Intracellular lymphocyte activation and carrier-mediated transport of C8-substituted guanine ribonucleosides

The studies described in this report constitute evidence substantiating that certain exogenous nucleoside derivatives can activate lymphocytes by acting intracellularly. These molecules, the C8-substituted guanine ribonucleosides, have recently been demonstrated to exert potent immunostimulatory and immunoregulatory activities both in vitro and in vivo. The current studies were undertaken to investigate whether the site of induction of mitogenesis in murine B lymphocytes by these compounds was intracellular or at the plasma membrane. Uptake of 8-bromoguanosine was found to proceed by carrier-mediated transport. Like that described for adenosine, the uptake system for 8-bromoguanosine could be resolved into high-affinity and low-affinity components. The hypothesis that the C8-substituted guanine ribonucleosides act intracellularly was tested in several ways. Immobilization of these substituted nucleosides, either on Sepharose beads or in the form of high molecular weight polymers, resulted in total loss of their mitogenicity. In addition, maneuvers designed to diminish plasma membrane fluidity interfered with transmembrane signaling by surface membrane-directed mitogens far more than they did with activation by the substituted nucleosides. Furthermore, modulation of surface membrane protein (IgM) with anti-IgM antibodies similarly resulted in differential inhibition of transmembrane signals with relatively little effect on activation by 8-mercaptoguanosine. Taken together, these data are consistent with the hypothesis that the C8-substituted guanine ribonucleosides trigger the cell at an intracellular site.