In sarcoidosis, which cell type is primarily responsible for the excess production of active vitamin D (1,25-dihydroxyvitamin D)?

In sarcoidosis, which cell type is primarily responsible for the excess production of active vitamin D (1,25-dihydroxyvitamin D)?

• A. T cells
• B. Hepatocytes
• C. Macrophages
• D. B cells

Answer: (C)

In sarcoidosis, granulomas contain activated macrophages that express 1-alpha-hydroxylase, the enzyme that converts 25-hydroxyvitamin D into 1,25-dihydroxyvitamin D (calcitriol). This production happens outside the kidney and is not tightly regulated by PTH or calcium levels, so calcitriol levels rise. The result is increased intestinal calcium absorption and often hypercalcemia. T cells help form granulomas but are not the source of the excess calcitriol; hepatocytes perform the initial 25-hydroxylation in the liver, not the final activation inside granulomas; B cells are not the primary source. Therefore, the macrophages within granulomas are responsible for the excess active vitamin D.

In sarcoidosis, granulomas contain activated macrophages that express 1-alpha-hydroxylase, the enzyme that converts 25-hydroxyvitamin D into 1,25-dihydroxyvitamin D (calcitriol). This production happens outside the kidney and is not tightly regulated by PTH or calcium levels, so calcitriol levels rise. The result is increased intestinal calcium absorption and often hypercalcemia. T cells help form granulomas but are not the source of the excess calcitriol; hepatocytes perform the initial 25-hydroxylation in the liver, not the final activation inside granulomas; B cells are not the primary source. Therefore, the macrophages within granulomas are responsible for the excess active vitamin D.