Sialic acid-binding Ig-like lectin-7 interacts with HIV-1 gp120 and facilitates infection of CD4posT cells and macrophages
© Varchetta et al.; licensee BioMed Central Ltd. 2013
Received: 26 June 2013
Accepted: 10 December 2013
Published: 13 December 2013
Sialic acid-binding Ig-like lectin-7 (Siglec-7) expression is strongly reduced on natural killer (NK) cells from HIV-1 infected viremic patients. To investigate the mechanism(s) underlying this phenomenon, we hypothesized that Siglec-7 could contribute to the infection of CD4pos target cells following its interaction with HIV-1 envelope (Env) glycoprotein 120 (gp120).
The ability of Siglec-7 to bind gp120 Env in a sialic acid-dependent manner facilitates the infection of both T cells and monocyte-derived macrophages (MDMs). Indeed, pre-incubation of HIV-1 with soluble Siglec-7 (sSiglec-7) increases the infection rate of CD4pos T cells, which do not constitutively express Siglec-7. Conversely, selective blockade of Siglec-7 markedly reduces the degree of HIV-1 infection in Siglec-7pos MDMs. Finally, the sSiglec-7 amount is increased in the serum of AIDS patients with high levels of HIV-1 viremia and inversely correlates with CD4pos T cell counts.
Our results show that Siglec-7 binds HIV-1 and contributes to enhance the susceptibility to infection of CD4pos T cells and MDMs. This phenomenon plays a role in HIV-1 pathogenesis and in disease progression, as suggested by the inverse correlation between high serum level of sSiglec-7 and the low CD4pos T cell count observed in AIDS patients in the presence of chronic viral replication.
KeywordsHIV-1 infection Siglec-7 CD4+ T cells Macrophages AIDS patients
The human immunodeficiency virus type 1 (HIV-1) envelope (Env) glycoprotein 120 (gp120) is extensively covered by carbohydrates that play an active role in the viral life cycle. Indeed, besides forming a protecting shield from antibody (Ab) recognition, this coat of N-linked glycans affects the folding of viral glycoproteins and virus infectivity [1–4]. gp120 N-linked glycans include 11 high-mannose or hybrid-type glycans and 13 complex-type glycans (containing terminal sialic acids) that are highly conserved in different isolates and clades [3, 5]. Sialic acids are a family of sugars with a 9-carbon backbone present at the ends of glycan chains in all cell types [6, 7]. They are recognized by many sugar-binding proteins, including sialic acid-binding lectins (Siglecs), a family of 16 I-type lectins showing a specific affinity for different sialic acids. In particular, Siglec-7 preferentially binds α(2,8)-linked disialic acid and α(2,6)-linked sialic acid . Siglec-7 is constitutively expressed on human natural killer (NK) cells, monocytes and a small subset of CD8pos T cells [9–13]. In regard to interactions with pathogens, Siglec-7 has been shown to bind Campylobacter jejuni and Pseudomonas aeruginosa expressing or displaying sialic acids on their surface, respectively [14–16].
Many human lectin-type receptors including galectins [17, 18], defensins [19–21] and others [22–27] have been shown to bind HIV-1 Env by recognizing glycans expressed on gp120. However, these molecules may affect HIV-1 infection in an opposite way as some, such as mannose-binding lectin (MBL)  and langerin , inhibit HIV-1 infection, while others, such as galectin-1, DC-SIGN , mannose receptor , syndecan-3  and DCIR , increase the susceptibility to HIV-1 infection.
Recently, it has been shown that different Siglecs, such as Siglec-1 and Siglec-7, recognize HIV-1 and enhance infection of monocytes , macrophages  and dendritic cells (DCs) . Indeed, sialic acids present on Env gp120 can be recognized by Siglecs either expressed on these immune cells or released in soluble forms, thus facilitating viral entry into target cells. Although Siglec-7 has been shown of being able to bind Env gp120 from different HIV-1 strains with lower affinity if compared to Siglec-1 , very little is know about the role of Siglec-7 in participating to the HIV-1 infections of CD4pos target cells. We have previously shown that the expression of Siglec-7 is significantly reduced on the surface of NK cell from HIV-1 infected viremic patients and that the successful suppression of viral replication by antiretroviral therapy (ART) restores Siglec-7 expression on these cells .
The present study demonstrates that, following its binding with HIV-1 Env gp120, Siglec-7 contributes to viral entry and infection of both CD4pos T cells and monocyte-derived macrophages (MDMs). Indeed, our results demonstrate that the treatment with soluble Siglec-7-Fc fusion protein increases the susceptibility to HIV-1 infection in Siglec-7neg/CD4pos T cells, while blockade of Siglec-7 with a specific Ab reduces the degree of infection in Siglec-7pos MDMs. Finally, the study shows that in the sera of viremic AIDS patients there are increased serum levels of soluble Siglec-7 (sSiglec-7) that inversely correlates with CD4pos T cell counts, thus suggesting a direct role of this glycan-binding protein in the modulation of HIV-1 infection and disease progression.
Soluble Siglec-7 binds HIV-1 envelope gp120 recombinant protein from different HIV-1 strains
The ability of Siglec-7 to directly bind HIV-1 was then assessed by using both R5 (BaL, SG) and X4 tropic (IIIB) isolates in a virion-binding assay. Goat anti-human Fc magnetic beads coated with Siglec-7-Fc or NKp44-Fc proteins were incubated with the HIV-1 infectious viral stocks, and the binding was evaluated by measuring the amount of captured p24 HIV-1 antigen by ELISA. In line with the experiments shown in the panels A-B-C of Figure 1, we found detectable amounts of p24 only in the presence of Siglec-7-Fc fusion protein coated beads and not after incubation with beads coated with NKp44-Fc chimera, regardless of the HIV-1 strains used. The binding of R5 or X4 HIV-1 isolates to beads coated with the anti-gp120 mAb 2G12 was used as positive internal control (Figure 2B).
Siglec-7 fusion protein enhances HIV-1 infection of CD4posT cells
We then investigated whether the Siglec-7 chimera was able to amplify the HIV-1 productive infection also in primary CD4pos T cells, the main target of HIV infection [34, 35]. To this end, we followed the same protocol used for PM1 cells. Again, our results showed a statistically significant increase of HIV-1 viral load in CD4pos T cells infected with HIV-1 IIIB pre-incubated with Siglec-7-Fc chimera compared to their counterpart infected with untreated HIV-1 strains, while pre-incubation of viral isolates with NKp44 fusion protein did not enhance the degree of infection of CD4pos T cells (Figure 3C). Although, we observed a higher HIV-1 viral load also in CD4pos T cells infected with HIV-1 BaL pre-incubated with Siglec-7-Fc chimera, this increase did not reach a statistical significance (Figure 3D).
Siglec-7 fusion protein increases HIV-1 viral entry in primary CD4posT cells
T cells are generally considered not to express substantial amounts of Fcγ receptors (FcγRs) although there are a few reports showing a positive expression of CD64 (FcγRI), CD32 (FcγRII) and CD16 (FcγRIII) on some CD4pos cell line or unique primary T cell subsets [36, 37]. Since our experimental approach relies on recombinant Siglec-7-Fc fusion protein as a surrogate of sSiglec-7, we proceeded to demonstrate that the enhanced HIV entry/infection in CD4pos T cell targets by Siglec-7-Fc is not associated with the expression of FcγRs. To this end, we measured the surface levels of CD64, CD32 and CD16 on PM1 cell lines and primary CD4pos T cells by flow cytometry and we did not detect any significant or bimodal expression of FcγRs on the aforementioned populations compared to positive controls (Additional files 1, 2 and 3).
Anti-Siglec-7 blocking Ab inhibits HIV-1 viral entry in MDMs
MDMs are susceptible to HIV-1 infection as they express both CD4 and the co-receptors CCR5 and CXCR4 [38, 39]. However, unlike primary CD4pos T cells and similar to circulating monocytes , MDMs constitutively express high levels of Siglec-7 (Figure 4C), thus representing a useful cellular model to study the Siglec-7/HIV-1 interaction. Since we found that sSiglec-7 promotes HIV-1 entry into CD4pos T cells, we asked whether blockade of this lectin-type receptor on the surface of MDMs could reduce the degree of HIV-1 infection. To validate this hypothesis, MDMs were cultured with HIV-1 BaL either with or without a previous incubation with anti-Siglec-7 blocking Ab. The amount of HIV-RNA transcripts was evaluated in MDM lysates 4 hours after infection to assess viral internalization. The results showed that the masking of Siglec-7 induced a statistically significant decrease of HIV-RNA copies compared to the infection performed in the absence of anti-Siglec-7 blocking Ab (Figure 4D).
Serum levels of sSiglec-7 correlate with viral load in HIV-I infected patients naïve for antiretroviral therapy (ART)
Demographic and clinical features of HIV-1 infected patients
(n = 42)
(n = 54)
(n = 13)
(n = 109)
Plasma HIV-1 RNA (copies/ml)
CD4pos T cell count (cells/mm3)
In this study, we show that the direct interaction between Siglec-7 and HIV-1 Env gp120 contributes to HIV-1 entry and infection of CD4pos cell targets, including T cells and macrophages. These findings are consistent with recent studies reporting that the interactions between sialic acids present on the viral envelope and Siglec receptors facilitate HIV-1 infection in several cellular targets [28–30].
Siglec molecules have been recognized as ligands of different sialic-acid bearing pathogens [13–15, 40–44], although it is still debated whether resting leukocytes with masked Siglecs are able to directly engage sialic acids on pathogens or if cell activation is required for unmasking these lectin-type receptors. Originally, the interactions between Siglec receptors and pathogens were reported to dampen host immune responses and to set appropriate activation thresholds for regulating cell proliferation and secretion of inflammatory mediators. Subsequently, it became evident that this biological phenomenon represents an advantage for pathogens that evolved to express sialic acids on their surface to evade host innate immune responses [45, 46]. However, recent evidence showed that sialic acid-expressing pathogens could also promote infection and mediate immune responses following their binding to Siglecs [13, 47, 48].
In line with a previous report , we demonstrate that Siglec-7 is able to directly interact with HIV-1 Env gp120 although we found a certain degree of variability among different HIV-1 strains. This phenomenon may have several explanations, including deletions or mutations of critical residues involved in Siglec-7 binding or, most likely, heterogeneity in HIV-1 glycosylation patterns [1–4, 13]. Evidence in favor of the latter hypothesis comes from our results showing that Siglec-7 interaction with HIV-1 is indeed sialic acid-dependent. In this regard, it has also been reported that sialic acid depletion from human CD4pos target cells enhances HIV-1 infection . This suggest that removing cis-interactions between sialic acids and Siglecs on the same cell leads to exposure of Siglecs that, in turn, become accessible to sialic acids exposed on gp120. Moreover, oligomeric modeling of the gp120 trimer shows that complex glycans are exposed on the outer rim, potentially accessible to Siglec recognition . Furthermore, it has recently been shown that different Siglecs, and Siglec-1 in particular, are able to recognize both R5 and X4 HIV-1 gp120 through interaction with sialic acids, thus contributing to HIV-1 infection of monocytes, macrophages and DCs [28–30].
The present study also provides evidence that Siglec-7 binds HIV-1 and enhances viral entry into CD4pos T cells when the soluble form of this lectin-type molecule is added to the culture. Indeed, our data clearly showed that Siglec-7-Fc fusion protein is able to enhance IIIB and BaL infections both in a CD4pos T cell line (PM1) and in primary CD4pos T cells, even though the levels of HIV-RNA were higher for IIIB compared to BaL. This latter phenomenon is likely due to the well-known preferential tropism of the IIIB HIV-1 strain for T cells . Further investigations focusing on the structures, biochemical properties and affinity of these interactions are needed to confirm this working hypothesis. Moreover, we also demonstrate that constitutive expression of Siglec-7 on MDMs markedly contributes to their HIV-1 infection, since specific Siglec-7 blockade induced a reduction of HIV-RNA in MDMs in a 4-hour entry assay. The degree of surface levels of Siglec-7 on monocytes and macrophages is still being debated, as there is one study showing that expression of this molecule on monocytes and MDMs is very low , while several other reports demonstrated that monocytes express high levels of Siglec-7 [9, 11, 12, 46, 52]. In our experiments, the constitutive expression of Siglec-7 on monocytes paralleled the ones observed on NK cells , and remained very high on MDMs even after 7 days of culture with GM-CSF although at lower levels if compared to freshly isolated monocytes.
Given the ability of Siglec-7 fusion protein to enhance in vitro HIV-1 infection of CD4pos T cells, we then determined whether different levels of sSiglec-7 released during the course of HIV-1 infection are associated with different clinical outcomes. We did not find any significant differences in the serum levels of sSiglec-7 between healthy controls and several different cohorts of HIV-1 infected patients. However, AIDS patients showed a trend toward increase of Siglec-7 in their sera compared to healthy donors and, in particular, we found a direct and statistically significant correlation between HIV-RNA and serum levels of Siglec-7 in naïve HIV-1 infected patients, suggesting that a higher viral burden could induce the shedding of Siglec-7 in serum. In this regard, it is conceivable to hypothesize that a possible source of sSiglec-7 are NK cells, since we demonstrated that the exposure of these cells to high chronic HIV-1 viremia induce the expansion of pathological NK cell subsets showing an aberrant receptor repertoire and an unusual negative expression of Siglec-7 [53, 54]. The fact that Siglec-7 fusion protein enhances the in vitro susceptibility of CD4pos T cells to be infected by HIV-1 is also consistent with the persistent high levels of HIV-1 viremia, with the high amount of sSiglec-7 and with low CD4pos T cell counts found in AIDS patients. Interestingly, a CD56neg/Siglec-7neg population associated with high levels of HIV-1 viremia has been described as a dysfunctional NK cell subset that is significantly expanded in the late stages of HIV-1 infection . In agreement with this, sSiglec-7 was found to be significantly increased in the sera of patients with viral load over 10,000 copies/ml, suggesting a possible role for this lectin-type receptor as a biological marker involved in the disease progression. We are currently testing this hypothesis in a larger patient cohort. Finally, an inverse correlation between the serum levels of Siglec-7 and CD4pos T cell counts was observed in AIDS patients, which were also the cohort of HIV-1 infected subjects with the highest serum levels of sSiglec-7. These findings lead us to hypothesize that the presence of remarkable amounts of sSiglec-7, together with a low CD4pos T cell count, high levels of HIV-1 viremia and expansion of Siglec-7neg NK cells, characterize the advanced stages of HIV infection.
For our experimental approaches testing the ability sSiglec-7 to enhance the infection of CD4pos PM1 cell line and primary T cells, we used a dose of fusion protein (10 μg/ml) much higher compared to amount of sSiglec-7 found in the sera of both healthy donors and HIV-1 infected patients (floating within a ng/ml range). We have chosen to use a higher sSiglec-7 concentration in our in vitro approach to maximize ligand-receptor binding. However, it must be emphasized that identical concentrations of an irrelevant protein such as soluble NKp44 was unable to promote infection using the same experimental conditions. Moreover, even though the serum concentrations of sSiglec-7 in HIV-1 infected patients or healthy donors are remarkably lower compared to those used in our in vitro approach, it remains to be determined the amounts of this soluble protein during acute phases of infection when HIV-1 viral load is in the ranges of several hundred-thousand copies . Finally, we need to assess whether higher levels of sSiglec-7 are also observed at gut or cervix mucosal levels, two of the main gates of viral entrance [55, 56]. Indeed, macrophages as well as NK and T cells play an active role within these tissue sites in context of innate and adaptive immune responses and establishment of viral reservoirs in the CD4pos targets [57–59].
Peripheral blood mononuclear cells (PBMCs) were obtained from buffy coat in accordance with the clinical protocol approved by the Institutional Review Board (IRB) of San Matteo Hospital, Pavia, Italy. The sera of HIV-1 infected patients (Table 1) were obtained in accordance with the clinical protocol approved by the IRB of Sacco Hospital, Milan, Italy. Donors from both protocols signed consent forms that were approved by the above-mentioned IRBs in accordance with Italian laws and with Declaration of Helsinki.
Recombinant human Siglec-7 and NKp44 fusion proteins and blocking anti-Siglec-7 polyclonal Ab were purchased from R&D system Inc. Commercially available ELISA kits were used for the detection of HIV-1 gp120 (Perkin Elmer) and sSiglec-7 (DuoSet, R&D system). Recombinant gp120s and anti-gp120 HIV-1 monoclonal Ab (mAb) 2G12 were obtained from the National Institutes of Health AIDS Research and Reference Reagent Program. Anti-Siglec-7 monoclonal Abs (mAbs) QA79 and Z176 were purchased from Instrumentation Laboratories.
Gp120 from HIV-1 IIIB strain was covalently conjugated to carboxyl microparticles using the PolyLink Protein coupling kit (Bangs Laboratories Inc.) following the manufacturer instructions. Siglec-7-Fc or NKp44-Fc fusion proteins were incubated with BioMag particles coated with goat anti-human IgG Fc Abs (Qiagen) in order to bind the beads.
Siglec-7 binding to gp120
260 ng of gp120-conjugated beads (130 μg/ml) were incubated with 1 μg of Siglec-7-Fc or NKp44-Fc protein for 30’ at RT. After three washing, a phycoerythrin (PE) polyclonal goat anti-human-Fc was added for 30’ at 4°C. Samples were then washed and analyzed with a Facscalibur flow cytometer (BD Biosciences). Data were analyzed using FlowJo software (TreeStar).
Sialidase treatment was performed incubating 0.16 units/ml of Vibrio cholerae neuraminidase (Sigma) in DMEM medium with gp120 coated beads for 2 hours at 37°C. Flicking of the tubes was done every 20’ to get a uniform treatment of sialidase. Beads were washed three times with DMEM 10% FCS.
Virus binding assay
10 μg of Siglec-7-Fc or NKp44-Fc proteins conjugated to magnetic beads were incubated with viral stocks of different HIV-1 strains (IIIB, SG, BaL; 5 × 105 copies/ml). After three washing steps with complete, culture medium samples were solubilized in lysis buffer (250 μl of 0.5% NP40 alternative, Calbiochem-Merck), bead debris were removed by magnetic adhesion and the concentration of p24 protein released in the buffer was measured by ELISA. The binding of R5 or X4 HIV-1 isolates to beads coated with the anti-gp120 mAb 2G12 was used a positive internal control.
PM1 cells were cultured in RPMI 1640 medium (Sigma) supplemented with 10% fetal calf serum (HyClone South Logan), 2 mM L-glutamine (Sigma) and antibiotics (100 U/ml penicillin, 0.1 μg/ml streptomycin, 0.25 μg/ml amphotericin B, Sigma). CD4+ T cells and CD14+ monocytes were isolated from PBMCs by negative magnetic selection (EasySep, Stemcell) from PBMCs obtained by healthy donors by standard density gradient centrifugation (Lympho Separation Medium, MP Biomedicals). The purity of both populations was always higher than 95%. Monocytes were cultured at 5 × 105 cells/ml in 48-well plates in RPMI 1640 supplemented with 10% fetal bovine serum (HyClone South Logan), 10% human serum AB (EuroClone S.p.A.), 2 mM L-glutamine (Sigma), antibiotics (100 U/ml penicillin, 0.1 mg/ml streptomycin, 0.25 mg/ml amphotericin B, Sigma) and 10 ng/ml GM-CSF for 7 days with medium replacement every 3 days. We analyzed the surface expression of Siglec-7, CD14 and CD33 (BD Pharmigen) on MDMs after 8 days of culture, in the presence of 20% human serum to avoid non-specific binding to Fc-receptors.
For multicolour (up to 8 colours) (FACS Fortessa, BD Pharmigen) flow cytometric analyses, PBMCs were stained with Brilliant Violet 421-labeled CD56, fluorescein isothiocyanate (FITC)-labeled CD3, Pacific Blue-labeled CD14 (Biolegend), allophycocyanin(APC)-labeled CD4 (R&D system), PerCP-labeled CD8, APC/C7-labeled 19, PC7-labeled CD16, phycoerythrin(PE)-labeled CD32 and CD64 (BD Pharmigen) monoclonal antibodies (mAbs). Viable cells were detected through the aqua live/dead staining, according to manufacturer instructions (Invitrogen). Within the lymphocyte gate, CD4pos T cells were defined as CD3pos, CD19neg, CD56neg, CD8neg cells, while monocytes were defined as CD14pos cells within the related gate. PM1 T cell cell lines were gated within the CD4pos cells [60, 61]. Data were analyzed by using FlowJo software (TreeStar).
Blocking of HIV-1 binding to MDMs by anti-Siglec-7 masking antibody
MDMs were treated with 10 μg/ml of anti-Siglec-7 blocking polyclonal Ab or IgG isotype control (R&D System) in the presence of 20% human serum for 1 hour at 4°C. After three washing with PBS 2% FCS cells were infected as described below.
HIV-1 infection of CD4+ cells
CD4+ PM1 T cells or primary CD4+ T cells were infected with either HIV-1BaL or HIV-1IIIB viruses. HIV-1 strains were propagated in phytohemagglutinin (PHA)-activated PBMC and collected from cell supernatant at peak times of HIV-RNA content. Cells (1 × 106/ml, 2 × 105/well) were inoculated with HIV-1 stock (5 × 105 HIV-RNA copies/well) previously treated (for 1 hour at 37°C) with Siglec-7- or NKp44-Fc protein (10 μg/ml) and cultured at 37°C in 20% FCS RPMI 1640 medium. HIV-RNA was evaluated in cell supernatants after 3 days of culture.
HIV-1 entry assay
After 4 hour incubation with HIV-virus (5 × 105 copies/well) CD4+ T cells (1 × 106/ml, 2 × 105/well) or MDMs (5 × 105 cells/ml, 5 × 105 cells/well) were washed twice and treated with trypsin (2.5 mg/ml) for 10’ at 37°C to remove the extracellular virus adherent to the cell surface. After three additional washing steps with complete culture medium, cell lysates were prepared with NP40 Alternative 0.5% (Calbiochem-Merck) and the concentration of HIV-RNA was measured by real-time PCR.
Real time PCR
The evaluation of HIV replication capacity was determined by measuring the HIV-RNA copy numbers in cell culture supernatants or, for entry assay, in the cell lysate after elimination of cell debris. In detail, viral HIV-1 RNA was extracted from supernatants or cell extracts using the automatic Easy Mag extractor (Biomerieux, Lyon, France) and quantified by real time PCR targeting a conserved region in the long terminal repeat (LTR) as previously reported .
Paired t test was used to compare the HIV IIIB-beads binding to Siglec-7 Fc-chimera protein. One-way ANOVA statistical test followed by a Bonferroni post-test was used to compare HIV-RNA copies/ml within the different experimental settings, while Kruskal Wallis statistical test followed by a Dunn post-test to compare the serum levels of sSiglec-7 in the different groups of HIV infected patients. Pearson correlation test was used to correlate sSiglec-7 with HIV-RNA levels or CD4+ cell numbers.
Sialic acid-binding immunoglobulin-like lectin 7
Human immunodeficiency virus type 1
Glycoprotein 120, AIDS, acquired immunodeficiency syndrome
C-C chemokine receptor type 5
C-X-C chemokine receptor type 4
Monocyte derived macrophages
Enzyme-linked immune-sorbent assay
Polymerase chain reaction.
This work was supported by the Italian Ministry of Health, Ricerca Finalizzata, Bando ISS, [Grant number RF-ICH-2009-1299677] to D. M., Ministero della Salute, by Ricerca Corrente Fondazione IRCCS Policlinico San Matteo: “Role of Innate Immune Response during Hepatitis C virus infection”, and by the intramural research program of National Institute of Allergy and Infectious Diseases, National Institutes of Health, USA.
We thank human donors and HIV-1 infected patients for their generosity and participation to this study. We are grateful to Huiyi Miao and Christina Guzzo for ELISA experiments and to Alessandro Moretta and Emanuela Marcenaro for providing reagents. We thank Carmine Tinelli for reviewing our statistical analyses.
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