The Elevated Secreted Immunoglobulin D Enhanced the Activation of Peripheral Blood Mononuclear Cells in Rheumatoid Arthritis

Abstract

Immunoglobulin D (IgD) is a surface immunoglobulin that is expressed as either membrane IgD (mIgD) or secreted IgD (sIgD). Researchers have shown that sIgD is often elevated in patients with autoimmune diseases. The possible roles of sIgD on the function of peripheral blood mononuclear cells (PBMCs) in rheumatoid arthritis (RA) are still unclear. In this study, we compared the expression of sIgD, mIgD and IgD receptor (IgDR) in RA patients and healthy controls, and investigated the effect of sIgD on the function of PBMCs. We found that the levels of sIgD, mIgD and IgDR were significantly higher in RA patients compared with healthy controls. The concentrations of sIgD were positively correlated with soluble receptor activator of nuclear factor-κB ligand (sRANKL), rheumatoid factor (RF) and C-reactive protein (CRP) in RA patients. Strikingly, IgD could enhance the proliferation of PBMCs and induce IL-1α, IL-1β, TNF-α, IL-6 and IL-10 production from PBMCs. Moreover, the percentage of activated T cell subsets (CD4+CD69+, CD4+CD154+) and activated B cell subsets (CD19+CD23+, CD19+CD21+, CD19+IgD+ and CD19-CD138+) were increased by IgD. The percentage of unactivated T cell subset (CD4+CD62L+) and immature B cell subset (CD19+IgM+IgD-) were decreased by IgD in PBMCs. Furthermore, the expressions of IgDR on T and B cells were significantly increased by treatment with IgD. Our results demonstrate that IgD enhanced the activation of PBMCs, which may contribute to RA pathogenesis. Therefore, IgD could be a potential novel immunotherapeutic target for the management of RA.

Fig 1. Levels of sIgD and mIgD in RA patients and healthy controls.

(A): Serum IgD concentrations were significantly different in RA patients (n = 28) and healthy controls (n = 22) (P < 0.001). (B): A dot plot showing the expression of IgD on T cells and B cells. (C) and (D): Box-plots representing the 10th, 25th, 50th (median), 75th and 90th percentiles of the frequencies of the CD3⁺IgD⁺, CD3⁺CD4⁺IgD⁺ and CD19⁺IgD⁺ (each as a percentage of lymphocytes) in the peripheral blood of healthy controls (n = 18, white bars) and RA patients (n = 20, grey bars). P < 0.05 was considered statistically significant.

Fig 2. The correlation of the serum IgD levels with biomarkers in RA patients.

Pearson correlation analyses were shown for the association of IgD with sRANKL (A), RF (B), CRP (C), ESR (D) and anti-CCP (E).

Fig 3. Expression of IgDR on T cells and B cells in RA patients and healthy controls.

(A): A histogram showing the expression of IgDR on CD3⁺ T cells and CD19⁺ gated B cells. The fluorescence profile of the stained cells was analyzed by flow cytometry. Red curve (a): isotype; blue curve (b): healthy control; orange curve (c): RA patient. (B): A dot plot showing the expression of IgDR on T cells and B cells. (C): Box-plots representing the 10th, 25th, 50th (median), 75th and 90th percentiles of the frequencies of the CD3⁺IgDR⁺, CD3⁺CD4⁺IgDR⁺ and CD19⁺IgDR⁺(each as a percentage of lymphocytes) in the peripheral blood of healthy controls (n = 15, white bars) and RA patients (n = 19, grey bars). P < 0.05 was considered statistically significant.

Fig 4. The stimulating effect of IgD on human PBMCs from RA patients and healthy controls.

PBMCs from RA patients or healthy controls cultured with 0.1, 0.3, 1, 3, 10 μg/ml of IgD in duration [24h (A), 48h (B), 72h(C)]. Data was expressed as mean±standard error of mean (n = 4), SI = absorbance at 450 nm in the cell cultures with drug divided by absorbance in the culture with medium alone.

Fig 5. The effect of IgD on levels of inflammatory cytokines in PBMCs culture supernatant.

PBMC culture supernatants were collected from RA patients and healthy controls and treated with IgD (1, 3, or 10 μg/ml) or PHA (4 μg/ml) for 24 h. Levels of IL-1α(A), IL-1β(B), TNF-α(C), IL-6(D), IL-10 (E) and IL-8(F) were measured by microarray scanner as described in the Materials and Methods. Data represent the mean ± standard error of the mean (n = 4). *P < 0.05, **P < 0.01 vs. the control from healthy controls; ^#<0.05, ^##P <0.01 vs. the control from RA.

Fig 6. The effects of IgD on subsets of T cells in PBMCs from RA patients and healthy controls.

Flow cytometry analysis was used to analyze the effects of IgD (1, 3, or 10 μg/ml) or PHA (4 μg/ml) on activated T cells (CD4⁺CD69⁺, CD4⁺CD154⁺) and unactivated T cells (CD4⁺CD62L⁺). Data are expressed as the mean ± standard error of the mean (n = 8). *P < 0.05, **P < 0.01 vs. the control from healthy controls; ^#P <0.05, ^##P <0.01 vs. the control from RA.

Fig 7. The effects of IgD on subsets of B cells in PBMCs from RA patients and healthy controls.

Flow cytometry analysis was used to analyze the effects of IgD (1, 3, or 10 μg/ml) or PHA (4 μg/ml) on the percentages of activated B cells (CD19⁺CD23⁺, CD19⁺CD21⁺), mature B cells (CD19⁺IgD⁺), plasma cells (CD19^-CD138⁺), and immature B cells (CD19⁺IgM⁺IgD^-). Data are expressed as the mean ± standard error of the mean (n = 8). *P < 0.05, **P < 0.01 vs. the control from healthy controls; ^#P <0.05, ^##P <0.01 vs. the control from RA.

Fig 8. The effects of IgD on IgDR expression in PBMCs from RA patients and healthy controls.

(A): A histogram showing the expression of IgDR on CD3⁺CD4⁺-gated T cells and CD19⁺-gated B cells. The fluorescence profile of the stained cells was analyzed by flow cytometry. Curve a: isotype control; curve b: control group; curve c: IgD (10 μg/ml) group. (B): Effects of IgD treatment (1, 3, or 10 μg/ml) on the percentages of CD3⁺IgDR⁺ T cells, CD3⁺CD4⁺IgDR⁺ T and CD19⁺IgDR⁺ B cells. Data are expressed as the mean ± standard error of the mean (n = 6). *P < 0.05, **P < 0.01 vs. the control from healthy controls; ^#P <0.05, ^##P <0.01 vs. the control from RA.