Calcitonin receptors on circulating normal human lymphocytes

Area of Science:

• Immunology
• Endocrinology
• Molecular Biology

Background:

• Circulating human lymphocytes express receptors for calcitriol and parathyroid hormone (PTH).
• The presence of calcitonin (CT) receptors on lymphocytes, however, remained uninvestigated.
• CT is the third classical calciotropic hormone, alongside calcitriol and PTH.

Purpose of the Study:

• To investigate the presence and characteristics of calcitonin (CT) receptors on human lymphocytes.
• To determine if human T-lymphocytes possess specific binding sites for CT.

Main Methods:

• Isolation of human peripheral blood mononuclear cells, including monocytes, total lymphocytes, and T-lymphocytes.
• Radiolabeling of salmon CT (sCT) with iodine-125 ([125I]sCT) for binding assays.
• Scatchard and Hill analyses to characterize binding affinity, capacity, and cooperativity.

Main Results:

• Saturable, specific, high-affinity binding sites for [125I]sCT were identified on unstimulated lymphocytes.
• Binding data from T- and B-lymphocytes suggested positive cooperativity (Hill coefficient > 1), which was not observed on isolated T-lymphocytes (Hill coefficient ≈ 1).
• CT binding sites exhibited down-regulation following preincubation with sCT or human CT, indicating receptor regulation.

Conclusions:

• Unstimulated normal human T-lymphocytes possess saturable, specific, high-affinity receptors for CT.
• These CT receptors on T-lymphocytes suggest they may be target sites for CT action.
• Potential roles for CT in regulating bone metabolism and the immune system are proposed.