Unaggregated serum IgA binds to neutrophil FcαR at physiological concentrations and is endocytosed but cross-linking is necessary to elicit a respiratory burst

Abstract

My43 is a monoclonal antibody directed against Fc alpha Rs on monocytes that also recognizes neutrophil (PMN) Fc alpha Rs and is able to elicit a respiratory burst in purified cells. It appears to be directed against the immunoglobulin A (IgA) binding site of Fc alpha Rs or an epitope in close proximity, since IgA and My43 compete for binding to PMNs. My43 immunoblotted Fc alpha R that had been affinity purified from PMN membranes and immunoprecipitated a 50-70 kDa protein from radiolabeled PMN membranes, apparently identical to that purified on IgA-Sepharose. These results suggest the presence of a single class of Fc alpha R on PMNs. Binding of unaggregated monomeric or dimeric serum IgA to Fc alpha Rs on PMNs occurs at physiological concentrations, suggesting that in vivo Fc alpha Rs are saturated with ligand. This binding does not elicit a respiratory burst. Purified PMNs do not express surface IgA, since receptor-bound IgA is lost from the cell surface by endocytosis at room temperature although not at 4 degree C. Rapid endocytosis of receptor-bound IgA has been demonstrated by flow cytometry and confocal microscopy. Unoccupied receptor that is able to bind IgA or My43 is subsequently reexpressed. Cross-linking of surface-bound IgA using F(ab')2 anti-alpha chain antibodies triggers an oxidative burst. After internalization of the surface IgA the same F(ab')2 anti-alpha chain antibodies do not trigger the burst.

The Interaction of FcαRI with IgA and Its Implications for Ligand Binding by Immunoreceptors of the Leukocyte Receptor Cluster

“…This corresponded to a change in K d from 0.25 M to 9.1 nM. The low affinity binding found with soluble receptor approximates previous studies where micromolar K d and K i values have been reported with soluble proteins and at the cell surface (10,15,25,46). The high affinity binding observed with immobilized receptor (K d ϭ 9 nM) more closely approximates the result from a study in which GM-CSF priming of neutrophils determined a K d of 6 nM, although in this study the resting K d was still high at 21 nM (43).…”

“…Fc␣RI is expressed on neutrophils, eosinophils, monocytes, and macrophages (10,(12)(13)(14)(15)(16)(17); Kupffer cells (2); and alveolar macrophages (18), and there have been several reports of possible expression on mesangial cells in the kidney (3)(4)(5)(6). IgA-dependant activation of cells via Fc␣RI is believed to be a key factor in host defense, and responses include phagocytosis, respiratory burst, degranulation, and cytokine release (2, 14 -19).…”

Characterization of FcαR-Triggered Ca2+ Signals: Role in Neutrophil NADPH Oxidase Activation

“…Higher concentrations of My43 than used in this study did not trigger larger [Ca 2ϩ ] i (not shown), suggesting that a finite, regulated, emptying of intracellular Ca 2ϩ stores occurs on Fc␣R crosslinking and that the Ca 2ϩ stores are not totally depleted by Fc␣R. Restimulation of neutrophils occurs because Fc␣R rapidly recycles and crosslinking causes rapid release of subplasma membrane vesicle Fc␣R to the cell surface (3,4).…”

“…Human neutrophil Fc␣R binds the Fc portion of immunoglobulin A (IgA) and recycles through an endocytic pathway on binding of monomeric IgA (3) but triggers physiological responses including activation of the NADPH oxidase and phagocytosis on crosslinking (4). Fc␣R crosslinking Fc␣R also triggers release of neutrophil granule enzymes and may therefore contribute to the pathology of infammatory autoimmune diseases (5).…”

Endocytosis of FcαR is clathrin and dynamin dependent, but its cytoplasmic domain is not required

“…It has been known for a long time that IgA and IgAcomplexed immune complex can be internalized by FcαR [7]. However, the underlying mechanism remains unknown.…”