A uremic patient developed hypercalcemia after tuberculosis infection, and his ionized calcium levels correlated with 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) levels. We performed further studies to determine whether monocytes are alternative sites of 1,25(OH)2D3 conversion beyond renal tubular cells. Using an ex vivo bioassay, in this study, we found that 1-α hydroxylase (CYP27B1) activity in monocytes is significantly higher in patients with active tuberculosis (TB) than in those with frequent TB contact. However, when monocytes from patients with active TB were restimulated with antigen derived from Mycobacterium tuberculosis, less 1,25(OH)2D3 was observed. In contrast, the level of 1,25(OH)2D3 was unchanged in those with frequent TB contact. We conclude that monocytes may be an alternative source of 1-α hydroxylase that could convert 25-hydroxyvitamin D3 to the more active 1,25(OH)2D3
