Microglia undergo sex-dimorphic transcriptional and metabolic rewiring during aging.

Microglia, the brain's resident macrophages, maintain brain homeostasis and respond to injury and infection. During aging they undergo functional changes, but the underlying mechanisms and their contributions to neuroprotection versus neurodegeneration are unclear. Previous studies suggested that microglia are sex dimorphic, so we compared microglial aging in mice of both sexes. RNA-sequencing of hippocampal microglia revealed more aging-associated changes in female microglia than male microglia, and more sex differences in old microglia than young microglia. Pathway analyses and subsequent validation assays revealed a stronger AKT-mTOR-HIF1α-driven shift to glycolysis among old female microglia and indicated that C3a production and detection was elevated in old microglia, especially in females. Recombinant C3a induced AKT-mTOR-HIF1α signaling and increased the glycolytic and phagocytic activity of young microglia. Single cell analyses attributed the aging-associated sex dimorphism to more abundant disease-associated microglia (DAM) in old female mice than old male mice, and evaluation of an Alzheimer's Disease mouse model revealed that the metabolic and complement changes are also apparent in the context of neurodegenerative disease and are strongest in the neuroprotective DAM2 subset. Collectively, our data implicate autocrine C3a-C3aR signaling in metabolic reprogramming of microglia to neuroprotective DAM during aging, especially in females, and also in Alzheimer's Disease.

Microbial Sensing by Hematopoietic Stem and Progenitor Cells.

Balanced production of immune cells is critical for the maintenance of steady-state immune surveillance, and increased production of myeloid cells is sometimes necessary to eliminate pathogens. Hematopoietic stem and progenitor cell (HSPC) sensing of commensal microbes and invading pathogens has a notable impact on hematopoiesis. In this review, we examine how commensal microbes regulate bone marrow HSPC activity to maintain balanced hematopoiesis in the steady state, and how HSPCs proliferate and differentiate during emergency myelopoiesis in response to infection. HSPCs express a variety of pattern recognition receptors and cytokine receptors that they use to sense the presence of microbes, either directly via detection of microbial components and metabolites, or indirectly by responding to cytokines produced by other host cells. We describe direct and indirect mechanisms of microbial sensing by HSPCs and highlight evidence demonstrating long-term effects of acute and chronic microbial stimuli on HSPCs. We also discuss a possible connection between myeloid-biased hematopoiesis and elevated levels of circulating microbiome-derived components in the context of aging and metabolic stress. Finally, we highlight the prospect of trained immunity-based vaccines that could exploit microbial stimulation of HSPCs.

Skin Wounding-Induced Monocyte Expansion in Mice Is Not Abrogated by IL-1 Receptor 1 Deficiency.

The aim of this study was to determine whether skin wounding induces monocyte (Mo) expansion in bone marrow and whether IL-1R1 signaling regulates this process. Our data show that skin wounding increases myeloid lineage-committed multipotent progenitors (MPP3 subset) and Mo in bone marrow, but this expansion is not impaired in Il1r1-/- mice. We also demonstrate that M-CSF-induced differentiation of myeloid progenitors into Mo is not impaired by the loss of IL-1R1 ex vivo, indicating that IL-R1 deficiency does not abrogate myeloid progenitor differentiation potential. In addition, we observed modestly delayed wound closure in Il1r1-/- mice associated with higher frequency of Ly6Clo Mo in the circulation at baseline and in wounds early after injury. Thus, in contrast to other models of inflammation that involve IL-1R1-dependent monopoiesis, our results demonstrate that skin wounding induces Mo progenitor and Mo expansion independently of IL-1R1 signaling.

Diabetes induces myeloid bias in bone marrow progenitors associated with enhanced wound macrophage accumulation and impaired healing.

Diabetes induces dysregulation throughout the spectrum of myeloid lineage cells from progenitors to terminally differentiated cells. Another complication of diabetes is persistent inflammation, including prolonged accumulation of macrophages, which contributes to impaired wound healing. However, it remains unclear whether diabetes disrupts the response of bone marrow progenitors to peripheral injury and whether such dysregulation leads to sustained inflammation and impaired healing. Here, we demonstrated that diabetic mice (db/db, referred to here as DB) exhibit myeloid lineage bias during homeostasis and following injury. In addition, cells in the LSK (Lin- Sca-1+ cKit+ ) population of DB mice are preprogrammed towards myeloid commitment at the transcriptional level, and cultured myeloid progenitors from DB mice produce more monocytes ex vivo than their non-diabetic counterparts. We also show via bone marrow transfer between interleukin-1 receptor 1 KO (Il1r1-/- ) and DB mice that IL-1R1 signaling is likely not involved in myeloid skewing in DB mice. Furthermore, in vitro experiments indicated that macrophage colony-stimulating factor receptor signaling is not likely involved in enhanced myeloid transcription factor expression in LSK cells of DB mice. Our findings indicate that myeloid lineage commitment in bone marrow may contribute to increased macrophage numbers observed in diabetic skin wounds, and that strategies to regulate monopoiesis during homeostasis or post-wounding may improve diabetic wound healing. © 2019 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

Oxidant Signaling Mediated by Nox2 in Neutrophils Promotes Regenerative Myelopoiesis and Tissue Recovery following Ischemic Damage.

Ischemic tissue damage activates hematopoietic stem and progenitor cells (HSPCs) in the bone marrow (BM)-generating myeloid cells, and persistent HSPC activity may drive chronic inflammation and impair tissue recovery. Although increased reactive oxygen species in the BM regulate HSPC functions, their roles in myelopoiesis of activated HSPCs and subsequent tissue recovery during ischemic damage are not well understood. In this paper, we report that deletion of Nox2 NADPH oxidase in mice results in persistent elevations in BM HSPC activity and levels of inflammatory monocytes/macrophages in BM and ischemic tissue in a model of hindlimb ischemia. Ischemic tissue damage induces oxidants in BM such as elevations of hydrogen peroxide and oxidized phospholipids, which activate redox-sensitive Lyn kinase in a Nox2-dependent manner. Moreover, during tissue recovery after ischemic injury, this Nox2-ROS-Lyn kinase axis is induced by Nox2 in neutrophils that home to the BM, which inhibits HSPC activity and inflammatory monocyte generation and promotes tissue regeneration after ischemic damage. Thus, oxidant signaling in the BM mediated by Nox2 in neutrophils regulates myelopoiesis of HSPCs to promote regeneration of damaged tissue.

Production of MHCII-expressing classical monocytes increases during aging in mice and humans.

Aging is associated with increased monocyte production and altered monocyte function. Classical monocytes are heterogenous and a shift in their subset composition may underlie some of their apparent functional changes during aging. We have previously shown that mouse granulocyte-monocyte progenitors (GMPs) produce "neutrophil-like" monocytes (NeuMo), whereas monocyte-dendritic cell progenitors (MDPs) produce monocyte-derived dendritic cell (moDC)-producing monocytes (DCMo). Here, we demonstrate that classical monocytes from the bone marrow of old male and female mice have higher expression of DCMo signature genes (H2-Aa, H2-Ab1, H2-Eb1, Cd74), and that more classical monocytes express MHCII and CD74 protein. Moreover, we show that bone marrow MDPs and classical monocytes from old mice yield more moDC. We also demonstrate higher expression of Aw112010 in old monocytes and that Aw112010 lncRNA activity regulates MHCII induction in macrophages, which suggests that elevated Aw112010 levels may underlie increased MHCII expression during monocyte aging. Finally, we show that classical monocyte expression of MHCII is also elevated during healthy aging in humans. Thus, aging-associated changes in monocyte production may underlie altered monocyte function and have implications for aging-associated disorders.

Macrophage Dysregulation and Impaired Skin Wound Healing in Diabetes.

Monocytes (Mo) and macrophages (Mϕ) play important roles in normal skin wound healing, and dysregulation of wound Mo/Mϕ leads to impaired wound healing in diabetes. Although skin wound Mϕ originate both from tissue resident Mϕ and infiltrating bone marrow-derived Mo, the latter play dominant roles during the inflammatory phase of wound repair. Increased production of bone marrow Mo caused by alterations of hematopoietic stem and progenitor cell (HSPC) niche and epigenetic modifications of HSPCs likely contributes to the enhanced number of wound Mϕ in diabetes. In addition, an impaired transition of diabetic wound Mϕ from "pro-inflammatory" to "pro-healing" phenotypes driven by the local wound environment as well as intrinsic changes in bone marrow Mo is also thought to be partly responsible for impaired diabetic wound healing. The current brief review describes the origin, heterogeneity and function of wound Mϕ during normal skin wound healing followed by discussion of how dysregulated wound Mϕ numbers and phenotype are associated with impaired diabetic wound healing. The review also highlights the possible links between altered bone marrow myelopoiesis and increased Mo production as well as extrinsic and intrinsic factors that drive wound macrophage dysregulation leading to impaired wound healing in diabetes.

The Ontogeny of Monocyte Subsets.

Classical and non-classical monocytes, and the macrophages and monocyte-derived dendritic cells they produce, play key roles in host defense against pathogens, immune regulation, tissue repair and many other processes throughout the body. Recent studies have revealed previously unappreciated heterogeneity among monocytes that may explain this functional diversity, but our understanding of mechanisms controlling the functional programming of distinct monocyte subsets remains incomplete. Resolving monocyte heterogeneity and understanding how their functional identity is determined holds great promise for therapeutic immune modulation. In this review, we examine how monocyte origins and developmental influences shape the phenotypic and functional characteristics of monocyte subsets during homeostasis and in the context of infection, inflammation, and cancer. We consider how extrinsic signals and transcriptional regulators impact monocyte production and functional programming, as well as the influence of epigenetic and metabolic mechanisms. We also examine the evidence that functionally distinct monocyte subsets are produced via different developmental pathways during homeostasis and that inflammatory stimuli differentially target progenitors during an emergency response. We highlight the need for a more comprehensive understanding of the relationship between monocyte ontogeny and heterogeneity, including multiparametric single-cell profiling and functional analyses. Studies defining mechanisms of monocyte subset production and maintenance of unique monocyte identities have the potential to facilitate the design of therapeutic interventions to target specific monocyte subsets in a variety of disease contexts, including infectious and inflammatory diseases, cancer, and aging.