The ability of normal human monocytes to phagocytose IgG-coated red blood cells is related to the number of accessible galactosyl and mannosyl residues in the Fc domain of the anti-red blood cell IgG antibody molecules

The percentage of normal human monocytes (MCs) that are able to form rosettes
with, and subsequently phagocytose, IgG-coated red blood cells (RBCs) has been
determined in vitro using five batches of anti-RBC IgG antibodies. These
antibodies differed from each other by their capacity to bind to lectins
recognizing two of the oligosaccharide structures of the Fc domain, namely,
peanut agglutinin (PNA) and concanavalin A (ConA) which specifically bind to
beta-galactosyl and alpha-mannosyl residues, respectively. The threshold between
high (H) and low (L) binding capacities (BC) was arbitrarily fixed at 15% of mean
specific binding. For each level of RBC sensitization tested (1500-6000 Ab
molecules/one RBC), the percentage of MCs binding at least three IgG-RBCs was
similar whatever the IgG Ab preparations used. In contrast, the percentage of MCs
capable of phagocytosing at least three IgG-RBCs coated with 3000, 4500 and 6000
IgG/cell, as well as the phagocytosis index (number of IgG-RBCs ingested/100 MCs)
of IgG-RBCs coated with 1500, 3000, 4500 and 6000 IgG/cell, were significantly
lower (P less than 0.01 at least) using IgG Ab molecules with either
[(PNA-H)(ConA-H)] BC, [(PNA-L) (ConA-H)] BC or with [(PNA-L)(ConA-L)] BC than the
corresponding values measured using RBCs coated with IgG Ab molecules exhibiting
[(PNA-H)(ConA-L)] BC. The binding to MCs of 125I-labelled anti-RBC IgG Ab
molecules exhibiting different binding profiles to PNA and to ConA was studied by
Scatchard plot analysis. A single class of binding sites was observed in each
case. MCs bound a mean of 23,000 IgG molecules with a mean association constant
(Ka) for IgG binding of about 1.4 X 10(8) M-1. These data indicate that terminal
(and/or accessible) galactosyl and mannosyl residues of IgG Ab molecules play a
role in the ingestion of IgG-RBCs by human MCs, despite the fact IgG Ab binding
to IgG(Fc) receptors is not significantly affected. Thus, when studying the
phagocytosis of IgG-coated RBC by human MC monolayers, the assay should be
performed not only using similar RBC/MC ratios and IgG coating values, but also
with IgG antibodies having comparable mean PNA and ConA binding capacities.