Potential PET tracers for imaging of tumor-associated macrophages

The increasing incidence of cancer over the years is one of the most challenging problems in healthcare. As cancer progresses, the recruitment of several immune cells is triggered. Infiltration of tumor-associated macrophages (TAMs) is correlated with poor patient prognosis. Since TAMs constitute a big portion of the tumor mass, targeting these cells seems to be an attractive approach for cancer immunotherapy. Additionally, TAM assessment using non-invasive imaging techniques, such as positron emission tomography (PET), might provide a better understanding of the role of TAMs in cancer, and a means for tumor profile characterization, patient selection for individualized immunotherapy and treatment monitoring. Imaging of TAMs using PET tracers is still in its infancy. TAMs have several characteristics that could be exploited as potential targets for imaging. Various PET tracers for these TAM biomarkers have been developed, although often in the context of (neuro)inflammatory diseases rather than cancer. Since macrophages in inflammatory diseases express similar biomarkers as TAMs, these PET tracers could potentially also be applied for the assessment of TAMs in the tumor microenvironment. Therefore, the present review provides an overview of the TAM biomarkers, for which potential PET tracers are available and discusses the status of these tracers.

Withholding of M-CSF Supplement Reprograms Macrophages to M2-Like via Endogenous CSF-1 Activation

Macrophage colony-stimulating factor (M-CSF or CSF-1) is known to have a broad range of actions on myeloid cells maturation, including the regulation of macrophage differentiation, proliferation and survival. Macrophages generated by M-CSF stimulus have been proposed to be alternatively activated or M2 phenotype. M-CSF is commonly overexpressed by tumors and is also known to enhance tumor growth and aggressiveness via stimulating pro-tumor activities of tumor-associated macrophages (TAMs). Currently, inhibition of CSF-1/CSF-1R interaction by therapeutic antibody to deplete TAMs and their pro-tumor functions is becoming a prevalent strategy in cancer therapy. However, its antitumor activity shows a limited single-agent effect. Therefore, macrophages in response to M-CSF interruption are pending for further investigation. To achieve this study, bone marrow derived macrophages were generated in vitro by M-CSF stimulation for 7 days and then continuously grown until day 21 in M-CSF absence. A selective pressure for cell survival was initiated after withdrawal of M-CSF. The surviving cells were more prone to M2-like phenotype, even after receiving interleukin-4 (IL-4) stimulation. The transcriptome analysis unveiled that endogenous CSF-1 level was dramatically up-regulated and numerous genes downstream to CSF-1 covering tumor necrosis factor (TNF), ras-related protein 1 (Rap1) and phosphatidylinositol 3-kinase (PI3K)-protein kinase B (AKT) signaling pathway were significantly modulated, especially for proliferation, migration and adhesion. Moreover, the phenomenal increase of miR-21-5p and genes related to pro-tumor activity were observed in parallel. In summary, withholding of CSF-1/CSF-1R interaction would rather augment than suspend the M-CSF-driven pro-tumor activities of M2 macrophages in a long run.

Glia Maturation Factor (GMF) Regulates Microglial Expression Phenotypes and the Associated Neurological Deficits in a Mouse Model of Traumatic Brain Injury

Traumatic brain injury (TBI) induces inflammatory responses through microglial activation and polarization towards a more inflammatory state that contributes to the deleterious secondary brain injury. Glia maturation factor (GMF) is a pro-inflammatory protein that is responsible for neuroinflammation following insult to the brain, such as in TBI. We hypothesized that the absence of GMF in GMF-knockout (GMF-KO) mice would regulate microglial activation state and the M1/M2 phenotypes following TBI. We used the weight drop model of TBI in C57BL/6 mice wild-type (WT) and GMF-KO mice. Immunofluorescence staining, Western blot, and ELISA assays were performed to confirm TBI-induced histopathological and neuroinflammatory changes. Behavioral analysis was done to check motor coordination ability and cognitive function. We demonstrated that the deletion of GMF in GMF-KO mice significantly limited lesion volume, attenuated neuronal loss, inhibited gliosis, and activated microglia adopted predominantly anti-inflammatory (M2) phenotypes. Using an ELISA method, we found a gradual decrease in pro-inflammatory cytokines (TNF-α and IL-6) and upregulation of anti-inflammatory cytokines (IL-4 and IL-10) in GMF-KO mice compared with WT mice, thus, promoting the transition of microglia towards a more predominantly anti-inflammatory (M2) phenotype. GMF-KO mice showed significant improvement in motor ability, memory, and cognition. Overall, our results demonstrate that GMF deficiency regulates microglial polarization, which ameliorates neuronal injury and behavioral impairments following TBI in mice and concludes that GMF is a regulator of neuroinflammation and an ideal therapeutic target for the treatment of TBI.

Microglia and Monocytes/Macrophages Polarization Reveal Novel Therapeutic Mechanism against Stroke

Stroke is a leading cause of morbidity and mortality worldwide, and consists of two types, ischemic and hemorrhagic. Currently, there is no effective treatment to increase the survival rate or improve the quality of life after ischemic and hemorrhagic stroke in the subacute to chronic phases. Therefore, it is necessary to establish therapeutic strategies to facilitate functional recovery in patients with stroke during both phases. Cell-based therapies, using microglia and monocytes/macrophages preconditioned by optimal stimuli and/or any therapies targeting these cells, might be an ideal therapeutic strategy for managing stroke. Microglia and monocytes/macrophages polarize to the classic pro-inflammatory type (M1-like) or alternative protective type (M2-like) by optimal condition. Cell-based therapies using M2-like microglia and monocytes/macrophages might be protective therapeutic strategies against stroke for three reasons. First, M2-like microglia and monocytes/monocytes secrete protective remodeling factors, thus prompting neuronal network recovery via tissue (including neuronal) and vascular remodeling. Second, these cells could migrate to the injured hemisphere through the blood–brain barrier or choroid–plexus. Third, these cells could mitigate the extent of inflammation-induced injuries by suitable timing of therapeutic intervention. Although future translational studies are required, M2-like microglia and monocytes/macrophages therapies are attractive for managing stroke based on their protective functions.

The Mincle ligand trehalose dibehenate differentially modulates M1-like and M2-like macrophage phenotype and function via Syk signaling

Introduction

Macrophages play a significant role in the progression of diseases, such as cancer, making them a target for immune‐modulating agents. Trehalose dibehenate (TDB) is known to activate M1‐like macrophages via Mincle, however, the effect of TDB on M2‐like macrophages, which are found in the tumor microenvironment, has not been studied.

Methods

qRT‐PCR, flow cytometry, cytokine ELISA, and Western Blotting were used to study the effect of TDB on GM‐CSF and M‐CSF/IL‐4 derived bone marrow macrophages (BMMs) from C57BL/6 and Mincle^−/− mice.

Results

TDB treatment up‐regulated M1 markers over M2 markers by GM‐CSF BMMs, whereas M‐CSF/IL‐4 BMMs down‐regulated marker gene expression overall. TDB treatment resulted in Mincle‐independent down‐regulation of CD11b, CD115, and CD206 expression by GM‐CSF macrophages and CD115 in M‐CSF/IL‐4 macrophages. GM‐CSF BMMs produced of significant levels of proinflammatory cytokines (IL‐1β, IL‐6, TNF‐α), which was Mincle‐dependent and further enhanced by LPS priming. M‐CSF BMMs produced little or no cytokines in response to TDB regardless of LPS priming. Western blot analysis confirmed that the absence of cytokine production was associated with a lack of activation of the Syk kinase pathway.

Conclusion

This study illustrates that TDB has the potential to differentially regulate M1‐ and M2‐like macrophages in the tumor environment.

Microglial/Macrophage Polarization Dynamics following Traumatic Brain Injury

Activated microglia and macrophages exert dual beneficial and detrimental roles after central nervous system injury, which are thought to be due to their polarization along a continuum from a classical pro-inflammatory M1-like state to an alternative anti-inflammatory M2-like state. The goal of the present study was to analyze the temporal dynamics of microglia/macrophage polarization within the lesion micro-environment following traumatic brain injury (TBI) using a moderate-level controlled cortical impact (CCI) model in mice. We performed a detailed phenotypic analysis of M1- and M2-like polarized microglia/macrophages, as well as nicotinamide adenine dinucleotide phosphate oxidase (NOX2) expression, through 7 days post-injury using real-time polymerase chain reaction (qPCR), flow cytometry and image analyses. We demonstrated that microglia/macrophages express both M1- and M2-like phenotypic markers early after TBI, but the transient up-regulation of the M2-like phenotype was replaced by a predominant M1- or mixed transitional (Mtran) phenotype that expressed high levels of NOX2 at 7 days post-injury. The shift towards the M1-like and Mtran phenotype was associated with increased cortical and hippocampal neurodegeneration. In a follow up study, we administered a selective NOX2 inhibitor, gp91ds-tat, to CCI mice starting at 24 h post-injury to investigate the relationship between NOX2 and M1-like/Mtran phenotypes. Delayed gp91ds-tat treatment altered M1-/M2-like balance in favor of the anti-inflammatory M2-like phenotype, and significantly reduced oxidative damage in neurons at 7 days post-injury. Therefore, our data suggest that despite M1-like and M2-like polarized microglia/macrophages being activated after TBI, the early M2-like response becomes dysfunctional over time, resulting in development of pathological M1-like and Mtran phenotypes driven by increased NOX2 activity.

Microglia in the TBI brain: The good, the bad, and the dysregulated

As the major cellular component of the innate immune system in the central nervous system (CNS) and the first line of defense whenever injury or disease occurs, microglia play a critical role in neuroinflammation following a traumatic brain injury (TBI). In the injured brain microglia can produce neuroprotective factors, clear cellular debris and orchestrate neurorestorative processes that are beneficial for neurological recovery after TBI. However, microglia can also become dysregulated and can produce high levels of pro-inflammatory and cytotoxic mediators that hinder CNS repair and contribute to neuronal dysfunction and cell death. The dual role of microglial activation in promoting beneficial and detrimental effects on neurons may be accounted for by their polarization state and functional responses after injury. In this review article we discuss emerging research on microglial activation phenotypes in the context of acute brain injury, and the potential role of microglia in phenotype-specific neurorestorative processes such as neurogenesis, angiogenesis, oligodendrogenesis and regeneration. We also describe some of the known molecular mechanisms that regulate phenotype switching, and highlight new therapeutic approaches that alter microglial activation state balance to enhance long-term functional recovery after TBI. An improved understanding of the regulatory mechanisms that control microglial phenotypic shifts may advance our knowledge of post-injury recovery and repair, and provide opportunities for the development of novel therapeutic strategies for TBI.

Microglia and neuroprotection

Microglia were first identified over a century ago, but our knowledge about their ontogeny and functions has significantly expanded only recently. Microglia colonize the central nervous system (CNS) in utero and play essential roles in brain development. Once neural development is completed, microglia function as the resident innate immune cells of the CNS by surveying their microenvironment and becoming activated when the CNS is challenged by infection, injury, or disease. Despite the traditional view of microglia as being destructive in neurological diseases, recent studies have shown that microglia maintain CNS homeostasis and protect the CNS under various pathological conditions. Microglia can be prophylactically activated by modeling infection with systemic lipopolysaccharide injections and these activated microglia can protect the brain from traumatic injury through modulation of neuronal synapses. Microglia can also protect the CNS by promoting neurogenesis, clearing debris, and suppressing inflammation in diseases such as stroke, autism, and Alzheimer's. Microglia are the resident innate immune cells of the CNS. Despite the traditional view of microglia as being destructive in neurological diseases, recent studies have shown that they maintain tissue homeostasis and protect the CNS under various pathological conditions. They achieve so by clearing debris, promoting neurogenesis, suppressing inflammation and stripping inhibitory synapses. This review summarizes recent advances of our understanding on the multi-dimensional neuroprotective roles of microglia.

Distinct role of FoxO1 in M-CSF- and GM-CSF-differentiated macrophages contributes LPS-mediated IL-10: implication in hyperglycemia

Macrophages are a heterogeneous population of immune cells that are essential for the initiation and containment inflammation. There are 2 well-established populations of inflammatory macrophages: classically activated M1 and alternatively activated M2 macrophages. The FoxO family of transcription factors plays key roles in a number of cellular processes, including cell growth, metabolism, survival, and inflammation. In this study, we determined whether the expression of FoxO1 contributes polarization of macrophages toward the M2-like phenotype by enhancing IL-10 cytokine expression. We identified that FoxO1 is highly expressed in M-CSF-derived (M2-like) macrophage subsets, and this M2-like macrophages showed a preferential FoxO1 enrichment on the IL-10 promoter but not in GM-CSF-derived (M1-like) macrophages during classic activation by LPS treatment, which suggests that FoxO1 enhances IL-10 by binding directly to the IL-10 promoter, especially in BMMs. In addition, our data show that macrophages in the setting of hyperglycemia contribute to the macrophage-inflammatory phenotype through attenuation of the contribution of FoxO1 to activate IL-10 expression. Our data identify a novel role for FoxO1 in regulating IL-10 secretion during classic activation and highlight the potential for therapeutic interventions for chronic inflammatory conditions, such as atherosclerosis, diabetes, inflammatory bowel disease, and arthritis.