Convenience versus Biological Significance: Are PMA-Differentiated THP-1 Cells a Reliable Substitute for Blood-Derived Macrophages When Studying in Vitro Polarization?

A Biodegradable Mg-Based Alloy Inhibited the Inflammatory Response of THP-1 Cell-Derived Macrophages Through the TRPM7-PI3K-AKT1 Signaling Axis

Mg-based alloys might be ideal biomaterials in clinical applications owing to favorable mechanical properties, biodegradability, biocompatibility, and especially their anti-inflammatory properties. However, the precise signaling mechanism underlying the inhibition of inflammation by Mg-based alloys has not been elucidated. Here, we investigated the effects of a Mg-2.1Nd-0.2Zn-0.5Zr alloy (denoted as JDBM) on lipopolysaccharide (LPS)-induced macrophages. THP-1 cell-derived macrophages were cultured on JDBM, Ti-6Al-4V alloy (Ti), 15% extract of JDBM, and 7.5 mM of MgCl2 for 1 h before the addition of LPS for an indicated time; the experiments included negative and positive controls. Our results showed JDBM, extract, and MgCl2 could decrease LPS-induced tumor necrosis factor (TNF) and interleukin (IL)-6 expression. However, there were no morphologic changes in macrophages on Ti or JDBM. Mechanically, extract and MgCl2 downregulated the expression of toll-like receptor (TLR)-4 and MYD88 compared with the positive control and inhibited LPS-induced nuclear factor-kappa B (NF-κB) and mitogen-activated protein kinase (MAPK) signaling pathways by inactivation of the phosphorylation of IKK-α/β, IKβ-α, P65, P38, and JNK. Additionally, the LPS-induced reactive oxygen species (ROS) expression was also decreased by extract and MgCl2. Interestingly, the expression of LPS-induced TNF and IL-6 could be recovered by knocking down TRPM7 of macrophages, in the presence of extract or MgCl2. Mechanically, the activities of AKT and AKT1 were increased by extract or MgCl2 with LPS and were blocked by a PI3K inhibitor, whereas siRNA TRPM7 inhibited only AKT1. Together, our results demonstrated the degradation products of Mg-based alloy, especially magnesium, and resolved inflammation by activation of the TRPM7-PI3K-AKT1 signaling pathway, which may be a potential advantage or target to promote biodegradable Mg-based alloy applications.

The interaction of aluminium-based adjuvants with THP-1 macrophages in vitro: Implications for cellular survival and systemic translocation

Within clinical vaccinations, recombinant antigens are routinely entrapped inside or adsorbed onto the surface of aluminium salts in order to increase their immunological potency in vivo. The efficacy of these immunisations is highly dependent upon the recognition and uptake of these complexes by professional phagocytes and their subsequent delivery to the draining lymph nodes for further immunological processing. While monocytes have been shown to internalise aluminium adjuvants and their adsorbates, the role of macrophages in this respect has not been fully established. Furthermore, this study explored the interaction of THP-1 macrophages with aluminium-based adjuvants (ABAs) and how this relationship influenced the survival of such cells in vitro. THP-1 macrophages were exposed to low concentrations of ABAs (1.7 μg/mL Al) for a maximum of seven days. ABA uptake was determined using lumogallion staining and cell viability by both DAPI (4',6-diamidino-2-phenylindole) staining and LDH (lactate dehydrogenase) assay. Evidence of ABA particle loading was identified within cells at early junctures following treatment and appeared to be quite prolific (>90% cells positive for Al signal after 24 h). Total sample viability (% LDH release) in treated samples was predominantly similar to untreated cells and low levels of cellular death were consistently observed in populations positive for Al uptake. It can thus be concluded that aluminium salts can persist for some time within the intracellular environment of these cells without adversely affecting their viability. These results imply that macrophages may play a role in the systemic translocation of ABAs once administered in the form of an inoculation.

Activation profiles of monocyte-macrophages and HDL function in healthy women in relation to menstrual cycle and in polycystic ovary syndrome patients

PURPOSE

Hormonal status and menopause affect human macrophage function and cardiometabolic risk. In polycystic ovary syndrome (PCOS) patients the cardiometabolic risk increases through mechanisms that are largely unknown. We tested the hypotheses that macrophage activation is influenced by menstrual cycle and that ovarian dysfunction in PCOS patients is associated with altered macrophage inflammatory responses and cholesterol efflux capacity of serum HDL.

METHODS

Blood samples were obtained in the follicular and luteal phases from cycling women (n = 10) and on a single visit from PCOS patients with ovarian dysfunction (n = 11). Monocyte-derived macrophage activation and monocyte subsets were characterized ex vivo using flow cytometry. The capacity of HDL to promote cell cholesterol efflux through the main efflux pathways, namely aqueous diffusion, ATP-binding cassette A1 and G1, was also evaluated.

RESULTS

Hormone and metabolic profiles differed as expected in relation to menstrual cycle and ovulatory dysfunction. Overall, macrophage responses to activating stimuli in PCOS patients were blunted compared with cycling women. Macrophages in the follicular phase were endowed with enhanced responsiveness to LPS/interferon-γ compared with the luteal phase and PCOS. These changes were not related to baseline differences in monocytes. HDL cholesterol efflux capacity through multiple pathways was significantly impaired in PCOS patients compared to healthy women, at least in part independent from lower HDL-cholesterol levels.

CONCLUSIONS

Regular menstrual cycles entailed fluctuations in macrophage activation. Such dynamic pattern was attenuated in PCOS. Along with impaired HDL function, this may contribute to the increased cardiometabolic risk associated with PCOS.

Quantitative 1H NMR Metabolomics Reveal Distinct Metabolic Adaptations in Human Macrophages Following Differential Activation

Macrophages (MΦs) are phagocytic immune cells that are found in nearly all human tissues, where they modulate innate and adaptive immune responses, thereby maintaining cellular homeostasis. MΦs display a spectrum of functional phenotypes as a result of microenvironmental and stress-induced stimuli. Evidence has emerged demonstrating that metabolism is not only crucial for the generation of energy and biomolecular precursors, but also contributes to the function and plasticity of MΦs. Here, 1D ¹H NMR-based metabolomics was employed to identify metabolic pathways that are differentially modulated following primary human monocyte-derived MΦ activation with pro-inflammatory (M1) or anti-inflammatory (M2a) stimuli relative to resting (M0) MΦs. The metabolic profiling of M1 MΦs indicated a substantial increase in oxidative stress as well as a decrease in mitochondrial respiration. These metabolic profiles also provide compelling evidence that M1 MΦs divert metabolites from de novo glycerophospholipid synthesis to inhibit oxidative phosphorylation. Furthermore, glycolysis and lactic acid fermentation were significantly increased in both M1 and M2a MΦs. These metabolic patterns highlight robust metabolic activation markers of MΦ phenotypes. Overall, our study generates additional support to previous observations, presents novel findings regarding the metabolic modulation of human MΦs following activation, and contributes new knowledge to the rapidly evolving field of immunometabolism.

Inhibition of inflammatory-molecule synthesis in THP-1 cells stimulated with phorbol 12-myristate 13-acetate by brefelamide derivatives

Plasma osteopontin (OPN) levels are elevated in tuberculosis patients and may involve granuloma formation. New inhibitors using brefelamide, an aromatic amide isolated from Dictyostelium cellular slime molds that may inhibit OPN transcription in A549 cells at 1 μM concentration, were synthesized as compounds C, D, and E. Their inhibitory activity against OPN synthesis in phorbol 12-myristate 13-acetate (PMA)-stimulated THP-1 cells was confirmed using enzyme-linked immunosorbent assay (ELISA), a multicolor immune-fluorescent microscope, and western blot. In the ELISA performed using full-length OPN, each compound showed significant inhibition in culture supernatants with half maximal inhibitory concentration (IC₅₀) values of 1.6, 1.8, and 2.2 μM for C, D, and E, respectively. In another ELISA to detect the immune-related form of OPN, IC₅₀ values were 0.6, 1.2, and 2.5 μM for compounds C, D, and E, respectively. The decreases in OPN expression and synthesis were confirmed using immunofluorescence and western blot studies using compound-treated cells or cell lysates. Luminex assay of the supernatants of PMA-treated THP-1 cells showed significant reduction in the synthesis of interleukin (IL)-1β, galectin-9, and tumor necrosis factor (TNF)-α. Elucidation of the detailed mechanisms of the biological activities of these compounds would be necessary; however, they may be used in clinical trials for infectious diseases, inflammatory disorders, and cancer.

Blister Fluid Induces MMP-9-Associated M2-Type Macrophages in Bullous Pemphigoid

Bullous pemphigoid (BP) is a cutaneous autoimmune disease, characterized by an inflammatory cascade leading to blister formation. Although macrophages were shown to participate in BP pathophysiology, their role in the blister formation process still needs to be investigated. We here addressed the influence of serum and blister fluid (BF) from patients with BP on the polarization status of macrophages with regards to the metalloproteinase-9 (MMP-9) expression. We demonstrated that several markers related to the alternatively activated macrophage phenotype (M2) including IL-10, TARC, arginase, TNFα, and IL-1RA were meaningfully increased in BF of patients with BP. We further showed that BF, but not serum from patients with BP, significantly induced the expression of CD163, CD206, and IL-10 in BP monocyte-derived macrophages (MDMs). Notably IL-10 was the only cytokine to be correlated to the reference clinical score, BP disease activity index (BPDAI), especially to the inflammatory BPDAI subscore evaluating urticarial and erythematous skin lesions (r = 0.57, p = 0.0004). We also found elevated levels of MMP-9 to M2-type macrophages ex vivo and highlighted the presence of CD163⁺ MMP-9⁺ macrophages histologically, at skin lesional site. Finally, we showed that methylprednisolone reduced MMP-9 levels in MDMs without modifying the other M2 markers. All together these results strongly support the presence of M2-phenotype macrophages with pro-inflammatory properties susceptible to favor blister formation in BP.

Degradation Products of Polydopamine Restrained Inflammatory Response of LPS-Stimulated Macrophages Through Mediation TLR-4-MYD88 Dependent Signaling Pathways by Antioxidant

Polydopamine (PDA) has a promising application as coating of biomaterials due to its favorable degradability and bioadaptability. However, its bioactivity, such as anti-inflammatory capacity, was still little known. Herein, we investigated whether degradable products of PDA could affect inflammatory response in lipopolysaccharide (LPS)-stimulated human THP-1-derived macrophages. The supernatants containing degradation products of PDA, annotated as PDA extracts, were collected after PDA being immersed in cell culture medium for 3 days. Wherein, the composition of the degradation products was analyzed by HPLC assay. Collected PDA extracts were diluted into 100%, 50%, and 25% of original concentration, respectively, to evaluate their anti-inflammatory ability on LPS-induced macrophages from the expression levels of pro-inflammatory cytokines to associated molecular mechanism. Our results showed that the PDA extracts were mainly composed of dopamine, quinine, and PDA segments. Furthermore, macrophages showed no cytotoxicity after PDA extract treatment with or without LPS, while the release levels of TNF-α and IL-6 by LPS-induced macrophages were decreased in dose-dependent by PDA extract treatment. Additionally, TLR-4 and MYD88 expression in protein and RNA level were downregulated by PDA extracts in LPS-induced macrophages. Similarly, PDA extracts effectively inhibited LPS-induced NF-κB trans-locating into nuclear by inactivation of the phosphorylation of IKK-α/β and IKβ-α. Of note, the production of LPS-induced ROS was reduced by PDA extracts in macrophages, while HO-1 expression, a critical protein of antioxidant signaling pathway, was increased. Based on these results, we proposed a potential mechanism by which degradation products of PDA suppressed inflammation of macrophages via downregulation TLR-4-MYD88-NFκB pathway and simultaneous activation HO-1 pathway, which might be a possible therapeutic target.

Cytotoxicity screening of emulsifiers for pulmonary application of lipid nanoparticles

INTRODUCTION

The pulmonary route is a non-invasive administration route that receives growing attention. The challenge for formulation development of orally inhaled formulations is, however, the limited number of approved excipients. Lipid nanoparticles are desired drug delivery systems for inhalation because lipids are biocompatible. However, addition of emulsifiers to stabilize the formulation may cause toxic effects. Alveolar epithelial cells and alveolar macrophages are the main cell types that get in contact with inhaled formulations in the deep lung. The different cell types are supposed to differ in the extent of particle uptake. Kolliphor RH40, Poloxamer 188, and Tween 80 are approved for use in oral formulations and widely used in the academic field for manufacturing of lipid nanoparticles. However, little is known about their pulmonary toxicity.

METHODS

Cytotoxicity of Kolliphor RH40, Poloxamer 188, and Tween 80 was studied by integration into solid lipid nanoparticles loaded with itraconazole as model drug. Cytotoxicity of the formulations was assessed in human alveolar epithelial cells and human and murine macrophages and correlated to cell uptake.

RESULTS

The tested emulsifiers showed overall low cytotoxicity with less pronounced adverse effects in human cells than in murine macrophages. Cellular uptake of Poloxamer 188 containing lipid nanoparticles was decreased in macrophages, while uptake of lipid nanoparticles with the other emulsifiers was similar in epithelial cells and phagocytes.

CONCLUSION

The tested emulsifiers appear suitable for use in pulmonary applications. Due to larger cell size and lower proliferation rate human cells showed lower cytotoxicity than the murine cells. Being human cells, they appear more suitable for the screening of adverse effects in human lungs.

Pro-inflammatory responses induced by A. fumigatus and A. versicolor in various human macrophage models

Exposure to mold-contaminated indoor air has been associated with various respiratory diseases, and there is a need for experimental data to confirm these associations. The pro-inflammatory properties of well-characterized aerosolized spores and hyphal fragments from Aspergillus fumigatus and Aspergillus versicolor were examined and compared using various human macrophage cell models including phorbol 12-myristate 13-acetate (PMA)-differentiated THP-1 macrophages (THP-1 Ma), primary peripheral blood monocyte-derived macrophages (MDM), and primary airway macrophages (AM) from induced sputum. X-ray treated samples of the two mold species induced different responses with A. fumigatus displaying the most potent induction of pro-inflammatory responses. While hyphal fragments from A. fumigatus were more potent than spores, similar responses were produced by the two growth stages of A. versicolor. THP-1 Ma was the most sensitive model releasing a broad range of cytokines/chemokines. MDM exhibited a similar cytokine/chemokine profile as THP-1 Ma, except for a low-quantity release of interleukin-1β (IL-1β). In contrast, AM appeared to be nonresponsive and yielded a different pattern of pro-inflammatory markers. Toll-like receptor (TLR)4, but also to a certain degree TLR2, was involved in several responses induced by spores and aerosolized hyphal fragments of A. fumigatus in MDM. Taken together, MDM seems to be the most promising experimental macrophage model. Abbreviations: AF: A. fumigatus, Aspergillus fumigatus; AV: A. versicolor, Aspergillus versicolor; AM: Airway Macrophage; CBA: Cytometric Bead Array; CD: Cluster of Differentiation; DTT: dithiothreitol; ELISA: Enzyme Linked Immunosorbent Assay; FBS: fetal bovine serum; GM-CSF: Granulocyte macrophage colony-stimulating factor; IL-1β: Interleukin-1beta; MDM: Monocyte-Derived Macrophages; NF-κB: Nuclear Factor kappa light chain enhancer of activated B cells; NLR: NOD-like Receptor; PAMP: Pathogen Associated Molecular Pattern; PMA: Phorbol 12-myristate 13-acetate; PRR: Pattern Recognition Receptor; THP-1: Human leukemia monocyte cell line; TLR: Toll-like Receptor; TNF-α: Tumor Necrosis Factor- alpha.

Annexin A1-mediated inhibition of inflammatory cytokines may facilitate the resolution of inflammation in acute radiation-induced lung injury

The present study evaluated the role of annexin A1 (ANXA1) in the treatment of acute radiation-induced lung injury (RILI) and investigated the mechanism of its action. The expression of ANXA1, interleukin-6 (IL-6) and myeloperoxidase (MPO) in the plasma of patients with RILI prior to and following hormonotherapy was assessed by enzyme-linked immunosorbent assay. The association of plasma ANXA1 concentration with clinical effect, and the correlation between the expression of ANXA1 and that of IL-6 and MPO were evaluated. ANXA1 was overexpressed or knocked down in a macrophage cell line, and its impact on IL-6 and MPO expression was measured. Following glucocorticoid hormonotherapy, patients with RILI exhibited a higher plasma concentration of ANXA1 compared with that prior to treatment, while IL-6 and MPO levels were lower. The concentration of ANXA1 in plasma was negatively correlated with IL-6 and MPO levels, with a correlation coefficient of -0.492 and -0.437, respectively (P<0.001). The increasing concentration of ANXA1 in plasma following treatment was associated with the clinical effect in patients with RILI (P=0.007). The expression levels of of IL-6 and MPO were inhibited both in the cytoplasm and in the culture solution, when ANXA1 expression was upregulated in a macrophage cell line. In conclusion, ANXA1 inhibited the synthesis and secretion of IL-6 and MPO inflammatory cytokines, indicating that ANXA1 may have therapeutic potential as a treatment target for RILI.

Material stiffness influences the polarization state, function and migration mode of macrophages

Biomaterial implantation is followed by an inflammatory cascade dominated by macrophages, which determine implant acceptance or rejection through pro- and anti-inflammatory polarization states (Anderson et al., 2008; Brown and Badylak, 2013). It is known that chemical signals such as bacterial endotoxins and cytokines (IL4) can direct macrophage polarization (Mantovani et al., 2004); however, recent evidence implicates biophysical cues in this process (McWhorter et al., 2015; Patel et al., 2012). Here we report that THP-1 derived macrophages cultured on collagen-coated polyacrylamide gels of varying stiffness adapt their polarization state, functional roles and migration mode according to the stiffness of the underlying substrate. Through gene expression and protein secretion analysis, we show that stiff polyacrylamide gels (323 kPa) prime macrophages towards a pro-inflammatory phenotype with impaired phagocytosis in macrophages, while soft (11 kPa) and medium (88 kPa) stiffness gels prime cells towards an anti-inflammatory, highly phagocytic phenotype. Furthermore, we show that stiffness dictates the migration mode of macrophages; on soft and medium stiffness gels, cells display Rho-A kinase (ROCK)-dependent, podosome-independent fast amoeboid migration and on stiff gels they adopt a ROCK-independent, podosome-dependent slow mesenchymal migration mode. We also provide a mechanistic insight into this process by showing that the anti-inflammatory property of macrophages on soft and medium gels is ROCK-dependent and independent of the ligand presented to them. Together, our results demonstrate that macrophages adapt their polarization, function and migration mode in response to the stiffness of the underlying substrate and suggest that biomaterial stiffness is capable of directing macrophage behaviour independent of the biochemical cues being presented to them. The results from this study establish an important role for substrate stiffness in directing macrophage behaviour, and will lead to the design of immuno-informed biomaterials that are capable of modulating the macrophage response after implantation. STATEMENT OF SIGNIFICANCE: Biomaterial implantation is followed by an inflammatory cascade dominated by macrophages, which determine implant acceptance or rejection through pro- and anti-inflammatory polarization states. It is known that chemical signals can direct macrophage polarization; however, recent evidence implicates biophysical cues in this process. Here we report that macrophages cultured on gels of varying stiffness adapt their polarization state, functional roles and migration mode according to the stiffness of the underlying substrate. The results from this study establish an important role for substrate stiffness in directing macrophage behaviour, and will lead to the design of immuno-informed biomaterials that are capable of modulating the macrophage response after implantation.

Functional suppression of macrophages derived from THP-1 cells by environmentally-relevant concentrations of arsenite

Environmental exposure to arsenic is known to induce immunotoxicity. Macrophages are the professional phagocytes that are important in the immune system. In this study, we utilized the macrophages derived from the THP-1 human monocyte cell line as the experimental model to study the functional suppression induced by arsenite (As⁺³), one of the most prevalent forms of inorganic arsenic, at environmentally-relevant concentrations. Apoptosis was observed in the THP-1 derived macrophages treated with 500 nM As⁺³ for 18 h. Suppression of phagocytosis was induced by 18 h As⁺³ treatment starting from 100 nM. Suppressive effects on the production of two pro-inflammatory cytokines, IL-1β and TNF-α, were also found with the treatment of low to moderate doses of As⁺³ in lipopolysaccharides-stimulated THP-1 derived macrophages. The nitric oxide production was also inhibited by As⁺³ treatments, which was negatively correlated with the production of superoxide. Collectively, the results from the study demonstrated that environmentally-relevant concentrations of As⁺³ induced cytotoxicity and suppressed the major cellular functions in THP-1 derived macrophages. The macrophages were showed to be relatively sensitive to As⁺³, and could be the essential target of the toxicity induced by environmental arsenic exposures.

Hyphae fragments from A. fumigatus sensitize lung cells to silica particles (Min-U-Sil): Increased release of IL-1β

Exposure to particulate matter (PM), such as mineral particles and biological particles/components may be linked to aggravation of respiratory diseases, including asthma. Here we report that exposure to Aspergillus fumigatus hyphae fragments (AFH) and lipopolysaccharide (LPS) induced both mRNA synthesis and release of pro-inflammatory interleukin-1 beta (IL-1β) in both human THP-1 monocytes (THP-1 Mo) and phorbol 12-myristate 13-acetate (PMA)-differentiated THP-1 monocytes (THP-1 macrophages; THP-1 Ma); while Min-U-Sil alone enhanced the release of IL-1β only in THP-1 Ma. Co-exposure to LPS or AFH with Min-U-Sil caused a synergistic release of IL-1β when compared to single exposures. In contrast, Min-U-Sil did not markedly change LPS- and AFH-induced release of tumor necrosis factor alpha (TNF-α). The combined exposures did not increase the LPS- and AFH-induced expression of IL-1β mRNA. Notably, the AFH- and LPS-induced IL-1β responses with and without co-exposure to Min-U-Sil in THP-1 Mo were found to be caspase-dependent as shown by inhibition with zYVAD-fmk. Furthermore, co-exposure with AFH and Min-U-Sil resulted in similar synergistic releases of IL-1β in primary human airway macrophages (AM; sputum), peripheral blood monocyte-derived macrophages (MDM) and in the human bronchial epithelial cell line (BEAS-2B). In conclusion, AFH induce both the synthesis and release of IL-1β. However, Min-U-Sil further enhanced the cleavage of the induced pro-IL-1β.

Comparative cytotoxicity of respirable surface-treated/untreated calcium carbonate rock dust particles in vitro

Calcium carbonate rock dust (RD) is used in mining to reduce the explosivity of aerosolized coal. During the dusting procedures, potential for human exposure occurs, raising health concerns. To improve RD aerosolization, several types of anti-caking surface treatments exist. The aim of the study was to evaluate cytotoxicity of four respirable RD samples: untreated/treated limestone (UL/TL), untreated/treated marble (UM/TM), and crystalline silica (SiO₂) as a positive control in A549 and THP-1 transformed human cell lines. Respirable fractions were generated and collected using FSP10 high flow-rate cyclone samplers. THP-1 cells were differentiated with phorbol-12-myristate-13-acetate (20 ng/ml, 48 h). Cells were exposed to seven different concentrations of RD and SiO₂ (0-0.2 mg/ml). RD caused a slight decrease in viability at 24 or 72 h post-exposure and were able to induce inflammatory cytokine production in A549 cells, however, with considerably less potency than SiO₂. In THP-1 cells at 24 h, there was significant dose-dependent lactate dehydrogenase, inflammatory cytokine and chemokine release. Caspase-1 activity was increased in SiO₂- and, on a lesser scale, in TM- exposed cells. To test if the increased toxicity of TM was uptake-related, THP-1 cells were pretreated with Cytochalasin D (CytD) or Bafilomycin A (BafA), followed by exposure to RD or SiO₂ for 6 h. CytD blocked the uptake and significantly decreased cytotoxicity of all particles, while BafA prevented caspase-1 activation but not cytotoxic effects of TM. Only TM was able to induce an inflammatory response in THP-1 cells, however it was much less pronounced compared to silica.

An In Vitro Model of Angiogenesis during Wound Healing Provides Insights into the Complex Role of Cells and Factors in the Inflammatory and Proliferation Phase

Successful vascularization is essential in wound healing, the histo-integration of biomaterials, and other aspects of regenerative medicine. We developed a functional in vitro assay to dissect the complex processes directing angiogenesis during wound healing, whereby vascular cell spheroids were induced to sprout in the presence of classically (M1) or alternatively (M2) activated macrophages. This simulated a microenvironment, in which sprouting cells were exposed to the inflammatory or proliferation phases of wound healing, respectively. We showed that M1 macrophages induced single-cell migration of endothelial cells and pericytes. In contrast, M2 macrophages augmented endothelial sprouting, suggesting that vascular cells infiltrate the wound bed during the inflammatory phase and extensive angiogenesis is initiated upon a switch to a predominance of M2. Interestingly, M1 and M2 shared a pro-angiogenic secretome, whereas pro-inflammatory cytokines were solely secreted by M1. These results suggested that acute inflammatory factors act as key inducers of vascular cell infiltration and as key negative regulators of angiogenesis, whereas pro-angiogenic factors are present throughout early wound healing. This points to inflammatory factors as key targets to modulate angiogenesis. The here-established wound healing assay represents a useful tool to investigate the effect of biomaterials and factors on angiogenesis during wound healing.