Paricalcitol (19-nor-1,25-dihydroxyvitamin D2) and calcitriol (1,25-dihydroxyvitamin D3) exert potent immunomodulatory effects on dendritic cells and inhibit induction of antigen-specific T cells
Introduction
A deficiency of an active form of vitamin D3 in patients suffering from advanced stages of chronic kidney disease (CKD) substantially contributes, among other factors, to the uremia-related immunodeficiency, to bone disease as well as to premature atherosclerosis and cardiovascular morbidity [1], [2], [3], [4]. A biologically active metabolite of vitamin D3, 1,25-dihydroxyvitamin D3 (calcitriol) exerts its effects via the nuclear vitamin D receptor (VDR). Vitamin D receptor has been identified in nearly all tissues and vitamin D is essential for human physiology [5]. Vitamin D3 and its analogues, vitamin D receptor activators (VDRA), in addition to their role in skeletal metabolism, are being increasingly recognized for their potent antiproliferative, prodifferentiative and immunomodulatory activities [6]. In the immune system, VDR is constitutively expressed in antigen-presenting cells (dendritic cells, monocytes and macrophages) and upon activation in T lymphocytes [7], [8], [9].
Since the recognition of the role of VDR activation in the regulation of parathyroid gland activity, synthetic calcitriol has been widely used in prevention and treatment of high-turnover bone disease as a manifestation of secondary hyperparathyroidism in patients with advanced CKD. However, both peroral and parenteral applications predispose to an increase of calcium and phosphate absorption from gut into the organism producing and worsening positive calcium and phosphate balance in patients lacking the renal excretory capacity [10], [11]. Hypercalcemic effect also limits the potential use of vitamin D3 in immunomodulatory indications in non-CKD patients [12].
Therefore, there has been an effort to find the synthetic analogue of calcitriol with sustained suppressive activity for the VDR-regulated parathyroid gene transcription, but without inducing the increase of intestinal absorption of calcium and possibly retaining its immunomodulatory capacity.
Paricalcitol (19-nor-1,25/OH2/D2) is a synthetic analogue of vitamin D approved by FDA for treatment of secondary hyperparathyroidism. Protein binding affinity, tissue distribution and whole body clearance of paricalcitol are equivalent to native vitamin D3, however, active intestinal transport of calcium and phosphate induced by paricalcitol is reduced to about one third compared to calcitriol [25].
Recent observations demonstrated important differences between paricalcitol and calcitriol. Renin suppression was more pronounced with paricalcitol [13]. Calcification process in experimental animals was enhanced by calcitriol and not by paricalcitol [14]. However, so far there has been a limited number of studies addressing the immunomodulatory effects of paricalcitol [15].
Dendritic cells (DCs) were demonstrated to be the primary targets for the immunomodulatory activity of calcitriol, as documented by the inhibition of DC differentiation and maturation, leading to the downregulated expression of MHC-II, costimulatory molecules and IL-12 [16], [17]. DCs, especially myeloid, exist in at least two functionally and phenotypically distinct stages, immature and mature DC. Immature DCs have high endocytic activity, specialize in antigen capture and processing and reside in peripheral tissues. Human mature DCs have high antigen-presenting capacity and T-cell stimulatory capacity due to expression of high levels of antigen-presenting (HLA-DR), adhesion (CD11c) and costimulatory molecules (CD80, CD86) as well as other DC-specific markers, such as CD83 or DC-LAMP [18].
In this study, we evaluated the immunomodulatory effects of paricalcitol and compared them to calcitriol. We focused on studying the in vitro effects of paricalcitol and calcitriol on a broad range of phenotypic and functional characteristics of monocyte-derived DCs and the capacity of DCs to induce antigen-specific T cells and/or regulatory T cells. We report that calcitriol and paricalcitol have comparable immunomodulatory effects on studied immunological parameters. Evidence that paricalcitol retains an immunomodulatory capacity similar to calcitriol could be of great practical importance because of its markedly lower calcemic effect.
Section snippets
Media and cell cultures
Complete culture medium was used for the culture of lymphocytes and dendritic cells and consisted of RPMI 1640 (Cambrex, Verviers, Belgium) supplemented with 10% heat-inactivated fetal bovine serum (Cambrex), 2 mM l-Glutamine (Cambrex), and 1% penicillin/streptomycin (Cambrex). Cells were cultured at 37 °C in a 5% CO2 atmosphere.
DC generation and treatment with VDRAs
Immature monocyte-derived DCs were generated from buffy coat monocytes. Monocytes were separated by negative selection according to manufacturer's instructions (EasySep
Characteristics of DCs generated in the presence of calcitriol and paricalcitol
In the first set of experiments we tested whether the presence of calcitriol or paricalcitol affects the process of differentiation of monocytes into immature DCs.
The presence of calcitriol or paricalcitol during 5 days of DC differentiation slightly decreased yield of immature DCs and their viability. However, the difference was not statistically significant (Fig. 1).
Modulation of DC maturation by VDRAs
Activation of immature DCs by Toll-like receptor agonists, such as lipopolysaccharide (LPS) induces an array of phenotypic and
Discussion
Vitamin D receptor serves as the regulator of gene transcription in many tissues and cells and appropriate activation of this receptor is essential for many biological functions. Consequently, lack of vitamin D is associated not only with bone pathology, but with a broad range of pathological processes and lack of VDR stimulation clearly is unfavorable in terms of clinical status and prognosis [22]. Immune system is an important target of VDR activation. Low vitamin D status increases the
Disclosure
Authors have no financial interest in the information contained in the manuscript.
Acknowledgments
Supported by research grants MSM 0021620812 and MSM 0021620819 from The Czech Ministry of Education and GAUK 7588/2007 from Charles University.
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These authors contributed equally.