Profiling target engagement and cellular uptake of cRGD-decorated clinical-stage core-crosslinked polymeric micelles.

Polymeric micelles are increasingly explored for tumor-targeted drug delivery. CriPec® technology enables the generation of core-crosslinked polymeric micelles (CCPMs) based on thermosensitive (mPEG-b-pHPMAmLacn) block copolymers, with high drug loading capacity, tailorable size, and controlled drug release kinetics. In this study, we decorated clinical-stage CCPM with the αvß3 integrin-targeted cyclic arginine-glycine-aspartic acid (cRGD) peptide, which is one of the most well-known active targeting ligands evaluated preclinically and clinically. Using a panel of cell lines with different expression levels of the αvß3 integrin receptor and exploring both static and dynamic incubation conditions, we studied the benefit of decorating CCPM with different densities of cRGD. We show that incubation time and temperature, as well as the expression levels of αvß3 integrin by target cells, positively influence cRGD-CCPM uptake, as demonstated by immunofluorescence staining and fluorescence microscopy. We demonstrate that even very low decoration densities (i.e., 1 mol % cRGD) result in increased engagement and uptake by target cells as compared to peptide-free control CCPM, and that high cRGD decoration densities do not result in a proportional increase in internalization. In this context, it should be kept in mind that a more extensive presence of targeting ligands on the surface of nanomedicines may affect their pharmacokinetic and biodistribution profile. Thus, we suggest a relatively low cRGD decoration density as most suitable for in vivo application.

Design, development and clinical translation of CriPec®-based core-crosslinked polymeric micelles.

Nanomedicines are used to improve the efficacy and safety of pharmacotherapeutic interventions. Unraveling the biological behavior of nanomedicines, including their biodistribution and target site accumulation, is essential to establish design criteria that contribute to superior performance. CriPec® technology is based on amphiphilic methoxy-poly(ethylene glycol)-b-poly[N-(2-hydroxypropyl) methacrylamide lactate] (mPEG-b-pHPMAmLacn) block copolymers, which are designed to upon self-assembly covalently entrap active pharmaceutical ingredients (API) in core-crosslinked polymeric micelles (CCPM). Key features of CCPM are a prolonged circulation time, high concentrations at pathological sites, and low levels of accumulation in the majority of healthy tissues. Proprietary hydrolysable linkers allow for tunable and sustained release of entrapped API, including hydrophobic and hydrophilic small molecules, as well as peptides and oligonucleotides. Preclinical imaging experiments provided valuable information on their tumor and tissue accumulation and distribution, as well as on uptake by cancer, healthy and immune cells. The frontrunner formulation CPC634, which refers to 65 nm-sized CCPM entrapping the chemotherapeutic drug docetaxel, showed excellent pharmacokinetic properties, safety, tumor accumulation and antitumor efficacy in multiple animal models. In the clinic, CPC634 also demonstrated favorable pharmacokinetics, good tolerability, signs of efficacy, and enhanced localization in tumor tissue as compared to conventional docetaxel. PET imaging of radiolabeled CPC634 showed quantifiable accumulation in âˆ¼50 % of tumors and metastases in advanced-stage cancer patients, and demonstrated potential for use in a theranostic setting even when applied at a companion diagnostic dose. Altogether, the preclinical and clinical results obtained to date demonstrate that mPEG-b-pHPMAmLacn CCPM based on CriPec® technology are a potent, tunable, broadly applicable and well-tolerable platform for targeted drug delivery and improved anticancer therapy.

Acoustic Cluster Therapy (ACT®) enhances accumulation of polymeric micelles in the murine brain.

The restrictive nature of the blood-brain barrier (BBB) prevents efficient treatment of many brain diseases. Focused ultrasound in combination with microbubbles has shown to safely and transiently increase BBB permeability. Here, the potential of Acoustic Cluster Therapy (ACT®), a microbubble platform specifically engineered for theranostic purposes, to increase the permeability of the BBB and improve accumulation of IRDye® 800CW-PEG and core-crosslinked polymeric micelles (CCPM) in the murine brain, was studied. Contrast enhanced magnetic resonance imaging (MRI) showed increased BBB permeability in all animals after ACT®. Near infrared fluorescence (NIRF) images of excised brains 1 h post ACT® revealed an increased accumulation of the IRDye® 800CW-PEG (5.2-fold) and CCPM (3.7-fold) in ACT®-treated brains compared to control brains, which was retained up to 24 h post ACT®. Confocal laser scanning microscopy (CLSM) showed improved extravasation and penetration of CCPM into the brain parenchyma after ACT®. Histological examination of brain sections showed no treatment related tissue damage. This study demonstrated that ACT® increases the permeability of the BBB and enhances accumulation of macromolecules and clinically relevant nanoparticles to the brain, taking a principal step in enabling improved treatment of various brain diseases.

Optical imaging of the whole-body to cellular biodistribution of clinical-stage PEG-b-pHPMA-based core-crosslinked polymeric micelles.

Core-crosslinked polymeric micelles (CCPM) based on PEG-b-pHPMA-lactate are clinically evaluated for the treatment of cancer. We macroscopically and microscopically investigated the biodistribution and target site accumulation of CCPM. To this end, fluorophore-labeled CCPM were intravenously injected in mice bearing 4T1 triple-negative breast cancer (TNBC) tumors, and their localization at the whole-body, tissue and cellular level was analyzed using multimodal and multiscale optical imaging. At the organism level, we performed non-invasive 3D micro-computed tomography-fluorescence tomography (µCT-FLT) and 2D fluorescence reflectance imaging (FRI). At the tissue and cellular level, we performed extensive immunohistochemistry, focusing primarily on cancer, endothelial and phagocytic immune cells. The CCPM achieved highly efficient tumor targeting in the 4T1 TNBC mouse model (18.6 %ID/g), with values twice as high as those in liver and spleen (9.1 and 8.9 %ID/g, respectively). Microscopic analysis of tissue slices revealed that at 48 h post injection, 67% of intratumoral CCPM were localized extracellularly. Phenotypic analyses on the remaining 33% of intracellularly accumulated CCPM showed that predominantly F4/80+ phagocytes had taken up the nanocarrier formulation. Similar uptake patterns were observed for liver and spleen. The propensity of CCPM to primarily accumulate in the extracellular space in tumors suggests that the anticancer efficacy of the formulation mainly results from sustained release of the chemotherapeutic payload in the tumor microenvironment. In addition, their high uptake by phagocytic immune cells encourages potential use for immunomodulatory anticancer therapy. Altogether, the beneficial biodistribution, efficient tumor targeting and prominent engagement of PEG-b-pHPMA-lactate-based CCPM with key cell populations underline the clinical versatility of this clinical-stage nanocarrier formulation.

Immune regulatory effects of high dose vitamin D3 supplementation in a randomized controlled trial in relapsing remitting multiple sclerosis patients receiving IFNß; the SOLARIUM study.

Multiple sclerosis (MS) is characterized by a disturbed immune homeostasis and low serum vitamin D levels are associated with an increased disease activity. While vitamin D has been hypothesized to promote the maintenance of immune homeostasis, vitamin D supplementation could be of benefit to patients with MS. The SOLAR study investigated the effects of high dose vitamin D3 supplementation on clinical outcomes in a randomized controlled trial. Here we present the immune regulatory effects, investigated in the SOLARIUM sub-study. Thirty Dutch relapsing remitting (RR) MS patients treated with IFNß-1a received high dose vitamin D3 supplementation and 23 patients received placebo during a period of 48weeks. Lymphocytes were phenotypically characterized by flow cytometry and in vitro cytokine secretion was assessed in the presence or absence of 1,25(OH)2D3 using Luminex technology. Changes in immune regulatory parameters were determined within subjects as well as between treatment groups. The proportion of cells in the immune regulatory cell compartment (nTreg, iTreg and Breg) was not altered upon high dose vitamin D3 supplementation. Proportions of T helper subsets were not affected by vitamin D3, except for the proportion of IL4+ Th cells, which decreased in the placebo but not in the vitamin D3 group. T cell cytokine secretion increased, most pronounced for IL5 and latency activated protein of TGFß, in the placebo group but not in the vitamin D3 group. Lymphocytes remained equally reactive to in vitro 1,25(OH)2D3. In conclusion, high dose vitamin D3 supplementation did not result in a relative increase in lymphocytes with a regulatory phenotype. However, this study supports the hypothesis that vitamin D contributes to the maintenance of immune homeostasis by preventing further disturbance of the T cell compartment early in the disease course of MS.

Induction of an altered CD40 signaling complex by an antagonistic human monoclonal antibody to CD40.

Blocking the interaction of CD40 with its ligand CD154 is a desirable goal of therapies for preventing and/or ameliorating autoimmune diseases and transplant rejection. CD154-blocking mAbs used in human clinical trials resulted in unanticipated vascular complications, leading to heightened interest in the therapeutic potential of antagonist mAbs specific for human CD40. Abs that do not require physical competition with CD154 to inhibit CD40 signaling have particular therapeutic promise. In this study, we demonstrate that the antagonist anti-human CD40 mAb PG102 fails to trigger CD40-mediated activation, as well as impairs CD154-mediated CD40 activation, via a distinct nonstimulatory CD40 signaling mechanism. PG102 did not induce early CD40-induced signaling events, and it inhibited early kinase and transcription factor activation by CD154 or agonist anti-CD40 mAbs. However, PG102 stimulated normal CD40-mediated TNFR-associated factor (TRAF)2 and TRAF3 degradation. PG102 induced the formation of a CD40 signaling complex that contained decreased amounts of both TRAF2 and TRAF3 and TRAF2-associated signaling proteins. Additionally, PG102-induced CD40 signaling complexes failed to recruit TRAF6 to detergent-insoluble membrane fractions. Fab fragments of PG102, while retaining CD40 binding, did not induce TRAF degradation, nor could they inhibit CD154-stimulated B cell signaling, indicating that CD40 aggregation is required for the signaling inhibition induced by PG102. The antagonistic impact of PG102 on CD40 signaling reveals that the manner of CD40 ligation can determine sharply different outcomes for CD40 signaling and suggests that such information can be used to therapeutically manipulate these outcomes.

Circulating vitamin D binding protein levels are not associated with relapses or with vitamin D status in multiple sclerosis.

BACKGROUND: A low vitamin D status has been associated with multiple sclerosis (MS). Most circulating vitamin D metabolites are bound to vitamin D binding protein (DBP). OBJECTIVES: The purpose of this study was to explore whether there is an association between MS and DBP. METHODS: We compared DBP concentrations in blood samples of controls (n = 30) and subjects with relapsing-remitting MS (RRMS) during remission (n = 29) and relapse (n = 15). Furthermore, we explored correlations of DBP with 25- hydroxyvitamin D (25(OH)D) and 1,25-dihydroxyvitamin D levels (1,25(OH)2D), and the effect of high-dose vitamin D3 supplementation on DBP levels in RRMS patients (n = 15). RESULTS: DBP-concentration did not differ between the sub-groups measured, and there was no correlation between DBP and vitamin D metabolite concentration within the physiological range. Upon supplementation of high doses vitamin D3, DBP concentration remained unaltered. After supplementation, serum 1,25(OH)2D(R = 0.517, p = 0.049), but not 25(OH)D, correlated positively with DBP. CONCLUSIONS: We found no association between DBP, MS, and vitamin D status within the physiological range. After high-dose vitamin D supplementation, DBP concentrations may be relevant for vitamin D metabolism.

Characterization of skin-resident T cells using a crawl-out method and flow cytometry.

A large fraction of the skin-homing T-cell population resides in the skin even under resting, non-inflammatory conditions. Here, we used a crawl-out culture method to retrieve T cells from human skin and characterized them using flow cytometric analysis. On average, 48000 viable, non-proliferating cells were retrieved per biopsy. We found that human skin contains a larger fraction of IL-17-, IL-4-, IL-10- and IL-22-positive T cells as compared with paired blood samples. Our research indicates that it is feasible to use the crawl-out method in combination with flow cytometry to characterize T-cell subpopulations in patient-derived skin biopsies. This method enables further study of the skin immune system and could function as a valuable tool for evaluation of the effects of immunotherapy in skin diseases.

Th17 expansion in granulomatosis with polyangiitis (Wegener's): the role of disease activity, immune regulation and therapy.

INTRODUCTION: In autoimmune diseases, IL-17 producing T-cells (Th17), a pro-inflammatory subset of T-cells, are pathophysiologically involved. There is little knowledge on the role of Th17 cells in granulomatosis with polyangiitis (GPA). In the present study, we investigated Th17 cells, Tregs and subsets of circulating Th17 cells in GPA and related results to disease activity. METHODS: 42 GPA patients in remission, 18 with active disease and 14 healthy controls (HC) were enrolled. Th17 cells, their subsets and regulatory T-cells were determined by intracellular fluorescence activated cell sorter (FACS). Data are given as mean percentage ±SD of total T-helper-cells. RESULTS: Th17 cells are expanded in active and quiescent GPA as compared to HC (1.7±1.4% vs. 0.7 ±0.3%, P = 0.006 and 1.9 ±1.5% vs. 0.7 ±0.3%, P<0.0001). Th17 expansion is stable over time and does not decline when remission is achieved. However, a negative association of Th17 cells and steroid dosage is observed (r=-0.46, P = 0.002). The Th17 expansion was not balanced by Tregs as indicated by skewed Th17/Treg ratios in active and quiescent GPA. Th17 subsets co-producing IFNγ or IL-10 are significantly increased in GPA. GPA patients in remission not receiving maintenance therapy have significantly more IL-10/IL-17A double positive T-cells than HC (0.0501 ±0.031% vs. 0.0282 ±0.016%, P = 0.007). CONCLUSIONS: We provide evidence for a persistent, unbalanced expansion of Th17 cells and Th17 subsets which seems to be independent of disease activity. Maintenance therapy reduces -but does not normalize- Th17 expansion.

Intracellular IL-10 detection in T cells by flowcytometry: the use of protein transport inhibitors revisited.

In the past two decades, interleukin-10 (IL-10) has gained much attention as an important regulatory cytokine involved in self-tolerance. Functional assessment of IL-10 producing immune cells is traditionally done by stimulation and measurement of cytokine production by flowcytometry. Thereby a protein transport inhibitor like monensin is used to accumulate the cytokine of interest intracellularly. In this study we elaborated on the monensin effect on cytokine detection and focused on IL-10 detection in human T cells. Peripheral blood mononuclear cells (PBMC) of 32 study subjects were isolated and stimulated with PMA/ionomycin, in the absence and presence of monensin, and stained intracellularly for IFN-γ, IL-4, IL-10 and IL-17A. Our results re-established that detection of IFN-γ+ and IL-4+ T cells benefited from the presence of monensin during stimulation. However, stimulation in the presence of monensin yielded lower proportions of IL-10+ T cells (0.45% (0.28-0.80) versus 0.80% (0.50-1.50) of CD4+ T cells, p<0.01), although monensin addition did result in an increased MFI (2431 (1273-4959) versus 1928 (1147-3760), p<0.01). Detectable fractions of IL-17A+ CD4+ T cells were not affected by monensin. A shorter incubation time, but not lower monensin concentrations, was effective in improving the detection of IL-10+ T cells. We found a strong correlation between the fraction of IL-10+ CD4+ T cells in the presence and absence of monensin (R=0.80 p<0.01). Next to this, also the detection of IL-10+ NK-T cells and IL-10+ monocytes, but not IL-10+ B cells, is impaired in the presence of monensin. This study shows that the effect of monensin on cytokine accumulation is time and cytokine dependent. Due to the use of monensin, previous research may have underestimated the number of IL-10+ leukocytes or may even have not been able to detect them at all. It is important to consider this for future research or when interpreting historical IL-10 data.

Expression of NLRP3 inflammasome and T cell population markers in adipose tissue are associated with insulin resistance and impaired glucose metabolism in humans.

Recent studies in rodents indicate that the nucleotide-binding domain, leucine-rich-containing family, pyrin domain-containing-3 (NLRP3) inflammasome and a proinflammatory shift in the T cell population in adipose tissue (AT) contribute to AT inflammation and insulin resistance. We investigated: (1) the interplay between the NLRP3 inflammasome and T cell populations in abdominal subcutaneous AT in obese and lean humans in relation to AT inflammatory processes, and (2) involvement of the NLRP3 inflammasome and T cell populations in insulin resistance. Abdominal subcutaneous AT biopsies were collected in 10 obese men with impaired glucose tolerance and 9 lean normal glucose tolerant age-matched controls. AT gene expression of NLRP3 inflammasome-related genes and markers of T cell populations, chemoattraction, macrophage infiltration and other aspects of inflammation were examined. Furthermore, we examined systemic adaptive immune activation and insulin sensitivity (hyperinsulinemic-euglycemic clamp). CASPASE-1 mRNA and the proportion of T(h)1 transcripts (TBX21/CD3ɛ) were significantly higher in AT from obese compared with lean subjects. CASPASE-1 expression and a relative increase in T(h)1 transcripts in AT were strongly associated with insulin resistance and impairments in glucose homeostasis. Gene expression of NLRP3, CASPASE-1, CD3ɛ (pan T cells), TBX21 (T(h)1 cells) and RORC (T(h)17 cells) was positively, whereas GATA3 (T(h)2 cells) was inversely correlated with AT inflammation. Our data suggest that NLRP3 inflammasome activation and a T(h)1 shift in the T cell population in AT of obese subjects is related to insulin resistance and impaired glucose metabolism, which may be explained by AT inflammatory processes.

Th17 expansion in MS patients is counterbalanced by an expanded CD39+ regulatory T cell population during remission but not during relapse.

In this study, percentages of CD39(+) Treg and Th17 cells were compared between relapsing-remitting MS patients and controls and were related to the vitamin D status. The Th17 cell population was expanded in about 40% of the MS patients. In MS patients in remission, but not during relapse, a positive association was found between Th17 cell and CD39(+) Treg percentages (r=0.468, p=0.007). Since CD39(+) Tregs have been shown to have Th17 suppressive capacities, we propose that a dysregulated Th17/CD39(+) Treg balance might contribute to disease exacerbation. A clear role for vitamin D in this regulation could not be established.

Reduction in IL-10 producing B cells (Breg) in multiple sclerosis is accompanied by a reduced naïve/memory Breg ratio during a relapse but not in remission.

In this study, we assessed B cell subsets, including Bregs, during stable and active disease in relapsing remitting multiple sclerosis (RRMS) patients and related B cell subsets to vitamin D status. We report that RRMS patients have a decreased percentage of both memory B cells and Bregs compared to healthy controls. During a relapse, the reduction in Bregs involved in particular naïve Bregs. We found no correlation between vitamin D status and B cell subsets. An effect of vitamin D on Bregs cannot be ruled out, since it might be the function that is interfered with instead of relative numbers.

Neutrophils and T cells: bidirectional effects and functional interferences.

Polymorphonuclear cells (PMN) are widely recognized as sophisticated killers during microbial infections. In recent years, PMN have been shown to interact and functionally interfere with other cells of the immune system. In this study, we investigated PMN-T cell interactions in an in vitro co-culture system. A relative increase in T cells in the co-culture was associated with the upregulation of CD66b expression on PMN. In addition, PMN were found to dose-dependently impair anti-CD3 induced CD4(+) T cell activation, proliferation and viability. In a transwell co-culture system, proliferation of T cells was, however, enhanced which illustrates that suppression was contact-dependent. The addition of an arginase-inhibitor or blocking antibodies against calprotectin, but not myeloperoxidase (MPO), partially restored T cell proliferation. Furthermore, the presence of PMN in the co-culture dose-dependently increased the fraction of IFN-γ and IL-17 producing T cells and decreased the percentage of IL-10 producing CD4(+) T cells. Altogether, these data show that there is cross-talk between PMN and T cells which, in non-inflammatory conditions, results in the effects described above. Further studies should investigate PMN-T cell functional interference in inflammatory situations and clarify the importance of this PMN-T cell cross-talk in the regulation of the immune response.

Effect of vitamin D(3) supplementation on peripheral B cell differentiation and isotype switching in patients with multiple sclerosis.

BACKGROUND: Vitamin D has been proposed as a promoter of immune homeostasis in multiple sclerosis (MS). During the past decade, the focus of the effects of vitamin D has been on dendritic cells and on T cells. Since there is an increasing interest in the role of B cells in the pathophysiology of MS, we studied the role of vitamin D on B cells in vivo in patients with MS. OBJECTIVE: We explored the effects of 12 weeks high-dose vitamin D(3) supplementation on peripheral B cell differentiation, immunoglobulin production and levels of B cell activating factor (BAFF) in 15 patients with MS. METHODS: Circulating B cell subsets were characterized by flow cytometry. Plasma immunoglobulin levels were assessed by nephelometry. Plasma BAFF levels were assessed by enzyme-linked immunosorbent assay (ELISA). RESULTS: Although a significant increase serum 25-hydroxyvitamin D was induced, we found no significant shift in B cell differentiation, isotype switching, or plasma BAFF levels. CONCLUSION: In patients with MS, supplementation of high doses vitamin D(3) does not have substantial effects on phenotypic markers of B cell differentiation in circulating B cells. Future studies may unravel more subtle changes in the B cell compartment, either in the circulation or in the central nervous system.

Effects of vitamin D on the peripheral adaptive immune system: a review.

Epidemiological studies have shown that a poor vitamin D status is associated with an increased risk of several diseases, including autoimmune diseases. The immune regulatory function of vitamin D is thought to have an important role in these associations. Cells of the adaptive immune system have shown to be direct targets of the vitamin D metabolites. Besides being direct targets, cells of the adaptive immune system express the enzymes involved in the metabolism of vitamin D, enabling them to locally convert 25(OH)D into its active metabolite 1,25(OH)2D. In this review, the effects of vitamin D on cells of the adaptive immune system are described. Experimental data in vitro show that vitamin D skews cells of the adaptive immune system toward a more tolerogenic status which might be exploited in the treatment of autoimmune diseases. However, it should be noticed that in vivo effects may differ from in vitro effects due to the cross-talk between different vitamin D sensitive cells, but data support the view that vitamin D is positively involved in maintaining or restoring immune homeostasis. Upcoming vitamin D supplementation trials will further elucidate the in vivo effects of vitamin D on the immune system and its potency to serve as an immune regulating agent in autoimmune diseases.

Vitamin D-related gene expression profiles in immune cells of patients with relapsing remitting multiple sclerosis.

An impaired vitamin D (vit-D) processing by immune cells of relapsing remitting multiple sclerosis (RRMS) patients may increase their vulnerability for a poor vit-D status. We assessed with qPCR the expression of vit-D related genes in PBMC and CD4+ T-cells. Gene expression profiles of vit-D receptor (VDR), CYP27B1 and CYP24A1 did not differ between RRMS patients and healthy controls. Interestingly, more VDR expression in PBMC correlated with less circulating IFN-γ+ CD4+ T-cells. Our results suggest that vit-D processing by immune cells is not impaired in RRMS, and is potentially relevant for the composition of the peripheral CD4+ T-cell compartment.

Abdominal fat mass is associated with adaptive immune activation: the CODAM Study.

Abdominal fat-related activation of the innate immune system and insulin resistance (IR) are implicated in the pathogenesis of cardiovascular diseases. Recent data support an important role of the adaptive immune system as well. In this study, we investigate the association between waist circumference and markers of systemic adaptive immune activation, and the potential mediating role of innate immune activation and/or IR herein. The study population consisted of 477 (304 men) individuals (mean age: 59.4 ± 7.0 years) in whom waist circumference, HOMA2-IR (IR derived from homeostasis model assessment), and markers of innate (C-reactive protein (CRP), interleukin (IL)-6, serum amyloid A (SAA)) and adaptive (neopterin, soluble CD25 (sCD25)) immune activation were measured. These markers were compiled into an adaptive and innate immune activation score by averaging the respective z-scores. After adjustments for age, sex, glucose metabolism, smoking status, prior cardiovascular disease, and other risk factors, waist circumference was associated with the adaptive (standardized regression coefficient ß = 0.12 (95% confidence intervals: 0.04-0.20)) and the innate immune activation scores (ß = 0.24 (0.17-0.31)), and with HOMA2-IR (ß = 0.49 (0.42-0.56)). The innate immune activation score and HOMA2-IR were also positively associated with the adaptive immune activation score (ß = 0.31 (0.21-0.40) and ß = 0.11 (0.02-0.21), respectively). The association between waist circumference and the adaptive immune activation score was completely abolished when further adjusted for innate immune activation and HOMA2-IR (to ß = -0.01 (-0.10-0.08)), and the specific mediation "effects" attributable to each of these variables were 58% and 42%, respectively. We conclude that abdominal obesity is associated with systemic adaptive immune activation and that innate immune activation and IR constitute independent and equally important pathways explaining this association.

Safety and T cell modulating effects of high dose vitamin D3 supplementation in multiple sclerosis.

BACKGROUND: A poor vitamin D status has been associated with a high disease activity of multiple sclerosis (MS). Recently, we described associations between vitamin D status and peripheral T cell characteristics in relapsing remitting MS (RRMS) patients. In the present study, we studied the effects of high dose vitamin D3 supplementation on safety and T cell related outcome measures. METHODOLOGY/PRINCIPAL FINDINGS: Fifteen RRMS patients were supplemented with 20,000 IU/d vitamin D3 for 12 weeks. Vitamin D and calcium metabolism were carefully monitored, and T cell characteristics were studied by flowcytometry. All patients finished the protocol without side-effects, hypercalcaemia, or hypercalciuria. The median vitamin D status increased from 50 nmol/L (31-175) at week 0 to 380 nmol/L (151-535) at week 12 (P<0.001). During the study, 1 patient experienced an exacerbation of MS and was censored from the T cell analysis. The proportions of (naïve and memory) CD4+ Tregs remained unaffected. Although Treg suppressive function improved in several subjects, this effect was not significant in the total cohort (P=0.143). An increased proportion of IL-10+ CD4+ T cells was found after supplementation (P=0.021). Additionally, a decrease of the ratio between IFN-γ+ and IL-4+ CD4+ T cells was observed (P=0.035). CONCLUSION/SIGNIFICANCE: Twelve week supplementation of high dose vitamin D3 in RRMS patients was well tolerated and did not induce decompensation of calcium metabolism. The skewing towards an anti-inflammatory cytokine profile supports the evidence on vitamin D as an immune-modulator, and may be used as outcome measure for upcoming randomized placebo-controlled trials. TRIAL REGISTRATION: Clinicaltrials.gov NCT00940719.