Inhibition of Ca2+ accumulation in mitogen-activated lymphocytes: role of membrane-bound plasma lipoproteins.

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Abstract
Low density lipoproteins (LDL) in which apoB [apolipoprotein B] is the only protein constituent inhibit [human] lymphocyte activation induced by the mitogen phytohemagglutinin. LDL suppress primary induction events, most notably enhanced Ca2+ accumulation, which occur as a result of mitogen-triggered alterations of the lymphocyte plasma membrane. Receptors in the lymphocyte membrane, designated immunoregulatory receptors, recognize and bind apoB-LDL. Under conditions of receptor saturation, about 22,500 .+-. 4500 LDL particles are bound to the lymphocyte surface at 4 and 37.degree. C. The Kd is 1.76 .times. 10-7 M and is independent of temperature. Suppression of phytohemagglutinin-induced 45Ca2+ accumulation by LDL is a receptor-dependent process; inhibition is contingent on attachment of LDL to the immunoregulatory receptors. This conclusion is supported by the following 3 lines of evidence: heparin restores the susceptibility of the lymphocytes to mitogenic challenge by displacing membrane-bound LDL; the immunoregulatory potency of LDL correlates directly with the amount of LDL bound at the cell surface; LDL-Sepharose complexes, which cannot be internalized by the cells, are as immunosuppressive as soluble LDL. Membrane-bound plasma lipoproteins apparently influence the behavior of intact cells.

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