Evidence for a role of accessible galactosyl or mannosyl residues of Fc domain in the in vivo clearance of IgG antibody-coated autologous erythrocytes in the rat

The potential role of accessible galactosyl or mannosyl residues of IgG in the
clearance of IgG-coated autologous red blood cells (IgG-RBCs) by the spleen and
the liver was investigated in the rat using rabbit anti-rat RBC IgG antibody
molecules differing from each other by their capacity to bind in vitro to peanut
agglutinin (PNA) and concanavalin A (Con A), i.e., two lectins that specifically
bind to beta-galactosyl and alpha-mannosyl residues of Fc domain, respectively.
Those IgG molecules [IgG(PNA) or IgG(Con A)] were separated from the starting
anti-RBC IgG antibody batch [IgG(total)] by affinity chromatography on lectin
columns. Each IgG-RBC preparation was labeled with 99mTc, and was reinjected iv
with autologous rat RBCs labeled with 111In to correct for 99mTc present in the
blood contained in each organ. The mean specific spleen uptakes per gram of the
three IgG-RBC preparations increased according to the level of RBC sensitization
but were at least 10 times higher in each instance than the mean specific liver
uptake per gram. Consistent with the clearance curves of IgG(PNA)-RBCs, the mean
specific splenic uptake per gram of those RBCs was significantly increased as
compared to the same parameters determined using either IgG(total)-RBCs or
IgG(Con A)-RBCs. In contrast, the mean specific liver uptakes per gram of
IgG(PNA)-RBCs, of IgG(Con A)RBCs, or of IgG(total)-RBCs were not significantly
different under otherwise identical experimental conditions. The increase in the
splenic removal of IgG(PNA)-RBCs was C3 independent. Furthermore, splenic
macrophages isolated from rats were able to bind in vitro significantly more
IgG(PNA)-RBCs than IgG(total)-RBCs or IgG(Con A)-RBCs. These data therefore
support the concept that, in the rat, the kinetic of removal of IgG-RBCs from the
bloodstream by the Fc receptors of splenic mononuclear phagocytes may vary
according to the nature of accessible carbohydrates located in the Fc domain of
IgG antibody molecules coating the erythrocytes.