Exposure to complement-bearing immune complexes enhances the in vitro sequestration of erythrocytes from young but not elderly donors

Understanding quasi-apoptosis of the most numerous enucleated components of blood needs detailed molecular autopsy

Erythrocytes are the most numerous cells in human body and their function of oxygen transport is pivotal to human physiology. However, being enucleated, they are often referred to as a sac of molecules and their cellularity is challenged. Interestingly, their programmed death stands a testimony to their cell-hood. They are capable of self-execution after a defined life span by both cell-specific mechanism and that resembling the cytoplasmic events in apoptosis of nucleated cells. Since the execution process lacks the nuclear and mitochondrial events in apoptosis, it has been referred to as quasi-apoptosis or eryptosis. Several studies on molecular mechanisms underlying death of erythrocytes have been reported. The data has generated a non-cohesive sketch of the process. The lacunae in the present knowledge need to be filled to gain deeper insight into the mechanism of physiological ageing and death of erythrocytes, as well as the effect of age of organism on RBCs survival. This would entail how the most numerous cells in the human body die and enable a better understanding of signaling mechanisms of their senescence and premature eryptosis observed in individuals of advanced age.

The anti-inflammatory role of the erythrocyte: impairment in the elderly

Human erythrocytes have significant anti-inflammatory capability. Via their complement receptor, CR1 (CD35), they function as the major carriers of immune complexes in the circulation and as a co-factor for factor-I in the cleavage of C3b and the resultant inactivation of C3- and C5-convertases. Erythrocytes of the elderly are defective in their control of C3- and C5-convertases and are thus defective in protecting the elderly from the inflammatory consequences of the activation of complement in the circulation. This defect stems from the reduced levels of CR1 and decay accelerating factor and from defective CR1 function. Erythrocytes of the elderly resemble senescent erythrocytes from young donors in that their CR1 can neither bind immune complexes nor function as a co-factor in the factor-I mediated cleavage of C3b. The erythrocytes of the elderly are defective in both promoting convertase decay and supporting C3b cleavage. The reduced levels and functional defects of erythrocyte CR1 should hamper the ability of the elderly individual to effectively clear the circulation of potentially inflammatory immune complexes as well as of micro-organisms which have bound complement via the alternative complement pathway. This defect in the ability to clear immune complexes and micro-organisms bearing C3b from the circulation should render the elderly individual susceptible to varied pathologies including infection, inflammation and concomitant damage to the vascular tissue commonly observed in the elderly.

Receptors involved in the phagocytosis of senescent and diamide-oxidized human RBCs

BACKGROUND

Senescent RBCs bear IgG and C3 opsonins that are three to four times less than required for similar phagocytosis of experimentally opsonized RBCs.

STUDY DESIGN AND METHODS

Studies were performed to determine the phagocyte receptors involved in phagocytosis in vitro. The effect of clustering of opsonins and oxidative damage in the sequestration of RBCs was studied by exposing RBCs to BS3 (bis[sulfosuccinimidyl]-suberate) and diamide (azodicarboxylic acid bis[dimethyl-amide]).

RESULTS

Sequestration of senescent RBCs was inhibited by the treatment of lymphokine-activated monocytes with N-acetyl-D-galactoseamine (GalNAc), arginine-glycine-aspartic acid (RGD), or antibodies to CR3, FcgammaRI, FcgammaRII, leukocyte response integrin (LRI), and integrin-associated protein (IAP). Exposure to BS3 alone did not enhance phagocytosis. The addition of serum resulted in opsonin binding. The level of opsonization required for sequestration was higher than on senescent RBCs and was only marginally inhibited by blocking CR3, FcgammaRI and FcgammaRII. Diamide treatment alone did not lead to sequestration. Diamide-treated RBCs exposed to serum bound opsonin much as did senescent RBCs, and sequestration was inhibited by GalNAc, RGD, and antibodies to CR3, FcgammaRI, FcgammaRII, LRI, and IAP.

CONCLUSION

Membrane alterations resulting in the binding of opsonins and the sequestration of senescent RBCs may be similar to those that occur on diamide-oxidized RBCs. They suggest the need for cooperative events among oxidation, clustering and cross-linking, and serum opsonization.

In vivo administration of intravenous immunoglobulin (IVIg) can lead to enhanced erythrocyte sequestration

Enhanced erythrocyte sequestration is one of the very few major adverse effects of intravenous immunoglobulin (IVIg). IVIg contains high molecular weight IgG complexes ( approximately 300 kDa) which, in the presence of serum, mimic immune complexes by activating complement, binding to CR1 of red blood cells (RBC) (CD35) and mediating erythrophagocytosis. Four of seven patients undergoing IVIg therapy showed significant drops in haematocrit and haemoglobin that were not due to isoantibodies in the IVIg. Prior to treatment, patients' RBC carried IgG and complement (C') 3d that were not bound as immune complexes via CR1 (CD35). The patients whose RBC bound immune complex-like moieties and showed drops in haematocrit and haemoglobin subsequent to IVIg were young adults (22-35 years); older patients (50-69 years) showed no ill effects. In the presence of complement, RBC of young patients bound IVIg complexes in vitro while those of older patients did not. It is not the absolute levels of erythrocyte-associated IgG or C'3 fragments, neither pre- nor post-therapy, which are predictive of IVIg associated decreases in haematocrit and haemoglobin levels. Patient age and RBC inability to bind the IVIg immune complex-like moieties in vitro both appear to be predictors of resistance to sequestration after in vivo treatment with IVIg.

Immune complex-like moieties in immunoglobulin for intravenous use (i.v.Ig) bind complement and enhance phagocytosis of human erythrocytes

Treatment with i.v.Ig can, on rare occasions, lead to detrimental effects such as enhanced erythrocyte sequestration and an increase in serum immune complexes with inflammatory sequellae such as exacerbation of glomerular nephritis. In this study, i.v.Ig (Sandoglobin) was examined for complement binding moieties which resemble immune complexes and can mediate the binding of IgG and C'3b to human erythrocytes via CR1 and enhance erythrocyte susceptibility to sequestration. Sephacryl S-200 HR separated i.v.Ig into two fractions: monomeric IgG (74%) and larger complexes of the molecular weight of an IgG dimer or greater (> or = 300 kD) (26%). In the presence of complement, the 'dimers' bound to human erythrocytes, rendering them susceptible to phagocytosis in vitro. Removal of erythrocyte-specific isoantibodies from the i.v.Ig had no effect on 'dimer' binding to the erythrocytes. Monomeric IgG contained virtually no complement-activating, erythrocyte-binding activity. Erythrocyte binding of complement-bearing IgG 'dimers' and subsequent phagocytosis resembles the binding of complement-bearing immune complexes to erythrocyte CR1. Exposure to Factor I leads to the release of complement-bearing IgG 'dimers' from erythrocyte CR1 and to the abrogation of erythrophagocytosis. Binding of complement-bearing IgG 'dimers' to the erythrocyte is blocked by To5, a CR1-specific monoclonal antibody.

Erythrocytes from young but not elderly donors can bind and degrade immune complex- and antibody-bound C3 in vitro

In situ aged erythrocytes (senescent erythrocytes from young donors and both young and old erythrocytes from elderly donors) demonstrate high levels of membrane-bound C3c and C3d and elevated susceptibility to in vitro phagocytosis. In this study we demonstrate that in situ aged erythrocytes are defective in their ability to degrade C3 fragments and clear them from the erythrocyte membrane. Erythrocytes from young donors bind complement-bearing immune complexes via CR1 and become susceptible to complement-mediated erythrophagocytosis ('innocent bystander' sequestration). Erythrocytes from elderly donors are defective in their ability to bind such immune complexes, as attested by the lack of an increment in membrane-associated C3 fragments as detected by flow cytometry and lack of an increment in in vitro sequestration. Factor I (serum)-dependent cleavage of C3 fragments and release of immune complexes from the erythrocytes of young donors lead to a drop in erythrocyte-associated C3 fragments and the disappearance of the 'innocent bystander' phenomenon. Inhibition of Factor I, and thus inhibition of C3b degradation and immune complex release for the erythrocyte membrane, enhances the levels of 'innocent bystander' sequestration of erythrocytes from young donors. Erythrocytes from elderly donors are defective in the dynamic process of CR1 binding of complement-bearing immune complexes and Factor I-associated release of membrane-associated C3 fragments. A defect in the ability of all in situ aged erythrocytes to clear their membranes of C3 fragments is also demonstrated for complement bound to the erythrocyte via IgM isoagglutinins. While erythrocytes from both young and elderly donors allow for the IgM-mediated binding of complement to the erythrocyte, only young erythrocytes from young donors demonstrate degradative activity with the release of membrane-associated C3c. Erythrocytes from elderly donors demonstrate lower levels of detectable CR1 (CD35), decay accelerating factor (DAF) (CD55) and Protectin (CD59) than do erythrocytes from young donors. Time course studies determine that the defect in handling of immune complexes observed for in situ aged erythrocytes is not due to differences in the kinetics of loading or releasing of immune complexes. These findings on the refractiveness of erythrocytes of elderly donors to uptake of immune complexes and the degradation of C3 fragments may be of importance not only in the understanding of erythrocyte sequestration, but also in the physiology of immune clearance in the elderly.