title: Vitamin D-binding protein controls T cell responses to vitamin D.
abstract: BACKGROUND
  In vitro studies have shown that the active form of vitamin D3, 1α,25-dihydroxyvitamin D3 (1,25(OH)2D3), can regulate differentiation of CD4+ T cells by inhibiting Th1 and Th17 cell differentiation and promoting Th2 and Treg cell differentiation. However, the serum concentration of 1,25(OH)2D3 is far below the effective concentration of 1,25(OH)2D3 found in in vitro studies, and it has been suggested that 1,25(OH)2D3 must be produced locally from the inactive precursor 25-hydroxyvitamin D3 (25(OH)D3) to affect ongoing immune responses in vivo. Although it has been reported that activated T cells express the 25(OH)D-1α-hydroxylase CYP27B1 that converts 25(OH)D3 to 1,25(OH)2D3, it is still controversial whether activated T cells have the capacity to produce sufficient amounts of 1,25(OH)2D3 to affect vitamin D-responsive genes. Furthermore, it is not known how the vitamin D-binding protein (DBP) found in high concentrations in serum affects T cell responses to 25(OH)D3.
  
  
  RESULTS
  We found that activated T cells express CYP27B1 and have the capacity to produce sufficient 1,25(OH)2D3 to affect vitamin D-responsive genes when cultured with physiological concentrations of 25(OH)D3 in serum-free medium. However, if the medium was supplemented with serum or purified DBP, DBP strictly inhibited the production of 1,25(OH)2D3 and 25(OH)D3-induced T cell responses. In contrast, DBP did not inhibit the effect of exogenous 1,25(OH)2D3. Actin, arachidonic acid and albumin did not affect the sequestration of 25(OH)D3 by DBP, whereas carbonylation of DBP did.
  
  
  CONCLUSIONS
  Activated T cells express CYP27B1 and can convert 25(OH)D3 to 1,25(OH)2D3 in sufficiently high concentrations to affect vitamin D-responsive genes when cultured in serum-free medium. However, DBP sequesters 25(OH)D3 and inhibits the production of 1,25(OH)2D3 in T cells. To fully exploit the immune-regulatory potential of vitamin D, future studies of the mechanisms that enable the immune system to exploit 25(OH)D3 and convert it to 1,25(OH)2D3 in vivo are required.
authors.forename: Martin
authors.initials: M
authors.lastname: Kongsbak
authors.forename: Marina Rode
authors.initials: MR
authors.lastname: von Essen
authors.forename: Trine Bøegh
authors.initials: TB
authors.lastname: Levring
authors.forename: Peter
authors.initials: P
authors.lastname: Schjerling
authors.forename: Anders
authors.initials: A
authors.lastname: Woetmann
authors.forename: Niels
authors.initials: N
authors.lastname: Ødum
authors.forename: Charlotte Menné
authors.initials: CM
authors.lastname: Bonefeld
authors.forename: Carsten
authors.initials: C
authors.lastname: Geisler
mesh_terms: D015090:25-Hydroxyvitamin D3 1-alpha-Hydroxylase / Q000378:metabolism; D051997:ADP-ribosyl Cyclase 1 / Q000378:metabolism; D000199:Actins / Q000494:pharmacology; D000418:Albumins / Q000494:pharmacology; D016718:Arachidonic Acid / Q000494:pharmacology; D001682:Biological Availability; D002112:Calcifediol / Q000378:metabolism; D005434:Flow Cytometry; D005786:Gene Expression Regulation / Q000187:drug effects; D006801:Humans; D008213:Lymphocyte Activation / Q000187:drug effects; D010873:Pinocytosis / Q000187:drug effects; D050050:Protein Carbonylation / Q000187:drug effects; D044967:Serum; D013601:T-Lymphocytes / Q000166:cytology / Q000187:drug effects / Q000276:immunology*; D014807:Vitamin D / Q000031:analogs & derivatives* / Q000494:pharmacology; D014809:Vitamin D-Binding Protein / Q000378:metabolism*
chemical_list: D000199:Actins; D000418:Albumins; D014809:Vitamin D-Binding Protein; C118756:dihydroxy-vitamin D3; D014807:Vitamin D; D016718:Arachidonic Acid; D015090:25-Hydroxyvitamin D3 1-alpha-Hydroxylase; D051997:ADP-ribosyl Cyclase 1; D002112:Calcifediol