- Regulation of TNF-α by 1α,25-dihydroxyvitamin D 3 in human macrophages from CAPD patients. Background We have previously reported that 1α,25-dihydroxyvitamin D 3 [1α,25(OH) 2 D 3 ] accumulates in the dialysis fluid of uremic patients treated by continuous ambulatory peritoneal dialysis (CAPD). It has been reported that this metabolite regulates the production of cytokines by monocytes/macrophages. Since tumor necrosis factor-α (TNF-α) initiates an inflammatory cascade during peritonitis, the aim of the present study was to investigate the effect of 1α,25(OH) 2 D 3 on the production of TNF-α by human peritoneal macrophages (HPMs). Methods HPMs were obtained from patients on CAPD. Cells were incubated with various concentrations of 1α,25(OH) 2 D 3 , 1α,24(S) dihydroxyvitamin D 2 [1α,24(S)(OH) 2 D 2 ] or 25-hydroxyvitamin D 3 (25-OH-D 3 ) for 16 hours. This was followed by lipopolysaccharide (LPS; 1 μg/mL) incubation for 2.5 to 6 hours. TNF-α protein production was determined by enzyme-linked immunosorbent assay. TNF-α mRNA was assayed by the reverse transcriptase-polymerase chain reaction procedure, using internal synthetic mRNA standards for quantitative results. Results Incubation of HPMs with 1α,25(OH) 2 D 3 prior to stimulation with LPS dose dependently inhibited the expression of TNF-α on both mRNA and protein levels. Similar results were obtained with the less calcemic vitamin D 2 analogue 1α,24(S)(OH) 2 D 2 . Incubation of HPMs with 25-OH-D 3 also revealed a down-regulation of TNF-α expression. Since this down-regulatory effect was blocked by ketoconazole, it is likely that this effect was caused by the conversion of 25-OH-D 3 into 1α,25(OH) 2 D 3 by HPMs. Conclusions 1α,25(OH) 2 D 3 has a potent inhibitory effect on the production of TNF-α by LPS-activated HPMs. We hypothesize that 1α,25(OH) 2 D 3 may constitute a regulatory mechanism that, by controlling the intensity of the inflammatory response of the peritoneum, will moderate tissue damage during peritonitis.