Calcitriol injections? Really? What relevance does that have to anything? I'll let my kidneys regulate 1,25D production, thank you.
Now for the real story on how 25D is used by the immune system for T-cell activation:
When the native T cell recognizes foreign molecules with its T cell receptor (TCR) it sends activation signals to the VDR gene. The VDR gene now starts the production of VDR. VDR binds vitamin D in the T cell and becomes activated. Vitamin D bound to activated VDR goes back into the cell nucleus and activates the gene for PLC-gamma1. PLC-gamma1 is produced and the T cells can get started.