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

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Abstract
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. 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 proceeded by carrier-mediated transport. Like that described for adenosine, the uptake system for 8-bromoguanoside 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 MW 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. 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. The C8-substituted guanine ribonucleosides apparently trigger the cell at an intracellular site.