ZIP8 exacerbates collagen-induced arthritis by increasing pathogenic T cell responses

Abstract

Zinc is a trace element that is essential for immune responses. Therefore, changes in cellular zinc levels in specific immune cells may influence inflammatory autoimmune diseases, such as rheumatoid arthritis (RA). However, the regulation of zinc mobilization in immune cells and its role in the pathogenesis of RA are not fully understood. Thus, we investigated the roles of zinc transporters in RA pathogenesis. We demonstrated that ZIP8 was specifically upregulated in CD4⁺ T cells that infiltrated the inflamed joint and that ZIP8 deficiency in CD4⁺ T cells abrogated collagen-induced arthritis. ZIP8 deficiency dramatically affected zinc influx in effector T cells and profoundly reduced T cell receptor (TCR)-mediated signaling, including NF-κB and MAPK signaling, which are pathways that are involved in T helper (Th) 17 cell differentiation. Taken together, our findings suggest that ZIP8 depletion in CD4⁺ T cells attenuates TCR signaling due to insufficient cellular zinc, thereby reducing the function of effector CD4⁺ T cells, including Th17 cells. Our results also suggest that targeting ZIP8 may be a useful strategy to inhibit RA development and pathogenesis.

Fig. 1. ZIP8 is upregulated in the inflamed joint tissues of CIA mice and RA patients.

a, b Zinc levels in the serum (a) and knee joint synovial fluid (b) of CIA mice and nonimmunized (NI) control mice (N = 10 mice per group). c Representative images of cellular zinc levels in the joint tissues of CIA and NI mice (N = 5 mice per group). c, cartilage; s, synovium. d Relative mRNA levels of ZIP family (zinc importer) and ZNT family (zinc exporter) members in total synovial cells were quantified by qRT-PCR analysis (N = 6 mice per group). The error bars represent ±SD, and unpaired two-tailed Student’s t-tests were performed to determine differences between groups (a–d). e Representative images of immunostaining for MMP3, ZIP8, MTF1, and MT1/MT2 in the synovium of CIA and NI mice (N = 5 mice per group). f Microarray analysis of the indicated ZIP family members in synovial tissues from patients with rheumatoid arthritis (RA; N = 33), patients with osteoarthritis (OA; N = 26), and healthy individuals (HE; N = 20). Microarray data were obtained from the Gene Expression Omnibus database at the National Center for Biotechnology Information (NCBI). The box indicates the 25th and 75th percentiles, with the centerline representing the mean; significance was analyzed by one-way ANOVA followed by Bonferroni’s post hoc comparison. ^*P < 0.05; ^**P < 0.005; ^***P < 0.0005; ns, not significant. Scale bars: 50 μm.

Fig. 2. ZIP8 regulates CIA in mice.

a Cumulative CIA incidence (score 1) on the indicated days after the first immunization of WT, Slc39a8^+/−, Mtf1^+/−, and Mt1^−/−Mt2^−/− mice. Mean ± SEM are shown. b Severity of CIA in the indicated CIA mice, as assessed by measuring the clinical score. c Type II collagen-specific autoantibody production under NI and CIA conditions in the sera of the indicated mice. d Scoring of synovial inflammation (synovitis) in the indicated CIA mice. e Representative images of Ki67 staining to detect proliferating cells (N = 8 mice per group). Scale bars: 50 μm. f Number of blood vessels in the synovium of the indicated mice under NI and CIA conditions. The numbers of mice used were as follows: 20 WT, Slc39a8^+/−, and Mtf1^+/− mice and 10 Mt1^−/−Mt2^−/− mice (a, b, d). Eight mice per group were used in the experiments presented in (c, e, and f). All parameters in (b–f) were determined on day 36 after the first immunization. The values are presented as mean ± 95% CI and were assessed with the Mann-Whitney U test (b, d); the values are presented as mean ± SEM. and were assessed with ANOVA and Bonferroni’s post hoc comparison (d, f). ^*P < 0.05, ^**P < 0.005, ^***P < 0.0005. ns: not significant.

Fig. 3. Among the infiltrated immune cells, ZIP8 is predominantly expressed in CD4⁺ T cells.

a, b Typical immunofluorescence microscopic images of DAPI, ZIP8, and markers for B cells (B220), T cells (CD4), macrophages (CD11b), and fibroblast-like synoviocytes (FLSs, vimentin) in synovial tissues (left) and primary cultures of total synovial cells isolated from CIA mice (right) (a). The percentage of ZIP8-positive cells was determined by immunofluorescence microscopic analysis of primary cultures of total synovial cells isolated from CIA mice (b N = 5 mice). c Flow cytometric analysis of ZIP8 expression on CD4⁺ cells, CD11b⁺ cells, and B220⁺ cells from total synovial cells isolated from the knee joints of CIA mice. The data shown in (a, c) are representative of five independent experiments.

Fig. 4. ZIP8 depletion in CD4⁺ T cells blocks CIA in mice.

a Cumulative CIA incidence on the indicated days after the first immunization of WT (Slc39a8^f/f), Slc39a8^+/−, Slc39a8^f/f;Lysm-Cre, and Slc39a8^f/f;CD4-Cre mice. b Clinical scores of Slc39a8^f/f, Slc39a8^+/−, Slc39a8^f/f;Lysm-Cre, and Slc39a8^f/f;CD4-Cre CIA mice. c Synovitis scoring of the indicated CIA mice. d Representative images of Ki67 staining to detect proliferating cells (N = 5 mice per group). e Type II collagen-specific autoantibody production under NI and CIA conditions in the sera of the indicated mice. f Numbers of blood vessels in the synovial tissues of the indicated mice under NI or CIA conditions. The numbers of mice were 20 (a, b, c) or 8 (e, f) mice per group. All CIA parameters in (b–f) were determined on day 39 after the first immunization. The values are presented as mean ± SEM, and the χ² test was used to analyze the incidence of CIA on day 39 (a); the values are presented as mean ± 95% CI and were assessed with the Mann-Whitney U test (b, c); the values are presented as mean ± SEM and were assessed with ANOVA followed by Bonferroni’s post hoc comparison (e, f). ^*P < 0.05, ^**P < 0.005, ^***P < 0.0005. ns: not significant.

Fig. 5. ZIP8 depletion in CD4⁺ T cells decreases the Th17 cell population in CIA mice.

a, b Flow cytometric analysis of IFN-γ- or IL-17A-producing CD4⁺ T cells in the spleens (a) and lymph nodes (b) of Slc39a8^f/f and Slc39a8^f/f;CD4-Cre mice on day 39 after the first immunization (N ≥ 4 mice per group). c, d Protein (c) and mRNA (d) levels of the indicated cytokines in the CD4⁺ T cells of Slc39a8^f/f and Slc39a8^f/f;CD4-Cre mice on day 39 after the first immunization (N ≥ 6 mice per group). e Populations of Th1 and Th17 cells differentiated from uncommitted CD4⁺ T cells isolated from Slc39a8^f/f and Slc39a8^f/f;CD4-Cre mice. f Tregs in the spleens of Slc39a8^f/f and Slc39a8^f/f;CD4-Cre CIA mice (N ≥ 4 mice per group; upper panel). Treg population differentiated from naive CD4⁺ T cells isolated from Slc39a8^f/f and Slc39a8^f/f;CD4-Cre mice (bottom panel). The data shown in (e) and (f) are representative of three independent experiments. The values are presented as mean ± SEM and were assessed with unpaired two-tailed Student’s t-tests. ^*P < 0.05, ^**P < 0.005. ns: not significant.

Fig. 6. ZIP8 is a key regulator of zinc influx in effector CD4⁺ T cells.

a CD4⁺ T cells were isolated from WT mice (C57BL/6 and DBA/1 J) and stimulated with 5 μg/mL anti-CD3 and anti-CD28 antibodies. The mRNA and protein levels of ZIP8 were determined at the indicated time points. b Representative confocal microscopic images of ZIP8 in primary cultures of mouse CD4⁺ T cells stimulated with anti-CD3 and anti-CD28 antibodies for 24 h. Nuclei were stained with 4,6-diamidino-2-phenylindole (DAPI). c, d Representative immunoblot image (c) and flow cytometric analysis (d) of ZIP8 in isolated CD4⁺ Tnaive and Tem cells. e Analysis of zinc influx in isolated Tnaive and Tem cells from Slc39a8^f/f and Slc39a8^f/f;CD4-Cre mice. The arrow indicates the time of ZnCl₂ treatment (45 μM). The data shown in (a–e) are representative of three independent experiments. The values are presented as mean ± SD. ^*P < 0.05, ^**P < 0.005.

Fig. 7. ZIP8 affects the effector stage of CD4⁺ T cell activation and proliferation.

a–c Tnaive and Tem cells were isolated from WT (Slc39a8^f/f) and cKO (Slc39a8^f/f;CD4-Cre) mice and stimulated with or without anti-CD3 and anti-CD28 antibodies. Representative images of immunoblots used to detect the binding of CD4 to Lck (a), to detect phospho-TCRζ, phospho-Zap70 and Shp-1 (b), and to examine ERK and JNK phosphorylation and IκBα degradation (c). d, e The proliferation of Tnaive and Tem cells from Slc39a8^f/f and Slc39a8^f/f;CD4-Cre mice stimulated with anti-CD3 and anti-CD28 antibodies was measured by [³H]thymidine incorporation (d) and CellTrace Violet (CTV) staining (e). The data in (d) were assessed with unpaired two-tailed Student’s t-tests. ^*P < 0.05, ^**P < 0.005. f The role of ZIP8 in CD4⁺ T cell-mediated RA pathogenesis. Effector CD4⁺ T cell activation and function require ZIP8 upregulation to increase zinc influx. Thus, ZIP8 depletion in CD4⁺ T cells blocked CD4⁺ T cell-mediated RA pathogenesis by inhibiting the supply of zinc to the enlarged CD4⁺ T cells.