Role of associated gamma-chain in tyrosine kinase activation via murine Fc gamma RIII.

Type III receptors for the Fc portion of IgG (Fc gamma RIII), initially characterized on macrophages and NK cells, are also expressed on several pre-B cell lines. Surface expression of Fc gamma RIII requires the association of the ligand binding alpha-chain with homodimeric gamma-chains. Type II Fc gamma R is homologous to Fc gamma RIII alpha-chain in the extracellular portion and differs in the transmembrane and cytoplasmic domains. The role of Fc gamma R in cell activation was investigated by expressing Fc gamma RIII and the lymphocyte-specific b1 isoform of Fc gamma RII (Fc gamma RIIb1) in an Fc gamma R-negative, sIgG-positive B-cell line. We found that, in contrast to Fc gamma RIIb1, Fc gamma RIII triggers the same events of cell activation as sIG i.e. Ca2+ mobilization, tyrosine phosphorylation and IL-2 secretion. By expressing cytoplasmic domain-lacking Fc gamma RIII alpha-chain in the absence or in the presence of gamma-chains, we demonstrated that cell activation via Fc gamma RIII requires the co-expression of gamma-chains, and is independent of the cytoplasmic portion of the alpha-chain. Furthermore, the cytoplasmic portion of the gamma-chain, fused to the extracellular and transmembrane domains of Fc gamma RII confers on the chimeric receptor the ability to trigger cell activation. Mutation of one tyrosine residue in the cytoplasmic domain of the gamma-chain prevented triggering of cytoplasmic signals. We therefore demonstrate that a tyrosine-containing motif, present in the cytoplasmic domain of the associated gamma-chain, is necessary and sufficient to trigger cell activation via Fc gamma RIII.     

Structure and mapping of the gene encoding mouse high affinity Fc gamma RI and chromosomal location of the human Fc gamma RI gene.

We describe the isolation and characterization of the gene encoding the mouse high affinity Fc receptor Fc gamma RI. Using a mouse cDNA Fc gamma RI probe four unique overlapping genomic clones were isolated and were found to encode the entire 9 kb of the mouse Fc gamma RI gene. Sequence analysis of the gene showed that six exons account for the entire Fc gamma RI cDNA sequences including the 5'- and 3'-untranslated sequences. The first and second exons encode the signal peptide; exons 3, 4, and 5 encode the extracellular Ig binding domains; and exon 6 encodes the transmembrane domain, the cytoplasmic region, and the entire 3'-untranslated sequence. This exon pattern is similar to Fc gamma RIII and Fc epsilon RI but differs from the related Fc gamma RII gene which contains 10 exons and encodes the b1 and b2 Fc gamma RII. Southern blot analysis had shown that the mouse Fc gamma RI gene is a single copy gene with no RFLP in inbred strains of mice, but analysis of an intersubspecies backcross of mice showed that unlike other mouse FcR genes which are on mouse chromosome 1 the locus encoding Fc gamma RI, termed Fcg1, is located on chromosome 3. Interestingly, the Fcg1 locus is located near the end of a region with known linkage homology to human chromosome 1. Analysis of human x rodent somatic cell hybrid cell lines indicates that the human FCG1 locus encoding the human Fc gamma RI maps to chromosome I and therefore possibly linked to other FcR genes on this chromosome. These results suggest that the linkage relationships among these genes in the human genome are not preserved in the mouse.     

Murine recombinant Fc gamma RIII, but not Fc gamma RII, trigger serotonin release in rat basophilic leukemia cells.

Murine Fc gamma RII and Fc gamma RIII have highly homologous extracellular domains, but unrelated transmembrane and intracytoplasmic (IC) domains. Murine Fc gamma RIIb1 and b2 are two isoforms of single-chain receptors which differ only by 47 aa in their IC domain. Murine Fc gamma RIII are composed of an IgG-binding alpha-chain, the intracellular portion of which is unrelated to that of Fc gamma RII, and of a homodimeric gamma-chain which also associates with Fc epsilon RI. Murine mast cells express Fc gamma RII, Fc gamma RIII, and Fc epsilon RI. They can be induced to degranulate by murine IgG immune complexes or by F(ab')2 fragments of the rat anti-murine Fc gamma RII/III mAb 2.4G2, complexed to mouse anti-rat (MAR) F(ab')2. In order to determine which murine Fc gamma R can activate mast cells, cDNA encoding murine Fc gamma RIIb1, Fc gamma RIIb2 or Fc gamma RIII alpha were stably transfected into RBL-2H3 cells. Murine Fc gamma RIII but not Fc gamma RIIb1 or Fc gamma RIIb2 induced serotonin release when aggregated by (2.4G2-MAR) F(ab')2 complexes. The respective roles of the IC domains of murine Fc gamma RIII subunits in signal transduction were investigated by stably transfecting cDNA encoding IC-deleted or chimeric murine Fc gamma R into RBL-2H3 cells. The substitution of the IC domain of murine Fc gamma RII for that of murine Fc gamma RIII gamma, but not that of murine Fc gamma RIII alpha, conferred the ability to trigger serotonin release. The deletion of IC sequences of the alpha subunit did not alter the ability of murine Fc gamma RIII to trigger serotonin release. It follows that 1) murine Fc gamma RIII, but not Fc gamma RII, can induce RBL cells to release serotonin, 2) the aggregation of the IC domain of the murine Fc gamma RIII gamma subunit is sufficient, but 3) the IC domain of the murine Fc gamma RIII alpha subunit is neither sufficient nor necessary for triggering serotonin release.     

Analysis of Fc gamma RIII (CD16) membrane expression and association with CD3 zeta and Fc epsilon RI-gamma by site-directed mutation.

Two genes encode Fc gamma RIII (CD16), a low affinity FcR for IgG. CD16-I is expressed as a phosphatidylinositol glycan-anchored membrane glycoprotein on neutrophils, whereas CD16-II is a transmembrane-linked glycoprotein on NK cells. Membrane anchoring is determined by codon 203. Site-directed mutation of codon 203 and transient expression of these cDNA in COS-7 cells indicated that Phe, Ile, Leu, and Val permit transmembrane expression, whereas Ser, Thr, Tyr, Asn, Gly, Ala, Asp and Lys enable phosphatidylinositol-glycan attachment. Thus, the involvement of amino acid 203 in membrane anchoring cannot be explained simply on the basis of size, charge, or polarity of the amino acid side groups at this site. Efficient expression of CD16-II in COS-7 cells requires co-transfection with either CD3 zeta or Fc epsilon RI-gamma. Truncation of the cytoplasmic segment of CD16 failed to affect association with CD3 zeta. CD3 zeta and Fc epsilon RI-gamma with truncated cytoplasmic segments were also able to facilitate membrane expression of CD16-II, implicating the transmembrane segments as the interaction site between CD16-II and CD3 zeta or Fc epsilon RI-gamma. Prior studies have suggested that the acidic residue in the CD3 zeta transmembrane segment may be important for the association of CD3 zeta complexes. Although site-directed mutation of CD3 zeta-Asp36 to Glu, Leu, or Val retained the ability to permit membrane expression of CD16-II, quantitatively the wild-type CD3 zeta-Asp36 provided optimal levels of expression, consistent with conservation of this amino acid in mouse and human CD3 zeta.     

Molecular cloning and expression of the mouse high affinity Fc receptor for IgG

Full length cDNA clones encoding the mouse Fc gamma RI were isolated by using redundant oligonucleotide probes based on previously determined amino acid sequence of protein bound to an IgG2a antibody column. Sequence analysis of cDNA clones indicates that mouse Fc gamma RI is a transmembrane glycoprotein that is composed of three disulfide bonded extracellular Ig binding domains unlike Fc gamma RII of man and mouse. These extracellular domains contain five potential sites of N-linked glycosylation; three sites in the first domain and one in each of the second and third domains. In addition a transmembrane region is present followed by a cytoplasmic tail of 84 amino acids. Analysis of the amino acid sequence of the first two extracellular domains of Fc gamma RI indicate that these are highly homologous to the extracellular domains of Fc gamma RII; the third domain is different and shows a lower level of homology to other FcR domains but is clearly related to the Ig super-family. Transfected cells expressing Fc gamma RI were shown to bind immune complexes of rabbit IgG; and monomeric IgG2a bound to transiently transfected cells with an affinity of approximately 5 x 10(7) M-1, i.e. the receptor was of high affinity and therefore was by definition Fc gamma RI. Northern analysis demonstrated that Fc gamma RI mRNA could be detected in the Fc gamma RI+ myeloid cell lines WEH1 3B and J774. Finally, Southern analysis indicated that Fc gamma RI is likely to be encoded by a single copy gene of approximately 9 kb.     

Alefacept, an immunomodulatory recombinant LFA-3/IgG1 fusion protein, induces CD16 signaling and CD2/CD16-dependent apoptosis of CD2(+) cells

Alefacept, an immunomodulatory recombinant fusion protein composed of the first extracellular domain of LFA-3 fused to the human IgG1 hinge, C(H)2, and C(H)3 domains, has recently been shown in phase II and III clinical trials to safely reduce disease expression in patients with chronic plaque psoriasis. Alefacept modulates the function of and selectively induces apoptosis of CD2(+) human memory-effector T cells in vivo. We have sought to gain further understanding of the mechanisms of action that influence the biological activity of alefacept and may contribute to its efficacy and patient responsiveness. Specifically evaluated is the ability of alefacept to activate intracellular signals mediated via CD2 and/or Fc gamma RIII (CD16). Experimentation using isoforms of alefacept engineered to have amino acid substitutions in the IgG1 C(H)2 domain that impact Fc gamma R binding indicate that alefacept mediates cognate interactions between cells expressing human CD2 and CD16 to activate cells, e.g., increase extracellular signal-regulated kinase phosphorylation, up-regulate cell surface expression of the activation marker CD25, and induce release of granzyme B. In the systems used, this signaling is shown to require binding to CD2 and CD16 and be mediated through CD16, but not CD2. Experimentation using human CD2-transgenic mice and isoforms of alefacept confirmed the requirement for Fc gamma R binding for detection of the pharmacological effects of alefacept in vivo. Thus alefacept acts as an effector molecule, mediating cognate interactions to activate Fc gamma R(+) cells (e.g., NK cells) to induce apoptosis of sensitive CD2(+) target cells.     

Transforming growth factor-beta in synovial fluids modulates Fc gamma RII (CD16) expression on mononuclear phagocytes.

Mononuclear phagocytes in the synovium of patients with arthritis, in contrast to blood monocytes, were found to express a third receptor for the constant region of Ig (Fc gamma RIII), in addition to Fc gamma RI and Fc gamma RII. Previously identified on mature mononuclear phagocytes or phagocytes exposed to transforming growth factor-beta (TGF-beta) in vitro, this study documents the presence of Fc gamma RIII (CD16) expressing cells at an inflammatory site. Furthermore, the presence of CD16 on the majority of the LeuM3 (CD14) positive synovial monocytic cells could be mimicked by exposing blood monocytes to synovial fluids from patients with rheumatoid arthritis (17 of 19) and synovial fluids from patients with osteoarthritis (4 of 4). In additional studies, the soluble factor in inflammatory synovial fluids responsible for regulating CD16 expression was found to be consistent with the presence of TGF-beta. Inhibition of the activity in synovial fluids with a neutralizing antibody to TGF-beta confirmed a role for this peptide in synovial phagocytic cell CD16 expression. Moreover, signal transduction through CD16 on synovial phagocytes resulted in augmented extracellular release of superoxide anion that may contribute to tissue damage and other inflammatory sequelae. Identification of TGF-beta and its association with upregulation of CD16 at sites of chronic inflammation may provide insight into the destructive lesions associated with inflammatory arthropathies.     

Cloned and expressed human Fc receptor for IgG mediates anti-CD3-dependent lymphoproliferation

We have utilized gene transfer experiments to investigate the role of a human monocyte receptor for IgG (Fc gamma RII) in mouse IgG1 anti-CD3 (Leu 4)-induced lymphoproliferation in vitro. Mouse Ltk- cells expressing human Fc gamma RII or a mutant of Fc gamma RII lacking the entire cytoplasmic domain of the receptor mediate anti-CD3-induced lymphoproliferation in cultures of adherent cell-depleted human PBMC. Expression of an Fc gamma RII mutant lacking transmembrane and cytoplasmic domains (soluble Fc gamma RII) in COS7 cells yielded a secreted receptor which retained affinity for IgG, even in the absence of the mutant receptor's N-linked oligosaccharides. Soluble Fc gamma RII inhibits rosette formation by human IgG-sensitized RBC and the Fc gamma RII-bearing cell line K562, but does not sitmulate anti-CD3-induced lymphoproliferation under the conditions tested.     

Evidence for cross-regulation of Fc gamma RIIIB (CD16) receptor-mediated signaling by Fc gamma RII (CD32) expressed on polymorphonuclear neutrophils.

Human polymorphonuclear neutrophils (PMN) express the low affinity receptors for the Fc domain of IgG (Fc gamma R), Fc gamma RII (CD32), and the glycosyl phosphatidylinositol-linked isoform of Fc gamma RIII (Fc gamma RIIIB, CD16) on their cell surface. Both of these receptors have been shown to be signal-transducing molecules. However, the mechanisms involved in such signaling are not clearly understood. In this report, we investigated intracellular Ca2+ ([Ca2+]i) signals triggered in PMN by both the receptors using aggregated human IgG (AggIgG) and specific mAb to Fc gamma RII (KuFc79) and Fc gamma RIII (3G8) as ligands. Addition of AggIgG as well as cross-linking of mAb KuFc79 and 3G8 bound to PMN induced [Ca2+]i flux. However, preincubation of PMN with mAb KuFc79 (whole Ig or Fab fragments) in the absence of cross-linking abrogated the [Ca2+]i flux induced by AggIgG and mAb 3G8, indicating that Fc gamma RII receptor occupancy by mAb KuFc79 can block signals mediated by Fc gamma RIIIB. KuFc79-isotype-matched control mAb (MOPC 195) did not abolish the signals generated by AggIgG and mAb 3G8. In addition, mAb KuFc79 did not abrogate [Ca2+]i responses elicited by the receptor for the chemotactic peptide FMLP indicating that modulation of signal transduction by Fc gamma RII-bound KuFc79 is selective for certain receptors. Immunofluorescence analysis of PMN initially treated with mAb KuFc79 followed by AggIgG showed that KuFc79 did not block the binding of AggIgG to PMN. Similarly, competitive binding studies revealed no stearic hindrance between mAb KuFc79 bound to Fc gamma RII and mAb 3G8 bound to Fc gamma RIIIB. Thus, the ability of mAb KuFc79 to modulate signals induced by AggIgG and 3G8 strongly suggests that Fc gamma RII may regulate Fc gamma RIIIB signaling. While previous studies on Fc gamma RII revealed a requirement for cross-linking of the receptor to induce its effector functions, the present study shows that binding of mAb KuFc79 to Fc gamma RII itself, even in a univalent form, results in cross-regulation of Fc gamma RIIIB-triggered signals. Treatment of PMN with protein tyrosine kinase inhibitors, genistein and herbimycin A, abrogated the [Ca2+]i signals elicited by both mAb KuFc79 and 3G8. These results suggest that tyrosine kinase enzyme(s) associated with these receptors may be crucial for positive/negative signals triggered by Fc gamma RII and Fc gamma RIIIB.     

High resolution mapping of the binding site on human IgG1 for Fc gamma RI, Fc gamma RII, Fc gamma RIII, and FcRn and design of IgG1 variants with improved binding to the Fc gamma R

Immunoglobulin G (IgG) Fc receptors play a critical role in linking IgG antibody-mediated immune responses with cellular effector functions. A high resolution map of the binding site on human IgG1 for human Fc gamma RI, Fc gamma RIIA, Fc gamma RIIB, Fc gamma RIIIA, and FcRn receptors has been determined. A common set of IgG1 residues is involved in binding to all Fc gamma R; Fc gamma RII and Fc gamma RIII also utilize residues outside this common set. In addition to residues which, when altered, abrogated binding to one or more of the receptors, several residues were found that improved binding only to specific receptors or simultaneously improved binding to one type of receptor and reduced binding to another type. Select IgG1 variants with improved binding to Fc gamma RIIIA exhibited up to 100% enhancement in antibody-dependent cell cytotoxicity using human effector cells; these variants included changes at residues not found at the binding interface in the IgG/Fc gamma RIIIA co-crystal structure (Sondermann, P., Huber, R., Oosthuizen, V., and Jacob, U. (2000) Nature 406, 267-273). These engineered antibodies may have important implications for improving antibody therapeutic efficacy.     

IL-4 decreases Fc gamma R membrane expression and Fc gamma R-mediated cytotoxic activity of human monocytes

Monocytes can express three classes of FcR for IgG: Fc gamma RI, Fc gamma RII, and Fc gamma RIII (CD64, CD32, and CD16, respectively) of which the Fc gamma RIII is expressed after prolonged culture. Fc gamma R expression is regulated by IFN-gamma. Because IFN-gamma and IL-4 have antagonistic effects on the expression of the FcR for IgE on human monocytes, we studied the effect of IL-4 on Fc gamma R expression and function. We show that IL-4 down-regulates Fc gamma RI, Fc gamma RII, and Fc gamma RIII expression of cultured monocytes and inhibits IFN-gamma enhanced Fc gamma RI expression. Exposure of monocytes to IL-4 for 40 h resulted in a dose-dependent decrease of the expression of all three Fc gamma R that persisted throughout the whole culture period (7 days). Anti-IL-4 antibodies completely reversed the IL-4 effect. In addition the impaired Fc gamma R expression correlated directly with reduced Fc gamma R-mediated function because monocytes cultured in the presence of IL-4 have a reduced capacity to lyse human E opsonized with human IgG anti-D or mouse antiglycophorin A antibodies. These observations, together with the previous finding that IL-4 induces Fc epsilon RIIb expression on monocytes, indicate that IL-4 and IFN-gamma may control the Fc gamma R-mediated immune response by differentially regulating Fc gamma R expression.     

Activation of cloned human natural killer cells via Fc gamma RIII

The Fc gamma RIII (CD16) Ag on human NK cells involved in antibody-dependent cellular cytotoxicity has been demonstrated to be an important activation structure. The present studies were carried out to further characterize the functional role of the CD16 Ag and the mechanisms whereby cytotoxicity is activated by using human NK clones. In phenotypic studies Fc gamma RIII was found to be expressed heterogeneously on various human cloned NK cells. Expression on CD3- and CD3+ clones varied with the donor and mAb used for detection. Functional data demonstrated that cytotoxicity against NK-resistant target cells can be induced in CD3-CD16+ NK clones and CD3+CD16+ clones with NK activity when various CD16 mAb were used. CD16 antibodies but not reactive isotype control antibodies induced cytotoxicity. In contrast to complete CD16 antibodies F(ab')2 fragments were not able to activate the cytotoxic mechanism. Both an antibody against FcR on the target cell (Fc gamma RII) and a CD11a antibody blocked induction of cytotoxicity. These results suggest that three steps are critical for activation of CD16+ cells via Fc gamma RIII: 1) specific binding of CD16 antibodies to Fc gamma RIII on effector cells irrespective of the epitope recognized; 2) cross-linking of effector cell CD16 Ag through binding of the Fc site of CD16 antibodies via corresponding FcR on the target cell membrane; and 3) interaction of CD11a/18 molecules with the target cell membrane.     

Functional association of CD9 with the Fc gamma receptors in macrophages

CD9, a member of the tetraspan family of proteins, is highly expressed on macrophages. Although a clear function for the molecule has yet to be described, we have found that the anti-CD9 mAb activates mouse macrophages. The rat anti-CD9 mAb, KMC8.8, but not the F(ab')(2), induced tyrosine phosphorylation of proteins including syk and cbl and induced cell aggregation in the mouse macrophage cell line, J774, suggesting that co-cross-linking of CD9 and Fc gamma R was required for the signal. Co-cross-linking of CD9-Fc gamma R with KMC8.8 on macrophages from three different FcR-deficient mice, FcR gamma-chain(-/-), Fc gamma RIIB(-/-), and Fc gamma RIII(-/-), revealed that Fc gamma RIII is specific and crucial for syk phosphorylation. Although both KMC8.8 and the anti-Fc gamma RIIB/III mAb, 2.4G2, evoked similar phosphorylation patterns, only KMC8.8 induced cell aggregation. Additionally, KMC8.8 treatment led to reduce levels of TNF-alpha production and p42/44 extracellular signal-related kinase phosphorylation relative to 2.4G2 stimulation. Immunofluorescence staining showed that co-cross-linking of CD9-Fc gamma R with KMC8.8 induced filopodium extension before cell aggregation, which was followed by simultaneous colocalization of CD9, Fc gamma RIIB/III, Mac-1, ICAM-1, and F-actin at the cell-cell adhesion site. Moreover, KMC8.8 treatment of Fc gamma R-deficient macrophages revealed that the colocalization of CD9, Fc gamma RIII, Mac-1, and F-actin requires co-cross-linking of CD9-Fc gamma RIII, whereas co-cross-linking of CD9-Fc gamma RIIB induced the colocalization of only CD9 and Fc gamma RIIB. Our results demonstrate that co-cross-linking of CD9 and Fc gamma Rs activates macrophages; therefore, CD9 may collaborate with FcRs functioning in infection and inflammation on macrophages.     

A subset of CD16-natural killer cells without antibody-dependent cellular cytotoxicity function

The low affinity IgG receptor, CD16 (Fc gamma RIII), is expressed on almost all peripheral blood natural killer (NK) cells. A small subset of CD3- CD16- CD56+ NK cells, representing less than 1% of peripheral blood lymphocytes, expands during in vivo IL-2 treatment. To analyze this CD16- NK cell subset in more detail, NK clones have been generated. One of them (TNK2) has been used to study the function of these cells in more detail. It is demonstrated that TNK2 exerts normal NK activity and displays large granular lymphocyte morphology. Since this clone lacks CD16 expression, antibody-dependent cellular cytotoxicity cannot be exerted. CD16 monoclonal antibodies fail to induce cytotoxic activity against NK-resistant target cells. These studies reveal that the lack of CD16 detection is not due to the modulation or the stage of activation of these NK cells. TNK2 is representative of this small subset of peripheral blood NK cells, expanded during IL-2 treatment, which does not express Fc gamma RIII and therefore cannot perform antibody-dependent cellular cytotoxicity.     

CR3 (Mac-1, alpha M beta 2, CD11b/CD18) and Fc gamma RIII cooperate in generation of a neutrophil respiratory burst: requirement for Fc gamma RIII and tyrosine phosphorylation

Cooperation among plasma membrane receptors in activating signal transduction cascades is not well understood. For almost 20 years, it has been clear that when a particulate foreign body is opsonized with complement as well as IgG, the efficiency of IgG effector functions is markedly enhanced. However, the molecular mechanisms involved in cooperation between IgG Fc receptors and complement receptors have not been elucidated. In this work, we show that when human neutrophils (PMN) are plated on a surface coated with both anti-CR3 and anti-Fc gamma RIII antibodies, the respiratory burst which occurs is equivalent to that stimulated by anti-Fc gamma RII. The CR3 ligand iC3b is as effective as anti-CR3 for cooperating with anti-Fc gamma RIII in generation of a respiratory burst. The synergy between CR3 and Fc gamma RIII for activating the NADPH oxidase is abolished by Fab of anti-Fc gamma RII. Nonetheless, the observed synergy is not an artifact of unintended Fc gamma RII ligation, since (a) only this combination of antibodies works to generate H2O2; (b) coating plates with either of the antibodies alone cannot activate the respiratory burst at any dose; (c) LAD (CR3 deficient) cells, which are perfectly competent to mount a respiratory burst when Fc gamma RII is engaged, are incapable of activating the respiratory burst when adherent to wells coated with anti-Fc gamma RIII and anti-CR3; (d) direct engagement of Fc gamma RII activates the respiratory burst by a pathway pharmacologically distinguishable from the synergistic respiratory burst. Fc gamma RIII/CR3 synergy is abolished by cytochalasin B and herbimicin, suggesting that both the actin cytoskeleton and tyrosine phosphorylation are necessary for activation of the synergistic respiratory burst. Further analysis shows that CR3 and Fc gamma RIII have distinct roles in activation of this Fc gamma RII-dependent assembly of the NADPH oxidase. Ligation of CR3 is sufficient to lead to Fc gamma RII association with the actin cytoskeleton on the adherent PMN surface. Coligation of Fc gamma RIII is required for tyrosine phosphorylation of Fc gamma RII. These data are consistent with a model in which phosphorylation of Fc gamma RII or a closely associated substrate initiates activation of a signal transduction pathway leading to oxidase assembly. These are the first data to demonstrate a molecular mechanism for synergy between IgG Fc and complement receptors in activation of phagocyte effector functions.     

Characterization of the family of dimers associated with Fc receptors (Fc epsilon RI and Fc gamma RIII).

The receptor for IgE (Fc epsilon RI) is a multimeric complex containing one alpha chain, one beta chain with four transmembrane domains and one homodimer of disulfide-linked gamma-chains. The Fc epsilon RI gamma-chains form additional disulfide-linked dimers with the homologous zeta- and eta-chains, as part of the TCR complex. The low affinity receptor for IgG (Fc gamma RIII)2 on NK cells is also associated with zeta-chains. Here we show that the gamma-chain is expressed in NK cells both as a group of heterogenous gamma gamma homodimers and also as a heterodimer bound to zeta. Fc gamma RIIIA is associated with three types of dimers zeta zeta, gamma zeta, and notably gamma gamma as well. In fact, gamma gamma appears to be the predominant species associating with Fc gamma RIIIA. The surface expressed Fc epsilon RI also associates with the same group of heterogenous gamma gamma homodimers. We also show that there is no C-terminal posttranslational cleavage of gamma occurring before its insertion into the plasma membrane as previously suggested. Thus, like the TCR, Fc gamma RIIIA may form a variety of receptor isoforms, though at present we do not understand the functional implications of these structures.