Metabolic breakdown of [3H]thymidine and the inability to measure human lymphocyte proliferation by incorporation of radioactivity.

Quantitative measurement of the incorporation of tritiated thymidine into cultures of phytohemagglutinin-stimulated lymphocytes is routinely used as an indication of the immunocompetence of the cells and of their proliferation. The present experiments show that several components of human blood catabolize nucleosides, including thymidine, extensively. Most of the radioactivity from tritiated thymidine, for example, is quickly rendered unincorporable as the compound is metabolized to thymine and further breakdown products. Thus, cells continue to proliferate without incorporating radioactivity from the medium. Furthermore, variability in the degree of catabolism has been found from person to person, so that neither measurement of the depletion of radioactivity from the medium nor measurement of the amount of label incorporated into the cultures can be used as a quantitative indicator of cell proliferation or immunocompetence.