Phagocytosis: How Macrophages Tune Their Non-professional Counterparts

Interplay Between Skeletal and Hematopoietic Cells in the Bone Marrow Microenvironment in Homeostasis and Aging

PURPOSE OF THE REVIEW

In this review, we discuss the most recent scientific advances on the reciprocal regulatory interactions between the skeletal and hematopoietic stem cell niche, focusing on immunomodulation and its interplay with the cell's mitochondrial function, and how this impacts osteoimmune health during aging and disease.

RECENT FINDINGS

Osteoimmunology investigates interactions between cells that make up the skeletal stem cell niche and immune system. Much work has investigated the complexity of the bone marrow microenvironment with respect to the skeletal and hematopoietic stem cells that regulate skeletal formation and immune health respectively. It has now become clear that these cellular components cooperate to maintain homeostasis and that dysfunction in their interaction can lead to aging and disease. Having a deeper, mechanistic appreciation for osteoimmune regulation will lead to better research perspective and therapeutics with the potential to improve the aging process, skeletal and hematologic regeneration, and disease targeting.

Diabetic Macrophage Exosomal miR-381-3p Inhibits Epithelial Cell Autophagy Via NR5A2

PURPOSE

To explore the mechanism underlying autophagy disruption in gingival epithelial cells (GECs) in diabetic individuals.

METHODS AND MATERIALS

Bone marrow-derived macrophages (BMDMs) and GECs were extracted from C57/bl and db/db mice, the exosomes (Exo) were isolated from BMDMs. qRT‒PCR and Western blotting were performed to analyse gene expression. The AnimalTFDB database was used to identify relevant transcription factors, and miRNA sequencing was utilised to identify relevant miRNAs with the aid of the TargetScan/miRDB/miRWalk databases. A dual-luciferase assay was conducted to verify intermolecular targeting relationships.

RESULTS

Similar to BMDMs, BMDM-derived Exos disrupted autophagy and exerted proinflammatory effects in GEC cocultures, and ATG7 may play a vital role. AnimalTFDB database analysis and dual-luciferase assays indicated that NR5A2 is the most relevant transcription factor that regulates Atg7 expression. SiRNA-NR5A2 transfection blocked autophagy in GECs and exacerbated inflammation, whereas NR5A2 upregulation restored ATG7 expression and ameliorated ExoDM-mediated inflammation. MiRNA sequencing, with TargetScan/miRDB/miRWalk analyses and dual-luciferase assays, confirmed that miR-381-3p is the most relevant miRNA that targets NR5A2. MiR-381-3p mimic transfection blocked autophagy in GECs and exacerbated inflammation, while miR-381-3p inhibitor transfection restored ATG7 expression and attenuated ExoDM-mediated inflammation.

CONCLUSION

BMDM-derived Exos, which carry miR-381-3p, inhibit NR5A2 and disrupt autophagy in GECs, increasing periodontal inflammation in diabetes.

Human intestinal epithelial cells can internalize luminal fungi via LC3-associated phagocytosis

Background

Intestinal epithelial cells (IECs) are the first to encounter luminal microorganisms and actively participate in intestinal immunity. We reported that IECs express the β-glucan receptor Dectin-1, and respond to commensal fungi and β-glucans. In phagocytes, Dectin-1 mediates LC3-associated phagocytosis (LAP) utilizing autophagy components to process extracellular cargo. Dectin-1 can mediate phagocytosis of β-glucan-containing particles by non-phagocytic cells. We aimed to determine whether human IECs phagocytose β-glucan-containing fungal particles via LAP.

Methods

Colonic (n=18) and ileal (n=4) organoids from individuals undergoing bowel resection were grown as monolayers. Fluorescent-dye conjugated zymosan (β-glucan particle), heat-killed- and UV inactivated C. albicans were applied to differentiated organoids and to human IEC lines. Confocal microscopy was used for live imaging and immuno-fluorescence. Quantification of phagocytosis was carried out with a fluorescence plate-reader.

Results

zymosan and C. albicans particles were phagocytosed by monolayers of human colonic and ileal organoids and IEC lines. LAP was identified by LC3 and Rubicon recruitment to phagosomes and lysosomal processing of internalized particles was demonstrated by co-localization with lysosomal dyes and LAMP2. Phagocytosis was significantly diminished by blockade of Dectin-1, actin polymerization and NAPDH oxidases.

Conclusions

Our results show that human IECs sense luminal fungal particles and internalize them via LAP. This novel mechanism of luminal sampling suggests that IECs may contribute to the maintenance of mucosal tolerance towards commensal fungi.

Protease-activated receptor 2 promotes clearance of Pseudomonas aeruginosa infection by inducing cAMP-Rac1 signaling in alveolar macrophages

Efficient phagocytosis of pathogens by the innate immune system during infectious injury is vital for restoring tissue integrity. Impaired phagocytosis, such as in the case of infection with Pseudomonas aeruginosa, a broad-spectrum antibiotic-resistant Gram-negative bacterium, can lead to a life threatening lung disorder, acute lung injury (ALI). Evidence indicates that loss of protease-activated receptor 2 (PAR2) impaired Pseudomonas aeruginosa clearance leading to non-resolvable ALI, but the mechanism remains unclear. Here, we focused on the alveolar macrophages (AMs), the predominant population of lung-resident macrophages involved in sensing bacteria, to understand their role in PAR2-mediated phagocytosis of Pseudomonas aeruginosa. We found that upon binding Pseudomonas aeruginosa, PAR2-expressing but not PAR2-null AMs had increased cAMP levels, which activated Rac1 through protein kinase A. Activated Rac1 increased actin-rich protrusions to augment the phagocytosis of Pseudomonas aeruginosa. Administration of liposomes containing constitutively active Rac1 into PAR2-null mice lungs rescued phagocytosis and enhanced the survival of PAR2-null mice from pneumonia. These studies showed that PAR2 drives the cAMP-Rac1 signaling cascade that activates Pseudomonas aeruginosa phagocytosis in AMs, thereby preventing death from bacterial pneumonia.

Immune-enhancing effect of water-soluble beta-glucan derived from enzymatic hydrolysis of yeast glucan

Immunostimulants play an important role in the treatment of immunodeficiency. Macrophages are the first line in our immune defense system and play a critical role in the immune response. Therefore, finding new and better substances to induce an immune response by activating macrophages is an attractive research topic, especially in the fields of immunopharmacology and cancer prevention. Keratinocytes actively crosstalk with immune cells during wound repair, so enhancing the function of keratinocytes is also an important part of improving immunity. Beta-glucans are naturally occurring polysaccharides, consisting of d-glucose monomers linked by beta-glycosidic bonds. Several studies have investigated the immunomodulatory effects of beta-glucan, such as its anti-inflammatory and antibacterial properties. However, the use of yeast cell wall glucan has been limited because it is not soluble in water. In this study, we produced low-molecular-weight water-soluble yeast glucan (WSY glucan) and confirmed various aspects of its immune-enhancing effect. The structure of the beta-(1→3) and (1→6) bonds of WSY glucan were confirmed by nuclear magnetic resonance spectroscopy (¹H-NMR) analysis. Our results showed that treatment with WSY glucan significantly and dose-dependently induced the production of inflammatory mediators (prostaglandin E₂ (PGE₂) and nitric oxide (NO)) and pro-inflammatory cytokines (tumor necrosis factor (TNF)-α and interleukin (IL)-6) in macrophages. In addition, WSY glucan treatment showed changes in the morphological structure of the macrophages and promoted phagocytic activity of the macrophages and wound healing in keratinocytes. Based on these results, WSY glucan is considered as a potential candidate for the treatment of diseases related to the weakening of the immune system without the limitation of insolubility.

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Soluble low-molecular-weight beta-glucan, WSY-glucan, was produced through enzymatic hydrolysis.

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WSY glucan significantly and dose-dependently induced the production of pro-inflammatory cytokines (TNF-α and IL-6) and inflammatory mediators (NO and prostaglandin E₂) in macrophages.

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WSY glucan treatment showed changes in the morphological structure of the macrophages and promoted phagocytic activity of the macrophages and wound healing in keratinocytes.

CX3CL1(+) Microparticles-Induced MFG-E8 Enhances Apoptotic Cell Clearance by Alveolar Macrophages

During the resolution phase of acute lung injury, apoptotic cells release CX3CL1 as a “find-me” signal to attract alveolar macrophage transmigration toward apoptotic cells for phagocytosis. However, it is still not clear whether CX3CL1 has pro-phagocytic activity on alveolar macrophage. In this study, we investigated the role of apoptotic NB4 cells-derived CX3CL1(+) microparticles (apo-MP) on the phagocytic activity of NR8383 cells. We demonstrate that exogenous CX3CL1 and apo-MP enhanced the phagocytic activity of NR8383 cells in a CX3 CR1-dependent manner. The apo-MP-enhanced phagocytic activity on NR8383 was attenuated when apo-MP and NR8383 cells were pre-treated with anti-CX3CL1 antibodies and anti-CX3CR1 antibody, respectively, before incubating both for phagocytic assay. Further studies demonstrate that exogenous CX3CL1 and apo-MP also enhanced NR8383 cells in their surface expression and release of MFG-E8 in a CX3CR1 dependent manner. The enhanced phagocytic activity of CX3CL1-treated NR8383 cells was attenuated when NR8383 cells were pre-treated with an anti-MFG-E8 antibody before CX3CL1 treatment. We conclude that apoptotic cell-derived CX3CL1(+) microparticles enhance the phagocytic activity of NR8383 cells by up-regulating their MFG-E8 as a bridge molecule, and these contribute to the formation of phagocytic synapses between apoptotic cells and alveolar macrophages for the subsequent phagocytic clearance of apoptotic cells.

Using blinking optical tweezers to study cell rheology during initial cell-particle contact

Phagocytosis is an important part of innate immunity and describes the engulfment of bacteria and other extracellular objects on the micrometer scale. The protrusion of the cell membrane around the bacteria during this process is driven by a reorganization of the actin cortex. The process has been studied on the molecular level to great extent during the past decades. However, a deep, fundamental understanding of the mechanics of the process is still lacking, in particular because of a lack of techniques that give access to binding dynamics below the optical resolution limit and cellular viscoelasticity at the same time. In this work, we propose a technique to characterize the mechanical properties of cells in a highly localized manner and apply it to investigate the early stages of phagocytosis. The technique can simultaneously resolve the contact region between a cell and an external object (in our application, a phagocytic target) even below the optical resolution limit. We used immunoglobulin-G-coated microparticles with a size of 2 μm as a model system and attached the particles to the macrophages with holographic optical tweezers. By switching the trap on and off, we were able to measure the rheological properties of the cells in a time-resolved manner during the first few minutes after attachment. The measured viscoelastic cellular response is consistent with power law rheology. The contact radius between particle and cell increased on a timescale of ∼30 s and converged after a few minutes. Although the binding dynamics are not affected by cytochalasin D, we observed an increase of the cellular compliance and a significant fluidization of the cortex after addition of cytochalasin D treatment. Furthermore, we report upper boundaries for the length- and timescale, at which cortical actin has been hypothesized to depolymerize during early phagocytosis.

Phosphatidylserine-Gold Nanoparticles (PS-AuNP) Induce Prostate and Breast Cancer Cell Apoptosis

Prostate and breast cancer are the current leading causes of new cancer cases in males and females, respectively. Phosphatidylserine (PS) is an essential lipid that mediates macrophage efferocytosis and is dysregulated in tumors. Therefore, developing therapies that selectively restore PS may be a potential therapeutic approach for carcinogenesis. Among the nanomedicine strategies for delivering PS, biocompatible gold nanoparticles (AuNPs) have an extensive track record in biomedical applications. In this study, we synthesized biomimetic phosphatidylserine-caped gold nanoparticles (PS-AuNPs) and tested their anticancer potential in breast and prostate cancer cells in vitro. We found that both cell lines exhibited changes in cell morphology indicative of apoptosis. After evaluating for histone-associated DNA fragments, a hallmark of apoptosis, we found significant increases in DNA fragmentation upon PS-AuNP treatment compared to the control treatment. These findings demonstrate the use of phosphatidylserine coupled with gold nanoparticles as a potential treatment for prostate and breast cancer. To the best of our knowledge, this is the first time that a phosphatidylserine-capped AuNP has been examined for its therapeutic potential in cancer therapy.

Murder on the Ovarian Express: A Tale of Non-Autonomous Cell Death in the Drosophila Ovary

Throughout oogenesis, Drosophila egg chambers traverse the fine line between survival and death. After surviving the ten early and middle stages of oogenesis, egg chambers drastically change their size and structure to produce fully developed oocytes. The development of an oocyte comes at a cost, the price is the lives of the oocyte’s 15 siblings, the nurse cells. These nurse cells do not die of their own accord. Their death is dependent upon their neighbors—the stretch follicle cells. Stretch follicle cells are nonprofessional phagocytes that spend the final stages of oogenesis surrounding the nurse cells and subsequently forcing the nurse cells to give up everything for the sake of the oocyte. In this review, we provide an overview of cell death in the ovary, with a focus on recent findings concerning this phagocyte-dependent non-autonomous cell death.

Differential engulfment of Staphylococcus aureus and Pseudomonas aeruginosa by monocyte-derived macrophages is associated with altered phagocyte biochemistry and morphology

Knowledge of changes in macrophages following bacterial engulfment is limited. U937-derived macrophages were incubated with Staphylococcus aureus or Pseudomonas aeruginosa. Morphological and biochemical changes in macrophages following host-pathogen interactions were visualized using Scanning Electron Microscopy (SEM) and Fourier-Transform Infrared Spectroscopy (FTIR) respectively. Principal Component Analysis (PCA) was used to assess the variability in the FTIR spectra. Following host-pathogen interactions, survival of S. aureus was significantly lower than P. aeruginosa (P<0.05) and cellular morphology of macrophages was different after incubation with S. aureus compared to P. aeruginosa. Following incubation with S. aureus macrophages were more globular and amorphous in shape whereas long linear pseudopodia were observed following incubation with P. aeruginosa. Distinct FTIR spectra were identified in macrophages post interaction with the different bacteria and PCA analysis demonstrated distinct biochemical differences in the phagocytes following engulfment of the bacteria, with > 99 % of variability in the FTIR spectra explained by the first two principal components. These findings demonstrated that there were clear morphological and biochemical changes in macrophages following engulfment of two different bacterial types suggesting that the biochemical components of the bacterial cell wall influenced the biochemical characteristics and hence the morphology of macrophages in distinct ways.

Are nanotechnological approaches the future of treating inflammatory diseases?

The current treatments for chronic inflammatory diseases cause severe side effects due to nonspecific drug accumulation. Nanotechnology opens the way to new therapeutic strategies that exploit the ability of immune cells, and especially of phagocytes, to internalize nanoparticles. The cellular uptake of nanoparticles requires specific interactions and is affected by the chemical and physical properties of the carriers. Therefore, optimizing these properties is crucial for designing nanodrugs for immunotherapy. In perspective, we discuss the nanoparticle-based approaches that have been proposed to induce tolerance in autoimmune disorders and lessen the symptoms of inflammatory diseases.

Correlated responses for DNA damage, phagocytosis activity and lysosomal function revealed in a comparison between field and laboratory studies: Fathead minnow exposed to tritium

Tritium entering the aquatic environment can confer a whole body internal radiological dose to aquatic organisms. Multiple stressors inherent in natural environments, however, confound estimates for observable radiation specific responses. To disentangle differences between field and laboratory outcomes to tritium exposures, a multivariate analysis comparing biomarkers for radiation exposure at the cellular level with changes in biological processes within tissues is described for fathead minnows (Pimephales promelas). Over tritium activity concentrations up to 180,000 Bq/L, DNA damage in the field were lower than DNA damage in the laboratory. This finding does not support an increase in morbidity of biota in field exposures. Energy deposited by tritium decay produces oxidised free radicals, yet the biological responses in brain, muscle and liver to oxidative stress differed between the studies and were not related to the tritium. For both studies, DNA damage in gonad and blood increased with increased tritium as did the fluorescence associated with lysosomal function in spleen. The studies differed in spleen phagocytosis activity were, in the laboratory but not the field, activity increased with increased tritium-and was correlatd with lysosomal function (Spearman coefficient of 0.98 (p = 0.001). The higher phagocytosis activity in the field reflects exposures to unmeasured factors that were not present within the laboratory. In the laboratory, DNA damage and lysosomal function were correlated: Spearman coefficients of 0.9 (Comet, p = 0.03) and 0.9 (micronuclei, p = 0.08). In the field, DNA damage by the Comet assay, but not by micronucleus frequency, correlated with lysosomal function: Spearman coefficients of 0.91 (Comet, p < 0.001) and 0.47 (micronuclei, p = 0.21). These observations highlight a need for better physiologic understanding of linkages between radiation-induced damage within cells and responses at higher levels of biological organization.

Macrophage Clearance of Apoptotic Cells: A Critical Assessment

As the body continues to grow and age, it becomes essential to maintain a balance between living and dying cells. Macrophages and dendritic cells play a central role in discriminating among viable, apoptotic, and necrotic cells, as selective and efficient phagocytes, without inducing inappropriate inflammation or immune responses. A great deal has been learnt concerning clearance receptors for modified and non-self-ligands on potential targets, mediating their eventual uptake, disposal, and replacement. In this essay, we assess current understanding of the phagocytic recognition of apoptotic cells within their tissue environment; we conclude that efferocytosis constitutes a more complex process than simply removal of corpses, with regulatory interactions between the target and effector cells, which determine the outcome of this homeostatic process.