RAPID TURNOVER OF THE RECIRCULATING LYMPHOCYTE POOL IN-VIVO

Lymphocytes are unique among blood cells in their capacity to continua lly recirculate between brood and the tissues via the lymph. Previous estimates of lymphocyte lifespan in vivo and the turnover of the recir culating lymphocyte pool have been deduced from indirect labeling tech niques. Using the fluorescent dye PKH-26, individual labeled cells hav e been tracked in sheep for periods >2 months. By direct measurement t heir lifespan was calculated. This label was found to be stable in viv o, allowing long-term analysis of the characteristics of the recircula ting lymphocyte pool. it is possible to calculate the rate of turnover of cells of the recirculating pool based on the rate at which labeled cells disappear from the lymphatic circulation. The recirculating lym phocyte pool was found to repopulate itself every 16.5 +/- 3.0 days. U sing this label, it was estimated that recirculating lymphocytes divid e on average once every 29.8 +/- 6.8 days. Labeled erythrocytes were a lso examined and found to have an average lifespan of 153 days, demons trating no dye loss over the 2 month period of observation. These data suggest that the recirculating lymphocyte pool is a highly dynamic co mpartment, with a high rate of turnover and peripheral cell division i n vivo. This is the first report of the direct measurement of the in v ivo turnover of recirculating lymphocyte pools, and this method may no w be used to further analyte the lifespan of individual lymphocyte sub sets and the in vivo lifespan of other cell types in vivo.