Alterations in plasma membrane lipid organization during lymphocyte differentiation

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Abstract
The fluorescent probe merocyanine 540, which binds preferentially to bilayers in which the lipids are loosely packed, was used to investigate changes in the organization of the lipids of the lymphocyte plasma membrane during primary and secondary lymphopoiesis. When mouse thymocytes were incubated with the dye, most immature cells stained, while most mature cells, about to enter the peripheral circulation, did not. Similarly, mature lymphocytes from both mouse and human peripheral blood did not stain, but these same cells did when activated by in vitro mitogenic stimulation. Freshly isolated splenic lymphocytes, presumably activated in vivo by antigen, also bound merocyanine 540, but after 48 hours of culture in the absence of stimulus they displayed only a low affinity for the dye, a phenotype that reverted to a high affinity upon mitogenic stimulation. These results suggest that changes in the organization of the lipids of the plasma membrane take place during lymphocyte differentiation: viz., immature cells possess a disordered membrane that becomes increasingly ordered as the cells mature and enter the peripheral circulation; then, upon antigen-induced differentiation, the plasma membrane again becomes disordered. These lipid organization changes are discussed in the context of their possible role in the regulation of lymphocyte circulation via intercellular interactions between lymphocytes and cells of the reticuloendothelial system.