Regulation of the formyl peptide receptor 1 (FPR1) gene in primary human macrophages

Abstract

The formyl peptide receptor 1 (FPR1) is mainly expressed by mammalian phagocytic leukocytes and plays a role in chemotaxis, killing of microorganisms through phagocytosis, and the generation of reactive oxygen species. A large number of ligands have been identified triggering FPR1 including formylated and non-formylated peptides of microbial and endogenous origin. While the expression of FPR1 in neutrophils has been investigated intensively, knowledge on the regulation of FPR1 expression in polarized macrophages is lacking. In this study we show that primary human neutrophils, monocytes and resting macrophages do express the receptor on their cell surface. Polarization of macrophages with IFNγ, LPS and with the TLR8 ligand 3M-002 further increases FPR1 mRNA levels but does not consistently increase protein expression or chemotaxis towards the FPR1 ligand fMLF. In contrast, polarization of primary human macrophages with IL-4 and IL-13 leading to the alternative activated macrophages, reduces FPR1 cell surface expression and abolishes chemotaxis towards fMLF. These results show that M2 macrophages will not react to triggering of FPR1, limiting the role for FPR1 to chemotaxis and superoxide production of resting and pro-inflammatory M1 macrophages.

Figure 1. FPR1 cell surface expression on human neutrophils, monocytes, and macrophages.

FACS analysis was performed to investigate cell surface expression of FPR1. A) Neutrophils. B) Monocytes. C) 9 day old macrophages. Autofluorescence of the cells is shown in light grey, the isotype control in grey and cells labeled with FPR1 Ab in black. D) Quantitative representation of the FPR1 median fluorescence intensity (antibody MFI minus isotype MFI) NG: neutrophils (n = 3). MO: monocytes (n = 5). MA: 9 day old macrophages (n = 9).

Figure 2. Regulation of FPR1 mRNA expression in human macrophages.

A) Relative mRNA expression of FPR1 after stimulation of human macrophages with different stimuli for 24 hours. IL-3 (20 ng/ml), IL-4 (10 ng/ml), IL-13 (10 ng/ml), IL-1β (5 ng/ml), IL-6 (10 ng/ml), INFγ (50 ng/ml), TNFα (1 ng/ml), CpG (100 ng/ml), LPS (100 ng/ml), Poly I:C (1 ng/ml), 3M-001 (3 µM), 3M-002 (3 µM). B) Time-course experiment of FPR1 mRNA expression in macrophages stimulated with IL-4 (10 ng/ml) (dotted) and IL-13 (10 ng/ml) (white) for 24, 48 and 72 hours. The values are normalized for GAPDH mRNA expression and are presented relative to non-stimulated control macrophages (black). Bars display the mean and the standard deviation (±SD) of three independent experiments. *p<0.05, **p<0.01.

Figure 3. Regulation of FPR1 cell surface expression in human macrophages.

FACS analysis was performed to investigate cell surface expression of FPR1 upon treatment with the indicated stimuli. A) FPR1 protein expression after stimulation of human macrophages for 48 hours with stimuli which were shown to regulate FPR1 mRNA expression (IL-4 (10 ng/ml), IL-13 (10 ng/ml), INFγ (50 ng/ml), LPS (100 ng/ml), 3M-002 (3 µM)). B) Time-course experiment of FPR1 protein expression in controls macrophages (black), or macrophages stimulated with IL-4 (10 ng/ml) (dotted) and IL-13 (10 ng/ml) (white) for 24, 48 and 72 hours. Values are presented relative to unstimulated macrophages. Bars display the mean and the standard deviation (±SD) of three independent experiments. *p<0.05, **p<0.01.

Figure 4. fLMF chemotaxis assays with human macrophages.

Control (Ctrl) and IL-4 (10 ng/ml), IL-13 (10 ng/ml), IFNγ (50 ng/ml), LPS (100 ng/ml), and 3M-002 (3 µM) stimulated macrophages were allowed to migrate towards the chemotactic factor fLMF (+). Medium without chemotactic factor (−) was used as control for basal migration in each experiment. Graphs show the mean migration index compared to each individual control (n = 3) and error bars display the standard deviation (±SD). *p<0.05; **p<0.01.