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FcγRs on Human Neutrophils: Emerging Patho-Physiological Concerns pp. 123-152 $0.00
Authors:  (Louis Marois, Emmanuelle Rollet-Labelle, Paul H. Naccache, Centre de Recherche en Rhumatologie et Immunologie, Centre de Recherche du CHUQ, Department of Microbiology-Infectiology and Immunology, Faculty of Medicine, Laval University, Québec, Canada)
Abstract:
In the immune system, phagocytes resolve a very large proportion of infections
subsequently to the recognition and the destruction of pathogens. This crucial function is
enhanced by the opsonisation of pathogens with immunoglobulins (mostly
immunoglobulin G (IgG)). The latter are recognized by Fcγ receptors (FcγRs) through the
Fc portion of IgGs. FcγRs also interact with IgG-containing immune complexes. Human
neutrophils, the most abundant circulating phagocytes, constitutively express two types of
FcγRs, FcγRIIa (CD32a) and FcγRIIIb (CD16b). This combination of isoforms is unique
to human neutrophils. FcγRIIa possesses an Immunoreceptor Tyrosine-based Activation
Motif (ITAM) in its cytoplasmic portion, whereas FcγRIIIb is a membrane-anchored
receptor which does not possess any cytoplasmic sequence. Neutrophils are the only
human phagocytes which do not express Immunoreceptor Tyrosine-based Inhibition
Motif (ITIM)-bearing FcγR, i.e. FcγRIIb, which is thought to downregulate the signal of
ITAM-bearing FcγRs. The absence of FcγRIIb on human neutrophils indicates that
alternative negative regulatory processes may be at play. A recent study from our
laboratory described a novel down-regulation mechanism of the activation of FcγRIIa
mediated by a proteasomal degradation process.
Little is known about the biological relevance of the co-expression of two FcγRs
with overlapping ligand-binding specificity on human neutrophils. Cholesterol-rich and
detergent-resistant plasma membrane micro-domains (DRMs or lipid rafts) appear to be
required for the transmission of the appropriate signal leading to the functional
responsiveness of neutrophils. Our previous studies provided evidence for a recruitment
of both FcγR isoforms in these micro-domains. FcγRs recruitment in DRMs is a very
early event observed following their cross-linking. These observations suggest that
cooperation between FcγRIIa and FcγRIIIb, leading to an optimal neutrophil response,
may take place in these domains. We are presently investigating this functional
cooperation and our results indicate that both isoforms are essential for an optimal
phagocytosis. 


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FcγRs on Human Neutrophils: Emerging Patho-Physiological Concerns pp. 123-152