Divergent Functions of Tissue-Resident and Blood-Derived Macrophages in the Hemorrhagic Brain.

Background and Purpose: Brain tissue-resident microglia and monocyte-derived macrophages (MDMs) are innate immune cells that contribute to the inflammatory response, phagocytosis of debris, and tissue repair after injury. We have previously reported that both microglia and MDMs transition from proinflammatory to reparative phenotypes over days after an intracerebral hemorrhage (ICH). However, their individual functional properties in the brain remain largely unknown. Here we characterized the differences between microglia and MDMs and further elucidate their distinct activation states and functional contributions to the pathophysiology and recovery after ICH. Methods: Autologous blood injection was used to model ICH in mice. Longitudinal transcriptomic analyses on isolated microglia and MDMs from mice at days 1, 3, 7 and 10 after ICH and naive controls identified core transcriptional programs that distinguish these cells. Imaging flow cytometry and in vivo phagocytosis assays were used to study phagocytic ability of microglia and MDMs. Antigen presentation was evaluated by ovalbumin-OTII CD4 T-cell proliferation assays with bone marrow­derived macrophages and primary microglia cultures. Results: MDMs had higher phagocytic activity and higher erythrophagocytosis in the ICH brain. Differential gene expression revealed distinct transcriptional signatures in the MDMs and microglia after ICH. MDMs had higher expression of MHCII (major histocompatibility complex class II) genes than microglia at all time points and greater ability to induce antigen-specific T-cell proliferation. Conclusions: The different ontogeny of microglia and MDMs lead to divergent responses and functions in the inflamed brain as these 2 cell populations differ in phagocytic functions and antigen-presenting capabilities in the brain after ICH.

Longitudinal transcriptomics define the stages of myeloid activation in the living human brain after intracerebral hemorrhage.

Opportunities to interrogate the immune responses in the injured tissue of living patients suffering from acute sterile injuries such as stroke and heart attack are limited. We leveraged a clinical trial of minimally invasive neurosurgery for patients with intracerebral hemorrhage (ICH), a severely disabling subtype of stroke, to investigate the dynamics of inflammation at the site of brain injury over time. Longitudinal transcriptional profiling of CD14+ monocytes/macrophages and neutrophils from hematomas of patients with ICH revealed that the myeloid response to ICH within the hematoma is distinct from that in the blood and occurs in stages conserved across the patient cohort. Initially, hematoma myeloid cells expressed a robust anabolic proinflammatory profile characterized by activation of hypoxia-inducible factors (HIFs) and expression of genes encoding immune factors and glycolysis. Subsequently, inflammatory gene expression decreased over time, whereas anti-inflammatory circuits were maintained and phagocytic and antioxidative pathways up-regulated. During this transition to immune resolution, glycolysis gene expression and levels of the potent proresolution lipid mediator prostaglandin E2 remained elevated in the hematoma, and unexpectedly, these elevations correlated with positive patient outcomes. Ex vivo activation of human macrophages by ICH-associated stimuli highlighted an important role for HIFs in production of both inflammatory and anti-inflammatory factors, including PGE2, which, in turn, augmented VEGF production. Our findings define the time course of myeloid activation in the human brain after ICH, revealing a conserved progression of immune responses from proinflammatory to proresolution states in humans after brain injury and identifying transcriptional programs associated with neurological recovery.

Bexarotene Enhances Macrophage Erythrophagocytosis and Hematoma Clearance in Experimental Intracerebral Hemorrhage.

Background and Purpose- Enhancement of erythrophagocytosis by macrophages in a timely manner can limit the toxic effects of erythrocyte metabolites and promote brain recovery after intracerebral hemorrhage (ICH). In the current study, we investigated the therapeutic effect of retinoid X receptor agonist, bexarotene, in facilitating erythrophagocytosis and neurobehavioral recovery in 2 mouse models of ICH. Methods- Bone marrow-derived macrophages and fluorescently labeled erythrocytes were used to study erythrophagocytosis in vitro with phenotypic changes quantified by gene expression. ICH was modeled in vivo using intrastriatal autologous blood and collagenase injection in mice with and without bexarotene treatment beginning 3 hours after ICH. In vivo phagocytosis, ability and hematoma clearance were evaluated by erythrophagocytosis assays, flow cytometry, and histological analysis. Neurological deficits and functional recovery were also quantified. Results- Bexarotene increased macrophage expression of phagocytosis receptors and erythrophagocytosis and reduced macrophage TNF (tumor necrosis factor) production in vitro. In vivo, bexarotene treatment enhanced erythrophagocytosis, reduced hematoma volume, and ultimately improved neurological recovery after ICH in 2 distinct models of ICH. Conclusions- Bexarotene administration is beneficial for recovery after ICH by enhancing hemorrhage phagocytosis, modulating macrophage phenotype, and improving functional recovery.