Blocking of lymphocyte surface binding sites for the soluble suppressor factor by protein-bound polysaccharide, PSK

The ability of protein-bound polysaccharide (PSK) to block the suppressive activity of soluble suppressor factor (SSF) was investigated. The suppressive activity of SSF derived from U-937 cells on phytohemagglutinin (PHA)-induced lymphocyte proliferative (LP) response was significantly reduced in the presence of PSK. The release of SSF was not inhibited by the treatment of U-937 cells with PSK. The suppressive activity of SSF on LP response to PHA was significantly decreased by the pretreatment of responder lymphocytes with PSK. Studies to determine lymphocyte receptor activity were performed. PSK competed with wheat germ agglutinin (WGA) which recognized the same receptor as SSF on the surface of the lymphocyte. Neither PSK nor serum competed with anti-CD4 monoclonal antibody. Thus, PSK may inhibit SSF-mediated suppression by competing for specific binding sites on the surface of responder lymphocytes.