Influence of 1,25-dihydroxy vitamin D3 on TLR4-induced activation of antigen presenting cells is dependent on the order of receptor engagement.

The vitamin D metabolite, 1,25-(OH)₂D₃, binds the vitamin D receptor (VDR) to exert its regulatory effects at the transcription level. VDR is expressed in professional antigen-presenting cells (pAPCs), such as macrophages (Mø) and dendritic cells (DCs). We show for the first time that the 24-hydroxylase enzyme is activated in bone marrow-derived dendritic cell (BMDC), due to 1,25(OH)₂D₃ stimulation which resulted in the induction of its gene, CYP24A1. Furthermore, we provide evidence that the influence of 1,25-(OH)₂D₃ on TLR-4-L-induced activation of pAPC is dependent on the order of VDR and TLR-4 engagement. Thus, pre-treatment of pAPC with 1,25-(OH)₂D₃ partially inhibited LPS-induced nitric oxide (NO) production. However, these inhibitory effects were not observed when LPS and 1,25-(OH)₂D₃ were added simultaneously or when LPS preceded 1,25-(OH)₂D₃. Moreover, we found that 1,25-(OH)₂D₃ pre-treatment of pAPCs did not cause general suppression since it interfered with NO levels but not with the cytokines IL-6 or TNF-α. Consequently, engagement of VDR by 1,25-(OH)₂D₃ can partially interfere with TLR-4-L-induced activation of pAPCs only when it occurs before TLR-4 stimulation.