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 1,10-dioctadecyl-3,3,30,30 tetramethyl-indocarboxycyanine perchlorate (LDL-DiI). The expression of LDLR, assessed by anti-LDLR, was: 3868% (n¼5) for fresh purified cells, 60610% (n¼12) for non-stimulated cells, 7965% (n¼10) for IL-2 (100U/ml)-stimulated cells and 9565% (n¼8) for pokeweed mitogen (PWM) (1:200 dilution)-stimulated cells. The optimal concentrations of agonist were 100U/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 mg of protein/ml (4868%). Scatchard analysis revealed a Kd of 3·260·22 ´ 10–8 M and 21806190 binding sites in non-stimulated cells, a Kd of 7·7360·36 ´ 10–9 M and 12 5006430 binding sites for IL-2 (100U/ml)-stimulated cells, and a Kd of 7·260·43 ´ 10–9 M and 13 2506450 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 1·360·11 ´ 10–8 M, and 9·260·2 ´ 10–9 M and 7·560·25 ´ 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 (7066%). The high expression of LDLR and the avid internalization of LDL suggest that LDL may be important for B cell physiological responses.