Leukocyte Labeling in Rats during and after Continuous Infusion of Tritiated Thymidine: Implications for Lymphocyte Longevity and DNA Reutilization

Leukocyte labeling was studied in rats during and after continuous intravenous infusion of H³-thymidine. The radioisotope was administered for varying periods up to 271 days. The results permit the following conclusions:

1. The median survival of small lymphocytes is about 1 month. Five to 8 per cent of small lymphocytes have a life span of more than 9 months.

2. Following the administration of H³-thymidine, reutilization of the tracer markedly delays the fall-off of labeled cells in the peripheral blood. Reutilization probably involves H³-thymidine released from labeled DNA during cell death, since suppression occurs with massive infusion of non-labeled thymidine.

3. Unlike granulocytes and large lymphocytes, small lymphocytes label nonuniformly, and appear to be comprised of at least two populations with different intensities of labeling and different turnover rates. The more heavily labeled cells have the faster turnover.

4. The complexity of the labeling process indicated by the present observations must be considered in the interpretation of H³-thymidine data. However, the survival of unlabeled cells during continuous H³-thymidine infusion remains a valid means of measuring the life spans of circulating blood cells.