Plasma membrane lipid organization and the adherence of differentiating lymphocytes to macrophages

Changes in the packing of phospholipids in the plasma membrane of lymphocytes occur during differentiation within primary and secondary lymphoid organs. As they differentiate, lymphocytes interact with a variety of reticuloendothelial cells, including macrophages. To investigate a possible relation between these two phenomena, the strength of the interactions between lymphocytes and macrophages was measured in vitro as a function of the tightness of packing of phospholipids on the lymphocyte surface. Strength of adherence was measured by the ability of lymphocytes to remain adherent to macrophages when subjected to increasing centrifugal forces. Phospholipid packing was assessed using the fluorescent lipophilic probe merocyanine 540 (MC540), which preferentially binds to bilayers in which the lipids are more loosely packed. Three subpopulations of murine thymocytes were resolved with respect to strength of adherence to peritoneal or thymic macrophages. To determine whether these subpopulations corresponded with the three classes of cells distinguishable by MC540 fluorescence, populations enriched for staining or non-staining cells, and cells sorted on the basis of MC540 fluorescence intensity, were examined. The least fluorescent cells were the least strongly adherent; the most fluorescent cells were the most strongly adherent; and cells of intermediate fluorescence had intermediate adherence. When splenic lymphocytes were examined with respect to adherence to peritoneal or splenic macrophages, similar patterns of fluorescence and adherence were seen. These results suggest that the organization of the plasma membrane lipid bilayer of lymphocytes may be involved in their interactions with macrophages during primary and secondary differentiation. The adherence signal for lymphocytes thus may be similar to that proposed for other blood cells.