- Open Access
Is vitamin D deficiency involved in the immune reconstitution inflammatory syndrome?
© Conesa-Botella et al; licensee BioMed Central Ltd. 2009
- Received: 13 February 2009
- Accepted: 21 April 2009
- Published: 21 April 2009
About 20–30% of persons with HIV infection, especially those living in countries with limited resources, experience an immune reconstitution inflammatory syndrome (IRIS) after starting antiretroviral treatment. The active form of vitamin D, 1,25-dihydroxyvitamin D, is a key player in the clearance of pathogens and influences the level of inflammation and macrophage activation.
Presentation of the hypothesis
We hypothesize that low availability of 1,25-dihydroxyvitamin D, either due to vitamin D deficiency or due to polymorphisms in the vitamin D receptor or in its activating/inactivating enzymes, contributes to the appearance of IRIS. Furthermore, drug interactions with the enzymatic pathways of vitamin D could favour the development of IRIS.
Testing the hypothesis
Our hypothesis could be explored by a case-control study to assess the prevalence of vitamin D deficiency in HIV-infected patients on antiretroviral treatment who develop and do not develop IRIS.
Implications of the hypothesis
If the role of vitamin D in IRIS is confirmed, we would be able to screen patients at risk for IRIS by screening for vitamin D deficiency. After confirmation by means of a clinical trial, vitamin D supplementation could be a cheap and safe way to reduce the incidence of IRIS.
- Human Immunodeficiency Virus
- Immune Reconstitution Inflammatory Syndrome
- Active Vitamin
- Human Immunodeficiency Virus Infected Patient
- Immune Restoration Disease
Highly active anti-retroviral therapy (HAART) decreases the mortality and improves the quality of life of persons living with human immunodeficiency virus (HIV) infection . Nevertheless, 17–32% of HIV infected persons living in countries with limited resources experience a temporary worsening of their clinical status after starting HAART despite immunological improvement [2, 3]. This paradoxical reaction occurs most frequently during the first 3 months after initiation of HAART and is known as immune reconstitution inflammatory syndrome (IRIS) or immune restoration disease (IRD) . To date, more than 20 different pathogens have been associated with IRIS [2, 3, 5, 6]. However, IRIS has also been described in association with autoimmune diseases, cancer, and some non-infectious granulomatous diseases such as sarcoidosis and Crohn's disease .
In countries with limited resources, Mycobacteria sp. are by far the most common pathogens involved .
There is now evidence that vitamin D plays a role in improving anti-tuberculosis immunity as well as in the regulation of immune responses [8–11], both of which are crucial steps in the development of IRIS. A double blind randomized controlled trial showed that a single dose of vitamin D significantly enhanced immunity to Mycobacteria tuberculosis (Mtb) among contacts of tuberculosis (TB)-infected patients . Liu et al showed later that vitamin D acts by increasing the level of the antimicrobial peptide cathelicidin produced by monocytes and macrophages [13, 14].
Low levels of vitamin D levels have been observed in African populations  as well as in HIV-infected persons (reviewed by Villamor ). A recent study in a cohort of HIV-positive patients in the Netherlands (73% white, 20% black) showed a prevalence of vitamin D deficiency of 29% in the total population, and 62% in black patients. Low levels of active vitamin D have been associated with low CD4 counts and AIDS progression .
TB treatment is also known to interfere with vitamin D metabolism and to cause osteomalacia . Vitamin D deficiency may be influenced by deficient substrate, but also by polymorphisms in its receptor or in the enzymes controlling the activation of this steroid.
Low levels of vitamin D could predispose HIV infected patients with a current or undiagnosed opportunistic infection (OI) to IRIS. Indeed, the active form of vitamin D, 1,25-(OH)2D, has anti-inflammatory activity  and there is now accumulating evidence for its role in the regulation of human T-cell and antigen-presenting cell (APC) functions [20, 21]. Furthermore, drug interactions with the enzymatic pathways of vitamin D  could favour the development of IRIS.
Pathogenesis of IRIS
HIV causes progressive depletion of CD4+ T-cells and impairs the immune system [2, 5]. In HIV/Mtb patients with severe immunodeficiency, impaired T-cell function impedes granuloma formation . When HAART is started, T-cell function is restored and granuloma formation is re-established, mainly in the lungs and lymph nodes, through activation of Mtb-infected macrophages by interferon-γ (IFN-γ) producing T-cells . Unfortunately, rapid or unbalanced restoration of the immune system against living or death organisms [7, 24] may also lead to uncontrolled antigen-specific responses  with reappearance of clinical symptoms  and development of IRIS.
Known risk factors for the development of IRIS include a low CD4 T-cell count when starting HAART, advanced OI with high OI antigen load, and a short time interval between OI treatment and the start of HAART [2, 25–28]. Other risk factors such as younger age, male gender, a higher CD8 T-cell percentage, a high viral load at baseline, a fast increase in CD4 T-cell count and fast decrease in viral load after the start of HAART, and a protease inhibitor (PI) based HAART regimen, were reported in some studies but not in others .
Vitamin D and the immune system
The main source of vitamin D stems from sun exposure: pre-vitamin D is converted by solar ultraviolet B radiation in the skin into vitamin D. Skin pigmentation is a known risk factor for hypovitaminosis D since melanin, responsible for the skin pigmentation, filters UV radiation [30–32].
To exert its actions, 1,25-(OH)2D binds to the vitamin D receptor (VDR) expressed in normal, malignant and immune cells . At least 36 tissues possess VDR and more than 10 tissues are able to produce 1,25-(OH)2D in a paracrine fashion . By the wide expression of VDR , 1,25-(OH)2D can regulate calcium homeostasis and bone metabolism as well as play an essential role in cell proliferation, differentiation and the above described regulation of the immune response [31, 33]. The variability in patients' susceptibility to immune dysfunction and thus IRIS could be explained by the polymorphisms of the VDR gene known to influence immune cell function , or by polymorphisms of hydroxylases regulating the production or the degradation of the bioactive vitamin D.
Vitamin D, HAART and IRIS
During HAART and immune reconstitution, pathogen-derived antigens are de novo recognized by APC, processed and presented to CD4+ T-cells leading to T-cell activation and secretion of macrophage-activating interferon-γ. Vitamin D inhibits this IFN-γ production .
In case of low 25-(OH)D levels prior to HAART, we hypothesize that a defective clearing of pathogens and a delayed negative feedback on macrophage activation due to low 1,25-(OH)2D production, can lead to excessive granuloma formation and an exacerbated inflammatory response described as IRIS.
To avoid macrophage-overstimulation, vitamin D should be given before their massive activation, i.e. before the initiation of the inflammation. Indeed vitamin D decreases immune stimulation , but if vitamin D is given when granulomas are already flourishing, the 1α-hydroxylase in activated macrophages can produce high amounts of 1,25-(OH)2D with systemic spillover  resulting in hypercalcemia  as described in Mtb-IRIS [39–41] and cryptococcus-IRIS . At that moment vitamin D supplementation could worsen the clinical status of the patient.
Our hypothesis could be explored by a case-control study to assess the prevalence of vitamin D deficiency in HIV infected patients on HAART who develop and do not develop IRIS. In cohort studies of patients initiated on HAART, the incidence of IRIS should be compared in patients with low, normal and high baseline vitamin D levels.
Moreover, polymorphisms of the VDR, the VDBP and the enzymes involved in vitamin D production should be investigated. We also propose to perform functional testing of vitamin D enzymes to find out if an increase of the vitamin D catabolism or a decrease of its production could contribute to low 1,25-(OH)2D concentrations at the site of inflammation, possibly leading to IRIS. Finally remains the issue whether the levels of vitamin D that are currently accepted as 'sufficient' for bone health apply to 'global health' and in particular anti-bacterial and anti-inflammatory properties of vitamin D .
There is an inter-relationship between vitamin D metabolism, HAART therapy and immunity. Impaired vitamin D metabolism in macrophages, whether caused by vitamin D deficiency or by HAART therapy, might be a determinant of IRIS in HIV-positive individuals. The potential role of vitamin D status in the pathogenesis of IRIS should be investigated. Indeed, if the role of vitamin D in IRIS is confirmed, vitamin D supplementation could be a cheap and safe way to prevent IRIS.
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