The immune response of spleen explants from primed rabbits to sheep red cells (SRC). I. DNA synthesis in the development of antibody-producing cells (PEC).

Explants of the spleens of rabbits previously primed to SRC and killed without boosting, were cultured with this antigen and the development of specific PFC (antibody-producing cells) was studied. The majority of PFC occurred in those cells which migrated from the explants into the medium and the antibodies produced by them did not require enhancement of their capacity to fix complement by developing antisera. The need for the continuous presence of antigen in the medium for optimal development of PFC was demonstrated. The majority of PFC arose from a DNA-synthesizing population of cells in these cultures, the migrating cells being very active in this respect. Only a small proportion of these DNA-synthesizing cells were found to be PFC, and it was also concluded that many cells were taking up radioactive material derived from other cells as well as labelled thymidine from the medium. Using colcemid to arrest cells in metaphase, no inhibition of the acute increase of PFC occurred during a 6-hour period of exposure indicating that this increase of these cells was not dependent on cell division. However this same increase of PFC was found to be highly radio-sensitive, indicating the involvement of DNA-dependent processes. It was therefore concluded that many cells including PFC, in these cultures were engaging in DNA synthesis without subsequent division. This was further emphasized by the observed rate of increase of PFC which was too rapid for the estimated rates of cell division, and the excessive number of cells labelling with [H]thymidine in relation to the mitotic rate.