Changes in structure and composition of lymphocyte nuclei during mitogenic stimulation.

Nuclei of lymphocytes stimulated in vitro with concanavalin A (Con A) were classified into three morphotypes: I--unstimulated; II--partially stimulated; III--fully stimulated, lymphoblastic nuclei. During the Con A-induced change from morphotype I to III nuclear volume increased up to sixfold, due to a near 10-fold increase in the interchromatinic region. At the same time, condensed chromatin rose in volume by only about 1.5-fold and became disaggregated in to small clumps. Regressive EDTA-uranyl staining demonstrated a large increase in interchromatinic fibrillar material in morphotypes II and III. Nuclear matrices isolated from stimulated murine lymphocytes showed structures comparable to the interchromatinic region of the morphotypes. The Con A-stimulated change in nuclear structure preceded onset of DNA replication and was unaffected by hydroxyurea or cytosine arabinoside. Cycloheximide blocked the structural change, even when given 20 hr after Con A. Autoradiography after [3H]leucine showed incorporation of label in the interchromatinic region of morphotype II and III nuclei, much of which remained stable during a 48-hr chase period. Nuclear structural activation was inhibited by alpha-amanitin but a significant stable nuclear RNA fraction was not detected. We conclude that an important event in lymphocyte activation is extensive synthesis of stable proteinaceous interchromatinic matrix which may be involved in chromatin remodeling and DNA replication and/or transcription.