Histamine type I (H1) receptor radioligand binding studies on normal T cell subsets, B cells, and monocytes

We have documented a single, specific binding site for [3H]pyrilamine on normal human T helper, T suppressor, B cells, and monocytes. The binding of the radioligand to its receptor is reversible with cold H1 antagonist, saturates at 40 to 60 nM, and binding equilibrium is achieved in 2 to 4 min. Using a computer program (Ligand), we calculated the dissociation constants, binding capacities, and numbers of receptors per cell for each of the different cell types. Monocytes were found to have the highest affinity (mean KD +/- SD; 3.8 +/- 4.8 nM) for [3H]pyrilamine, followed by T helper cells (KD = 5.0 +/- 6.6 nM), B cells (KD = 14.2 +/- 2.0 nM), and T suppressor cells (KD = 44.6 +/- 49.4 nM). T suppressor cells were found to express the higher number of H1 receptors per cell (35,697 +/- 15,468), followed by B cells (10,732 +/- 9060), T helper cells (6838 +/- 8167), and monocytes (5589 +/- 2266). The kinetics of binding for this radioligand was carried out in resting and mitogen-stimulated T cells over a 48-hr period. We found that the binding affinity for [3H]pyrilamine increased over the 48-hr period, whereas the number of receptors per T cell was essentially unchanged. In contrast, T cells stimulated with Con A or PHA were shown to have a greater than fourfold increase in the number of receptors per cell, whereas the binding affinity for [3H]pyrilamine decreased over the 48-hr period. Preincubation of T cells with unlabeled histamine before carrying out the radioligand binding assay resulted in a decrease in the binding affinity of the receptors to [3H]pyrilamine, but the number of receptors per cell did not change significantly. Although the function of H1 receptors on T cells, B cells, and monocytes has not been completely defined, this receptor has the potential of playing an important role in modulating the immune response.