Abstract
The immunological role of a naturally proliferating subpopulation of splenic T cells was investigated using the graftโvs.โhost (GvH) reaction in the mouse. Normal parental spleen cells, purified splenic T cells or lymph node cells were pulseโtreated for one hour in vitro with tritiated thymidine of high specific activity ([^3^H]dThd, โthymidine suicideโ). The treatment specifically and selectively kills proliferating cells which are actively synthesizing DNA, i.e. cells in S phase. Following treatment, the cells were transferred to F~1~ recipients and the GvH reaction measured by the splenomegaly assay. The results showed that the GvH effector cells in the donor spleen and lymph node are nonproliferating T cells. Furthermore, donor spleen cells treated with [^3^H]โdThd consistently had enhanced GvH reactivity when compared to the controls, while the phytohemagglutinin response of these same treated cell suspensions was significantly inhibited. When purified splenic T cells were used, treatment with [^3^H]dThd also caused an increase in the GvH reaction, showing that a T cell population was being affected by the cycleactive agent. These results indicated that some naturally proliferating T cells have suppressor functions, and their specific inactivation allows nonproliferating effector T cells to mount a more vigorous GvH reaction.