PHAGOCYTOSIS OF SICKLE ERYTHROCYTES - IMMUNOLOGICAL AND OXIDATIVE DETERMINANTS OF HEMOLYTIC-ANEMIA

Abstract

Hemolytic anemia in sickle disease involves both intravascular and extravascular destruction of erythrocytes. Since the latter presumably involves the RES, interactions were examined between sickle erythrocytes and macrophages. In erythrophagocytosis assays, 18.9 .+-. 7.2% of human marrow macrophages ingest sickle RBC [red blood cell], while only 3.1 .+-. 2.1% ingest normal RBC. This abnormality is not explained by reticulocytosis, and it is strongly dependent upon RBC density. The interaction between sickle RBC and macrophages appears to be partly immunologic, since it is partially blocked by Fc receptor blockade. Admixture of sickle RBC (pretreated with rabbit anti-human-Ig) and Fc-receptor-bearing K562 cells results in 15.6 .+-. 10.6% K562-RBC rosette formation compared with only 0.5 .+-. 1.2% for normal RBC. Regarding other factors that might promote erythrophagocytosis, sickle RBC are found to spontaneously generate twice-normal amounts of dialdehyde byproducts of lipid peroxidation (malondialdehyde or MDA). Peroxide or reagent-MDA treatment of normal RBC significantly enhances their phagocytosis, and MDA is at least 50 times more potent than other aldehydes studied here. Oxidative and immunologic effects may be related, since exposure of MDA-treated RBC to Ig-containing human sera results in a further significant enhancement of erythrophagocytosis. For comparison of different sickle patients, an adherence assay demonstrates that sickle RBC are 1.03-6.85 times more adherent to macrophages than are normal RBC and degree of adherence correlates significantly with irreversibly sickled cell (ISC) counts and hematologic variables reflecting hemolytic rate. Propensity for RBC interaction with macrophages is likely to be a determinant of hemolytic rate in sickle disease. Pertinent mechanisms appear to involve modification of RBC membranes by dialdehyde byproducts of excessive autoxidation and the abnormal acquisition of surface Ig on sickle RBC, although participation of other membrane defects was not excluded. Appearance of the senescence antigen in old normal RBCs apparently represents modification of the membrane by MDA.