Autologous IgM, IgA, and complement binding to sickle erythrocytes in vivo. Evidence for the existence of dense sickle cell subsets.

We have previously reported that sickle erythrocytes sedimenting at high specific density after gradient centrifugation exhibit increased IgG binding in vivo as compared with low-density paired samples. We have performed the present study to determine whether the opsonization of dense sickle cells in vivo could also involve autologous IgM, IgA, and complement. IgA, IgM, and complement binding in vivo to the surface of density-separated sickle erythrocytes was detected by flow cytometric analyses. IgM and complement C3 fragment binding was detected primarily on high-density sickle erythrocytes. With the exception outlined below, IgA binding was detected for all sickle cell fractions that sediment at densities > 1.085 g/mL. IgM, IgA, and complement C3 fragment binding was increased on high-density sickle erythrocytes as compared with low-density paired samples and exceeded that binding to normal erythrocytes by 30% +/- 10% (mean +/- range), 50% +/- 10%, and 41% +/- 5%, respectively. Two-color flow cytometry indicates that high-density sickle cell fractions contain at least two heterogeneous RBC subsets. One is an RBC subset that binds IgA in combination with IgM and C3, and the second subset is devoid of IgA yet binds IgM and C3. These findings indicate that high-density sickle cells exhibit a greater heterogeneity than has been reported in previous studies, which is based on autologous Ig binding in vivo; and suggest that RBC components of this most severely dehydrated sickle cell subpopulation could have heterogeneous origin and pathophysiologic significance. Although the functional role of IgA binding to human RBCs is unclear, our findings that IgM and complement bind to the same high-density sickle cell fractions suggest that both the IgM and the sickle erythrocyte-bound IgG determined in previous studies could mediate the deposition of complement on dense sickle cells in vivo. These findings support the hypotheses that irreversibly sickled cell-enriched high-density sickle RBC subpopulations could be removed from the circulation by erythrocyte phagocytosis that is enhanced by the presence of complement.