beta Galactoside binding protein inhibits B cell growth and induces cell apoptosis.

P galactoside binding protein (PGBP) is a novel physiological regulator of the cell cycle secreted by mouse embryo fibroblasts (MEFs) [l]. Addition of PGBP to MEFs has been shown to induce cell cycle arrest. PGBP is a monomeric protein of 134 amino acids long. PGBP's cytostatic effect is independent of its carbohydrate-binding activity and it operates by binding to high affinity receptors on target cells with a Kd of 10-'% [I]. The effects of PGBP on growth and viability of a range of human B cell lines as well as on the activation of primary tonsillar human B cells has been investigated in the present study. PGBP, in a dose-dependent manner, inhibited growth of JMl (pre-B), Ramos (Burkitt lymphoma), Raji (Burkitt lymphoma) and Daudi (Burkitt lymphoma) B cell lines. Maximal growth inhibition was achieved with concentrations of between 4Ong/d and 40Ong/ml which was similar to the concentrations required for growth inhibition of MEFs [I]. Growth inhibition was evident 24h after exposure to PGBP and growth arrested cells were larger than untreated cells, perhaps reflecting a block in a late stage of the cell cycle (late S or G2) and/or induced differentiation as has been observed for IL-6 [3,4]. PGBP treated cells remained viable for up to three days after treatment and then underwent apoptosis.