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CUTTING EDGE |

* Department of Hygiene, Microbiology and Social Medicine, Innsbruck Medical University and Ludwig-Boltzman-Institute for AIDS Research, Innsbruck, Austria; and
Department of Immunology and Research Group of the Hungarian Academy of Sciences, Eötvös Loránd University, Budapest, Hungary
| Abstract |
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-chain of complement receptor 3, receptor for iC3b), proving the importance of complement receptor 3 in this process. Inhibition of complement activation by EDTA also prevents enhanced infection, further demonstrating the role of complement in virus uptake and productive infection. Since HIV is, even in the absence of Abs, regularly opsonized by complement, most probably the above-described mechanism plays a role during in vivo primary infection. | Introduction |
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In general, microorganisms coated with C3 fragments acquire the capacity for high binding to CR14 (3, 4, 5, 6, 7). CR3 and CR4 are members of the
2-integrin family, are expressed on several cell types, including phagocytic cells, and play an important role in phagocytosis of opsonized pathogens. It has been shown that dendritic cells (DCs) in normal human skin as well as monocyte-derived DCs (MDCs) differentiated in vitro express CD11b (8). Regarding the expression of CD11c, two major populations are present in the body: the CD11c+ myeloid DCs, and the CD11c plasmacytoid DCs. CR3 and adhesion molecules are documented to facilitate viral entry into different target cells (9, 10).
CD14+ human monocytes can develop into MDCs in vitro by culturing in medium containing GM-CSF and IL-4. By day 5 in culture, the cells have the characteristics of immature DCs (imMDCs) and can be further induced to mature DCs by inflammatory stimulus such as LPS. Maturation rapidly induces DCs to express costimulatory molecules, such as CD80 and CD86, necessary for the activation of naive T cells (11). Targets of HIV during primary in vivo infection are macrophages and DCs, which are predominantly permissive for R5 viruses (12). imMDCs can be productively infected with R5 viruses and the virus produced by these cells is able to infect T cells. Mature DCs do not produce viral particles, still they are able to transmit both R5 and X4 viruses to T cells (13).
Our observation that complement opsonization of HIV causes a distinctively productive infection of DCs has not yet been documented. In this study, we report that CR3 plays a crucial role in the productive infection of imMDCs by C3-opsonized HIV.
| Materials and Methods |
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Anti-human (hu) C3c, anti-huC3d, and anti-huIgG rabbit polyclonal Abs and FITC-conjugated goat anti-mouse Ig polyclonal Ab were obtained from DakoCytomation (Glostrup, Denmark). Mouse mAbs used in this work were as follows: anti-CD4, anti-CD86, anti-CD83, anti-CD14, anti-CCR5, anti-CD11c (BD Pharmingen, San Diego, CA) anti-CD18 (OKM-1); hybridoma supernatants and mouse IgG1 isotype control were used at 10 µg/ml. mAb TMG6-5 blocking anti-CD11b (14) was kindly provided by I. Andó (Biological Research Center, Szeged, Hungary).
DC cultures
MDCs were generated in vitro by culturing monocytes (15, 16). Briefly, PBMCs were obtained by the standard Ficoll-Hypaque (Pharmacia Biotech, Uppsala, Sweden) method. Monocytes were isolated by adherence using gelatin-coated petri dishes (17). The purity of the monocyte suspension was 9095%, as judged by FACS analysis. Differentiation of monocytes into imMDCs was induced by culturing in RPMI 1640 medium (Invitrogen Life Technologies, Grand Island, N.Y.) supplemented with 1600 U/ml recombinant human (rh) GM-CSF (Novartis, Vienna, Austria), and 1500 U/ml rhIL-4 (PromoCell, Heidelberg, Germany), 10% heat-inactivated FCS (Invitrogen Life Technologies), 2 mM L-glutamine, and 50 µg/ml gentamicin (Sigma-Aldrich, St. Louis, MO) in 24-well tissue culture plates (Costar, Cambridge, MA). Culture medium was refreshed at days 5 and 7 and supplemented with cytokines at days 2, 5, and 7. imMDCs were CD14, CD80+, CD86+, CD83, CD11b+, CD11c+, HLA-DR+, CD4low, and CCR5low (12, 18). imMDCs were used for virus binding and infectious experiments on day 7 of culture. Occasionally, in vitro maturation of MDCs was induced by 100 ng/ml LPS (Sigma-Aldrich).
HIV sources, p24 ELISA
The M-tropic HIV-1 primary isolate 92UG037 (subtypeA/A, R5 tropism) was kindly provided by G. Stiegler (Institute of Applied Microbiology, Vienna, Austria). Virus stock was produced by PHA-activated PBMCs of healthy donors and maintained in RPMI 1640 supplemented with 10% FCS, 2 mM L-glutamine, 100 U/ml penicillin, and 100 µg/ml streptomycin. The virus was pelleted at 20,000 rpm by ultracentrifugation and stored at 80°C. For assessment of virus quantity, the p24 capture ELISA was used, developed at the Institute of Applied Microbiology in Vienna (19); it was performed as described earlier (18).
Opsonization of HIV, virus capture assay
Opsonized virus was prepared in the presence of anti-HIV IgG (purified from pooled sera of HIV+ patients) and normal human serum as complement source (HIVIgG + C) or in the presence of anti-HIV IgG alone (HIVIgG). Control samples were prepared in medium only (HIV). For opsonization, 1 µg/ml p24 HIV was incubated with 50 µg/ml anti-HIV IgG (HIVIgG) or with anti-HIV IgG and 10-fold diluted normal human serum (HIVIgG + C) for 1 h at 37°C. In some experiments, EDTA (20 mM) was used to inhibit complement activation (HIVIgG + C + EDTA). The opsonized virus was washed and pelleted by ultracentrifuge; aliquots were stored at 80°C and thawed once before the experiments. Opsonization was detected by a virus capture ELISA system. Preparations containing opsonized HIV were incubated in wells coated by Abs specific for huIgG, or for complement fragments C3c and C3d, respectively. Captured HIV was lysed in detergent and transferred to anti-p24-coated plates. After incubation with anti-HIV IgG, the virus bound intensively to the anti-IgG coat. As a result of classical pathway activation of the complement system, strong C3 and low (if any) IgG deposition were detected in the HIVIgG + C samples, suggesting poor accessibility of the IgG on HIVIg + C.
Binding of HIV to DC
To demonstrate binding of HIV, 5 x 105 imMDCs were incubated with 10 ng/ml p24 HIV, HIVIgG, HIVIgG + C, and HIVIgG + C + EDTA for 1 h at 37°C. Then nonbound virus was removed by washing two times with warm medium. The cells were lysed with 1% Nonidet P-40 and the amount of p24 Ag was determined by p24 ELISA. To determine the effect of proteinase K (Sigma-Aldrich) on HIV entry, the cells were held in the presence of 10 µg/ml enzyme before determination of p24. All data are presented as mean values ± SD.
Infection of MDCs with HIV-1
To determine the infectivity of HIV, 5 x 105 imMDCs were incubated with 10 ng/ml p24 HIV, HIVIgG, HIVIgG + C, and HIVIgG + C + EDTA, respectively, for 8 h, then the cells were washed two times with warm medium and placed back into culture in 24-well plates for an additional 15 days. Twenty-five percent of medium was replaced with rhIL-4 and rhGM-CSF every third day. The amount of virus produced by the cells was estimated by p24 ELISA from the supernatants every other day.
Flow cytometry
The expression of surface molecules of cells was determined at day 7 of culture. Briefly, 5 x 104 cells were washed with 1% BSA-0,1% NaN3-PBS and incubated with Abs reacting with CD80, CD86, CD83, CD11b, CD11c, CD18, HLA-DR, CD14, CD4, and CCR5, followed by FITC-labeled anti-mouse IgG. As control, samples were labeled with isotype-matched control Abs. After staining, the cells were washed and fixed in PBS containing 1% Formalin and 0,1% NaN3 and analyzed by flow cytometry (FACScan; BD Biosciences, Mountain View, CA).
| Results |
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It has been demonstrated in earlier studies that DCs do not possess CR1 and CR2, while they express CR3 and CR4 (18, 20, 21). Since opsonization is known to influence viral infection (4, 5, 22) we aimed to clarify whether CR3, mediating the uptake of C-coated particles by several cell types, is involved in a similar process of MDCs. For these experiments we used the Ab TMG6-5, which interacts with the receptors ligand binding site. Fig. 1 shows that TMG6-5 binds to imMDCs and mature MDCs (maMDCs) in a similar fashion. It is also seen in Fig. 1 that the percentage of CR3+ cells is decreased in maMDCs.
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To investigate the binding of C-opsonized HIV, imMDCs were incubated with HIVIgG + C and for control with HIV, HIVIgG, and HIVIgG + C + EDTA. After removing nonbound virus, cells were lysed and the amount of p24 Ag was determined by ELISA. Fig. 2 shows that HIVIgG + C binds more efficiently to imMDCs than HIVIgG. This enhancement was abolished when complement activation and opsonization were inhibited by 20 mM EDTA.
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Infection of human monocytes and macrophages with HIV is greatly facilitated by opsonization of the virus with C3 fragments (5, 22). To test whether infection of imMDCs by HIV is influenced by opsonization, the cells were cultured with the HIV-1 primary isolate pretreated with IgG and complement. Culture supernatants were collected every other day, centrifuged, and the concentration of free virions was estimated by p24 ELISA. As demonstrated in Fig. 3, opsonization with HIV-specific IgG and human complement caused >10-fold enhancement in HIV production by MDCs, as compared with nonopsonized HIV or HIV opsonized with specific IgG only. Virus particles treated with serum in the presence of 20 mM EDTA caused low productive infection similar to the medium-treated control.
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To investigate the role of CR3 in the uptake of the opsonized virus, the cells were preincubated with mAb TMG6-5, which interacts with the ligand binding site of CD11b. As shown in Fig. 4A, the Ab reduced binding of HIVIgG + C to background level. Also, as seen in Fig. 4B, the productive infection of MDCs tested on day 14 was almost completely abolished by the CR3-specific Ab, while the isotype-matched control Ab showed no effect. To demonstrate that uptake of opsonized HIV was the key to productive infection, cells were treated with proteinase K before p24 measurement. As shown in Fig. 4A, this enzyme treatment resulted in a significant loss of HIV associated with MDCs.
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| Discussion |
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Opsonization of HIV by complement occurs in human serum by direct binding of C1q already in the absence of anti-HIV Abs (1, 2), but is much more pronounced after binding of Abs on the surface of HIV. Complement is also known to be activated by HIV upon direct binding of MBL; the activation of the alternative pathway in HIV-infected patients has also been observed (24). Other studies, however, show that intact HIV activates complement only in the presence of antiviral Abs (25, 26). Sensitivity to complement-mediated lysis is influenced by the actual composition of the viral membrane, as in vitro studies demonstrated that some primary isolates of HIV are highly resistant to complement-mediated lysis due to the incorporation of host cell-derived complement control proteins, including CD46, CD55, and CD59 (27, 28).
Since HIV particles in infected patients are early on coated with complement at the site of infection, imDCs could be the first target of infection. As reported by Shi et al. (29), clustering of CR3 on monocytic cells recruits a Toll/IL-1R family-like signaling cascade, interacting with IL-1R-associated kinase 1 to induce NF-
B activity. HIV opsonized with C3 fragments represents a multimerized ligand. In our studies its binding most probably leads to clustering of CR3 and thus supports NF-
B signaling. This may then direct regulation of genes encoding for cytokines, chemokines, adhesion molecules, and enzymes, thereby modulating several cellular functions.
Although the precise mechanism through which complement coating of HIV leads to a 30% increase in HIV binding to CR3 on imMDCs but to a 10-fold increase in virus production remains unclear. The following hypothesis may be offered: Binding of complement-coated HIV to CR3 may induce specific signaling cascades and direct HIV to compartments in DCs which are favorable for productive infection. Both aspects are subjects of future research.
| Acknowledgments |
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| Footnotes |
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1 Address correspondence and reprint requests to Prof. Manfred P. Dierich, Department of Hygiene, Microbiology and Social Medicine, Innsbruck Medical University and Ludwig-Boltzman-Institute for AIDS Research, Fritz-Pregl-Strasse 3, A-6020 Innsbruck, Austria. E-mail address: hygiene{at}uibk.ac.at ![]()
2 Abbreviations used in this paper: CR, complement receptor; DC, dendritic cell; MDC, monocyte-derived DC; imMDC, immature MDC; hu, human; rh, recombinant human; maMDC, mature MDC. ![]()
Received for publication June 8, 2004. Accepted for publication August 18, 2004.
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