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Control of human B-lymphocyte replication. I. Characterization of novel activation states that precede the entry of G0 B cells into cycle.

Tonsillar B lymphocytes of a particularly high buoyant density were prepared essentially free of contaminating monocytes and T cells. When exposed to anti-immunoglobulin, such cells initiated the hydrolysis of inositol phospholipids. This provides a postulated 'dual signal' for growth through the liberation of intracellular calcium stores and the activation of protein kinase C. Nevertheless, neither anti-immunoglobulin nor direct agonists of this bifurcating pathway (respectively, calcium ionophore and the phorbol ester TPA) were capable, when used alone, of driving cells out of G0 and into RNA synthesis. All three agents did, however, induce two activation antigens at the surface of G0 B cells, which included CD23, p45 and a lineage-unrestricted antigen identified by the monoclonal antibody BK.19.9. Cells that had been exposed to calcium ionophore, but not those activated with either TPA or anti-immunoglobulin, revealed further change indicated by an increased accessibility of their native DNA for the intercalating dye acridine orange. Cells receiving full mitogenic signals in the form of Staphylococcus aureus Cowan Strain I (SAC) or a combination of TPA and calcium ionophore showed the same initial sequelae but continued to enter the cell cycle and progress through to DNA synthesis. The observations identify two phases in the early activation of human B cells, both in terms of various temporal events, and the signals required to promote each activation state. before entering the proliferative cycle. Thus, the exit of human B cells from G0 appears subject to multiple controls that precede those associated with G1 and later phases of the cell cycle.