The participation of FcRIIIb in the degradation of FcRIIa cannot be seen using monoclonal antibodies for cross-linking but only upon stimulation by HA-IgG
The participation of FcRIIIb in the degradation of FcRIIa cannot be seen using monoclonal antibodies for cross-linking but only upon stimulation by HA-IgG. of calcium that was blocked only by Sodium sulfadiazine antibodies against FcRIIIb. We also observed that this calcium influx as well as the IgG-dependent phagocytosis were dependent on the integrity of the plasma membrane detergent-resistant microdomains to which both isoforms were recruited following activation by heat-aggregated IgGs. These data clarify the mechanisms that regulate the FcRs constitutively expressed on human neutrophils, describe a specific contribution of FcRIIIb at the level of the mobilization of calcium, and provide evidence for a crucial role of detergent-resistant microdomains in this process. Keywords:Calcium, Lipid Raft, Neutrophil, Phagocytosis, Superoxide Ion, Fc Receptors, Calcium Influx, Calcium Mobilization == Introduction == Fc receptors (FcRs)3are important key activators of the immune system. They play major functions in host resistance linking humoral and cellular responses, in particular in the contexts of phagocytosis, antibody-dependent cell cytotoxicity, enhanced antigen presentation and clearance of immune complexes (13). More and more studies suggest also a role for FcRs in systemic auto-immune diseases such as, among others, rheumatoid arthritis, vasculititis, and lupus erythematosus (4). Immune cells express different members of this receptor family (2). Human neutrophils are unique in that they constitutively express two types of FcRs (5,6): FcRIIa (CD32a), a transmembrane protein that possesses a noncanonical immunoreceptor tyrosine-based activation motif in its intracellular portion, and FcRIIIb (CD16b), a GPI-anchored protein whose surface expression is 10-fold higher than that of FcRIIa (135,000versus10,000 receptors/cell, respectively) (7). FcRIIIb was thought to be exclusively expressed by human neutrophils (8), but a recent publication demonstrated that this receptor is also expressed at a low level by human basophils (9). The expression of these two FcRs represents a combination that is a signature of human neutrophils. Under resting conditions, the affinities of these two receptors for the Fc portion of human monomeric IgG are similarly low. Despite the fact that numerous lines of evidence indicate that this engagement of each of these two FcRs stimulates signaling pathways, it is more than likely that, under patho-physiological conditions (phagocytosis, clearance of immune complexes), they are both simultaneously engaged and activated. Phagocytosis is an essential function of neutrophils. This mechanism of clearance of pathogens or immune complexes allows this leukocyte to make an important contribution to the innate immune response. Opsonization of microbial pathogens by antibodies or match Sodium sulfadiazine fragments favors the engulfment of the targets. Phagocytosis of IgG-opsonized pathogens or IgG-containing immune complexes is usually mediated in Sodium sulfadiazine great part by the ligation of FcRs. Several studies show that FcRIIa is usually directly involved in the phagocytic process (1013), and the results of different studies indicate that this expression of FcRIIa (14), but not that of FcRIIIb (15), is sufficient to confer phagocytic ability to transfected fibroblasts. These observations explain why FcRIIa was considered as the major, if not the unique, FcR isoform involved in the IgG-dependent phagocytosis in human neutrophils. However, a synergistic enhancement of phagocytosis is usually observed when these two receptors are present and brought on (16), and recent Sodium sulfadiazine publications report decreased phagocytic activity in neutrophils from FcRIIIb-deficient donors, despite the presence of functional FcRIIa (17,18). These data illustrate the complexity Sodium sulfadiazine of the poorly understood roles of the FcRIIIb in FcR-dependent phagocytosis in human neutrophils. Most of the previous studies were performed using activation with FcR isoform-specific monoclonal antibodies, which makes it hard to clearly delineate the specific contributions Nkx2-1 of FcRIIa-dependentversusFcRIIIb-dependent signals to the functional responses of the neutrophils as well as providing little information about potential cooperative between these two receptors..