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The immune modulating capacity of vitamin D₃ is well-recognized. Ultra-violet (UV) exposure determines production of vitamin D₃in vivo and varies through the course of the year, especially in temperate regions. However, it is not known whether the human innate immune response differs due to seasonality. To validate the seasonal effects of vitamin D₃, the effect of 1,25(OH)₂D₃ on peripheral blood mononuclear cells (PBMC) cytokine response was first determined in vitro. 1,25(OH)₂D₃ decreased interleukin (IL)-6 and tumour necrosis factor (TNF)-α release by PBMC stimulated with tripalmitoyl-S-glycerylcysteine (Pam3Cys) or lipopolysaccharide (LPS). Subsequently, ex-vivo stimulation studies were performed in 15 healthy volunteers through the course of the four seasons of the year. PBMC were isolated and stimulated with Toll-like receptor (TLR)-2 and TLR-4 ligands Pam3Cys and LPS, respectively …