The question of the biologic significance of asialo-GM1 (aGM1) expression by a limited number of cells including natural killer cells has been raised by the recent demonstrations that aGM1 is expressed by activated macrophages and activated T cells as well as the proliferating thymoblast and functionally mature subpopulations of thymus. The current report demonstrates that the expression of aGM1 on aGM1-negative lymphocytes can be induced by stimulation with mitogens under activating conditions. In addition, activation of the aGM1-negative tumor lines EL-4/F and P388D1/B1 under conditions that result in interleukin 2 or interleukin 1 production, respectively, also result in a significant increase in aGM1 expression by the tumor cell lines. Expression of aGM1 therefore appears to be associated with events occurring as a prelude to or during activation of effector function. Since the aGM1-negative cells do display sialylated versions of aGM1 which can be converted by neuraminidase treatment into serologically recognizable aGM1, it is suggested that the expression of aGM1 might reflect a change in the level of glycolipid sialylation rather than a change in membrane lipid composition per se. The small percentage of lymphocytes in normal, nonstimulated spleen and thymus populations which express significant levels of aGM1 were sorted and analyzed for total cellular protein and RNA. Increases in RNA synthesis and in total cellular protein and RNA above basal levels of resting lymphocytes are considered indicators of a G0----G1 transition. The aGM1-positive populations displayed a larger mean population size (as indicated both by higher forward light scatter and by higher total cellular protein content), and a higher mean population RNA content than the aGM1-negative populations. These data are discussed in the context of the hypothesis that the expression of aGM1 may be associated with early events in the activation of resting cells which prepare the cells for subsequent induction of effector function.

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