TREM1 and influenza: To determine the functionality of VDR, we measured the expression of human antibacterial peptide cathelicidin; triggering receptor expressed on myeloid cells 1 (TREM-1), a receptor of the innate immune system which is known to be induced by vitamin D [35]; retinoic acid inducible gene (RIG)-I and interferon (IFN)-β genes, which play an important role in the response to viral challenges including influenza [36].