Inhibition by Phytohemagglutinin of DNA Synthesis in Cultured Mouse Lymphomas

Summary

Crude extracts of a variety of plants possess agglutinins for both normal and malignant cells. Phytohemagglutinin (PHA), the most widely used plant lectin, both stimulates macromolecular synthesis and causes agglutination of normal lymphoid cells. We showed that PHA, at concentrations stimulating DNA synthesis in normal mouse splenic lymphocytes, inhibited DNA synthesis in cultured Gross ascites lymphoma cells. The inhibition was partially reversible and affected RNA as well as DNA synthesis. Another plant lectin, pokeweed mitogen, did not inhibit lymphoma cells but did inhibit DNA synthesis in myeloma cells. Evidence presented suggests that neoplastic cells have an increased density of mitogen receptor sites on their surface. The specificities of the receptor sites on lymphoid tumors may be related to the differentiation influences which characterize the thymus-dependent and the thymus-independent developmental pathways. While the precise mechanism of the inhibitory action of PHA on lymphoid cells is not known, it may be qualitatively identical to, but quantitatively an exaggeration of, its stimulatory action on normal lymphoid cells.