Nucleoside Transport and Proliferative Rate in Human Thymocytes

The thymus is a site of active T-Iymphoid cell proliferation and DNA synthesis. In this study, the capacity of human thymocytes for nucleoside transport was assessed both by cytosine arabinoside influx and by equilibrium binding of nitrobenzylmercaptopurine riboside (NBMPR). a specific ligand for the equilibrative nucleoside transporter of leukocytes. The proportion of freshly isolated thymocytes synthesizing DNA was 8.6% ± 2.1 % (n = 12) by 3H-thymidine labeling index and 7.8% ± 2.9% (n = 4) S-phase cells by flow cytometric analysis of DNA content. In comparison, both methods gave proliferation S-phase values <1 % for peripheral blood lymphocytes (PBLs). Thymocytes expressed a high density of specific NBMPR binding sites (26.068 ± 8,776 sites per cell. n = 1 2) as compared with PBLs (1.123 ± 553 sites per cell. n = 8). The initial influx of cytosine arabinoside into thymocytes was 14-fold greater than into PBLs, and in both cell types the influx of nucleoside was totally inhibited by 0.5 smol/L NBMPR, which is