Incorporation of [3H] thymidine into the spleens of intact mice during the immune response to sheep erythrocytes (SRC).

The incorporation of [H]thymidine, administered shortly before killing, into the spleens of intact mice during the primary immune response to SRC has been studied, using autoradiography with exposure periods of 184 days before development. A mixture of weakly and heavily labelled nuclei were situated in well-defined areas of the follicles. A marked increase of heavily labelled nuclei coincided with an increased uptake of [H]thymidine into the spleen DNA during the first 3 days of the immune response. At this time the number of lightly labelled nuclei in the follicles was reduced. The heavily labelled nuclei were first apparent in the periarteriolar zone, then in germinal centres spreading out into the red pulp. After the peak of [H]thymidine incorporation was over (day 3–4) weakly-labelled nuclei accumulated in the red pulp and persisted until day 9 after the injection of SRC. It was concluded that many non-dividing cells in mouse spleen were incorporating small amounts of [H]thymidine into their nuclear DNA. In view of the accumulation of such cells in the red pulp during the course of the immune response to SRC, it was considered that this evidence of DNA synthesis was a manifestation of metabolic turnover of this molecule and relevant to the immune process. From the data presented it was also concluded that only a small proportion of the total spleen population engaged in DNA synthesis and proliferation were actually induced to produce specific antibodies. This preliminary investigation showed the complex nature of the immune process leading to antibody synthesis, which requires much further detailed study.