Hemolysis of human erythrocytes is a new bioactivity of gangliosides

The monosialoganglioside GM1a protects against complement attack

The complement system is a part of the innate immune system in the fluid phase and efficiently eliminates pathogens. However, its activation requires tight regulation on the host cell surface in order not to compromise cellular viability. Previously, we showed that loss of placental cell surface sialylation in mice in vivo leads to a maternal complement attack at the fetal-maternal interface, ultimately resulting in loss of pregnancy. To gain insight into the regulatory function of sialylation in complement activation, we here generated trophoblast stem cells (TSC) devoid of sialylation, which also revealed complement sensitivity and cell death in vitro. Glycolipid-analysis by multiplexed capillary gel electrophoresis coupled to laser-induced fluorescence detection (xCGE-LIF) allowed us to identify the monosialoganglioside GM1a as a key element of cell surface complement regulation. Exogenously administered GM1a integrated into the plasma membrane of trophoblasts, substantially increased binding of complement factor H (FH) and was sufficient to protect the cells from complement attack and cell death. GM1a treatment also rescued human endothelial cells and erythrocytes from complement attack in a concentration dependent manner. Furthermore, GM1a significantly reduced complement mediated hemolysis of erythrocytes from a patient with Paroxysmal nocturnal hemoglobinuria (PNH). This study demonstrates the complement regulatory potential of exogenously administered gangliosides and paves the way for sialoglycotherapeutics as a novel substance class for membrane-targeted complement regulators.

Human platelets express gangliosides with LKE activity and ABH blood group activity

BACKGROUND

Platelets express several neutral glycosphingolipids with ABH and P blood group activity that may play a role in infectious, autoimmune, and alloimmune thrombocytopenia. In RBCs, sialylated glycosphingolipids or gangliosides with blood group activity have also been reported. To determine whether similar antigens are expressed by platelets, the total platelet ganglioside fraction was isolated and screened for blood-group-active glycosphingolipids.

STUDY DESIGN AND METHODS

Platelet gangliosides were isolated by organic extraction, base hydrolysis, anion exchange, silicic acid, and high-performance liquid chromatography. Gangliosides were identified and characterized by high-performance thin-layer chromatography-immunostaining with blood group-specific MoAbs and glycosidase digestion.

RESULTS

Group A, but not group O, platelets express five gangliosides with group A activity. Of five A MoAbs and lectins examined, only MoAbs Birma-1 and MHO4 recognized all five sialyl A bands. The sialyl A bands were sensitive to endoglycoceramidase and neuraminidase. One sialyl A band may represent a branched ganglioside with sialyl-I and group A activity. Platelets also express an LKE-active ganglioside consistent with sialyl-galactosylgloboside.

CONCLUSION

In addition to sialyl-iI and sialyl-Le(x) gangliosides, group A platelets express gangliosides with LKE activity and group A activity. Like RBCs, group A-active gangliosides may act as alloantigens and autoantigens to naturally occurring isohemagglutinins.

Mechanism of intravascular hemolysis in paroxysmal nocturnal hemoglobinuria (PNH)

Paroxysmal nocturnal hemoglobinuria (PNH) hemolysis requires both intravascular complement activation and affected erythrocytes susceptible to complement. This susceptibility is explained by a deficiency in complement regulatory membrane proteins that are attached to the membrane by a glycosylphosphatidylinositol (GPI) anchor. Affected cells lack a series of GPI-anchored membrane proteins with various functions. The lack is caused by a synthetic defect of the anchor due to an impaired transfer of N-acetylglucosamine to phosphatidylinositol which is an early metabolic precursor in the anchor synthesis. Moreover, PIG-A gene responsible for the membrane defect was recently cloned. Further, a possible mechanism of complement activation has been proposed, especially for an infection-induced hemolytic precipitation which is clinically crucial. Thus, the molecular events, leading to intravascular hemolysis characteristic of PNH, has been virtually clarified. Next major concern is the nature of PIG-A: How does PIG-A explain the complex pathophysiology of PNH which exhibits various clinical manifestations?

Expression of cryptantigen Th on paroxysmal nocturnal hemoglobinuria erythrocytes in association with a hemolytic exacerbation

Paroxysmal nocturnal hemoglobinuria (PNH) erythrocytes lack complement regulatory membrane proteins and are susceptible to complement. Although the critical role of complement in intravascular hemolysis in PNH is accepted, the precise mechanism of complement activation in vivo is unknown. Accordingly, in a PNH patient who was suffering from a hemolytic precipitation soon after a common cold-like upper respiratory infection, we analyzed the erythrocytes with lectins and by flow cytometry to detect membrane alteration that lead to complement activation. The lectin reactivity of erythrocytes showed the expression of cryptantigen Th. The patient serum at the time of the hemolysis induced the expression of Th on erythrocytes from PNH patients and from healthy volunteers in vitro, whereas neither the patient serum after recovery from the hemolysis nor blood type-matched control serum from healthy donor showed this activity. Moreover, autologous serum selectively hemolyzed Th+ PNH erythrocytes, but not Th- PNH erythrocytes, or Th+ control erythrocytes. Hemolysis was not observed either in complement-inactivated serum or in blood type-matched cord blood serum, which lacks natural antibodies to cryptantigens. These findings indicate that the immunoreaction of infection-induced Th with natural antibody on PNH erythrocytes is a trigger of the complement activation, leading to intravascular hemolysis.

Purification and characterization of a human membrane protein that activates the alternative complement pathway and allows the deposition of homologous complement C3

A human myeloid cell subline, P39+, is found to be a target for human complement (C) via the alternative pathway and to allow the deposition of multiple C3 fragments on its membranes, though expressing the complement regulatory proteins decay-accelerating factor and membrane cofactor protein. The parent cell line, P39-, which is phenotypically similar to the P39+ subline, does not allow the deposition of homologous C3 fragments. In this study, we established a monoclonal antibody, M161 Ab, which reacted with P39+ but not P39- cells. This Ab recognized a 43-kD protein in P39+ cell lysate transblotted onto nitrocellulose. Using this Ab as a probe, we purified the 43-kD protein, namely, M161 antigen (Ag). M161 Ag had a basic isoelectric point (pI), 9.3-9.4 by chromatofocusing, and was precipitated as an insoluble material at the pI point. The purified M161 Ag was a single-chain protein and did not possess N- or O-linked carbohydrates. When the purified M161 Ag was transblotted onto nitrocellulose and incubated with Mg(2+)-EGTA serum, human C3 fragments were efficiently deposited on M161 Ag. The major species of the deposited C3 fragments was C3b. Furthermore, the C3 fragments bound to the M161 Ag were detached by 1 M hydroxylamine, suggesting that a covalent ester linkage sustains M161 Ag-C3b interaction. NH2-terminal amino acid analysis revealed that M161Ag is a novel membrane protein. Hence, it appeared that M161 Ag is a potent activator of human alternative complement pathway on human cells that activates homologous C3 and allows the deposition of C3b on itself. Thus, under some conditions, homeostasis of complement is maintained even on human cells, not only by the complement regulatory proteins, but also by membrane C3-activating molecules on which C3b is deposited.