Macrophage plasticity in experimental atherosclerosis

Abstract

As in human disease, macrophages (MØ) are central players in the development and progression of experimental atherosclerosis. In this study we have evaluated the phenotype of MØ associated with progression of atherosclerosis in the apolipoprotein E (ApoE) knockout (KO) mouse model.We found that bone marrow-derived MØ submitted to M1 and M2 polarization specifically expressed arginase (Arg) II and Arg I, respectively. This distinct arginase expression was used to evaluate the frequency and distribution of M1 and M2 MØ in cross-sections of atherosclerotic plaques of ApoE KO mice. Early lesions were infiltrated by Arg I(+) (M2) MØ. This type of MØ favored the proliferation of smooth muscle cells, in vitro. Arg II(+) (M1) MØ appeared and prevailed in lesions of aged ApoE KO mice and lesion progression was correlated with the dominance of M1 over the M2 MØ phenotype. In order to address whether the M2->M1 switch could be due to a phenotypic switch of the infiltrated cells, we performed in vitro repolarization experiments. We found that fully polarized MØ retained their plasticity since they could revert their phenotype. The analysis of the distribution of Arg I- and Arg II-expressing MØ also argued against a recent recruitment of M1 MØ in the lesion. The combined data therefore suggest that the M2->M1 switch observed in vivo is due to a conversion of cells already present in the lesion. Our study suggests that interventional tools able to revert the MØ infiltrate towards the M2 phenotype may exert an atheroprotective action.

Figure 1. Phenotypic and functional features of M1 and M2 macrophages.

A: The effect of an overnight IFNγ priming step was tested on C57Bl/6 mouse bone marrow-derived MØ subjected to M1- or M2-polarizing conditions. The expression of iNOS, ArgI, ArgII, and Ym1/2 were determined by real time RT-PCR on RNA extracted 10 hours after the induction of polarization. Data were calculated using the 2^−ΔΔCt Pfaffl formula in which experimental conditions (M1 and M2) are compared to Ct values obtained in M0 MØ and normalized to the Ct values of the HPRT house-keeping gene. B: Primary aortic vascular smooth muscle cells (VSMCs) from C57Bl/6 mice were cultured for 48 hours in the presence of media conditioned by C57Bl/6 M1 or M2 MØ which were polarized in the presence of the PPARγ agonist pioglitazone (Pio), the PPARγ antagonist GW9662 (GW), or the arginase inhibitor Nor-NOHA. As controls, VSMCs were also cultured with the same concentration of the polarizing agents, of the PPARγ agonists and antagonists, or of the arginase inhibitor. At the end of the assay, the number of viable cells in each condition was evaluated by the MTT assay and by using a standard curve established with known numbers of cells. *; **: p<0.05; p<0.01 vs matched medium conditioned by MØ polarized in the standard way (−). Note that the number of cells obtained with the M2-conditioned medium were significantly greater than with M1-conditioned medium (p<0.01 vs matched condition).

Figure 2. Kinetic expression of M1 and M2 markers.

Bone marrow-derived MØ from C57BL/6 mice (MØ^B6, closed circles) and ApoE KO mice (MØ^ApoE, open circles) were differentiated, primed with IFNγ and polarized or not (No polarization) towards the M1 (M1 polarization) or the M2 phenotype (M2 polarization). Expression of Arg I, iNOS, Arg II were determined by real time PCR and normalized by HPRT (au: arbitrary unit). IL-6 was monitored in cell culture supernatant by ELISA. Results are representative of three independent experiments.

Figure 3. Macrophages of early atherosclerotic lesions in ApoE KO mice express Arg I while Arg II predominates in late stages.

A: Immunofluorescence of atherosclerotic lesions from 20 and 55 week-old ApoE KO mice. MØ were identified as Mac3⁺ (red) cells. Co-expression of Mac3 and Arg I (violet) and/or Arg II (green) were identified by image overlay. Merge: overlay of bright field, DAPI, Mac3, Arg I and Arg II stainings. Inset: magnification of the zone delimited in the “Merge” frame. B: Arg I, Arg II and Arg I⁺ Arg II⁺ double positive surface areas within 3 random fields/plaque of 20 (white) and 55 (grey) week-old mice. C, D: Regression analysis between the surface area of plaques (X-axis) and total MØ area (Mac3, C) or the Arg I⁺/Arg II⁺ ratio (D). R²: regression correlation coefficient. E: Expression of IL-4 and IFNγ transcripts on microdissected atherosclerotic lesions from 20 and 55 week-old ApoE KO mice determined by real time PCR and normalized by HPRT (au: arbitrary unit). **; ***: p<0.001; p<0.0001 vs 20 w. F–I: Microdissection of aortic atherosclerotic lesions. Lesions can be detected (delimited by black dashes) through the vascular wall in the dissected aortic root (white dashes). The vascular wall was opened longitudinally to expose the luminal side of the vessel (G). The aortic cusps are readily identified. Arrow heads indicate the plane of fracture at the lesion/media interface, which allows the separation of the lesion from the media. The arrows in H indicate the movement performed with tweezers to detach the lesion. Appearance of the vessel after the dissection of the lesion is shown in I.

Figure 4. ApoE modulates the expression of PPARγ.

A: Expression of IL-4Rα1 and of IL-13Rα1 in non-polarized MØ^B6 and MØ^ApoE determined by real time PCR and normalized by HPRT (au: arbitrary unit). B: Expression of PPARγ in non-polarized MØ from MØ^B6 (black) and MØ^ApoE (white) mice with increasing doses of recombinant ApoE (rApoE) determined by real time PCR and normalized by HPRT (au: arbitrary unit). **: p<0.001; ***: p<0.0001 vs MØ^B6. C: Expression of ArgI in M1 and M2 MØ^B6 (black) and MØ^ApoE (white) mice polarized with or without (−) pioglitazone (Pio) or GW9662 (GW) determined by real time PCR. The Results are expressed as 2^−ΔΔCt . *: p<0.05; **: p<0.001 vs matched MØ^B6 condition; †: p<0.05; ††: p<0.01 vs matched M1 condition.

Figure 5. Distribution of Arg I⁺ and Arg II⁺ macrophages in atherosclerotic lesions.

A: A new morphometric method was set up to analyze the distribution of Arg I⁺ and Arg II⁺ MØ based on profiles (n = 10 per lesion, 3 lesions per mouse) outlined accross the lesions between the internal elastic lamina (IEL) and the lumen border. To compare the profiles regardless of the lesion thickness, they were adjusted and represented according to the percentage of the IEL-lumen border distance. Two profiles and are plotted as representative examples. B: Adjusted profiles showing the Arg I and Arg II labeling distributions in the lesion in early (20w) and advanced (55w) aortic cusps of ApoE KO mice.

Figure 6. Preserved plasticity of fully polarized macrophages.

MØ^ApoE (white) and MØ^B6 (black) were primed and subjected to a first M1 or M2 polarization. After ten hours, the culture conditions were switched during 10 additional hours in order to induce the opposite phenotype. Expression of iNOS, Arg I and ArgII was evaluated by real time PCR and normalized by HPRT (au: arbitrary unit). Results are representative of three distinct experiments.