Highly Dynamic Transcriptional Signature of Distinct Macrophage Subsets during Sterile Inflammation, Resolution, and Tissue Repair

Abstract

Macrophage gene expression determines phagocyte responses and effector functions. Macrophage plasticity has been mainly addressed in in vitro models that do not account for the environmental complexity observed in vivo. In this study, we show that microarray gene expression profiling revealed a highly dynamic landscape of transcriptomic changes of Ly6CposCX3CR1lo and Ly6CnegCX3CR1hi macrophage populations during skeletal muscle regeneration after a sterile damage. Systematic gene expression analysis revealed that the time elapsed, much more than Ly6C status, was correlated with the largest differential gene expression, indicating that the time course of inflammation was the predominant driving force of macrophage gene expression. Moreover, Ly6Cpos/Ly6Cneg subsets could not have been aligned to canonical M1/M2 profiles. Instead, a combination of analyses suggested the existence of four main features of muscle-derived macrophages specifying important steps of regeneration: 1) infiltrating Ly6Cpos macrophages expressed acute-phase proteins and exhibited an inflammatory profile independent of IFN-γ, making them damage-associated macrophages; 2) metabolic changes of macrophages, characterized by a decreased glycolysis and an increased tricarboxylic acid cycle/oxidative pathway, preceded the switch to and sustained their anti-inflammatory profile; 3) Ly6Cneg macrophages, originating from skewed Ly6Cpos cells, actively proliferated; and 4) later on, restorative Ly6Cneg macrophages were characterized by a novel profile, indicative of secretion of molecules involved in intercellular communications, notably matrix-related molecules. These results show the highly dynamic nature of the macrophage response at the molecular level after an acute tissue injury and subsequent repair, and associate a specific signature of macrophages to predictive specialized functions of macrophages at each step of tissue injury/repair.

Macrophage PPARγ, a Lipid Activated Transcription Factor Controls the Growth Factor GDF3 and Skeletal Muscle Regeneration

“…Although alternatively activated macrophages have been implicated in tissue repair and PPARγ has been reported to be a regulator of alternative macrophage polarization (Odegaard et al, 2007), we have previously reported that muscle Ly6C + and Ly6C − macrophages do not correspond to canonical alternatively polarized macrophage populations (Varga et al, 2016) in the CTX model. Therefore it is not surprising that, in this model PPARγ is controlling genes other than alternative macrophage related ones, reported to be PPARγ–dependent in other tissue compartments and contexts (Odegaard et al, 2007).…”

Immunohistochemical Identification of Human Skeletal Muscle Macrophages

“…This, coupled with the macrophage continuum, has led to inconsistencies with regard to identification and nomenclature across fields and across species (Murray et al , 2014). Skeletal muscle macrophages have primarily been studied in rodent models of injury, where the M1 versus M2 macrophage classification has proven useful (Smith et al , 2008; Chazaud et al , 2009; Tidball and Villalta, 2010, Kharraz et al , 2013; Novak and Koh, 2013; Saclier et al , 2013b; Rigamonti et al , 2014; Tidball et al , 2014; Wang et al , 2014; Sciorati et al , 2016; Varga et al , 2016; Mackey and Kjaer, 2017). The skeletal muscle response to injury is characterized by highly orchestrated temporal processes.…”

Selective recruitment of non-classical monocytes promotes skeletal muscle repair

“…Macrophages designated as classical inflammatory “M1” macrophages predominate early after injury, while alternatively-activated “M2” macrophages (AAMs) orchestrate the later phases of tissue repair. Inflammatory macrophages respond to tissue damage signals and upregulate genes associated with alarmins and acute inflammatory-phase proteins [10]. Later during the inflammatory response, AAMs are a potent source of insulin-like growth factor (IGF-1) required for muscle regeneration [11] and highly express genes encoding extracellular matrix (ECM) and ECM remodeling proteins [10].…”

“…Inflammatory macrophages respond to tissue damage signals and upregulate genes associated with alarmins and acute inflammatory-phase proteins [10]. Later during the inflammatory response, AAMs are a potent source of insulin-like growth factor (IGF-1) required for muscle regeneration [11] and highly express genes encoding extracellular matrix (ECM) and ECM remodeling proteins [10]. Though some resting tissues such as the lung, liver, and brain have macrophage populations that are seeded embryonically [12, 13], skeletal muscle possesses very few macrophages during homeostasis [3].…”