Investigating the roles of regulatory T cells, mast cells and interleukin-9 in the control of skin inflammation by vitamin D

Topical application of biologically active vitamin D [1,25-dihydroxyvitamin D (1,25(OH)₂D)], or low-calcemic analogues, curb skin inflammation through mechanisms that involve migratory dendritic cells (DCs) and regulatory T (T_Reg) cells. 1,25(OH)₂D also promotes immunoregulation by mast cells, and inhibits the development of T helper type-9 (Th9) cells that secrete interleukin-9 (IL-9). Here, we investigated the ability of topical 1,25(OH)₂D to suppress contact dermatitis through an IL-9-dependent process, examining mast cells and IL-9-secreting T cells. Contact dermatitis was modelled in adult BALB/c female mice by initiating a “biphasic ear swelling response” following a single application of 2,4-dinitrofluorobenzene (DNFB). Topical 1,25(OH)₂D (125 ng) applied to ear pinnae prior to (but not after) DNFB sensitisation suppressed the efferent phase of the ear swelling response. This dose of 1,25(OH)₂D did not cause hypercalcemia. At the peak of the efferent ear swelling response, proportions of T_Reg (CD3 + Foxp3+) cells and numbers of mast cells were increased in ear skin of 1,25(OH)₂D-treated mice. Topical 1,25(OH)₂D increased the proportion of Foxp3 + IL-9 + T_Reg cells and the capacity of T_Reg cells to secrete IL-9 ex vivo. However, the proportion of the IL-9 + cells of the total T_Reg cell population was small (< 1%), and the amount of IL-9 secreted by T_Reg cells from mice treated with IL-9 was low (< 50 pg/ml). Furthermore, injection of anti-IL-9 neutralising antibody (100 µg, intraperitoneally) prior to sensitisation did not significantly reverse the suppressive effects of 1,25(OH)₂D. In conclusion, topically applied 1,25(OH)₂D suppressed the efferent phase of a biphasic cutaneous ear swelling response through mechanism(s) that may be dependent on mast cells and T_Reg cells; however, the role of IL-9 in mediating these responses is uncertain. More studies are needed to further characterise the mechanisms by which topical 1,25(OH)₂D modulates cell-mediated immune responses central to its suppressive effects upon contact dermatitis.