Schwartz R S, Tanaka Y, Fidler I J, Chiu D T, Lubin B, Schroit A J
J Clin Invest. 1985 Jun;75(6):1965-72. doi: 10.1172/JCI111913.
The precise mechanism by which sickle erythrocytes (RBC) are removed from the circulation is controversial, although it is possible that enhanced recognition of these cells by circulating mononuclear phagocytes could contribute to this process. We investigated this possibility by interacting sickle cells with cultured human peripheral blood monocytes. Our results show that both irreversibly sickled cells (ISC) and deoxygenated reversibly sickled cells (RSC) had a higher avidity for adherence to monocytes than did oxygenated sickle and normal RBC. ISC were the most adherent cell type. Adherence of RSC to monocytes was found to be reversible; reoxygenation of deoxygenated RSC resulted in a significant decrease in RSC--monocyte adherence. Concomitant with alterations in sickle RBC adherence were alterations in the organization and bilayer distribution of membrane phospholipids in these cells. Specifically, enhanced adherence was associated with increased exposure of RBC membrane outer leaflet phosphatidylserine (PS) and phosphatidylethanolamine, whereas lack of adherence was associated with normal patterns of membrane phospholipid distribution. To investigate the possibility of whether the exposure of PS in the outer membrane leaflet of these cells might be responsible for their recognition by monocytes, the membranes of normal RBC were enriched with the fluorescent PS analogue 1-acyl-2[(N-4-nitro-benzo-2-oxa-1,3-diazole)aminocaproyl]-phosphatidy lse rine (NBD-PS) via transfer of the exogenous lipid from a population of donor phospholipid vesicles (liposomes). RBC enriched with NBD-PS exhibited enhanced adherence to monocytes, whereas adherence of RBC enriched with similar amounts of NBD-phosphatidylcholine (NBD-PC) was not increased. Furthermore, preincubation of monocytes with PS liposomes resulted in a approximately 60% inhibition of ISC adherence to monocytes, whereas no inhibition occurred when monocytes were preincubated with PC liposomes. These findings strongly suggest that erythrocyte surface PS may be a ligand recognized by receptors on human peripheral blood monocytes and that abnormal exposure of PS in the outer leaflet of the RBC membrane, as found in sickle RBC, might serve to trigger their recognition by circulating monocytes. Our results further suggest that abnormalities in the organization of erythrocyte membrane phospholipids may have significant pathophysiologic implications, possibly including shortened cell survival.
镰状红细胞（RBC）从循环中被清除的确切机制存在争议，尽管循环单核吞噬细胞对这些细胞的识别增强可能促成了这一过程。我们通过使镰状细胞与培养的人外周血单核细胞相互作用来研究这种可能性。我们的结果表明，不可逆镰状细胞（ISC）和脱氧可逆镰状细胞（RSC）对单核细胞的黏附亲和力均高于氧合镰状细胞和正常RBC。ISC是最易黏附的细胞类型。发现RSC与单核细胞的黏附是可逆的；脱氧RSC再氧化导致RSC与单核细胞的黏附显著减少。与镰状RBC黏附的改变相伴的是这些细胞中膜磷脂的组织和双层分布的改变。具体而言，黏附增强与RBC膜外层小叶磷脂酰丝氨酸（PS）和磷脂酰乙醇胺的暴露增加相关，而缺乏黏附则与膜磷脂分布的正常模式相关。为了研究这些细胞外膜小叶中PS的暴露是否可能是它们被单核细胞识别的原因，通过从一群供体磷脂囊泡（脂质体）转移外源性脂质，使正常RBC的膜富含荧光PS类似物1-酰基-2-[(N-4-硝基苯并-2-恶唑-1,3-二唑)氨基己酰基]-磷脂酰丝氨酸（NBD-PS）。富含NBD-PS的RBC对单核细胞的黏附增强，而富含等量NBD-磷脂酰胆碱（NBD-PC）的RBC的黏附并未增加。此外，用PS脂质体预孵育单核细胞导致ISC对单核细胞的黏附受到约60%的抑制，而当用PC脂质体预孵育单核细胞时未发生抑制。这些发现强烈表明，红细胞表面PS可能是人类外周血单核细胞上受体识别的配体，并且如在镰状RBC中发现的，RBC膜外层小叶中PS的异常暴露可能触发循环单核细胞对它们的识别。我们的结果进一步表明，红细胞膜磷脂组织的异常可能具有重要的病理生理意义，可能包括细胞存活时间缩短。
