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Journal of Virology, October 2004, p. 10833-10836, Vol. 78, No. 19
0022-538X/04/$08.00+0 DOI: 10.1128/JVI.78.19.10833-10836.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.
Department of Immunology/Microbiology, Rush University Medical Center, Chicago, Illinois
Received 4 February 2004/ Accepted 18 May 2004
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) (22), the present study was performed to determine the effect of neutrophil activation on the capacity of neutrophils to bind HIV-1 and transfer infection to CD4-positive lymphocytes.
Neutrophils were isolated from the venous blood of healthy adults by density gradient centrifugation as described previously (21). The study protocol was approved by the Institutional Review Board of Rush University Medical Center. The neutrophils (3 x 106) were incubated in triplicate with 1 µM formylated peptide (fMLP), 10 ng of TNF-
/ml, 100 ng of Escherichia coli lipopolysaccharide (LPS) serotype O55:B5/ml, or in HEPES-buffered Hanks' balanced salt solution, pH 7.4, containing 30 µg of human serum albumin (Calbiochem, La Jolla, Calif.)/ml alone for 15 min at 37°C or, as a control for nonspecific effects, at 4°C. The cells were collected by centrifugation and were incubated in RPMI 1640 with HIV-1BaL [CCR5-tropic] containing 2,000 pg of p24 for 2 h at 4°C. The HIV-1 was produced in PHA-stimulated PBMC as described previously (16). The total incubation volume was 0.1 ml. Incubations were stopped by centrifugation, and the cells were washed extensively to remove unbound virus and were then lysed in 125 µl of 0.5% Triton X-100 containing a cocktail of protease inhibitors to inhibit degradation of the p24 (16). Cell counting demonstrated that similar numbers of neutrophils (approximately 1.3 x106) were recovered from resting and stimulated samples. HIV-1 was quantified by measurement of p24 antigen content in 100 µl of the cell lysate by using an HIV-1 p24 antigen capture enzyme-linked immunosorbent assay (AIDS Vaccine Program, National Cancer Institute-Frederick Cancer Research and Development Center, Frederick, Md.). The lower limit of the sensitivity of the assay as performed in our laboratory is 19 pg/ml. Results are presented as the net total amount of bound HIV-1 after the subtraction of nonspecific values for neutrophil lysate alone and for free virus alone, which were routinely <5 and 1 pg, respectively. The level of neutrophil binding of HIV-1BaL was significantly increased by activation with 1 µM fMLP or 10 ng of TNF-
/ml (two- or threefold, respectively) (Fig. 1A and B). In contrast, incubating the neutrophils with 100 ng of LPS/ml or with any of the stimuli at 4°C did not increase the level of HIV-1BaL binding. Parallel experiments confirmed that the brief incubation with fMLP or TNF-
, but not LPS, activated the neutrophils, as measured by the increased expression of CD11b (Fig. 2). Additional results demonstrated that incubating neutrophils with 3 to 30 ng of TNF-
/ml stimulated a concentration-dependent increase in HIV-1 binding, with 30 ng of TNF-
/ml causing an approximately sevenfold increase in binding (Fig. 1C). In results not shown, all three concentrations of TNF-
, however, stimulated the same increase in CD11b expression as that shown for 10 ng of TNF-
/ml in Fig. 2. The latter finding indicates that increased expression of CD11b is not directly responsible for the increased binding of HIV-1, although an increase in CD11b affinity cannot be excluded.
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FIG. 1. Neutrophil activation increases HIV-1 binding. (A and B) Neutrophils were incubated with or without 1 µM fMLP, 10 ng of TNF- /ml, or 100 ng of LPS/ml for 15 min at 37°C (filled bars) or at 4°C (open bars) and then were incubated with 2,000 pg of HIV-1BaL for 2 h at 4°C. Cells were lysed, and binding was quantified by measurement of viral p24 antigen content in the lysate. Results obtained in two separate experiments (A and B), each using neutrophils of a different individual, are shown as the means ± standard errors (SE) for triplicate samples. Similar results were obtained for two additional experiments. (C) Neutrophils were incubated with or without the indicated concentrations of TNF- for 15 min at 37°C, and binding of HIV-1BaL was determined as described for panels A and B. Results are presented as the means ± SE for triplicate samples in a single experiment. Similar results were obtained in two additional experiments. *, P < 0.05 compared with the value for resting neutrophils (None) by Student's paired t test.
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FIG. 2. TNF- and fMLP increase neutrophil expression of CD11b. Neutrophils were incubated without (thick line) or with (filled peak) 1 µM fMLP, 10 ng of TNF- /ml, or 100 ng of LPS/ml for 15 min at 37°C. The cells were washed and incubated with phycoerthrin (PE)-conjugated mouse anti-CD11b (BD Biosciences, San Diego, Calif.) (thick line; filled peak) or the phycoerthrin-conjugated isotype (immunoglobulin G2a) control (thin line) as described previously (4). Results of a single experiment are shown. Similar results were obtained in three additional experiments, each using neutrophils from a different donor.
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/ml (results not shown). Thus, HIV-1 binds to activated neutrophils independently of CD4. Other surface molecules, including C-type lectin receptors on dendritic cells (7, 23) as well as glycosaminoglycans (14) and galactosyl ceramides (1, 6), are also targets for gp120-mediated binding of HIV-1 to cells. The results presented in Fig. 3A indicate that C-type lectins do not mediate HIV-1 binding by the activated neutrophils, as chelation of calcium and other divalent cations by EDTA did not impair the binding. More directly, the results shown in Fig. 3B demonstrate that HIV-1 binds to the neutrophils independently of gp120. Activation of the neutrophils by 10 ng of TNF-
/ml increased the binding of gp120/41-negative HIV-1NL4-3 produced in HEK293 cells (American Type Culture Collection, Manassas, Va.) as described previously (20). The results presented in Fig. 3C indicate that a host-derived molecule(s) common at least to lymphocytes and HEK293 cells mediates the binding of HIV-1 to neutrophils, as TNF-
stimulation produced similar increases in the binding of HIV-1NL4-3 propagated either in lymphocytes or in HEK293 cells (20). Lastly, incubating resting and TNF-
-stimulated neutrophils with HIV-1 at 37°C yielded the same levels of HIV-1 binding as observed after incubating neutrophils with the virus at 4°C (Fig. 3D). This finding suggests that the binding of HIV-1 by the neutrophils is relatively specific, as incubation at 37°C would facilitate nonspecific binding. This finding also argues against the internalization of HIV-1, at least during the 2-h incubation period.
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FIG. 3. HIV-1 binding to activated neutrophils occurs independently of calcium, gp120/41, and incubation temperature. Neutrophils were incubated without (None) or with 10 ng of TNF- /ml at 37°C for 15 min. The neutrophils were then incubated for 2 h with 2,000 pg of HIV-1BaL in the presence or absence of 10 mM EDTA at 4°C (A), 2,000 pg of wild-type HIV-1NL4-3 (ENV+) or gp120/41-negative HIV-1NL4-3 particles (ENV) at 4°C (B), 2,000 pg of HIV-1NL4-3 produced in PBMC or HEK293 cells at 4°C (C), or 2,000 pg of HIV-1BaL for 2 h at 4°C or at 37°C (D). The results shown were obtained in separate experiments and are presented as the means ± SE for triplicate samples in a single experiment. Similar results were obtained in two additional experiments for each protocol. *, P < 0.05 compared with the value to resting neutrophils (None) by Student's paired t test.
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/ml for 15 min at 37°C and were then incubated with 2,000 pg of HIV-1BaL for 2 h at 4°C in the standard manner. After washing, the neutrophil-HIV-1BaL conjugates were cultured with heterologous PBMC that had been pre-activated by PHA as described previously (16). As a control, the PHA-stimulated PBMC were also cultured with 300 pg of free HIV-1BaL, a concentration previously established to cause maximum infection after 7 days of culture (16), alone and in combination with a freeze-thaw lysate of 2 x 106 neutrophils. HIV-1 bound to the activated neutrophils continued to cause a significantly greater level of virus production after 5 days of culture, but no further increase was noted after 7 days of culture, when virus production reached a plateau. As a consequence, HIV-1 bound to either resting or activated neutrophils produced similar levels of p24 after 7 days of culture. HIV-1BaL bound to TNF-
-stimulated neutrophils also accelerated the infection of autologous PHA-stimulated PBMC (n = 2; results not shown). Although several neutrophil products (9, 10, 12), including some preformed mediators (15, 19), stimulate HIV-1 replication in infected cells, the addition of a freeze-thaw lysate of 2 x 106 neutrophils to the PBMC cultures did not increase the level of replication caused by the free virus. Instead, it is likely that the increased infection levels observed with HIV-1 bound to activated neutrophils result from the enhanced transfer of the HIV-1 by the neutrophils to the lymphocytes. The possibility that the enhanced transfer simply reflects the increased amount of HIV-1 bound to the activated neutrophils (approximately 117 pg for the experiment in Fig. 4) cannot be excluded. Similarly, because HIV-1 production in activated PBMC may not be increased linearly, the possibility that HIV-1 bound to activated neutrophils has an even greater effect on infection of PBMC per se cannot be excluded.
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FIG. 4. Neutrophil activation accelerates infection of activated lymphocytes by neutrophil-bound HIV-1. Neutrophils (Neut) were incubated in the absence or presence of 10 ng TNF- /ml at 37°C and were then incubated with 2,000 pg of HIV-1BaL as described in the legend for Fig. 1. The cells were then cultured with PHA-activated PBMC for 7 days. The PHA-activated PBMC were also cultured with 300 pg of free HIV-1BaL alone and in the presence of a freeze-thaw lysate of neutrophils (2 x 106 cells). Virus replication was measured as p24 antigen content in the culture supernatants at days 3 (A), 5 (B), and 7 (C). Results are presented as the means ± SE for triplicate samples in a single experiment. Similar results were obtained in three additional experiments. *, P < 0.05 compared with the value for the resting neutrophil-HIV-1 conjugates by Student's paired t test.
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We thank Maria Miclea and Deborah Oconer for technical assistance.
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-dependent pathway. J. Immunol. 171:3734-3741.
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