Interaction of phytohemagglutinin with plasma membranes of pig lymphocytes and thymus cells

Phytohemagglutinin (PHA) stimulated pig small lymphocytes and thymus cells to transform and synthesise DNA. The lymphoagglutinating and lymphocyte-stimulating activities of PHA were adsorbed by whole lymphocytes and by the purified plasma membranes of lymphocytes and thymus cells. The lymphocyte plasma membrane was 22 times as effective as whole cells (on a dry wt basis) in removing the lymphocyte-stimulating activity. The capacities of the thymus plasma membrane to adsorb the PHA activities were similar to those of the lymphocyte plasma membrane. It was concluded that thymus cells are stimulated to transform by PHA, and that the surface membranes of lymphocytes and thymus cells possess a similar number of receptors for PHA. Sucrose density gradient centrifugation of lymphocyte plasma membrane dissolved in 2% sodium deoxycholate gave a fraction which inhibited the lymphoagglutinating and lymphocyte-stimulating activities of PHA; the sedimentation coefficient of the inhibitory material was between those of immunoglobulin G and ovalbumin. The results suggest that the initial event in the transformation of lymphocytes by PHA is the selective binding of the PHA mitogen by a receptor on the cell surface.