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The Journal of Immunology, 2001, 166: 21-25.
Copyright © 2001 by The American Association of Immunologists

Negative Regulation of Expression and Function of Fc{gamma}RIII by CD3{zeta} in Murine NK Cells1

Hisashi Arase2, Tadahiro Suenaga, Noriko Arase3, Yoshimitsu Kimura4, Katsuhiko Ito, Ritsuko Shiina, Hiroshi Ohno5 and Takashi Saito6

Department of Molecular Genetics, Chiba University Graduate School of Medicine, Chiba, Japan


    Abstract
 Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 
Fc{gamma}RIII is involved in Ab-dependent cell-mediated cytotoxicity (ADCC) and cytokine production by NK cells. Signaling and expression of Fc{gamma}RIII are dependent on FcR{gamma}. Although NK cells express not only FcR{gamma} but also CD3{zeta}, the role of CD3{zeta} in NK cell function remains unclear. Here, we found that the expression of Fc{gamma}RIII on NK cells from CD3{zeta}-deficient mice is unexpectedly up-regulated compared with that on cells from normal mice. Furthermore, ADCC and IFN-{gamma} production upon Fc{gamma}RIII-cross-linking by NK cells from CD3{zeta}-deficient mice were also up-regulated. Up-regulation of the surface expression of Fc{gamma}RIII on CD3{zeta}-deficient NK cells is not mediated by transcriptional augmentation of either Fc{gamma}RIII or FcR{gamma} gene because there was no significant difference in the expression of mRNA for Fc{gamma}RIII and FcR{gamma}. Transfection of CD3{zeta} into a cell line expressing Fc{gamma}RIII and FcR{gamma} induced a decrease in the cell surface expression of Fc{gamma}RIII. These findings reveal a negative regulatory role of CD3{zeta} in Fc{gamma}RIII-mediated function of murine NK cells.


    Introduction
 Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 
Natural killer cells are activated upon recognition of a variety of target cells and exhibit natural cytotoxicity. Recently, a number of receptors were found to be involved in the activation of NK cells, although the exact features of the receptors responsible for natural cytotoxicity remain unclear (1, 2). NK cells also show Ab-dependent cell-mediated cytotoxicity (ADCC)7 upon cross-linking of IgG Fc{gamma}R with Ab (3, 4, 5). Among different Fc{gamma}Rs, NK cells mainly express the low affinity receptor for IgG termed Fc{gamma}RIII (CD16). Recently, human Fc{gamma}RIII on NK cells has been shown to be involved in direct recognition of specific targets in the absence of Ab (6), although its physiological function is still unclear. Therefore, Fc{gamma}RIII expressed on NK cells is involved in ADCC and partly in Ab-independent natural cytotoxicity.

Fc{gamma}RIII is expressed on the cell surface in association with FcR{gamma} (Fc{epsilon}RI{gamma}), which was originally identified as a signaling component of the high affinity IgE receptor (Fc{epsilon}RI) complex (7, 8). Because Fc{gamma}RIII cannot be expressed on the cell surface in the absence of FcR{gamma}, NK cells obtained from FcR{gamma}-deficient (-/-) mice do not express Fc{gamma}RIII or show ADCC function (9, 10). Furthermore, NK cells from Fc{gamma}RIII-/- mice also fail to exhibit ADCC, confirming that Fc{gamma}RIII is the IgG Fc receptor responsible for this function in NK cells (11).

The CD3{zeta}-chain is one of the components of the TCR-CD3 complex and possesses three tyrosine-based activation motifs (ITAM) in its cytoplasmic domain. ITAMs of CD3{zeta} are rapidly tyrosine phosphorylated upon TCR cross-linking and transduce activation signals in T cells (12). In addition, CD3{zeta} is required for the cell surface expression of the TCR complex and plays a crucial role in the regulation of the assembly and intracellular transport of the TCR-CD3 complex. Indeed, the expression of TCR is severely impaired in CD3{zeta}-deficient cells and mice (13, 14, 15, 16, 17).

CD3{zeta} is also expressed in both human and murine NK cells despite the fact that NK cells do not express TCR and seem to be involved in NK cell activation (18, 19). CD3{zeta} is phosphorylated upon cross-linking of Fc{gamma}RIII and is thought to be involved in signal transduction through Fc{gamma}RIII in human NK cells (20, 21). However, it is known that there is a significant difference between human and murine CD3{zeta} in Fc{gamma}RIII expression. Human CD3{zeta} as well as FcR{gamma} can be associated with Fc{gamma}RIII and are involved in the surface expression of Fc{gamma}RIII, whereas murine Fc{gamma}RIII can associate only with FcR{gamma}, not with CD3{zeta}. (22). In addition, no significant functional defects have been reported in NK cells from CD3{zeta}-/- mice (17). From these analyses, it has been widely believed that CD3{zeta} does not play any important role in the activation of murine NK cells.

In the present study we found that Fc{gamma}RIII expression on NK cells from CD3{zeta}-/- mice is greatly enhanced. Furthermore, Fc{gamma}RIII-mediated functions of NK cells from CD3{zeta}-deficient mice were also up-regulated. These findings demonstrate a novel function of CD3{zeta} in murine NK cells in regulation of the expression and function of Fc{gamma}RIII.


    Materials and Methods
 Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 
Mice

The establishment and characterization of CD3{zeta}-/- mice were described previously (14). CD3{zeta}-/- mice were maintained in our animal facility and back-crossed to C57BL/6 mice seven times. FcR{gamma}-/- mice were generated from C57BL/6-derived embryonic stem cells as previously described (10).

Preparation of NK cells

NK cells were purified as previously described (23). Briefly, splenocytes were mixed with anti-CD4 mAb (GK1.5) and anti-CD8 mAb (53.6.7) followed by incubation with magnetic beads (Advanced Magnetics, Cambridge, MA) coupled with goat anti-mouse IgG Ab and goat anti-rat IgG Ab (Cappel, Organon Teknika, West Chester, PA) to remove surface Ig+ (sIg+) B cells and CD4+ and CD8+ T cells. The residual cells were then incubated with PE-anti-NK1.1 mAb and FITC-anti-CD3 mAb (PharMingen, San Diego, CA). The stained cells were sorted into NK1.1+ CD3- cells by FACStarPlus (Becton Dickinson, Mountain View, CA). The purity of the sorted cells was always >99%.

Cell culture and stimulation

Purified NK cells were cultured in RPMI 1640 supplemented with 10% FCS, kanamycin (100 µg/ml), and 5 x 10-5 M 2-ME. NK cells were expanded by culturing freshly purified NK cells for 5–7 days in the presence of 1000 U/ml human rIL-2 (Ajinomoto, Kawasaki, Japan) and were used for further analysis. For the analysis of IFN-{gamma} production, 5 x 104 NK cells were stimulated with immobilized mouse IgG1 mAb (anti-biotin, Zymed, South San Francisco, CA) or with recombinant mouse IL-12 (4.9 x 106 U/mg; supplied by Genetics Institute, Cambridge, MA) for 2 days. Mouse IgG1 mAb was immobilized on a 96-well flat-bottom sterile ELISA plate (Corning, Corning, NY) by incubation for 2 h at 37°C in 0.1 M NaHCO3.

Measurement of IFN-{gamma}

The amount of IFN-{gamma} produced in the culture supernatants was measured by ELISA using a standard protocol with rIFN-{gamma} as the standard (24). Anti-IFN-{gamma} mAb (R4-6A2, PharMingen) was used to capture IFN-{gamma}. Biotinylated anti-IFN-{gamma} mAb (XMG1.2, PharMingen) was used to detect captured IFN-{gamma}.

ADCC activity

ADCC activity was analyzed as previously described (25). P815 cells were surface biotinylated (26), followed by labeling with PKH-2-green fluorescence dye (Sigma, St. Louis, MO). Thereafter, PKH-2-labeled P815 cells were cultured with various numbers of NK cells at 37°C for 4 h in the presence of various concentrations of rabbit anti-biotin Ab (Rockland, Gilbertsville, PA). Dead cells were stained with propidium iodide, and the proportion of dead cells in PKH-2-stained target cells was determined by flow cytometry. Data are presented as the mean ± SD from triplicate cultures.

Flow cytometry

Cells were stained with fluorescence-labeled Abs and analyzed with FACScan or FACScalibur (Becton Dickinson). Fluorescence-labeled Abs used for flow cytometry were FITC- or PE-2.4G2 (PharMingen) and PE-anti-NK1.1 (PharMingen).

RT-PCR

Total cellular RNA was extracted by the guanidinium-isothiocyanate method. Single-strand cDNA was synthesized with reverse transcriptase from 0.5 µg of RNA and was used for PCR. Primer sequences used were as follows: {beta}-actin: 5' primer, TGGAATCCTGTGGCATCCATGAAAC; 3' primer, TAAAACGCAGCTCAGTAACAGTCCG; Fc{gamma}RIII: 5' primer, GTTTAAGGCCACAGTCAATG; 3' primer, GGTTGGCTTTTGGGATAG; and FcR{gamma}: 5' primer, ATGATCTCAGCCGTGATCTTG; 3' primer, AGTCTCATATGTCTCCTGGCT. Various amounts of cDNAs were amplified in PCR under the following conditions: 94°C for 1 min, 57°C for 1 min, and 72°C for 1.5 min with 20 cycles for {beta}-actin or with 26 cycles for Fc{gamma}RIII and FcR{gamma}. After amplification, PCR products were separated by electrophoresis on 1.5% agarose gel containing ethidium bromide and visualized by UV illumination.

Surface biotinylation, immunoprecipitation, and Western blotting

NK cells were surface biotinylated as previously described (26). Biotinylated cells were lysed with a lysis buffer containing 1% digitonin, 50 mM Tris-HCl (pH 7.6), 150 mM NaCl, 10 µg/ml aprotinin, 10 µg/ml leupeptin, 1 mM PMSF, and 10 mM iodoacetamide, at a concentration of 1 x 107 cell/ml. Immunoprecipitation was performed with anti-CD3{zeta} (H146) or anti-Fc{gamma}RIII (2.4G2) mAbs. Immunoprecipitates were separated on two-dimensional nonreducing (16%) and reducing (18%) SDS-PAGE and transferred onto a polyvinylidene difluoride membrane (Immobilon-P; Millipore, Bedford, MA). The biotinylated proteins were detected using streptavidin-peroxidase (Vecstain Elite ABC kit; Vector, Burlingame, CA), an ECL system (Amersham International, Aylesbury, U.K.).

Transfection

cDNAs for Fc{gamma}RIII and FcR{gamma} were subcloned into pMx retrovirus expression vector (provided by Dr. T. Kitamura, University of Tokyo, Tokyo, Japan). These cDNAs were transiently transfected into BOSC23 packaging cells using Lipofectamine Plus (Life Technologies, Gaithersburg, MD). Culture supernatants were collected at 2 days after transfection, and NIH-3T3 cells were infected by addition of the supernatants. Fc{gamma}RIII-expressing cells were purified by FACStarPlus, and a single-cell clone stably expressing Fc{gamma}RIII was obtained. CD3{zeta} was subcloned into internal ribosomal entry site (pIRES)-green fluorescence protein (EGFP) expression vector (Clontech, Palo Alto, CA). Mutant CD3{zeta} in which isoleucine in the transmembrane region was substituted to leucine (I46L-CD3{zeta}) was generated by recombinant PCR and subcloned into pIRES-EGFP vector. CD3{zeta}-IRES-GFP, I46L-CD3{zeta}-IRES-GFP, and IRES-GFP (vector only) were transiently transfected into the Fc{gamma}RIII transfectants. At 36 h after transfection, the expressions of GFP and Fc{gamma}RIII on the transfectants were analyzed.


    Results
 Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 
Up-regulation of Fc{gamma}RIII expression on CD3{zeta}-/- NK cells

To analyze the role of CD3{zeta} in NK cells, we purified NK cells from splenocytes from wild-type, CD3{zeta}-/-, and FcR{gamma}-/- mice. Purified NK cells were expanded for 5 days in the presence of IL-2, and the expression of NK1.1 and Fc{gamma}RIII on the cell surface was analyzed (Fig. 1Go). As previously reported, FcR{gamma}-/- NK cells were deficient in Fc{gamma}RIII expression, because FcR{gamma} is required for the surface expression of Fc{gamma}RIII (9, 10). In contrast, Fc{gamma}RIII expression on the cell surface of CD3{zeta}-/- NK cells was significantly higher than that on wild-type NK cells. Similar results were obtained when Fc{gamma}RIII expression was analyzed on freshly isolated NK cells (data not shown). In contrast to Fc{gamma}RIII, the surface expression of NK1.1, another FcR{gamma}-associated molecule on NK cells (23), did not differ between wild-type and CD3{zeta}-/- NK cells, suggesting that CD3{zeta} specifically down-regulates Fc{gamma}RIII expression on NK cells.



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FIGURE 1. Expression of Fc{gamma}RIII and NK1.1 on the cell surface of NK cells from wild-type, CD3{zeta}-/-, and FcR{gamma}-/- mice. Purified NK cells were stained with FITC-2.4G2 and PE-anti-NK1.1 mAbs, and the expression of Fc{gamma}RIII and NK1.1 is shown. Because NK cells do not express Fc{gamma}RII, the anti-Fc{gamma}RII/III mAb, 2.4G2, reacts only with Fc{gamma}RIII on NK cells.

 
Next, we addressed the question of whether the function of Fc{gamma}RIII in NK cells from CD3{zeta}-/- mice is affected by the elevated expression of the surface Fc{gamma}RIII. Because NK cells show ADCC and are involved in Ab-mediated immune responses (4, 5), we first analyzed the ADCC activity by NK cells from CD3{zeta}-/- mice. CD3{zeta}-/- NK cells exhibited no significant difference from wild-type NK cells in cytotoxicity against biotinylated P815 cells in the absence of anti-biotin Ab (Fig. 2GoA). However, CD3{zeta}-/- NK cells, which express a higher level of Fc{gamma}RIII, showed significantly more potent cytotoxicity in the presence of the anti-biotin Ab than wild-type NK cells (Fig. 2GoB). The ADCC activity was correlated with the Ab concentration (Fig. 2GoC). Interestingly, the differences in ADCC function between wild-type and CD3{zeta}-/- NK cells were greater at lower concentrations of Ab. While CD3{zeta}-/- NK cells showed almost maximum cytotoxicity in the presence of 6.3 µg/ml of Ab, wild-type NK cells did not reach maximum cytotoxicity with the same concentration of Ab, and the cytotoxicity continued to gradually increase as Ab concentrations were increased. In contrast, there was no significant difference between wild-type and CD3{zeta}-/- NK cells in natural cytotoxicity against NK-sensitive target cells, such as YAC-1 (17) (Fig. 2GoD). These results indicate that CD3{zeta} down-regulates ADCC activity, but not natural cytotoxicity by NK cells.



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FIGURE 2. Cytotoxicity of NK cells from wild-type and CD3{zeta}-/- mice. Cytotoxic activity by NK cells from wild-type ({circ}) and CD3{zeta}-/- mice (•). Cytotoxicity by NK cells against biotinylated P815 cells was analyzed in the absence (A) or the presence (B) of anti-biotin Ab (3.1 µg/ml) and in the presence of various concentrations of anti-biotin Ab at a 3:1 E:T cell ratio (C). Cytotoxicity against YAC-1 cells was analyzed (D).

 
The triggering of NK cells through Fc{gamma}RIII induces not only ADCC but also IFN-{gamma} production. Wild-type and CD3{zeta}-/- NK cells were stimulated with various concentrations of immobilized IgG1 mAb or recombinant murine IL-12, and the amount of IFN-{gamma} produced in culture supernatants was measured by ELISA 2 days later. As shown in Fig. 3GoA, the amount of IFN-{gamma} produced by CD3{zeta}-/- NK cells upon stimulation with immobilized IgG1 mAb was severalfold higher than that produced by wild-type NK cells. In contrast, there was no difference in IFN-{gamma} production by these NK cells upon stimulation with IL-12 (Fig. 3GoB), demonstrating that the increased production of IFN-{gamma} by CD3{zeta}-/- NK cells was not due to an elevated capacity for IFN-{gamma} production. These results indicate that not only the expression level of Fc{gamma}RIII but also its function are up-regulated in NK cells lacking CD3{zeta}.



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FIGURE 3. IFN-{gamma} production by NK cells from wild-type and CD3{zeta}-/- mice. IFN-{gamma} production by NK cells from wild-type ({circ}) and CD3{zeta}-/- (•) mice. NK cells were stimulated with various concentrations of IgG1 Ab immobilized on culture plate (A) or IL-12 (B) for 2 days, and the amount of IFN-{gamma} produced in the culture supernatants was determined.

 
Equivalent expression of the transcripts for Fc{gamma}RIII and FcR{gamma} in both wild-type and CD3{zeta}-/- NK cells

To address the question of whether the enhanced expression of Fc{gamma}RIII on the cell surface of CD3{zeta}-/- NK cells can be attributed to an increase in mRNA for Fc{gamma}RIII or FcR{gamma}, we analyzed the amounts of transcripts for Fc{gamma}RIII, FcR{gamma}, and {beta}-actin in NK cells from wild-type, CD3{zeta}-/-, and FcR{gamma}-/- mice by semiquantitative RT-PCR. As shown in Fig. 4Go, we could not observe any significant difference between wild-type and CD3{zeta}-/- NK cells in the expression levels of mRNA for Fc{gamma}RIII and FcR{gamma}. This observation suggests that the augmented expression of Fc{gamma}RIII on CD3{zeta}-/- NK cells was not due to the increased expression of mRNA of Fc{gamma}RIII or FcR{gamma} and that CD3{zeta} regulates the expression of Fc{gamma}RIII by a post-transcriptional mechanism.



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FIGURE 4. RT-PCR analysis of Fc{gamma}RIII and FcR{gamma} mRNA expression. mRNA expression of FcR{gamma}, Fc{gamma}RIII, and {beta}-actin in NK cells from wild-type, CD3{zeta}-/-, and FcR{gamma}-/- mice were analyzed by semiquantitative RT-PCR analysis. Sequentially diluted cDNAs from NK cells (x1, x3, x10, and x30) were amplified.

 
Surface expression of the CD3{zeta}-FcR{gamma} heterodimer on NK cells

FcR{gamma} is required for the surface expression of Fc{gamma}RIII on mouse NK cells (8, 9, 10). Because NK cells express both CD3{zeta} and FcR{gamma}, it is postulated that CD3{zeta} forms heterodimers with FcR{gamma} similarly in T cells (27, 28), and the formation of the heterodimers may affect the amount of the FcR{gamma} homodimers required for efficient expression of Fc{gamma}RIII. Therefore, we analyzed the expression of dimers containing CD3{zeta} and FcR{gamma} on the cell surface of NK cells.

NK cells from wild-type mice were expanded in the presence of IL-2 and surface biotinylated. The cell lysates were immunoprecipitated with anti-CD3{zeta} and anti-Fc{gamma}RIII mAbs, and the precipitates were analyzed on two-dimensional SDS-PAGE. As shown in Fig. 5Go, immunoprecipitation with anti-CD3{zeta} mAb revealed that CD3{zeta} was expressed mainly as CD3{zeta}-FcR{gamma} heterodimers on the surface of NK cells, and CD3{zeta} homodimers were barely observed. By contrast, when Fc{gamma}RIII was precipitated, only FcR{gamma} homodimers, but not CD3{zeta}-FcR{gamma} heterodimers, were detected. These results indicate that Fc{gamma}RIII associates only with FcR{gamma} homodimers and that NK cells express CD3{zeta}-FcR{gamma} heterodimers on the cell surface, but these heterodimers do not associate with Fc{gamma}RIII, suggesting that CD3{zeta} regulates the level of the FcR{gamma} homodimer through the formation of CD3{zeta}-FcR{gamma} heterodimers.



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FIGURE 5. Fc{gamma}RIII associates with FcR{gamma} homodimers, but not CD3{zeta}-FcR{gamma} heterodimers, on the cell surface of NK cells. CD3{zeta} and FcR{gamma} were immunoprecipitated from cell lysates of surface biotinylated NK cells from normal mice with anti-CD3{zeta} mAb (left panel) or anti-Fc{gamma}RIII mAb (right panel). Precipitated proteins were analyzed by two-dimensional SDS-PAGE under nonreducing (NR) and reducing (R) conditions. CD3{zeta}-FcR{gamma} heterodimers ({gamma}-{zeta}) and FcR{gamma} homodimers ({gamma}-{gamma}) are indicated. Molecular size markers are indicated at the left margin.

 
Down-regulation of Fc{gamma}RIII expression by CD3{zeta} on Fc{gamma}RIII- and FcR{gamma}-transfected NIH-3T3 cells

We next examined the direct effect of CD3{zeta} on Fc{gamma}RIII expression by transfection of CD3{zeta} into cells expressing both Fc{gamma}RIII and FcR{gamma}. We transfected Fc{gamma}RIII and FcR{gamma} cDNAs into NIH-3T3 cells and isolated a clone that stably expresses Fc{gamma}RIII on their cell surface. Thereafter, the wild-type CD3{zeta} and a mutant CD3{zeta} containing a mutation within the transmembrane region were transfected into the Fc{gamma}RIII-expressing NIH-3T3 clone.

It has been reported that the leucine 46 in the transmembrane region of human CD3{zeta} is crucial for the association with Fc{gamma}RIII because the substitution of leucine to isoleucine, which is an equivalent substitution to murine CD3{zeta}, abrogated the interaction of CD3{zeta} with Fc{gamma}RIII (22). Indeed, the cell surface expression of mouse Fc{gamma}RIII was induced when Fc{gamma}RIII was transfected into COS cells with murine CD3{zeta} possessing the substitution of isoleucine 46 to leucine (I46L-CD3{zeta}; data not shown). According to these observations, wild-type CD3{zeta} as well as I46L-CD3{zeta} were transfected into an Fc{gamma}RIII-expressing NIH-3T3 clone.

Because Ab against the extracellular region of CD3{zeta} is not available, CD3{zeta} cDNAs were transfected using an expression vector containing IRES-GFP gene to monitor CD3{zeta}-expressing cells by analyzing GFP expression. We confirmed that the expression level of CD3{zeta} was correlated with the amount of GFP in this system (data not shown). When wild-type CD3{zeta} was transfected into an Fc{gamma}RIII-expressing NIH-3T3 clone, the surface expression of Fc{gamma}RIII was significantly decreased in the GFP+ population, but not in the GFP- population (Fig. 6Go). In contrast, transfection of I46L-CD3{zeta} and vector alone did not change the Fc{gamma}RIII expression on the cell surface of GFP+ cells. These observations provide evidence that the down-regulation of cell surface expression of Fc{gamma}RIII is dependent on the inability of CD3{zeta} to associate with Fc{gamma}RIII.



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FIGURE 6. Suppression of the cell surface expression of Fc{gamma}RIII by murine CD3{zeta}, but not I46L-CD3{zeta}. CD3{zeta}-IRES-GFP (thick line), I46L-CD3{zeta}-IRES-GFP (thin line), and IRES-GFP (dotted line) were transiently transfected into an NIH-3T3 cell line stably transfected with both Fc{gamma}RIII and FcR{gamma}. Cells were stained with PE-anti-Fc{gamma}RIII mAb 36 h after transfection and analyzed by FACScaliber. The cell surface expressions of Fc{gamma}RIII on GFP-positive and GFP-negative cells are shown.

 

    Discussion
 Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 
In the present study, we found that CD3{zeta} plays an important role in the regulation of Fc{gamma}RIII expression on murine NK cells. In the absence of CD3{zeta}, the expression and function of Fc{gamma}RIII in NK cells were augmented. Furthermore, transfection of murine CD3{zeta} into transfectants expressing Fc{gamma}RIII and FcR{gamma} significantly reduced Fc{gamma}RIII expression on the cell surface.

Both ADCC activity and IFN-{gamma}-producing capacity were up-regulated in CD3{zeta}-/- NK cells. Particularly, CD3{zeta}-/- NK cells showed significant cytotoxicity even with low concentrations of Ab, suggesting that the overall avidity of Fc{gamma}RIII on CD3{zeta}-/- NK cells toward Ab-coated target cells is up-regulated. The up-regulation of Fc{gamma}RIII-mediated function could be simply attributed to the increased expression of Fc{gamma}RIII on the cell surface. The alternative possibility is that the signaling capacity of Fc{gamma}RIII was up-regulated by increasing the FcR{gamma} homodimer in the absence of CD3{zeta}-FcR{gamma} heterodimers in CD3{zeta}-/- mice on the basis that Fc{gamma}RIII associates only with FcR{gamma} homodimers. Contrary to our findings, Liu et al. (17) previously reported that CD3{zeta}-/- NK cells exhibit normal ADCC function. However, they measured ADCC activity at a single concentration of Ab. Indeed, our data demonstrated that CD3{zeta}-/- NK cells exhibit similar ADCC activity to that in wild-type NK cells at high concentrations of Ab (Fig. 2GoC). Therefore, it is likely that the previous report used such a high concentration of Ab and that any significant difference in ADCC activity could not be observed.

Although precise mechanism for CD3{zeta}-mediated down-regulation of the cell surface expression of Fc{gamma}RIII remains unclear, our data suggest that the regulatory function is dependent on the inability of murine CD3{zeta} to associate with Fc{gamma}RIII. FcR{gamma} is required for the cell surface expression of Fc{gamma}RIII, and human, but not murine, CD3{zeta} can substitute for this function of FcR{gamma} (8, 22). Because CD3{zeta} forms heterodimers with FcR{gamma} (27, 28), our data suggest that wild-type CD3{zeta} interferes with the association of Fc{gamma}RIII with FcR{gamma} homodimers by the formation of CD3{zeta}-FcR{gamma} heterodimers.

Surface biotinylation of NK cells demonstrated that FcR{gamma} homodimers, but not CD3{zeta}-FcR{gamma} heterodimers, were associated with Fc{gamma}RIII on the cell surface despite the fact that CD3{zeta}-FcR{gamma} heterodimers were readily detected on the cell surface of NK cells (Fig. 5Go). In addition, the I46L-CD3{zeta} mutant failed to interfere with the surface expression of Fc{gamma}RIII, and this mutant CD3{zeta} exhibits an affinity sufficient to associate with Fc{gamma}RIII (Fig. 6Go; our unpublished observation). These observations suggest that CD3{zeta} down-regulates the expression of Fc{gamma}RIII by forming CD3{zeta}-FcR{gamma} heterodimers that cannot associate with Fc{gamma}RIII in NK cells.

We have previously shown that FcR{gamma} is also associated with NK1.1, and this association is necessary for signal transduction through NK1.1 (23). In contrast to Fc{gamma}RIII, the surface expression of NK1.1 did not increase on CD3{zeta}-/- NK cells (Fig. 1Go). Our previous finding that NK1.1 does not require FcR{gamma} for its surface expression unlike Fc{gamma}RIII (23), may explain the reason why the surface expression of NK1.1 was not increased on CD3{zeta}-/- NK cells. Therefore, the down-regulatory effect by CD3{zeta} is specific for receptors that require FcR{gamma} for their cell surface expression.

Because CD3{zeta} has been thought to be a signal-transducing molecule for Ag and Ab cell surface receptors, our study provides a novel negative regulatory role of the immune response by CD3{zeta}. Recently, FcR{gamma} has been reported to associate with paired Ig-like receptor A (29, 30) and Ig-like transcripts 1 (31) and seems to be involved in transducing activation signal through these molecules. Therefore, it might be possible that CD3{zeta} also negatively regulates the functions of these FcR{gamma}-associated molecules in a manner similar to that found for Fc{gamma}RIII. In contrast, because human CD3{zeta} can associate with Fc{gamma}RIII, a similar regulatory mechanism of CD3{zeta} observed in this study for the mouse system will not be simply applicable in human. However, the present findings might contribute to develop new therapeutic methods to down-regulate the function of Fc{gamma}RIII or other FcRs in humans.


    Acknowledgments
 
We thank Dr. J. Hamuro for the gift of rIL-2, Dr. J. V. Ravetch for Fc{gamma}RIII cDNA, Dr. T. Kitamura for pMx vector and for IL-12, Drs. S. Taki and L. L. Lanier for critical reading of the manuscript, M. Sakuma for technical help, and H. Yamaguchi and Y. Kurihara for secretarial assistance.


    Footnotes
 
1 This work was supported by Grants-in-Aid for Scientific Research from the Ministry of Education, Science, and Culture. Back

2 Current address: Department of Microbiology and Immunology, University of California San Francisco, CA 94143. Back

3 Current address: Hooper Foundation, University of California, San Francisco, CA 94143. Back

4 Current address: Mitsui Pharmaceuticals Inc., Mobara 297-0017, Japan. Back

5 Current address: Division of Molecular Membrane Biology, Cancer Research Institute, Kanazawa University, Kanazawa 920-0934, Japan. Back

6 Address correspondence and reprint requests to Dr. Takashi Saito, Department of Molecular Genetics, Chiba University Graduate School of Medicine, Chiba 260-8670, Japan. Back

7 Abbreviations used in this paper: ADCC, Ab-dependent cell-mediated cytotoxicity; GFP, green fluorescence protein; IRES, internal ribosomal entry site; ITAM, immunoreceptor tyrosine-based activation motif; sIg, surface Ig. Back

Received for publication July 7, 2000. Accepted for publication September 27, 2000.


    References
 Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 

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