A pair of naturally occurring antibodies may dampen complement-dependent phagocytosis of red cells with a positive antiglobulin test in healthy blood donors.
OBJECTIVE
It is known that red blood cells (RBC) from healthy blood donors with a positive direct antiglobulin test (DAT) for IgG continue to circulate despite carrying elevated numbers of IgG molecules. To unravel the properties of these RBC-bound IgG, we studied them not only on whole RBC populations, but also on density-fractionated RBCs.


METHODS
The properties of acid-eluted RBC-bound IgG and plasma IgG were studied by ELISA for binding to RBC proteins and opsonins, and by blotting. In vitro phagocytosis was studied on density-separated RBCs.


RESULTS
IgG-DAT-positive blood donors carried most IgG molecules on dense RBCs and had more RBCs of high density than DAT-negative controls. Their densest RBCs were older than the oldest RBCs of DAT-negative controls, based on the band 4.1a/b ratio. In vitro phagocytosis of senescent RBCs from IgG-DAT-positive donors was 1.5 to 2 fold higher than that of senescent control cells, but the same or less in the presence of physiological IgG concentrations, implying that RBC-bound IgGs impaired complement-dependent uptake. The IgG molecules on these DAT-positive RBCs comprised anti-band 3 naturally occurring antibodies (NAbs) and were two- to fivefold enriched in anti-C3 and framework-specific anti-idiotypic NAbs as compared to controls. Correspondingly, anti-C3 and framework-specific anti-idiotypic NAbs were proportionally elevated in the plasma of two-thirds of DAT+ donors.


CONCLUSIONS
Extra-binding of anti-C3 together with anti-idiotypic NAbs to senescent RBC-associated C3 fragments may suppress complement-dependent RBC phagocytosis and may prolong the in vivo life span of RBCs.
Alaia
V
V
Institute of Biochemistry, Department of Biology, ETH Zurich, Zurich, Switzerland.
Frey
B M
BM
Siderow
A
A
Stammler
P
P
Kradolfer
M
M
Lutz
H U
HU
D000888:Antibodies, Anti-Idiotypic / Q000276:immunology*; D001782:Blood Donors; D003176:Complement C3 / Q000276:immunology*; D003298:Coombs Test; D004912:Erythrocytes / Q000276:immunology*; D006801:Humans; D007074:Immunoglobulin G / Q000276:immunology*; D010587:Phagocytosis / Q000276:immunology*
D000888:Antibodies, Anti-Idiotypic; D003176:Complement C3; D007074:Immunoglobulin G