Expression of low-density lipoprotein receptors in peripheral blood and tonsil B lymphocytes

Abstract

B lymphocytes, purified from peripheral leucocytes from young normolipaemic humans, expressed and internalized low-density lipoprotein receptors (LDLR). The expression was assessed by a monoclonal anti-LDLR. The internalization of LDL was assessed by LDL labelled with 125I (125ILDL) and 1,1'-dioctadecyl-3,3,3',3' tetramethyl-indocarboxycyanine perchlorate (LDL-DiI). The expression of LDLR, assessed by anti-LDLR, was: 38 ± 8% (n = 5) for fresh purified cells, 60 ± 10% (n = 12) for non-stimulated cells, 79 ± 5% (n = 10) for IL-2 (100 U/ml)-stimulated cells and 95 ± 5% (n = 8) for pokeweed mitogen (PWM) (1:200 dilution)-stimulated cells. The optimal concentrations of agonist were 100 U/ml of IL-2, and 1:200 dilution of PWM. IL-2 and PWM increased the internalization of LDL-DiI by 1.5-fold. The internalization of LDL-DiI was maximal at 60 μg of protein/ml (48 ± 8%). Scatchard analysis revealed a Kd of 3.2 ± 0.22 x 10-8 M and 2180 ± 190 binding sites in non-stimulated cells, a Kd of 7.73 ± 0.36 x 10-9 M and 12500 ± 430 binding sites for IL-2 (100 U/ml)-stimulated cells, and a Kd of 7.2 ± 0.43 x 10-9 M and 13 250 ± 450 binding sites for PWM (1:200 dilution)-stimulated cells. Lineweaver-Burk analysis of LDL binding (LDL- DiI) revealed that the apparent Kd for non-stimulated cells was 1.3 ± 0.11 x 10-8M, and 9.2 ± 0.2 x 10-9 M and 7.5 ± 0.25 x 10-9 M for IL-2- and PWM-stimulated cells, respectively. B lymphocytes from tonsils also showed a high expression of LDLR assessed with anti-LDLR (70 ± 6%). The high expression of LDLR and the avid internalization of LDL suggest that LDL may be important for B cell physiological responses.