Identification of a lymphocyte surface receptor for low density lipoprotein inhibitor, an immunoregulatory species of normal human serum low density lipoprotein

Abstract

The present study demonstrates the existence on human peripheral blood lymphocytes of a saturable cell surface receptor for low density lipoprotein inhibitor (LDL-In), a subset of normal human serum low density lipoprotein (LDL) that has been previously demonstrated to suppress selected lymphocyte functions in vivo and in vitro. The binding of radioiodinated LDL-In of demonstrable biological activity occurs rapidly and is quantitatively augmented by prior cultivation of the lymphocytes in lipoprotein-depleted serum, suggesting regulation of receptor density by lipoproteins in vivo. Binding is temperature dependent, facilitated by calcium ions, saturable at 4 degrees C within 40-60 min, and blocked by prior exposure to unlabeled LDL-In. The lymphocyte receptor is trypsin sensitive and regenerates in vitro with a t1/2 of 3.6 h. LDL-In receptors are calculated to have a maximum density of 4,860 +/- 460 per cell if uniformly distributed on all lymphocyte subsets. These receptors have an estimated average association constant of 1.47 X 10(7) liters/mol. When considered in context of the estimated concentration of LDL-In in blood, the receptors should be partially occupied in vivo by endogenous plasma LDL-In. Prior site occupancy inhibition experiments designed to analyze the specificity of LDL-In binding demonstrate that (a) LDL-In is 13.7-fold more effective than whole LDL in blocking the subsequent binding of 125I-LDL-In to cells; and that (b) LDL is 11-fold more effective than LDL-In in blocking the binding of 125I-LKL. This is consistent with the degree of contamination of each lipoprotein with the other lipoprotein. An independent identity of the LDL-In receptor is also supported by observations that in contrast to the previously described LDL receptor, synthesis and expression of the LDL-In receptor on lymphocytes are not suppressed by cultivation of the cells in the presence of 25-hydroxycholesterol and cholesterol. These findings suggest the existence of a previously undescribed and discrete receptor on lymphocytes for LDL-In, and that the modulation of lymphocyte function by LDL-In may be mediated by a specific cell surface receptor pathway.