Autoradiographic studies of DNA synthesis in lymphoid tissues

Using long exposures of stripping film autoradiographs before processing, mixtures of weakly and strongly labelled nuclei were seen in different areas of the mouse spleen. Previous results (Harris et al., 1973) led to the conclusion that many cells, not in division cycle, were labelling with (3H) thymidine and that this process was important for the development of specific antibody-producing cells following stimulation with an antigen such as sheep red cells (SRC). The present data are an analysis of the (3H) thymidine labelling kinetics in the spleens of mice reared in conventional or germ-free conditions. The labelling seen in the 24 h following an injection of (3H) thymidine could best be interpreted on the basis of synthesis of unstable DNA. The changes in the pattern, and distribution of labelled nuclei as well as the intensity of their labelling was not compatible with cell division only, but was also the result of movement of labelled material between the lymphoid cells of the organ. Germ-free mice were followed for 24 days following a single injection of (3H) thymidine. The rate of uptake of label into the spleen was much slower than has been found previously in mice reared in conventional conditions. When SRC were injected 2 h after giving (3H) thymidine the labelling of lymphoid cells in the spleen and blood was quite different to controls given (3H) thymidine alone. Detailed analysis indicated that turnover of labelled material, presumably DNA, as well as cells was involved. This turnover of DNA could be considered to be metabolic in the sense that renewal, increase in amount, loss, and transfer to other cells were involved. These, and other studies, in vivo (Harris & Olsen, 1973) and in vitro (Harris et al. 1975) indicate that such processes, involving DNA, are highly relevant to the development of antibody-producing capacity by cells responding to antigenic challenge.