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The primate erythrocyte complement receptor (CR1) as a privileged site:
binding of immunoglobulin G to erythrocyte CR1 does not target erythrocytes
for phagocytosis
ML Reinagel, M Gezen, PJ Ferguson, S Kuhn, EN Martin and RP Taylor
Department of Biochemistry, University of Virginia School of Medicine,
Charlottesville 22908, USA.
The primate erythrocyte (E) complement receptor, CR1, is a transmembrane
glycoprotein located in clusters on the surface of E. In vivo studies have
demonstrated that during processing and clearance of complement-opsonized
immune complexes, large amounts of immunoglobulin G (IgG) can be bound to
primate E via CR1 with no E loss or lysis. However, when comparable amounts
of IgG are bound to other sites on E, in many cases the E are cleared from
the circulation by the mononuclear phagocytic system. Therefore, due to its
role in immune complex processing, CR1 may represent a privileged site on
the primate E. To delineate further this property of E CR1, we performed in
vitro phagocytosis assays in the absence of complement and examined the
ingestion of E, opsonized at various sites with IgG, by peripheral blood
monocytes. When either human or rhesus monkey E were opsonized at sites
other than CR1, with between 1,000 and 15,000 IgG per E, substantial
phagocytosis of E was evident. However, when comparable amounts of IgG were
bound exclusively via CR1, little, if any, phagocytosis was observed. The
key to the low phagocytic level of E opsonized via CR1 may be related to
the requirements of a "zipper mechanism" for phagocytosis first annunciated
by Griffin et al. Based on their findings, we suggest that due to the
presence of preexisting clusters of CR1 on the E membrane, large amounts of
IgG can be bound to E under conditions that preclude circumferential
engagement (and phagocytosis) of the entire E by Fc receptors on the
monocyte.
Volume 89,
Issue 3,
pp. 1068-1077,
02/01/1997
Copyright © 1997 by The American Society of Hematology
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