Association of High Calcitriol Serum Levels and Its Hydroxylation Efficiency Ratio with Disease Risk in SLE Patients with Vitamin D Deficiency

Abstract

Vitamin D (calcidiol) deficiency in systemic lupus erythematosus (SLE) is more frequent than in healthy subjects (HS); it is associated with clinical activity and damage in SLE. Although calcidiol is considered the best indicator of the vitamin D serum status, its deficiency could not reflect its hydroxylation efficiency ratio and calcitriol serum status. This study was aimed at assessing the association of calcidiol and calcitriol serum levels and its hydroxylation efficiency ratio with the risk to clinical and renal disease activities in SLE patients. A cross-sectional study was conducted in 308 SLE and HS women; calcidiol and calcitriol serum levels were evaluated by immunoassays. SLE patients showed lower calcidiol serum levels vs. HS (21.2 vs. 24.2 ng/mL; p < 0.001). Active SLE patients presented higher calcidiol/calcitriol ratio scores vs. inactive SLE patients (2.78 vs. 1.92 pg/ng; p = 0.02), and SLE patients with renal disease activity showed a pattern of calcidiol-deficient levels (19.5 vs. 25.3 ng/mL; p < 0.04) with higher calcitriol levels (47 pg/mL vs. 41.5 pg/mL; p = 0.02) and calcidiol/calcitriol ratio scores (2.13 vs. 1.54 pg/ng; p < 0.02) compared to SLE patients without renal disease activity. Calcidiol levels were negatively correlated with calcitriol levels (r = -0.26; p = 0.001) and urine proteins (mg/dL) (r = -0.39; p < 0.01). Regarding calcitriol levels, it was positively correlated with the blood lymphocyte count (r = 0.30; p < 0.001) and negatively correlated with the glomerular filtration rate (r = -0.28; p = 0.001). Moreover, the calcitriol/calcidiol ratio was positively correlated with urine proteins (r = 0.38; p < 0.01). The calcidiol deficiency (OR = 2.27; 95% CI = 1.15-4.49; p < 0.01), high calcitriol levels (T3^rd, OR = 4.19, 95% CI = 2.23-7.90; p < 0.001), and a high calcitriol/calcidiol ratio score (T3^rd, OR = 5.93, 95% CI: 3.08-11.5; p < 0.001) were associated with the risk for SLE. In conclusion, a pattern of calcidiol deficiency with high calcitriol serum levels and a high vitamin D hydroxylation efficiency ratio was associated with disease risk in SLE patients.

Figure 1

Vitamin D serum metabolites and vitamin D hydroxylation efficiency ratio from SLE patients vs. HS. (a) Calcidiol serum levels of SLE patients (5.1–40.4 ng/mL) vs. HS (14.8–40.2 ng/mL); (b) calcitriol serum levels of SLE patients (22.1–103 pg/mL) vs. HS (8.92–89.8 pg/mL); (c) efficiency of vitamin D hydroxylation by the calcitriol/calcidiol ratio of SLE patients (0.73–10.5 pg/ng) vs. HS (0.30–4.31 pg/ng). Data provided in median (p5^th–p95^th in parenthesis); Mann–Whitney U test. SLE: systemic lupus erythematosus; HS: healthy subjects.

Figure 2

Vitamin D serum metabolites and vitamin D hydroxylation efficiency ratio from SLE patients stratified by renal disease activity. (a) Calcidiol serum levels (ng/mL) from SLE patients with renal activity (3.26–36.6 ng/mL) and no renal activity (12.7–40.4 ng/mL); (b) calcitriol serum levels (pg/mL) from SLE patients with renal activity (19.0–53.2 pg/mL) and no renal activity (21.8–51.2 pg/mL); (c) calcitriol/calcidiol ratio (pg/ng) from SLE patients with renal activity (0.76–15.3 pg/ng) and no renal activity (0.71–3.21 pg/ng). Data provided in median (p5^th–p95^th in parenthesis), Mann–Whitney test. SLE: systemic lupus erythematosus.

Figure 3

Calcitriol serum levels and vitamin D hydroxylation efficiency ratio stratified by the calcidiol reference values. (a) Calcitriol serum levels (pg/mL) from SLE patients vs. HS, (b) Efficiency of vitamin D hydroxylation (pg/ng) from SLE patients vs. HS; (c) calcitriol serum levels (pg/mL) from SLE patients with clinical activity vs. SLE patients with clinical inactivity; (d) efficiency of vitamin D hydroxylation (pg/ng) from SLE patients with clinical activity vs. SLE patients with clinical inactivity; (e) calcitriol serum levels (pg/mL) from SLE patients with renal activity vs. SLE patients with no renal activity, (f) efficiency of vitamin D hydroxylation (pg/ng) from SLE patients with renal activity vs. SLE patients with no renal activity. Data provided in median, Mann–Whitney test. Calcidiol reference values: deficiency (<20 ng/mL), insufficiency (≥20 to <30 ng/mL), and sufficiency (≥30 ng/mL).

Figure 4

Correlations of vitamin D metabolite serum levels and vitamin D hydroxylation efficiency ratio with clinical and renal variables in SLE patients. (a) Correlation of calcidiol serum levels and calcitriol serum levels; (b) correlation of calcidiol serum levels and urine proteins; (c) correlation of calcidiol serum levels and serum calcium; (d) correlation of serum calcium and Mex-SLEDAI; (e) correlation of calcitriol serum levels with blood lymphocyte count; (f) correlation of calcitriol serum levels and GFR; (g) correlation of calcitriol/calcidiol ratio and urine proteins; (h) correlation of GFR with the SLICC ACR-DI score. r: Spearman's correlation coefficient; GFR: glomerular filtration rate.

Figure 5

Association of vitamin D metabolite serum levels and vitamin D hydroxylation efficiency ratio with the SLE and renal disease activity. (a) Association of vitamin D metabolite serum levels and calcitriol/calcidiol ratio with the SLE (SLE patients vs. HS); (b) association of vitamin D metabolite serum levels and calcitriol/calcidiol ratio with the presence of renal activity. HS: healthy subjects. OR: odds ratio, Fisher's exact test, blue (protective OR < 1) and orange (risk OR ≥ 1) graphics, p values < 0.05, 95% confidence interval (CI). Black graphics: not significant differences (p > 0.05) or high confidence intervals. Calcitriol/calcidiol ratio tertiles: T3^rd (≥2.23 to 23.6 pg/ng), T2^nd (≥1.36 to ≤2.23 pg/ng), and T1^st (0.01 to <1.36 pg/ng). Calcidiol reference values: deficiency (<20 ng/mL), insufficiency (≥20 to <30 ng/mL), and sufficiency (≥30 ng/mL). Calcitriol tertiles: T3^rd (≥48.7 to 157.3 pg/mL), T2^nd (≥33.6 to <48.7 pg/mL), and T1^st (0.33 to <33.6 pg/mL).

Figure 6

Biphasic effect of calcitriol on health and autoimmunity. (a) In health, the beneficial effect of calcitriol could be achieved at moderate concentrations presented in the calcidiol sufficiency status. Adequate concentrations of calcidiol and calcitriol (vitamin D sufficiency) allow adequate immunomodulation that improves the immune response and tolerance, favoring polarization towards the tolerogenic Th profiles, reducing the maturation of autoreactive B cells and the antigenic presentation of the dendritic cells, with an increase of IL-10 and TGF-β cytokines. (b) In SLE, lower calcidiol serum levels (vitamin D deficiency) increase the vitamin D hydroxylation efficiency ratio and the extrarenal calcitriol synthesis; this event may occur due to a compensatory mechanism where the immune cell system in a proinflammatory autoimmune context increases the extrarenal hyperactivity of the CYP27B1 enzyme, contributing to an increase in the calcitriol serum concentrations by calcidiol hydroxylation and promotes the exacerbation of the pathological autoimmune response, characterized by a positive feedback loop of the aberrant self-antigenic presentation, the plasma cell differentiation, autoantibody production, autoreactive Th2 polarization, tolerance loss, and damage to tissues.