Qualitative differences in the lymphocyte phytohemagglutinin (PHA) response between the bone marrow and other lymphoid organs

Abstract

Bone marrow exhibits a limited response to the “T cell mitogen” phytohemagglutinin (PHA) which differs qualitatively from the response obtained with other populations of lymphocytes. Since conventional techniques were not sufficiently sensitive for analyzing the meager response of the marrow, a highly sensitive microculture assay system was devised which relies on purification of bone marrow lymphocytes by sucrose gradient centrifugation and on strict adherence to culture conditions optimal for the marrow. Using these techniques in CBA mice, PHA responsive cells in the marrow were found to differ qualitatively from responsive cells of other lymphoid organs in spite of the fact that they were morphologically identical. Maximal stimulation of bone marrow lymphocytes by PHA occurred at a mitogen concentration five times the optimal dose for thymus, lymph node and spleen cells. Furthermore, the BML PHA response peaked in a broad plateau between Days 4 and 7 of incubation whereas the other lymphoid organs demonstrated an early “spike” response to PHA maximal at 2–3 days of culture. The unique kinetics of the BML PHA response are compatible with a hypothesis which suggests that the marrow response to PHA is mediated by precursor T cells, whereas the responding cells in other lymphoid organs are mature T cells.