A Critical Role of PCSK9 in Mediating IL-17-Producing T Cell Responses in Hyperlipidemia

Young Uk Kim; Patrick Kee; Delia Danila; Ba-Bie Teng

doi:10.4110/in.2018.19.e41

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Kim, Kee, Danila, and Teng: A Critical Role of PCSK9 in Mediating IL-17-Producing T Cell Responses in Hyperlipidemia

Original Article

Immune Netw 2019;19(6):e41.

Published online: 4 December 2019

DOI: https://doi.org/10.4110/in.2018.19.e41

A Critical Role of PCSK9 in Mediating IL-17-Producing T Cell Responses in Hyperlipidemia

Young Uk Kim¹, Patrick Kee², Delia Danila², Ba-Bie Teng^3,^4,^*

¹Center for Immunology and Autoimmune Diseases, The Brown Foundation Institute of Molecular Medicine, McGovern Medical School, The University of Texas Health Science Center at Houston, Houston, TX 77030, USA

²Department of Internal Medicine, McGovern Medical School, The University of Texas Health Science Center at Houston, Houston, TX 77030, USA

³Center for Human Genetics, The Brown Foundation Institute of Molecular Medicine, McGovern Medical School, The University of Texas Health Science Center at Houston, Houston, TX 77030, USA

⁴Graduate School of Biomedical Sciences, The University of Texas Health Science Center at Houston, Houston, TX 77030, USA

^*Correspondence to Ba-Bie Teng Center for Human Genetics, The Brown Foundation Institute of Molecular Medicine for the Prevention of Human Diseases, McGovern Medical School, The University of Texas Health Science Center at Houston, 1825 Pressler St., Suite 530D, Houston, TX 77030, USA. E-mail: babie.teng@uth.tmc.edu

Conflict of Interest The authors declare no potential conflicts of interest.

Received 5 September 2019 Revised 12 October 2019 Accepted 15 October 2019

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/4.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

We previously demonstrated that atherogenic Ldlr^−/− Apobec1^−/− (LDb) double knockout mice lacking both low-density lipoprotein receptor (LDLR) and apolipoprotein B mRNA-editing catalytic polypeptide-1 (Apobec1) had increased serum IL-17 levels, with T cell programming shifted towards Th17 cells. In this study, we assessed the role of proprotein convertase subtilisin/kexin type 9 (PCSK9) in T cell programming and atherogenesis. We deleted the Pcsk9 gene from LDb mice to generate Ldlr^−/− Apobec1^−/− Pcsk9^−/− (LTp) triple knockout mice. Atherosclerosis in the aortic sinus and aorta were quantitated. Lymphoid cells were analyzed by flow cytometry, ELISA and real-time PCR. Despite of dyslipidemia, LTp mice developed barely detectable atherosclerotic lesions. The IL-17, was very low in plasma and barely detectable in the aortic sinus in the LTp mice. In the spleen, the number of CD4⁺ CD8⁻ cells and splenocytes were much lower in the LDb mice than LTp mice, whereas, the IL-17-producing cells of γδ TCR⁺ T cells and effector memory CD4⁺ T cells (CD44 ^hi CD4⁺) in the spleen were significantly higher in the LDb mice than in the LTp mice. The Rorc mRNA expression levels were elevated in LDb mice compared to LTp mice. When re-stimulated with an anti-CD3 Ab, CD44 ^hi CD4⁺ T cells from LDb mice secreted more IL-17 than those from LTp mice. T cells from LDb mice (with PCSK9) produce more IL-17 at basal and stimulated conditions when compared with LTp mice (without PCSK9). Despite the dyslipidemic profile and the lack of LDLR, atherogenesis is markedly reduced in LTp mice. These results suggest that PCSK9 is associated with changes in T cell programming that contributes to the development of atherosclerosis.

Keywords: PCSK9, IL-17, Atherosclerosis, T-cells

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Figure 1.

The effect of PCSK9 on atherosclerosis development and on the levels of plasma cholesterol and triglyceride in LDb and LTp male mice at 5 months of age. (A) The atherosclerotic lesions using en face quantification method on the aorta of LDb (n=10) and LTp (n=10) mice are expressed as % lesions, which were calculated as the ratio of aortic surface covered by plaques (in square millimeters) divided by the total surface of the whole aorta (in square millimeters). The results of % plaques of each aorta and mean±SD are shown. (B) The plaque area (in square microns) of each aortic sinus (LDb=5 and LTp=6) were quantified using AxioVision release 4.8 software (Zeiss). Each data point represents an average of 6 sections of aortic sinus for 1 animal. The plaque size (µm²) of each animal and mean±SD are shown. We used 2-tailed unpaired t-test with Welch's correction to analyze the difference between LDb versus LTp mice. The p values are shown. (C) The concentrations (mg/dl) of plasma cholesterol and triglyceride in C57BL/6J (n=7), LDb(n=7), and LTp (n=7) are presented. The mean±SD of each group is shown. Statistical analysis were performed using 2-tailed unpaired t-tests with Welch's correction. The p values are listed. One-way ANOVA was also used to analyze the results.

Figure 2.

Analysis of T cell in the thymus and the spleen of C57BL/6J, LDb and LTp mice. (A) A representative result of flow cytometry analysis showing the frequencies of T cell populations in the thymus of WT (C57BL/6J), LDb and LTp mice is shown. (B) The frequencies of CD4⁺ CD8⁻ T cells from the thymus of WT (C57BL/6J), LDb and LTp mice (male mice at 5-months of age, n=6 for each strain) were analyzed by flow cytometry. The results of the analyses and mean±SD are shown. (C) The total number of splenocytes in the spleen of WT (C57BL/6J), LDb and LTp mice (male mice at 5-months of age, n=6 for each strain) were analyzed by flow cytometry. The results of the analyses and mean±SD are shown. (D) The total number of CD4⁺ T cells in the spleen of WT (C57BL/6J), LDb and LTp mice (male mice at 5-months of age, n=6 for each strain) were analyzed by flow cytometry. The results of the analyses and mean±SD are shown. Statistical analysis of results was performed using 2-tailed unpaired t-tests with Welch's correction. The p values are listed. The p<0.05 is considered significantly. ns, not significant.

Figure 3.

The effect of PCSK9 on IL-17 in C57BL/6J (WT), LDb and LTp mice. (A) LTp mice had barely detectable plasma levels of IL-17. IL-17 levels in serum obtained from C57BL/6J (n=4), LDb (n=5), and LTp (n=5) mice were measured by ELISA and expressed as pg/ml. (B) LTp mice had barely detectable expression of IL-17 in the aortic sinus. Aortic sinus sections obtained from LDb (n=6) and LTp (n=6) mice were stained with isotype control or polyclonal anti-IL-17 (red) and DAPI (blue). We quantified the intensity of immunofluorescence images expressing IL-17 from each section. LDb mice show increased expression of IL-17 in the aortic sinus, compared to LTp mice. The densities of the expressions of IL17 were determined. (C). LTp mice had decreased numbers of γδ TCR⁺ T cells, compared to LDb mice. The absolute numbers of γδ TCR⁺ T cells in the spleen of WT (C57BL/6J, n=6), LDb (n=6), and LTp (n=6) mice were analyzed by flow cytometry. (D) LTp mice had decreased % of γδ TCR⁺ T cells, compared to LDb mice. The frequency of γδ TCR⁺ T cells in the spleen of WT (C57BL/6J, n=6), LDb (n=6), and LTp (n=6) mice were analyzed by flow cytometry. All results are presented as mean±SD in this figure. Statistical analysis of the esults was performed using 2-tailed unpaired t-tests with Welch's correction. The p values are listed. The p<0.05 is considered significantly. ns, not significant.

Figure 4.

Deficiency of PCSK9 reduces the expression and secretion of IL-17 from Th subset cells (CD44 ^hi CD4⁺) obtained from the spleens of C57BL/6J (WT), LDb, and LTp mice. (A) The frequencies of CD44 ^hi CD4⁺ T cells from the spleen of WT (C57BL/6J), LDb, and LTp mice were analyzed using flow cytometry. The results of the frequencies of CD44 ^hi CD4⁺ T cells and mean±SD are shown on the right. (B) The frequencies of IFN-γ and IL-17 expressing cells from CD44 ^hi CD4⁺ T cells in the spleen of WT (C57BL/6J, n=6), LDb (n=6), and LTp (n=6) mice are analyzed by flow cytometry. The frequencies and total number of IFN-γ and IL-17 expressing cells from CD44 ^hi CD4⁺ T cells are presented and mean±SD are also shown. (C) The relative expression levels of mRNA transcripts of Rorc and Tbx21 genes from the flow cytometry sorted CD44 ^hi CD4⁺ T cells of the spleen of WT (C57BL/6J), LDb, and LTp mice (n=6 from each strain) were determined using real-time qPCR. The sorted CD44 ^hi CD4⁺ T cells were stimulated with anti-CD3 (1 µg/ml) for 4 h, followed by RNA extraction and qPCR determination. The RNA levels from each animal and the mean±SD are shown. Statistical analyses of results in figures A to C was performed using 2-tailed unpaired t-tests with Welch's correction. The p values are listed. The p<0.05 is considered significantly. ns, not significant. (D) Re-stimulated CD44 ^hi CD4⁺ T cells from LDb mice secret more IL-17. The flow cytometry sorted CD44 ^hi CD4⁺ T cells (2×10⁵ cells) from the spleen of WT (C57BL/6J), LDb, and LTp mice were re-stimulated with different concentrations of anti-CD3 (0, 0.5, and 5 µg/ml) for 3 days. The IL-17 (ng/ml) secreted to the media was measured using ELISA. The results are presented as mean±SD at each concentration. Data represents 3 independent experiments. Statistical analysis was performed using 2-way ANOVA. The p values are listed. ns, not significant.

Figure 5.

A schematic diagram illustrates the role of PCSK9 mediates atherosclerosis via immune cells. This diagram displays the role of PCSK9 in atherosclerosis and immune response mediated via IL-17 cells and other immune cells including Th1, Treg and Tfh cells. Under normal lipidemic condition with low PCSK9 levels, the hepatocytes produce and secrete normal LDLs, these LDLs do not influence the development of atherosclerosis. Under hyperlipidemia condition with increased PCSK9 levels, the hepatocytes produce increased amounts of modified LDLs, which are cholesterol ester and phospholipid enriched. These modified LDLs contribute to the development of atherosclerosis by possibly altering T cells programing shifted towards IL-17 producing T cells to increase IL-17 production or via induce cytokines production to modulating immune cells including Th1, Treg or Tfh T cells to influence IL-17 production. Increased IL-17 contributes to the development of atherosclerosis.

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