Stereospecific receptors for substance P on cultured human IM-9 lymphoblasts

The neuropeptide substance P (SP), which has been demonstrated to bind specifically to human blood T lymphocytes and to stimulate their uptake of [3H]thymidine and [3H]leucine, now is shown to bind stereospecifically to cultured human lymphoblasts of the IM-9 line. The specific binding of [3H]SP by IM-9 lymphoblasts increases linearly with the concentration of IM-9 lymphoblasts, achieves a plateau after approximately 15 to 20 min at 4 degrees C and 4 to 6 min at 37 degrees C, and is rapidly reversible at both 4 degrees C and 37 degrees C. The binding of [3H]SP at steady-state conditions demonstrates a dissociation constant (KD) of 0.65 +/- 0.19 nM (mean +/- SD, n = 5) and 22,641 +/- 6143 receptors per IM-9 lymphoblast. Maximal specific binding of [3H]SP to IM-9 lymphoblasts is observed at pH 7.4 and is dependent on the presence of Mg2+, but not Ca2+, in the medium. The peptide structural determinants of the inhibition of binding of [3H]SP to IM-9 lymphoblasts by substituent peptides and homologs of SP indicate that the receptors recognize predominantly the carboxy-terminal portion of SP. The characteristics of the interaction of SP with IM-9 lymphoblasts suggests a receptor-directed mechanism by which neuropeptides may modulate specifically the contributions of lymphocytes to immunity.