Determination of lymphocyte turnover by continuous infusion of H3 thymidine.

ECENTLY, the life span of the lymphocyte has become a subject of considerable interest as well as controversy among hematologists. The large numbers of lymphocytes entering the general circulation from the thoracic duct point to a short life span of less than a day,1 while isotopic experiments24 indicate either long life spans of weeks or months or reutilization of large fragments of nuclear components. Gowans’ experiments favor extensive recirculation of lymphocytes5 and a long life span, but reutilization remains a possible alternative. \Ve suggest that the life span of lymphocytes from birth to final disappearance can be determined unequivocally by application of the concept advanced by Osgood,6 that continuous availability of an isotopic precursor of DNA should label the DNA in every newborn cell. Cells that leave the circulation at the end of their lives would then be replaced by labeled cells. According to Osgood, the specific activity of DNA isolated from suitable samples should, therefore, increase continuously until all cells originally present are replaced by labeled cells. At this time, a plateau should be reached, provided the level of available precursors and the rate of its incorporation into DNA remain constant. In this manner, the maximum life span of the cells present at the beginning of the experiment would be defined. This general approach can be simplified and refined by the use of H thymidine,7 which is incorporated into DNA only and permits autoradiographic identification of the individual labeled cells. On the death of the last cell born prior to the continuous availability of thymidine, 100 per cent of cells should be labeled. Cell types that are morphologically distinguishable in autoradiographs, such as large and small lymphocytes, can be traced separately without need for physical separation. Moderate variation of the daily dose of H thymidine results in variation of the grain count, but not the percentage of labeled cells. Consequently, reutilization does not affect the estimate of the life span. It should be noted that the life span is defined here as the interval from birth to final disappearance from the blood stream. This includes any initial sojourn in the lymph nodes subsequent to the last division and prior to release into the peripheral circulation as well as any subsequent temporary extravascular period.