Relative contributions of osteal macrophages and osteoclasts to postnatal bone development in CSF1R-deficient rats and phenotype rescue following wild-type bone marrow cell transfer.

Macrophage and osteoclast proliferation, differentiation and survival are regulated by colony-stimulating factor-1 receptor (CSF1R) signaling. Osteopetrosis associated with Csf1 and Csf1r mutations has been attributed to the loss of osteoclasts and deficiency in bone resorption. Here we demonstrate that homozygous Csf1r mutation in rat leads to delayed postnatal skeletal ossification associated with substantial loss of osteal macrophages (osteomacs) in addition to osteoclasts. Osteosclerosis and site-specific skeletal abnormalities were reversed by intraperitoneal transfer of wild-type bone marrow cells (BMT) at weaning. Following BMT, IBA1+ macrophages were detected before TRAP+ osteoclasts at sites of ossification restoration. These observations extend evidence that osteomacs independently contribute to bone anabolism and are required for normal postnatal bone growth and morphogenesis.

Therapeutic potential of macrophage colony-stimulating factor in chronic liver disease.

Resident and recruited macrophages control the development and proliferation of the liver. We have previously shown in multiple species that treatment with a macrophage colony stimulating factor (CSF1)-Fc fusion protein initiated hepatocyte proliferation and promoted repair in models of acute hepatic injury in mice. Here, we investigated the impact of CSF1-Fc on resolution of advanced fibrosis and liver regeneration, using a non-resolving toxin-induced model of chronic liver injury and fibrosis in C57BL/6J mice. Co-administration of CSF1-Fc with exposure to thioacetamide (TAA) exacerbated inflammation consistent with monocyte contributions to initiation of pathology. After removal of TAA, either acute or chronic CSF1-Fc treatment promoted liver growth, prevented progression and promoted resolution of fibrosis. Acute CSF1-Fc treatment was also anti-fibrotic and pro-regenerative in a model of partial hepatectomy in mice with established fibrosis. The beneficial impacts of CSF1-Fc treatment were associated with monocyte-macrophage recruitment and increased expression of remodelling enzymes and growth factors. These studies indicate that CSF1-dependent macrophages contribute to both initiation and resolution of fibrotic injury and that CSF1-Fc has therapeutic potential in human liver disease.

CSF1R-dependent macrophages control postnatal somatic growth and organ maturation.

Homozygous mutation of the Csf1r locus (Csf1rko) in mice, rats and humans leads to multiple postnatal developmental abnormalities. To enable analysis of the mechanisms underlying the phenotypic impacts of Csf1r mutation, we bred a rat Csf1rko allele to the inbred dark agouti (DA) genetic background and to a Csf1r-mApple reporter transgene. The Csf1rko led to almost complete loss of embryonic macrophages and ablation of most adult tissue macrophage populations. We extended previous analysis of the Csf1rko phenotype to early postnatal development to reveal impacts on musculoskeletal development and proliferation and morphogenesis in multiple organs. Expression profiling of 3-week old wild-type (WT) and Csf1rko livers identified 2760 differentially expressed genes associated with the loss of macrophages, severe hypoplasia, delayed hepatocyte maturation, disrupted lipid metabolism and the IGF1/IGF binding protein system. Older Csf1rko rats developed severe hepatic steatosis. Consistent with the developmental delay in the liver Csf1rko rats had greatly-reduced circulating IGF1. Transfer of WT bone marrow (BM) cells at weaning without conditioning repopulated resident macrophages in all organs, including microglia in the brain, and reversed the mutant phenotypes enabling long term survival and fertility. WT BM transfer restored osteoclasts, eliminated osteopetrosis, restored bone marrow cellularity and architecture and reversed granulocytosis and B cell deficiency. Csf1rko rats had an elevated circulating CSF1 concentration which was rapidly reduced to WT levels following BM transfer. However, CD43hi non-classical monocytes, absent in the Csf1rko, were not rescued and bone marrow progenitors remained unresponsive to CSF1. The results demonstrate that the Csf1rko phenotype is autonomous to BM-derived cells and indicate that BM contains a progenitor of tissue macrophages distinct from hematopoietic stem cells. The model provides a unique system in which to define the pathways of development of resident tissue macrophages and their local and systemic roles in growth and organ maturation.

The Mononuclear Phagocyte System of the Rat.

The laboratory rat continues to be the model of choice for many studies of physiology, behavior, and complex human diseases. Cells of the mononuclear phagocyte system (MPS; monocytes, macrophages, and dendritic cells) are abundant residents in every tissue in the body and regulate postnatal development, homeostasis, and innate and acquired immunity. Recruitment and proliferation of MPS cells is an essential component of both initiation and resolution of inflammation. The large majority of current knowledge of MPS biology is derived from studies of inbred mice, but advances in technology and resources have eliminated many of the advantages of the mouse as a model. In this article, we review the tools available and the current state of knowledge of development, homeostasis, regulation, and diversity within the MPS of the rat.

Analysis of homozygous and heterozygous Csf1r knockout in the rat as a model for understanding microglial function in brain development and the impacts of human CSF1R mutations.

Mutations in the human CSF1R gene have been associated with dominant and recessive forms of neurodegenerative disease. Here we describe the impacts of Csf1r mutation in the rat on development of the brain. Diffusion imaging indicated small reductions in major fiber tracts that may be associated in part with ventricular enlargement. RNA-seq profiling revealed a set of 105 microglial markers depleted in all brain regions of the Csf1rko rats. There was no evidence of region or sex-specific expression of microglia-associated transcripts. Other than the microglial signature, Csf1rko had no effect on any neuronal or region-specific transcript cluster. Expression of markers of oligodendrocytes, astrocytes, dopaminergic neurons and Purkinje cells was minimally affected. However, there were defects in dendritic arborization of doublecortin-positive neurogenic precursors and expression of poly-sialylated neural cell adhesion molecule (PS-NCAM) in the dentate gyrus of the hippocampus. Heterozygous Csf1rko rats had no detectable brain phenotype. We conclude that most brain developmental processes occur normally in the absence of microglia and that CSF1R haploinsufficiency is unlikely to cause leukoencephalopathy.

Analysis of the impact of CSF-1 administration in adult rats using a novel Csf1r-mApple reporter gene.

Macrophages are present in large numbers in every tissue in the body where they play critical roles in development and homeostasis. They exhibit remarkable phenotypic and functional diversity, underpinning their adaptation to specialized roles in each tissue niche. CSF1, signaling through the CSF1 receptor, which is restricted to monocyte-macrophage lineage cells in adults, is a critical growth factor controlling macrophage proliferation, differentiation, and many aspects of mature macrophage function. We have generated a macrophage reporter rat, utilizing a construct containing elements of the mouse Csf1r promoter and the highly conserved Fms intronic regulatory element to drive mApple fluorescent protein expression. Csf1r-mApple was robustly expressed in monocyte-macrophage lineage cells in rat bone marrow (BM), peripheral blood, and tissues, with detectable expression in granulocytes and B cells and no evidence of expression in hematopoietic precursors or non-hematopoietic cells. Here, we use the Csf1r-mApple transgene to highlight and dissect the abundance and heterogeneity of rat tissue macrophage populations, and to demonstrate parallel increases in blood monocytes and multiple tissue macrophage populations, including BM, liver, spleen, and lung, in response to CSF1 treatment in vivo. The Csf1r-mApple rat is a novel tool enabling analysis of rat macrophages in situ by direct imaging and providing an additional phenotypic marker to facilitate exploration of rat tissue macrophage phenotypic and functional heterogeneity.

Phenotypic impacts of CSF1R deficiencies in humans and model organisms.

Mϕ proliferation, differentiation, and survival are controlled by signals from the Mϕ CSF receptor (CSF1R). Mono-allelic gain-of-function mutations in CSF1R in humans are associated with an autosomal-dominant leukodystrophy and bi-allelic loss-of-function mutations with recessive skeletal dysplasia, brain disorders, and developmental anomalies. Most of the phenotypes observed in these human disease states are also observed in mice and rats with loss-of-function mutations in Csf1r or in Csf1 encoding one of its two ligands. Studies in rodent models also highlight the importance of genetic background and likely epistatic interactions between Csf1r and other loci. The impacts of Csf1r mutations on the brain are usually attributed solely to direct impacts on microglial number and function. However, analysis of hypomorphic Csf1r mutants in mice and several other lines of evidence suggest that primary hydrocephalus and loss of the physiological functions of Mϕs in the periphery contribute to the development of brain pathology. In this review, we outline the evidence that CSF1R is expressed exclusively in mononuclear phagocytes and explore the mechanisms linking CSF1R mutations to pleiotropic impacts on postnatal growth and development.

Deletion of a Csf1r enhancer selectively impacts CSF1R expression and development of tissue macrophage populations.

The proliferation, differentiation and survival of mononuclear phagocytes depend on signals from the receptor for macrophage colony-stimulating factor, CSF1R. The mammalian Csf1r locus contains a highly conserved super-enhancer, the fms-intronic regulatory element (FIRE). Here we show that genomic deletion of FIRE in mice selectively impacts CSF1R expression and tissue macrophage development in specific tissues. Deletion of FIRE ablates macrophage development from murine embryonic stem cells. Csf1rΔFIRE/ΔFIRE mice lack macrophages in the embryo, brain microglia and resident macrophages in the skin, kidney, heart and peritoneum. The homeostasis of other macrophage populations and monocytes is unaffected, but monocytes and their progenitors in bone marrow lack surface CSF1R. Finally, Csf1rΔFIRE/ΔFIRE mice are healthy and fertile without the growth, neurological or developmental abnormalities reported in Csf1r-/- rodents. Csf1rΔFIRE/ΔFIRE mice thus provide a model to explore the homeostatic, physiological and immunological functions of tissue-specific macrophage populations in adult animals.