Multiple Regions of Human FcγRII (CD32) Contribute to the Binding of IgG

Abstract

The low affinity receptor for IgG, Fc␥RII (CD32), has a wide distribution on hematopoietic cells where it is responsible for a diverse range of cellular responses crucial for immune regulation and resistance to infection. Fc␥RII is a member of the immunoglobulin superfamily, containing an extracellular region of two Ig-like domains. Cell surface receptors for the Fc portion of IgG (Fc␥R) are expressed on most hematopoietic cells, and through the binding of IgG they play a key role in homeostasis of the immune system and host protection against infection. Three structurally related but functionally distinct classes of Fc␥R have been defined: Fc␥RI, Fc␥RII, and Fc␥RIII (1-3). Fc␥RII is a low affinity receptor for IgG that binds only IgG immune complexes and is expressed on a diverse range of cells such as monocytes, macrophages, neutrophils, eosinophils, platelets, and B cells (1-3). Fc␥RII is involved in a number of immune responses including antibody-dependent cell-mediated cytotoxicity, clearance of immune complexes, release of inflammatory mediators, and regulation of antibody production (1-6).The extracellular region of Fc␥RII comprises two Ig-like disulfide-bonded extracellular domains that are related to the Ig superfamily proteins and are most closely related to the C2 set of Ig domains (7-12). The two Ig-like domain extracellular region of Fc␥RII is structurally conserved in all of the Ig superfamily leukocyte FcRs (including Fc␥RI, Fc␥RIII, Fc⑀RI, and Fc␣RI) and presumably represents an Ig-interactive motif (13-17). The elucidation of the molecular basis of FcR-Ig interactions is fundamental for understanding the mechanisms by which these receptors mediate biological functions such as activation of inflammatory cells, induction of cytokine release, and destruction of pathogens. In previous studies we utilized chimeric Fc receptors to identify the IgG binding region of human Fc␥RII (18,19 (20). However, despite the direct demonstration of only a single region involved in the binding of IgG, there is compelling evidence to suggest that other regions of Fc␥RII contribute to binding. A genetic polymorphism of human Fc␥RIIa, the so called "responder/non-responder" system, results in an amino acid substitution in domain 2 at residue 131 (Arg 3 His), which has been shown to influence the binding of mouse . Similarly, in the mouse a genetic polymorphism of Fc␥RII, identified as differences at residues 116 and 161, defines the epitope of the anti-Ly17.2 mAb 1 that blocks the binding of IgG to this receptor (24, 25). Our previous molecular modeling studies of Fc␥RII domain 2 (wherein the Asn 154 -Ser 161 binding region was located to an exposed loop region; the F/G loop) suggest that these functionally important amino acid changes are situated in the B/C and CЈ/E loops (containing residues 116 and 131, respectively), which are juxtaposed to the F/G loop (contains residue 161) at or near the interface with domain 1 (20). Furthermore, the studies using chimeric Fc␥RII/Fc⑀RI receptors have identified three regions i...

Fine Structure Analysis of Interaction of FcεRI with IgE

“…Cells transfected with the ␥154 -161⑀ chimeric receptor bound IgE-EA ( Fig. 2A, Table I), and the binding was specific, since mock-transfected cells or cells transfected with Fc␥RIIa did not bind IgE-EA (Table I) (25), has been replaced with the homologous Fc⑀RI␣ sequence (Table I). Similar experiments demonstrated that the ␥129 -134⑀ chimeric receptor could also specifically bind IgE-EA (Fig.…”

“…4). The three loop regions identified herein as involved in the binding of IgE by Fc⑀RI␣, namely the F-G, CЈ-E, and B-C, have also been implicated as crucial in the binding of IgG by Fc␥RIIa (25,29) or Fc␥RIII (30,31) (Fig. 4).…”

“…To aid in the localization of putative IgE binding regions of Fc⑀RI␣, we have generated a three-dimensional model of the second extracellular domain (D2) of Fc⑀RI␣ based on the known structure of a related domain, CD4-2. CD4-2 belongs to the C2 set of Ig superfamily members, and sequence alignment of Fc⑀RI␣-D2 with CD4-2 suggests that this domain will adopt a similar folding pattern (25,28). The structure of the Fc⑀RI␣-D2 model is characteristic of the C2 Ig-fold, comprising seven ␤-strands (A, B, C, CЈ, E, F, G) that form two antiparallel ␤-sheets of ABE and CCЈFG (Fig.…”

“…Fc␥RI (37), Fc␥RIIa (25,29), and Fc␥RIII (30,31), reveal a number of similarities in the molecular basis of how these receptors interact with their respective ligands. The two Ig-like domains of the extracellular regions of Fc⑀RI␣, Fc␥RIIa, and Fc␥RIII and the first two domains of the three domains of Fc␥RI clearly represent a structurally conserved Ig binding motif of this receptor family.…”

“…The model of Fc⑀RI␣-D2 was constructed by mutation of our previously described model of human Fc␥RIIa-D2 (25), which was based on the crystal structure of domain 2 of CD4 (protein data base file pbd2cd4.ent; Brookhaven National Laboratory, Upton, NY) (26,27). Briefly, using a sequence alignment of Fc⑀RI␣-D2 with Fc␥RIIa-D2 and CD4-2 (28), regions of Fc␥RIIa-D2 aligned with the ␤-sheet residues of CD4-2 were designated as structurally conserved residues, with other residues designated as loops.…”