Immunoregulatory effects of vitamin D3 on mast cells during immunoglobulin E-dependent immune responses.

Mast cells (MCs) can exert anti-inflammatory effects via production of interleukin (IL)- 10 in a number of Immunoglobulin (Ig)E-independent immune responses. Recently, we reported that 1α,25-dihydroxyvitamin D₃ (1α,25(OH)₂D₃), the biologically active form of vitamin D₃ (VitD₃), can induce IL-10 production from mouse bone marrow-derived cultured MCs (mBMCMCs). For the current project, we further investigated if the well-recognised pro-inflammatory properties of MCs in IgE-dependent immune settings can be reduced upon 1α,25(OH)₂D₃ administration and, if so, which mechanisms are likely to be responsible. In the presence of 1α,25(OH)₂D₃, IgE + specific antigen (sAg)-stimulated mBMCMCs exhibited reduced degranulation, as well as decreased production of the pro-inflammatory cytokines, TNFα and IL-6, in a vitamin D receptor (VDR)-dependent manner. Concomitantly, 1α,25(OH)₂D₃ significantly up-regulated the production of IL-10. In addition, we demonstrated for the first time the expression of CYP27B1, the enzyme that generates 1α,25(OH)₂D₃ from its inactive precursor 25-hydroxyvitamin D3 (25OHD₃ ) in both mBMCMCs and human cord-blood-derived MCs (hCBMCs). This enables mBMCMCs to produce endogenous 1α,25(OH)₂D₃ and thus granting 25OHD₃ similar VDR-dependent immunosuppressive effects to 1α,25(OH)₂D₃ on activated MCs either directly or indirectly. By employing a mouse IgE-mediated MC-dependent passive cutaneous anaphylaxis (PCA) model as well as four mouse groups with different cutaneous MC profiles in the ears, including wild-type (WT) C57BL/6 mice, MC-deficient C57BL/6-Kitʷ⁻ˢʰʹʷ⁻ˢʰ mice and C57BL/6-Kitʷ⁻ˢʰʹʷ⁻ˢʰ mice engrafted with either WT or VDR-deficient (VDR⁻ʹ⁻) mBMCMCs, we found that topical application of either 1α,25(OH)₂D₃ or 25OHD₃ significantly curtailed the magnitude of PCA-associated ear swelling, potentially by reducing the extent of MC degranulation and/or the secretion of various MC-derived cytokines. Notably, these PCA-suppressive effects required the presence of dermal MCs and their expression of VDR. Taken together, data presented in this thesis provide evidence that 1α,25(OH)₂D₃ and 25OHD₃, the latter likely via its conversion to the active metabolite by MCs, can suppress IgE + sAg-mediated MC activation in a VDR-dependent manner both in vitro and in vivo. This suggests the therapeutic potential for various VitD₃ analogues to treat MC-dependent IgE-associated allergic disorders.Thesis (Ph.D.) -- University of Adelaide, School of Molecular and Biomedical Science, 201