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Title: The immunomodulatory effects of vitamin D on mononuclear phagocytes
Author: Evans, R.
Awarding Body: University College London (University of London)
Current Institution: University College London (University of London)
Date of Award: 2014
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Expression of the vitamin D receptor on mononuclear phagocytes renders them susceptible to modulation by 1,25(OH)2D3, which is metabolised from its precursor 25(OH)D3 by the enzyme CYP27B1. Modulation of dendritic cells (DC) with 1,25(OH)2D3 has been shown in vitro to suppress their ability to prime proinflammatory T cells responses, suggesting extra renal synthesis of 1,25(OH)2D3 from the precursor 25(OH)D3 may promote tolerance in vivo. I sought to gain a greater mechanistic understanding of these phenomena at molecular, cellular and intercellular levels in the human system. I found that 1,25(OH)2D3 stimulation of DC results in a selective preservation of NFκB-mediated transcriptional responses to LPS, although these gene expression changes were diminished in magnitude. I also found enhanced p38 signalling in 1,25(OH)2D3 treated ex vivo monocytes, as well as those from 25(OH)D3 supplemented vitamin D deficient patients. 1,25(OH)2D3 suppresses expression of genes relating to antigen presentation which can be rescued by IFNγ stimulation. This treatment also results in a rescue of phenotypic maturation in DC in response to LPS, but without rescue of T cell activation. Preliminary experiments suggest there may be a defect in MDDC-T cell contact formation and maintenance. I find that DC are incapable of synthesising 1,25(OH)2D3 sufficient to inhibit their maturation, unlike macrophages which are able to bring about gene expression changes in DC by paracrine activation. This is due to reduced expression of CYP27B1 protein in DC due to significant expression of truncated transcript, which can be rescued by LPS stimulation. DC also readily induce CYP24A1 to degrade bioactive 1,25(OH)2D3 to limit VDR-driven transcription. Overall macrophages are a possible physiological source of 1,25(OH)2D3, and as such sufficient 1,25(OH)2D3 to inhibit DC activity will only arise after initiation of adaptive immunity and monocyte recruitment. Overall, my thesis provides deeper mechanistic insight into the role of vitamin D in immunomodulation of DC with regard to their interactions with T cells in the human system.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available