Dual effect of vasoactive intestinal peptide on the mitogenic response of rabbit spleen lymphocytes

The effects of vasoactive intestinal peptide (VIP) have been investigated on the mitogenic response of rabbit spleen cells. Specific binding of 125I-VIP to these mononuclear cells is rapid and saturable. Analysis of binding reveals two classes of binding sites, a class of high-affinity binding sites with KD = 0.93 +/- 0.11 nM and maximal binding capacity of 2000 +/- 560 sites/cell, and a class of low-affinity binding sites with KD = 225 +/- 58 nM and maximal binding capacity of 280,000 +/- 60,000 sites/cell. The VIP regulatory effect on mitogen-stimulated rabbit spleen cell proliferation appears to be time dependent and bimodal. When VIP was added simultaneously with mitogens, it induced an inhibition of the proliferative response. With concanavalin A (Con A) or pokeweed mitogen (PWM), addition of 10(-8) M VIP resulted in a maximal 30% inhibition of [3H]thymidine incorporation after 96 h of culture. This inhibitory effect was significant at concentrations from 10(-8)-10(-6) M and half-maximal inhibition was obtained with 1.2 x 10(-9) M VIP. By contrast, when rabbit spleen cells were preincubated for 18 h with VIP alone, the lymphocyte proliferative response to Con A was increased. However, this increase was mitogen-selective, since it was only observed when the T-cell mitogen Con A was used. The maximal response was obtained after 96 h of culture in the presence of Con A. The VIP stimulatory effect was dose-dependent with a maximal effect at 10(-7) M and a half-maximal effect at 1.7 x 10(-9) M VIP. The effect of VIP was also time-dependent, since a 6 h preincubation was sufficient to induce a significant increase in the proliferative response which was maximal after an 18 h preincubation.