Ligation of Fc gamma receptor IIB inhibits antibody-dependent enhancement of dengue virus infection

Abstract

The interaction of antibodies, dengue virus (DENV), and monocytes can result in either immunity or enhanced virus infection. These opposing outcomes of dengue antibodies have hampered dengue vaccine development. Recent studies have shown that antibodies neutralize DENV by either preventing virus attachment to cellular receptors or inhibiting viral fusion intracellularly. However, whether the antibody blocks attachment or fusion, the resulting immune complexes are expected to be phagocytosed by Fc gamma receptor (FcγR)-bearing cells and cleared from circulation. This suggests that only antibodies that are able to block fusion intracellularly would be able to neutralize DENV upon FcγR-mediated uptake by monocytes whereas other antibodies would have resulted in enhancement of DENV replication. Using convalescent sera from dengue patients, we observed that neutralization of the homologous serotypes occurred despite FcγR-mediated uptake. However, FcγR-mediated uptake appeared to be inhibited when neutralized heterologous DENV serotypes were used instead. We demonstrate that this inhibition occurred through the formation of viral aggregates by antibodies in a concentration-dependent manner. Aggregation of viruses enabled antibodies to cross-link the inhibitory FcγRIIB, which is expressed at low levels but which inhibits FcγR-mediated phagocytosis and hence prevents antibody-dependent enhancement of DENV infection in monocytes.

Dengue is the most common mosquito-borne viral disease globally. The lack of an effective preventive measure, especially a licensed vaccine, has resulted in the global spread of this virus (1, 2). Although neutralizing antibodies can confer lifelong immunity against reinfection by one of the four dengue virus (DENV) serotypes, subneutralizing antibody levels or cross-reactive antibodies appear to enhance the risk of severe dengue in subsequent infections (3–6). DENV bound with subneutralizing concentrations of antibody has been shown to result in increased virus uptake and replication in Fc gamma receptor (FcγR)-bearing cells such as monocytes/macrophages (4, 7). Thus, defining the determinants for virus neutralization will be important for the design of an effective dengue vaccine that protects against all four DENV serotypes while minimizing the risk of antibody-dependent enhancement of DENV infection.

Neutralization of flavivirus infection is a multiple-hit phenomenon. Recent stoichiometric studies have shown that both antibody affinity and epitope accessibility are important determinants for virus neutralization (8–10). Antibodies neutralize DENV by either preventing virus attachment to cellular receptors (11) or inhibiting viral fusion intracellularly (12). However, whether the antibody blocks attachment or fusion, the resulting immune complex is expected to be cleared from the circulation by professional phagocytes, especially the FcγR-bearing cells. This suggests that only antibodies that are able to block fusion intracellularly would be able to neutralize DENV upon FcγR-mediated uptake by monocytes. We thus set out to examine the early events of this interaction between the DENV immune complex and monocytic cells. However, instead, we serendipitously identified a mechanism that inhibits dengue virus infection where antibodies aggregate viruses in a concentration-dependent manner, which in turn allows for cross-linking of Fc gamma receptor IIB (FcγRIIB) that inhibits uptake of the DENV immune complex.

Results

Convalescent Sera Neutralize Homologous DENV Serotypes at Levels That Mediate Uptake of Immune Complexes but Neutralize Heterologous DENV Serotypes at Levels That Inhibit Uptake.

To address the interactions involved in antibody-mediated neutralization in monocytes, we obtained early convalescent sera from patients with primary DENV infection. Confirmation of the primary infection status, along with the identification of the DENV serotype with which these patients have been infected, was carried out in the corresponding acute serum sample. The results are shown in Table S1. Using the plaque reduction neutralization test (PRNT) on BHK cells, we observed that cross-reactive antibodies were present in these early convalescent sera (Table S2), which is consistent with previous findings (13, 14). These sera were also able to neutralize the four DENV serotypes, albeit at varying titers, when the FcγR-bearing THP-1 cells were used instead of BHK cells (Fig. S1). However, using DiD (1, 1′-dioctadecyl-3, 3, 3′,3′-tetramethylindodicarbocyanine, 4-chlorobenzenesulfonate salt)-labeled DENV (15, 16), we observed distinct differences in the early events when DENV was reacted with the highest dilution of serum that resulted in complete virus neutralization (hereafter referred to as the DENV immune complex) to THP-1 (Fig. 1A). In a serum sample that fully neutralized both DENV-1 and -2, uptake of neutralized immune complexes was observed to be significantly higher, as measured by flow cytometry, when DENV-2 (homologous) instead of DENV-1 (heterologous) was used (Fig. 1B). Immunofluorescence showed that the neutralized DENV-2 immune complexes were colocalized to LAMP-1 compartments (Fig. 1C). In contrast, no subcellular trafficking of the neutralized DENV-1 immune complexes was observed (Fig. 1C). These observations were reproduced in a panel of sera from DENV-2 primary patients where uptake of the DENV immune complex was observed only when DENV-2 but not the other serotypes were used (Fig. 1 D–F). This observation was also not limited to DENV-2. Uptake of the DENV immune complex was observed only when convalescent serum samples from patients with primary DENV-1, DENV-3, or DENV-4 were reacted with the homologous but not the heterologous serotypes (Fig. S2). These findings indicate that neutralization of homologous DENV serotypes can occur at levels that mediate uptake of immune complexes, but neutralization of heterologous DENV serotypes occurs only at levels where FcγR-mediated uptake is inhibited.

Fig. 1.

Convalescent primary DENV-2 human sera neutralize homologous serotypes at levels permissible for internalization but neutralize heterologous serotypes at levels that inhibit uptake. (A) Summary of method used to investigate early events of neutralized DENV immune complexes in THP-1 cells. (B) Percentage of internalized DiD-labeled DENV virus (DiD⁺ cells) in THP-1 cells at 30 min post infection when in complex with convalescent serum (3136) at the respective neutralizing titers, analyzed by flow cytometry. Data are normalized against cells infected with only virus (without antibodies) to account for differences in uptake for different DENV serotypes. (C) Fate of neutralized immune complexes when 3136 is in complex with DENV-2 or DENV-1. LAMP-1 is green, DiD-labeled DENV is blue, and h3H5 is red. (Scale bar, 7.5 μm.) (D–F) Same analysis as depicted in B and C, but with three other convalescent primary DENV-2 sera (6583, 3111, 3598). Neutralization titers < 10 were not considered for this analysis. Data are represented as mean ± SD. **P < 0.01.

Antibody Concentration Effects on FcγR-Mediated Uptake of Immune Complexes.

The observation that neutralized viruses were taken up and trafficked to the late endosome/lysosome compartment is consistent with the known function of monocytes in removing immune complexes from circulation. However, the inhibition of the uptake of immune complexes is intriguing. Neutralization of the heterologous DENV serotypes appeared to occur at lower dilutions of the convalescent sera than that needed for the homologous serotype (Fig. 1). This observation suggests that inhibition of FcγR-mediated uptake is affected by antibody concentration. However, early convalescent sera also contain IgM antibodies that could complex DENV without interacting with FcγRs. To address this potential interaction, we titrated a serum from a volunteer who was infected with DENV-2 over 30 y ago, reacted this to DENV-2, and determined its fate in THP-1. The results showed that lower serum dilution (Fig. S3) resulted in a similar reduction of DiD-labeled DENV-2 uptake whereas increasing serum dilutions resulted in FcγR-mediated uptake of the immune complex without DENV replication (Fig. 2 A and B).

Fig. 2.

Increasing antibody concentration inhibits immune complex internalization by THP-1 cells. (A) Percentage of internalized DiD-labeled DENV-2 virus (DiD⁺ cells) in THP-1 cells at 30 min post uptake when complexed with different neutralizing dilutions of convalescent DENV-2 serum, analyzed by flow cytometry. Dashed line indicates DiD⁺ cells in the presence of DiD-labeled DENV-2 only. (B) Subcellular localization of DENV-2, DENV-2 in complex with 1:8 serum, and DENV-2 in complex with undiluted serum in THP-1 cells. LAMP-1 is labeled green, DiD-labeled DENV-2 blue and h3H5 red. (C) Same as A, but with various neutralization concentrations of h3H5. (D) Same as B, but with DENV-2 only, DENV-2 in complex with 1.56 μg/mL h3H5, or DENV-2 in complex with 400 μg/mL h3H5. (E) Subcellular localization of Alexa594-labeled DENV-2 in complex with 400 μg/mL h3H5. LAMP-1 is green, Alexa594-labeled DENV-2 is red, and h3H5 is blue. Data are represented as mean ± SD. (Scale bar, 7.5 μm.)

To reduce variability resulting from the use of different sera, we investigated the mechanism for the observed concentration-dependent effects using the monoclonal antibody (mAb) 3H5, which is specific against DENV-2 (17, 18). Because FcγR engagement is required for FcγR-mediated phagocytosis of immune complexes (19), we generated a mouse–human chimeric antibody of 3H5 (h3H5) consisting of mouse V_H and V_L sequences and human γ1 and κ constant sequences (20). These antibodies were indistinguishable from the parent 3H5 mAb in their ability to bind to DENV-2 (Fig. S4A). Complete DENV-2 neutralization was observed from 1.56 to 400 μg/mL of h3H5. Subneutralizing concentrations of h3H5 enhanced viral infection to a greater extent after humanization into IgG1 but not IgG4 (Fig. S4B), indicating that specific interactions with human FcγR were attained (21). Using h3H5, we observed that increasing levels of antibody concentration along the range that fully neutralized DENV-2 resulted in the reduction of DiD⁺ cells (Fig. 2C). Likewise, as observed with convalescent sera, the immune complexes were trafficked to LAMP-1 compartments at 30 min post infection although increasing the h3H5 concentration to 400 μg/mL resulted in inhibition of uptake of the immune complexes (Fig. 2D). However, because the DiD signal is quenched before fusion with cellular membranes (16), it is not possible to visualize DENV outside of the cell. To overcome this limitation, we labeled DENV with Alexa Fluor (22) and demonstrated the presence of antibody-bound viruses clustered outside THP-1 cells (Fig. 2E). Similar findings were also made when primary monocytes were used instead of THP-1 cells (Fig. S5). Overall, these results indicate that immune complexes formed with neutralizing antibody can be rapidly internalized via the FcγR but this process is inhibited with increasing levels of antibody.

Size of DENV Immune Complex Is Dependent on the Concentration of Antibody.

One possible explanation for the antibody concentration-dependent inhibition of the immune complex uptake is that excess antibodies competed with the immune complexes for limited FcγR. To test this possibility, DENV complexed with h3H5 at 400 μg/mL was compared with that at 1.56 μg/mL but with an addition of isotype control antibodies to give a total antibody concentration of 400 μg/mL The addition of isotype control antibodies did not inhibit internalization of DENV-2 (Fig. 3 A and B), indicating that inhibition of FcγR internalization cannot be explained by competition for limited receptors by free antibodies. Instead, increasing antibody concentration could have cross-linked DENV, resulting in the formation of viral aggregates. To test this hypothesis, we used a sucrose density gradient consisting of layers extending from 60% sucrose to 10% sucrose in 10% increments to separate immune complexes of different sizes (23). Equal volume fractions were removed from the bottom of each tube for quantitative PCR analysis to detect for DENV RNA; immune complexes in the earlier fractions would thus have greater density (Fig. 3C). The size of aggregates in fractions that showed peak viral RNA copy numbers was determined using dynamic light scattering (Fig. 3D). The average diameter of DENV-2 in the experiment was 51.9 nm, which is consistent with previous observations that showed that DENV is ∼50 nm in diameter (24). Reacting 33.3 μg/mL of the Fab fragments of 3H5 Fab or 3 μg/mL of h3H5 with DENV-2 did not result in significantly different particle sizes. However, with 100 μg/mL of h3H5, peak DENV RNA copies shifted to fraction 9 (Fig. 3C), and this corresponded to an increased immune complex size of 148.2 nm in diameter (Fig. 3D). A similar increase in immune complex size was also observed with DENV-2 convalescent serum from Fig. 2A. When undiluted serum was used, the peak DENV RNA copies shifted significantly to the smaller fractions (Fig. 3E), which corresponded with an increased immune complex size of 182.3 nm in diameter (Fig. 3F). Taken together, these findings support the notion that larger aggregates were formed with increasing antibody concentrations.

Fig. 3.

Inhibition of immune-complex internalization is not due to FcγR competition but due to increased immune-complex size. (A) Percentage of DiD⁺ cells at 30 min post uptake when in complex with 400 μg/mL h3H5 or 1.56 μg/mL h3H5 with addition of 398.44 μg/mL of human isotype control. (B) Subcellular localization of DENV-2 complexed with 1.56 μg/mL h3H5 and with the addition of 398.44 μg/mL human IgG1 isotype control. LAMP-1 is green, DiD-labeled DENV-2 is blue, and human antibodies are red. (Scale bar, 7.5 μm.) (C) Proportion of total viral RNA extracted from the various sucrose fractions. Proportion of viral RNA in each fraction was determined by dividing the viral RNA copy number in that fraction by the viral RNA copy number in the entire gradient. Shown are the moving averages of free virus and 33.3-μg/mL Fab fragments of h3H5 or virus in complex with 3 μg/mL h3H5 or with 100 μg/mL h3H5 using qPCR. (D) Diameter of the immune complexes measured using dynamic light scattering in the respective sucrose fractions with peak viral RNA copy number. (E and F) Similar to C and D, but with 1:10 or undiluted serum. Data are represented as mean ± SD. **P < 0.01.

Aggregation of DENV Enables Antibodies to Cross-Link the Inhibitory FcγRIIB.

Given the increase in size when higher concentrations of antibodies were reacted with DENV, it is possible that the resultant larger viral aggregates enable antibodies to cross-link inhibitory FcγR that is expressed at lower levels on the cell membrane. One such candidate is FcγRIIB, which is known to exert an inhibitory effect on FcγR-mediated phagocytosis (25, 26). Because cross-linking of FcγRIIB has been previously shown to result in SHP-1 phosphorylation that down-regulates phagocytosis (27), we tested for SHP-1 phosphorylation when DENV-2 was complexed with 1.56 or 100 μg/mL of h3H5. Interestingly, increased phosphorylation was observed when DENV-2 was complexed with 100 μg/mL h3H5 (Fig. 4A). To confirm that FcγRIIB is functionally involved in the inhibition of immune complex uptake, we knocked down its expression in THP-1 using siRNA. Reduction of FcγRIIB but not FcγRI or FcγRIIA (Fig. 4B) resulted in significantly increased uptake of viral aggregates even with high h3H5 concentration (Fig. 4C). DENV-2 remained neutralized despite higher levels of uptake in the FcγRIIB knockdown cells (Fig. S6). Conversely, overexpression of FcγRIIB in THP-1 (Fig. 4D) resulted in reduced uptake of DENV immune complexes across all neutralizing antibody concentrations (Fig. 4E) and lower levels of infection when subneutralizing concentrations of h3H5 were used (Fig. 4F), compared with the mock-transfected cells. Taken collectively, our data indicate that FcγRIIB is important in the inhibition of the uptake of larger viral aggregates.

Fig. 4.

FcγRIIB is involved in the inhibition of immune-complex internalization of larger viral aggregates. (A) THP-1 cells exposed to media (mock), DENV-2 in complex with 1.56 μg/mL h3H5, and DENV-2 complexed with 100 μg/mL h3H5 30 min post infection. Cell lysates were immunoblotted with anti-SHP-1 and anti-phospho-SHP-1 (p-SHP-1). (B) THP-1 cells transfected with a control siRNA or siRNA against FcγRIIB. Cell lysates were immunoblotted with anti-FcγRIIB, anti-FcγRI, anti-FcγRIIA, and LAMP-1 antibodies. LAMP-1 served as a loading control. (C) Percentage of internalized DiD-labeled DENV-2 in THP-1 cells transfected with control siRNA or siRNA against FcγRIIB when in complex with various h3H5 concentrations. Dashed line indicates DiD⁺ cells in the presence of DiD-labeled DENV-2 only, without antibodies. (D) THP-1 cells subjected to mock transfection or transfected with FcγRIIB DNA. Cell lysates were immunoblotted with anti-FcγRIIB, anti-FcγRI, anti-FcγRIIA, and LAMP-1 antibodies (loading control). (E) Same as C, but using THP-1 cells with mock transfection or with transfected FcγRIIB DNA. (F) Plaque titers at 72 h post infection on cells with mock transfection or with overexpression of FcγRIIB. Dashed line indicates plaque titers in the presence of DENV-2 only, without h3H5. No significant differences between control and treated cells were observed with virus-only infection. Data are represented as mean ± SD.

Discussion

How the neutralizing antibody–virus complex interacts with monocytes is poorly understood, even though it represents an important piece of the puzzle of how dengue pathogenesis and immunity interact. Previous investigations into the fate of the neutralizing antibody–virus complexes have made use of kidney cell lines, such as LLC-MK2, Vero, and BHK-1 cells (28). These studies have shown that antibodies neutralize either by blocking viral fusion following uptake or by viral attachment (29). These cells, however, neither express FcγR naturally nor are primary targets of dengue virus in human infections. Monocytes, on the other hand, play a central role in DENV replication (30) as well as in the removal of the antibody–virus complex in vivo (31–33).

We have explored the initial events following the introduction of the DENV immune complex to THP-1 monocytic cells using early convalescent sera. On average, these convalescent sera were obtained 22 d post illness onset. Antibodies at this early convalescent period have been shown to have a broad cross-neutralization activity against the DENV serotypes that only became more serotype-specific with time (13, 14). Using such an approach, we observed two distinct patterns of virus neutralization. At the highest antibody titers that produced 100% DENV neutralization, homologous DENV serotypes were internalized by THP-1 cells and trafficked to the late endosome/lysosome whereas heterologous DENV serotypes were neutralized only at titers where FcγR-mediated uptake was inhibited.

Although the uptake of neutralized DENV was expected, the inhibition of FcγR-mediated uptake of DENV or other viral immune complexes has not been previously described as a means of viral neutralization. Our data indicate that, in addition to blocking attachment or fusion, antibodies can also neutralize by forming large viral aggregates, which in turn cross-links FcγRIIB to inhibit phagocytosis. This was observed with both h3H5 and human polyclonal antibodies in a convalescent serum sample. The aggregated viruses allow the resulting immune complex to be sufficiently large to cross-link FcγRIIB.

Unlike the receptors FcγRI and FcγRIIA, which contain the immunoreceptor tyrosine-based activation motif that activates phagocytosis, FcγRIIB is an inhibitory receptor with the immunoreceptor tyrosine-based inhibitory motif, which, when engaged, can activate phosphatases such as SHP-1 to down-regulate phagocytosis (34). FcγRIIB's role in the immune response to infection has not been studied extensively. Most work has involved Streptococcus pneumoniae, a Gram-positive bacterium that is a major cause of pneumonia. In these studies, FcγRIIB-deficient mice have been shown to have improved FcγR-mediated phagocytosis of the antibody-bound bacterium (35). Likewise, FcγRIIB deficiency is associated with an increased resistance to Staphylococcus aureus (36) and Mycobacterium tuberculosis (37) infection in mice. The only study that has examined the role of FcγRIIB in response to viral infection showed that FcγRIIB-mediated inhibition of phagocytosis by dendritic cells was associated with reduced antibody and T-cell responses to human papilloma virus-like particles (38). The involvement of FcγRIIB in virus neutralization is thus unique and shows that ligation of FcγRIIB impairs phagocytosis and antibody-dependent enhancement in monocytes. Our observations may also explain previously reported observations of peak antibody titers following acute influenza or dengue infections being associated with transient reduction in the phagocytic activity of macrophages in experimental mouse models (39, 40).

That cross-linking of FcγRIIB can inhibit antibody-dependent enhancement of DENV infection would suggest that, as long as an antibody can bind to an epitope in a manner that allows for the formation of concentration-dependent viral aggregates, DENV can be neutralized in vivo. If immunity were to be mediated by this mechanism alone, then antibodies would protect against all four DENV serotypes. However, epidemiological observations indicate that lifelong immunity is specific to the serotype with which a person has been infected. This suggests instead that, for long-lasting immunity, antibodies must be able to prevent infection even at levels that do not form viral aggregates that cross-link FcγRIIB, a scenario expected as antibody levels wane following acute infection. In contrast, inhibition of theh uptake of heterologous serotypes by the early convalescent sera from patients with primary DENV infection was observed only at low serum dilutions. This process may be the reason why a person can be transiently immune to the remaining three serotypes of DENV following an acute primary DENV infection (41, 42). Further studies, however, would be needed to establish this notion.

Our demonstration that antibodies aggregate viruses also raises several unexplored questions. In addition to interacting with FcγRs directly, immune complexes can also bind to complement components and interact with complement receptor 1 (CR1) on red blood cells in vivo (43–45). The determinants of the outcome of the simultaneous interaction between DENV immune complexes and the CR1 receptor along with the various types of FcγR on macrophages in the liver and spleen are unknown. Even if phagocytosis was not inhibited in this interaction, it would be interesting to determine if aggregation of DENV could sufficiently reduce the stoichiometric requirement for virus neutralization by reducing the epitopes available on each virion to interact with cellular receptors. In addition, we observed that only a subset of monocytes can uptake DENV, which is consistent with previous reports indicating that only a minor population of monocytes can uptake and replicate DENV (46, 47). These observations suggest that additional factors may influence the uptake of the DENV immune complex.

In conclusion, aggregation of DENV by antibodies that results in the engagement of FcγRIIB is a mechanism that inhibits DENV infection of monocytes.

Materials and Methods

Antibodies and Human Sera.

3H5 chimeric human/mouse IgG1 and IgG4 antibodies were constructed as previously described (20). Human sera were obtained from early dengue infection and control (EDEN) study as previously described (48).

Cells.

BHK-21, C6/36, THP-1, and Vero cells were purchased from the American Type Culture Collection (ATCC) and cultured according to ATCC recommendation. Primary monocytes were isolated from the principal investigator and cultured as described in SI Materials and Methods.

Virus Stock.

DENV-1 (07K3640DK1) and DENV-3 (05K863DK1) are clinical isolates obtained from the EDEN study (36). DENV-2 (ST) is a clinical isolate from the Singapore General Hospital, and DENV-4 (H241) was obtained from ATCC. Viruses were propagated in the Vero cell line and harvested 96 h post infection and purified through 30% sucrose. Virus pellets resuspended in 5 mM Hepes, 150 mM NaCl, and 0.1 mM EDTA (HNE) buffer were stored at −80 °C until use. Infectious titer was determined by plaque assay.

Virus Infection in THP-1 Cells.

Serial twofold dilutions of h3H5 or human sera were incubated for 1 h at 37 °C before being added to THP-1 cells at a multiplicity of infection (moi) of 10. At 72 h after infection, the culture was clarified by centrifugation, and the infectious titer of dengue virus in the culture supernatant was quantified with plaque assay. The antibody dilution required to mediate full virus neutralization was then determined.

Fluorescent Labeling of Virus.

The method for DiD labeling and Alexa594 labeling of DENV was as previously described (22) and as detailed in SI Materials and Methods.

Virus Internalization in THP-1.

Neutralizing concentrations of h3H5/human sera were incubated with DiD-labeled DENV or AF584-DENV for 1 h at 37 °C before adding to THP-1 cells (moi 10). Cells were then subjected to 20 min of synchronization on ice, followed by 30 min of infection at 37 °C, and then fixed with paraformaldehyde. Flow cytometry was used to determine the percentage of cells with internalized virus complexes, and confocal immunofluorescence was used to determine localization of antibody–virus complexes in the cell. Detailed description is provided in SI Materials and Methods.

Sucrose Gradient Analysis of DENV Immune Complex Sizes.

Sucrose gradient was formed by careful layering of 10–60 sucrose solutions (in HNE buffer) in 10% increments, starting with the densest at the bottom, in 13.5-mL Ultra-Clear tubes (Beckman Coulter). The gradient was allowed to linearize overnight at 4 °C. Equal amounts of purified DENV were incubated with humanized antibodies at 3 or 100 μg/mL h3H5 IgG or h3H5 Fab (h3H5 enzymatically digested to give only one arm of the Fab fragment) for 1 h at 37 °C. The samples were then carefully layered on top of the linearized 10–60% sucrose gradient and centrifuged overnight at 25,000 × g at 4 °C in a SW41Ti rotor for 17 h. Each gradient was then harvested in 0.25-mL fractions from the bottom of the tube. Subsequently, viral RNA was extracted from each fraction and quantified by real-time PCR. The proportion of virus present in each fraction was then tabulated by dividing it to the total viral RNA loaded in these fractions. Each fraction was then plotted as an average with two neighboring fractions to obtain smoother curves.

Dynamic Light Scattering of Peak Sucrose Fractions.

The sucrose fraction that showed the highest virus genome copy number was identified as the peak fraction and subjected to dynamic light scattering to determine the DENV immune complex diameter. Twenty microliters of each fraction was loaded into a Quartz cuvettete for analysis by Zetasizer Nano S machine (Malvern) at 37 °C using 50% sucrose in HNE buffer as the dispersant. Data were then analyzed using Zetasizer Nano software version 6.01. The diameter reports generated are an average of more than 10 readings.

SHP-1 Phosphorylation in THP-1 Cells.

DENV-2 was incubated with 1.56 or 100 μg/mL h3H5 and added to THP-1 cells for 30 min at 37 °C. For mock infection, only RPMI media was added. Cells were then extracted and lysed before performing Western blot as described in SI Materials and Methods.

siRNA Transfection into THP-1 Cells.

Human FcγRIIB siRNA (Qiagen) and AllStars negative control siRNA (Qiagen) duplexes (50 nM) were incubated with DharmaFect2 (Dharmacon) in serum-free media for 20 min and then added to cells at a density of 2 × 10⁵ cells/mL After 6 h incubation, cells were replaced with RPMI growth media for 2 d to allow recovery. This was followed by a second round of siRNA transfection. Knockdown efficiency was determined by Western blot as described in SI Materials and Methods.

FcγRIIB Transfection of THP-1.

FcγRIIB cDNA was purchased from Origene and transfected using Lipofectamine LTX and Plus reagent (Invitrogen) in accordance with the manufacturer's instructions. Cells were subjected to two rounds of transfection, and transfection efficiency was determined by Western blot as described in SI Materials and Methods.

Statistical Analysis.

Two-tailed unpaired Student's t test was used to determine if the difference in the mean observed was statistically significant (P < 0.05). All calculations were done using GraphPad Prism v5.0 (GraphPad Software Inc.).