25-Hydroxyvitamin-D levels in Sjögren’s syndrome: is it the right time to dismiss the case or not?

Introduction

Sjögren syndrome (SjS) is a gradually progressing, inflammatory, autoimmune condition that mostly affects the exocrine glands and manifests clinically as dry mouth and eyes. In SjS, extra glandular involvements including arthritis, pulmonary disease, renal disease, and vasculitis may also occur. SjS is classified as ‘primary’ (pSjS) when it occurs without related comorbidity and ‘secondary’ when it occurs with another autoimmune disease [1]. Many factors are associated with the pathogenesis and progression of the disease, such as genetics, infectious factors, or hormonal factors such as gonadal steroids 2], [3], [4. Although there are strenuous efforts to elucidate the elaborate mechanisms and possible causes of the disease, the exact mechanisms remain shrouded today.

Vitamin D is considered a steroidal hormone, as it is synthesized by the human organism itself as well as it is obtained exogenously, and has many effects on the body through its receptors [5]. One of the aspects of vitamin D is its immunomodulatory effects [6, 7]. Based on these effects, there are many studies conducted to evaluate the autoimmune disease and vitamin D relationship. These studies have shown that vitamin D, especially hypovitaminosis, is related to the occurrence of some autoimmune diseases, such as diabetes mellitus type 1, inflammatory bowel disease, multiple sclerosis, rheumatoid arthritis, behçet’s disease, or systemic lupus erythematosus. The role of vitamin D in the regulation of the inflammatory systems was also studied [8].

The role of vitamin D in SjS has been a topic for research, especially in recent years. As SS also has an inflammatory component, it was hypothesized that vitamin D deficiency may contribute to the aggravation of the disease. Moreover, even in the absence of a possible contribution to the inflammation, vitamin D is thought to contribute to the tear quality and affect tear break-up times in patients with non-Sjögren dry eye [9]. Some studies report that lower levels of vitamin D are associated with the disease activity and some of its manifestations [10]. Still, these results can not be reproduced by other studies, such as Zilahi et al. [11]. Surprisingly, a study by Sandhya et al. found that lower vitamin D levels were associated with lower disease activity [12]. Many complex mechanisms are thought to underlie the possible connection between vitamin D levels and connective tissue diseases, as they interact in multiple pathways in the immune regulatory system [13]. Even in the presence of studies investigating the role of vitamin D in Sjögren’s disease and its activity, the present data is little and often contradictory, raising the need for further studies to elucidate the possible role of vitamin D in these patients [13].

This study aimed to investigate whether patients with Sjögren Syndrome had lower vitamin D levels when compared to healthy controls, and to show whether there was a correlation between vitamin D levels and clinical course and manifestations of the disease.

Materials and methods

This study included 88 female patients with primary SjS who fulfilled the United States-European Consensus Criteria [14]. The patients were followed up at the Outpatient Clinic of the Department of Rheumatology of a university hospital. Controls consisted of 3,338 age-matched healthy females who were admitted to outpatient clinics of the hospital with acute symptoms (e.g., fatigue, pain…), and with no history and an eventual diagnosis of a chronic disease. To assess any clinical manifestations that were present, a thorough review of all the patients’ medical records and a physical examination of each patient were conducted. Moreover, patients were carefully examined for their comorbidities by a rheumatologist, especially for the presence of other autoimmune diseases, osteoporosis, or diseases that may affect vitamin D metabolism. The study was approved by the local board of ethics. Moreover, patients were carefully examined for their comorbidities by a rheumatologist, especially for the presence of other autoimmune diseases, osteoporosis, or diseases that may affect vitamin D metabolism. The study was approved by the local board of ethics.

None of the enrolled patients were found to be receiving vitamin D supplementation before and during the time of blood sampling, as it was routinely inquired during their evaluations.

All patients and controls inhabited the western region of Turkey with similar weather conditions. To minimize the seasonal variability, all samples were taken during October and November.

Exclusion criteria consisted of a history of pregnancy, lactation, parathyroid problems, and calcium or vitamin D supplement use.

Disease-related laboratory tests included Schirmer’s test, salivary gland biopsy, anti-nuclear antibody, rheumatoid factor, anti-Ro (SS-a), anti-La (SS-b), C-reactive protein (CRP), complete blood count, complement (C3–C4) levels. A positive Schirmer tear production test was defined as scores below 5 mm [15]. Minor salivary gland biopsies were considered positive if the focus score was equal to or more than 1 [16]. Rheumatoid factor (RF) cutoff values for positivity were set as >25 mIU/mL [17].

Results

Discussion

The results of the study show that patients with SS had no difference regarding their vitamin D levels when compared to healthy populations. Moreover, the presence of vitamin D deficiency or insufficiency did not contribute to the clinical characteristics of the disease, with no associations found between them and vitamin D levels. Finally, patients with lower or higher disease activities were not found to differ in terms of vitamin D levels.

The relationship between vitamin D and Sjögren Syndrome has been debated in recent years. The studies conducted had mixed results, often inconsistent and unable to show a healthy direction. Thus, these inconsistencies called for a study to show whether this patient group had the problem of lower vitamin D levels, and if the disease indices and findings were associated with this issue.

Preclinical studies show that diseases related to impaired immunomodulation, including SjS, may be related to vitamin D deficiency or insufficiency. It has been discovered that dendritic cells (DC), monocytes, macrophages, CD4+ cells, activated B cells (Act B), and the converting enzyme, 1-a-hydroxylase, are all cells that express the vitamin D receptor [26]. When the Toll-like receptor1/2 (TLR1/2) is activated in macrophages and monocytes (Mw/Mono), 1.25-(OH)2D causes the synthesis of the antibiotic cathelicidin. By blocking the synthesis of IL-12 by Mw/Mono and DCs as well as the presentation of co-stimulatory receptors (CD40, CD80, and CD86), 1.25-(OH)2 D prevents the recruitment of CD4+ cells [6]. The studies have also shown that vitamin D promotes the synthesis of anti-inflammatory cytokines (IL-4, IL-5, IL-10, FoxP3), while inhibiting the production of proinflammatory ones (Il-2, IL-16, IL-23, IFN-g), thus contributing to the maintenance of the balance of inflammation [27, 28].

Clinical studies have also been conducted to find out whether such findings affect SjS. Bang et al. have shown the inverse correlations found between levels of 25 OH-D3 and measures of clinical and immunoinflammatory status, pointing out that vitamin D might be responsible for some of the pathogenetic events in disease [10]. Erten et al. have found that patients with Sjögren Syndrome had lower levels of vitamin D when compared to healthy controls [29]. vitamin D deficiency has also been linked to reduced tear break-up time and worse Schirmer test results in patients with dry eye, but not with SjS, which could be interpreted as it could be a factor in exacerbation of main symptoms indirectly [9]. Interestingly, Baldini et al. proposed that vitamin D may play a role in the early course of SjS, associating the lower vitamin D levels with different disease phenotypes [30]. While some studies show positive results, a study by Zilahi et al. failed to associate vitamin D receptor polymorphisms with the development of SjS [11].

This study points out that vitamin D might not play a significant role in the development and course of the disease. Patients included in the study didn’t show any difference in vitamin D levels in age-matched controls, which were consistent with the findings of Agmon-Levin et al. and Szodoray et al. [31, 32]. The differences found in the prior studies may be interpreted as a result of a chronic inflammatory process or a lack of sun exposure, pointing out the possibility of an inverse causality. Still, our study was not able to show this association.

There have also been studies investigating a possible relationship between clinical phenotypes of Sjögren Syndrome and vitamin D levels. These studies mostly stem from preclinical evidence on 25 OH-D3’s effects on the immune system, a deficiency of which may cause altered immune function. A study by Agmon-Levin et al. correlated lower levels of vitamin D with neuropathy and lymphoma incidence in patients with SjS [31]. Similarly, one of the studies associated hypovitaminosis D with a different clinical course and manifestations, particularly leukopenia [30]. In our study, we did not show a relationship between clinical manifestations or biochemical and immunologic markers that are relevant to SjS. While none of our patients had a history of lymphoma, only lymphadenopathy, we were also not able to show a relationship between neuropathy and vitamin D levels. It is hard to show possible relationships between relatively rare manifestations of the disease, requiring higher numbers of patients. Thus, our numbers may not have been enough to show an association between lymphadenopathy and vitamin D, as all three patients with lymphadenopathy belonged to the lower vitamin D group. Still, incidences of peripheral neuropathy were similar between groups (1.9 vs. 2.9 %), and while our small numbers can be a limitation for this finding, the findings in the literature can also be incidental with minimal differences, warranting studies with higher numbers of patients.

Strengths of this study include a considerable amount of control patients in number, which were all age-matched and one of the highest to date. Patients with Sjögren Syndrome were small when compared to the pool of the control group since the disease is not very common. Still, the use of appropriate statistical methods provided a more accurate estimation in such a scenario. We have also had a detailed evaluation of these patients, covering many aspects of the disease. We have also conducted analyses for different vitamin D level groups alongside correlation analyses, which proved to be consistent.

There are also weaknesses in this study. While the patient numbers are adequate for a decision, more patients could have resulted in a firm conclusive result. Moreover, most of our patients had a low disease activity, with only a few patients with higher disease activity. Both the vitamin D statuses and disease activity are dynamic in nature and are due to change with time, so a cross-sectional evaluation might not have been enough in showing some possible associations. As some of the manifestations of SS are rare, and our patients had a milder form of the disease, most of our analyses on the rarer involvements of SS are done with small numbers. Thus, these findings should be interpreted carefully. Moreover, these findings are mostly based on patients with mild disease, hindering the generalizability of the results. The design of this study is also cross-sectional and retrospective, thus being relatively weaker in showing cause and effect relationships. Observational studies which include the follow-ups of patients such as cohorts usually give more accurate results. While most of these patients were being followed up in institutions, it was lacking for the chosen control group, thus making such a study design impossible for this case. Finally, we have only evaluated 25 OH-D3 levels in these patients and did not perform any additional tests to show the levels of 1-25-OH form of vitamin D. As it is the active form of vitamin D, possible differences regarding its levels may have contributed to the disease activity in these patients [10].

Conclusions

This study shows vitamin D might not play a significant role in the development and course of SjS, and patients with different vitamin D levels did not show significant differences in clinical courses. While the literature shows heterogeneous findings with the vitamin D levels of this patient group, this study includes a considerable number of SS patients, as well as a high number of healthy controls from the same population, differing from the other studies. Still, these findings should be evaluated carefully, especially for the rarer manifestations of the disease, and should not be generalized to all SjS patients since our groups mostly consisted of patients with mild disease. Based on the inconsistency of the studies in this field, more studies with prospective designs and more patients might give findings with better precision.