Lipid Synthesis: An Indicator of Antigen-Induced Signal Transduction in Antigen-Binding Cells

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Abstract
A biochemical parameter of lymphocyte activation, lipid synthesis, has been measured in a purified specific antigen-binding cell population (ABC). ABC isolated from immune and nonimmune animals by sequential centrifugation on buoyant density and sedimentation velocity gradients have a 2- to 7-fold higher rate of 14C-choline incorporation into phospholipid than either unfractionated spleen cells or cells depleted of ABC. Also ABC from immune animals were shown to have a 4- to 7-fold higher rate of 14C-acetate incorporation into their neutral lipids than nonbinding controls. The elevated lipid synthesis seen in both nonimmune SRBC-ABC and TNP-SRBC ABC indicates that antigenic contact via the B cell immunoglobulin receptor results in signal transduction and activation of the specific receptor-bearing lymphocyte population. Binding of the same particle (SRBC) to B cells via their Fc receptors did not regularly result in activation of lipid synthesis. The magnitude of the increased lipid synthesis in ABC populations approached that seen in LPS-stimulated spleen cells. We propose that the measurement of early activation events in purified ABC may be a more appropriate criterion for antigen-induced signals than later events such as thymidine incorporation or antibody secretion.