The effect of normal spleen cells on 3H-thymidine uptake by target cells in vitro

When tumour cells (line L cells) were grown in culture with syngeneic normal non-immune C3H/He mouse spleen cells or in the cell free supernatant of these spleen cells they incorporated less tritiated thymidine (3H-Tdr) than when grown by themselves. Despite this effect there was no depression in either cell growth or DNA synthesis. Autoradiographic studies revealed that the decrease of 3H-Tdr incorporation into tumour cells in the presence of spleen cells was not due to inhibition of cell entry into S phase but due to the amount of 3H-Tdr the tumour cells incorporated. Since the medium of spleen cell cultures was found to contain DNA and the addition of calf thymus DNA to fresh growth medium also resulted in decreased 3H-Tdr uptake by the tumour cells, it was concluded that line L cells utilize DNA released by spleen cells into the medium for de novo DNA synthesis. On the basis of these findings, it is suggested that caution be used in interpreting decreased 3H-Tdr uptake as determined by scintillation counting as evidence for a cytostatic effect exerted by lymphoid cells or their supernatants in vitro.