains the leading cause of nonrelapse mortality after allogeneic stem cell transplantation for hematological malignancies. Manifestations of GVHD in the central nervous system (CNS) present as neurocognitive dysfunction in up to 60% of patients; however, the mechanisms driving chronic GVHD (cGVHD) in the CNS are yet to be elucidated. Our studies of murine cGVHD revealed behavioral deficits associated with broad neuroinflammation and persistent Ifngupregulation. By flow cytometry, we observed a proportional shift in the donor-derived T-cell population in the cGVHD brain from early CD8 dominance to later CD4 sequestration. RNA sequencing of the hippocampus identified perturbations to structural and functional synapse-related gene expression, together with the upregulation of genes associated with interferon-γ responses and antigen presentation. Neuroinflammation in the cortex of mice and humans during acute GVHD was recently shown to be mediated by resident microglia-derived tumor necrosis factor. In contrast, infiltration of proinflammatory major histocompatibility complex (MHC) class II+donor bone marrow (BM)–derived macrophages (BMDMs) was identified as a distinguishing feature of CNS cGVHD. Donor BMDMs, which composed up to 50% of the CNS myeloid population, exhibited a transcriptional signature distinct from resident microglia. Recipients of MHC class II knockout BM grafts exhibited attenuated neuroinflammation and behavior comparable to controls, suggestive of a critical role of donor BMDM MHC class II expression in CNS cGVHD. Our identification of disease mediators distinct from those in the acute phase indicates the necessity to pursue alternative therapeutic targets for late-stage neurological manifestations.

Chronic graft-versus-host disease (cGVHD) remains a significant complication of allogeneic hematopoietic stem cell transplantation. Central nervous system (CNS) involvement is becoming increasingly recognised, where brain-infiltrating donor MHC class II +bone marrow-derived macrophages (BMDM) are implicated mediators of pathology. Colony-stimulating factor 1 receptor (CSF1R)-dependent BMDM are established mediators of cutaneous and pulmonary cGVHD, and clinical trials assessing the efficacy of CSF1R antibody blockade to deplete macrophages are promising. We hypothesized that CSF1R antibody blockade may also be a useful strategy to prevent/treat CNS cGVHD.

Definition of the heterogeneity of monocyte‐derived cells in the neurogenic olfactory neuroepithelium, their turnover, and the role of CX3CR1 in this process. Macrophages in the olfactory neuroepithelium are thought to play major roles in tissue homeostasis and repair. However, little information is available at present about possible heterogeneity of these monocyte‐derived cells, their turnover rates, and the role of chemokine receptors in this process. To start addressing these issues, this study used Cx3cr1gfpmice, in which the gene sequence for eGFP was knocked into the CX3CR1 gene locus in the mutant allele. Using neuroepithelial whole‐mounts from Cx3cr1gfp/+mice, we show that eGFP+cells of monocytic origin are distributed in a loose network throughout this tissue and can be subdivided further into two immunophenotypically distinct subsets based on MHC‐II glycoprotein expression. BM chimeric mice were created using Cx3cr1gfp/+donors to investigate turnover of macrophages (and other monocyte‐derived cells) in the olfactory neuroepithelium. Our data indicate that the monocyte‐derived cell population in the olfactory neuroepithelium is actively replenished by circulating monocytes and under the experimental conditions, completely turned over within 6 months. Transplantation of Cx3cr1gfp/gfp(i.e., CX3CR1‐deficient) BM partially impaired the replenishment process and resulted in an overall decline of the total monocyte‐derived cell number in the olfactory epithelium. Interestingly, replenishment of the CD68lowMHC‐II+subset appeared minimally affected by CX3CR1 deficiency. Taken together, the established baseline data about heterogeneity of monocyte‐derived cells, their replenishment rates, and the role of CX3CR1 provide a solid basis to further examine the importance of different monocyte subsets for neuroregeneration at this unique frontier with the external environment.

Injured neurons in the mammalian central nervous system (CNS) do not normally regenerate their axons after injury. Neurotrauma to the CNS usually results in axonal damage and subsequent loss of communication between neuronal networks, causing long-term functional deficits. For CNS regeneration, repair strategies need to be developed that promote regrowth of lesioned axon projections and restoration of neuronal connectivity. After spinal cord injury (SCI), cystic cavitations are often found, particularly in the later stages, due to the loss of neural tissue at the original impact site. Ultimately, for the promotion of axonal regrowth in these situations, some form of transplantation will be required to provide lesioned axons with a supportive substrate along which they can extend. Here, we review the use of olfactory ensheathing cells: their location and role in the olfactory system, their use as cellular transplants in SCI paradigms, alone or in combination with gene therapy, and the unique properties of these cells that may give them a potential advantage over other cellular transplants.

The advent of tools enabling the direct manipulation of microglia has furthered our understanding of their role in health and disease. Here, we present a detailed protocol allowing for microglia turnover in adult CX3CR1creERT2× iDTR or CX3CR1creERT2× iDTR × tdTomatofloxmice, either in a brain-wide or region-specific manner, and their subsequent isolation for downstream applications. This protocol may be used to explore microglia biology and their putative region-specific heterogeneous functional diversity, expanding our understanding of their importance in various neuroinflammatory conditions.