Role of the tubulin-microtubule system in lymphocyte activation

The role of the tubulin-microtubule system was examined in human peripheral blood leukocytes after activation with phytohemagglutinin (PHA). Soluble tubulin and microtubules were measured with a [³H]colchicine-binding assay. It was found that the tubulin content of PHA-activated lymphocytes was consistently increased relative to total protein content after 36 h of culture. There was no increase in the proportion of total tubulin which was present as microtubules at 36 h. Nevertheless, as a result of increased tubulin synthesis, there was a two- to three-fold increase in total microtubular mass. Colchicine, which disrupts microtubules, was used to assess the role of microtubule assembly in the sequence of events which follow lymphocyte activation, namely lymphokine release, protein synthesis, RNA synthesis, and DNA synthesis. Colchicine consistently inhibited DNA synthesis but did not inhibit release of the lymphokine, osteoclast activating factor (OAF). Protein and RNA syntheses were inhibited much less than DNA synthesis. The fact that some effects of PHA on lymphocytes appear to require intact microtubules and at least one does not suggest that the microtubule-dependent step in PHA-stimulated lymphocyte activation occurs at a stage after propagation of the signal from the membrane to the cell interior.