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

Abstract

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.

Idiotypic Networks and Other Preconceived Ideas

“…In rats, about 10% of blood lymphocytes were found to be labelled after one day, 30% after 5 days. The rate of increase of this percentage then slowed down, and 5-8% of blood lymphocytes remained unlabelled after 9 months of thymidine infusion (Robinson et al 1965). TTiese results indicate that about one third of the lymphocytes present in the blood of mice or rats have newly arrived from the generating tissues.…”

“…These authors (Ropke & Everett 1975) furthermore concluded that a majority of emerging small T cells die within a few days, and pointed out that only a very small fraction of newly formed lymphocytes are needed to maintain the pool of long-lived lymphocytes in the body. If we imagine that 20% of all lymphocytes present in a mouse (the same percentage as found [Robinson et al 1965] for lymphocytes in the blood of rats) have lifespans exceeding 100 days and that another 20% have Hfespans of a few days only, then clearly the lifespan distribution of the newly formed cells must be quite different from the lifespan distribution of the total lymphocyte population. To maintain a steady state, the population of newly formed cells must contain manyfold more cells having lifespans of a few days than cells having lifespans of more than 100 days.…”

“…Occasionally, these "stable" lymphocytes must proliferate and secrete immunoglobulin. The 5-8% unlabelled cells found (Robinson et al 1965) after 9 months of thymidine infusion represent only that fraction of the "stable" lymphocytes that have not divided in that period. This contradicts the view that the large turnover of lymphocytes concerns the entire lymphocyte population (Freitas et al 1982).…”

“…Only 2 x 10'', or 20%, of these remain unlabelled. This is reflected by the finding that, in the blood of rats, 5-8% of lymphocytes remain unlabelled after 9 months of thymidine infusion (Robinson et al 1965). About half of the lymphocytes transported by the blood of my mouse are newly formed, short-lived cells coming from the bone marrow and from their temporary deposits in the spleen and elsewhere (Robinson et ai.…”

Lymphocyte Transformation in Megaloblastic Anaemia: Morphology and DNA Synthesis

“…Recent studies in mammals and humans have shown that the great majority of the small lymphocytes of peripheral blood are resting cells with a relatively long life span and slow turnover in vivo (Schooley et al, 1959;Robinson et al, 1965;Norman et al, 1968;Buckton et al, 1967). During this phase, DNA replication is insignificant and metabolic activities are relatively slow.…”