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J Virol. 2003 February; 77(4): 2762–2767.
PMCID: PMC141110

Human Immunodeficiency Virus Type 1 Attachment, Coreceptor, and Fusion Inhibitors Are Active against both Direct and trans Infection of Primary Cells


Inhibitors of human immunodeficiency virus type 1 attachment (CD4-immunoglobulin G subclass 2), CCR5 usage (PRO 140), and fusion (T-20) were tested on diverse primary cell types that represent the major targets both for infection in vivo and for the inhibition of trans infection of target cells by virus bound to dendritic cells. Although minor cell-type-dependent differences in potency were observed, each inhibitor was active on each cell type and trans infection was similarly vulnerable to inhibition at each stage of the fusion cascade.

Human immunodeficiency virus type 1 (HIV-1) entry proceeds via the sequential steps of gp120-CD4 attachment, gp120-coreceptor interactions, and gp41-mediated fusion (9, 21). CD4-immunoglobulin G subclass 2 (CD4-IgG2) (PRO 542) is an HIV-1 attachment inhibitor that has shown promise in phase I/II testing (18, 26, 31). PRO 140 (PA14) is an anti-CCR5 monoclonal antibody (27, 37) that is entering phase I testing. The fusion inhibitor T-20 is a peptide derived from HIV-1 gp41; it targets a transient gp41 conformation formed after coreceptor binding (5). T-20 has demonstrated promising safety and antiviral effects in phase III testing (B. Clotet, A. Lazzarin, D. Cooper, J. Reynes, K. Arasteh, M. Nelson, C. Katlama, J. Chung, L. Fang, J. Delehanty, and M. Salgo, 14th Int. AIDS Conf., abstr. LbOr19A, 2002; K. Henry, J. Lalezari, M. O'Hearn, B. Trottier, J. Montaner, P. Piliero, S. Walmsley, J. Chung, L. Fang, J. Delehanty, and M. Salgo, 14th Int. AIDS Conf., abstr. LbOr19B, 2002).

Studies of entry inhibitors typically have utilized cultured cell lines or peripheral blood mononuclear cells (PBMC). However, HIV-1 replicates in additional cell types and tissues in vivo (4, 12, 33, 40) and these have important implications for therapy.

Entry inhibitors could also be used prophylactically. Dendritic cells (DC) at surface epithelia may disseminate virus to susceptible cells in draining lymphoid tissue (16, 35). Cord blood mononuclear cells (CBMC) provide an available source of cells relevant to vertical transmission.

Whereas immature DC replicate HIV-1 efficiently, mature DC are poorly infectible (14). However, mature DC can bind virus and mediate the infection of cocultured CD4+ T cells in trans. There are conflicting reports on the susceptibility of cell-bound virus to neutralizing antibodies (13, 15, 34), and other classes of entry inhibitors have not been similarly studied. Here, we explore the breadth of activity in vitro of the three main classes of HIV-1 entry inhibitors.

Direct infection of diverse primary cell types.

We first tested the inhibitors for activity on diverse primary cells. CD4-IgG2, murine PRO 140, and T-20 were prepared as described previously (1, 24, 37). RANTES (PeproTech, Rocky Hill, N.J.), a natural chemokine ligand for CCR5, was tested for comparison. The studies examined the subtype B CCR5-using (R5) primary isolates HIV-1JR-FL (25), HIV-1SF162 (7), and HIV-1Case C 1/85 (8).

PBMC and CBMC (36), macrophages (37), and mature and immature DC (11) were isolated and cultured according to published methods. The inhibition assay was performed essentially as described previously (38). Briefly, inhibitors were combined for 1 h at 37°C with virus (CD4-IgG2 and T-20) or target cells (PRO 140 and RANTES). Inhibitors, virus, and cells were then incubated for 5 to 7 days at 37°C, and the extent of viral replication was determined by p24 antigen enzyme-linked immunosorbent assay of the culture supernatants. The viral inocula in 50% tissue culture infective doses were 400 to 1,000 for PBMC and CBMC, 1,000 for macrophages, and 5,000 to 10,000 for DC.

Table Table11 summarizes the concentrations required for 90% (IC90) and 50% (IC50) inhibition for the various cell types. The median IC90s for CD4-IgG2, PRO 140, RANTES, and T-20 in PBMC cultures were 13, 31, 16, and 62 nM, respectively (Table (Table1).1). These values are consistent with those from prior studies of these agents (24, 37-39).

Interisolate variation in inhibitor activity for direct and trans infection of primary cells

CD4-IgG2, PRO 140, and T-20 each mediated 90% inhibition of viral replication in PBMC, CBMC, macrophages, and immature DC (Table (Table1).1). In contrast, RANTES was ineffective in macrophage cultures, as reported previously (2, 3, 10, 28-30, 32, 37). Although RANTES blocked virus replication by 50% at moderate concentrations (Table (Table1),1), higher concentrations often led to the enhancement of infection (data not shown). Inhibition studies were not conducted on mature DC, which were poorly susceptible to infection.

For comparison of cell type differences in inhibitor potency, we used log-transformed IC50s and IC90s, which more closely followed a Gaussian distribution than did the raw values, facilitating comparison of the means by two-tailed t tests. ICs observed for PBMC were compared with those for other cell types. Since the t test was performed four times for each drug, the threshold P value was adjusted from 0.05 to 0.013 in accordance with Bonferroni's correction.

For a given inhibitor, the mean ICs for the other cell types tended to cluster about the values observed for PBMC (Fig. (Fig.1).1). Thus CD4-IgG2, PRO 140, and T-20 are broadly active in blocking the entry of HIV-1 into PBMC, CBMC, macrophages, and immature DC. These findings are consistent with those of a similarly designed study employing RANTES, T-20, and the CCR5 antagonist SCH-C (19a).

FIG. 1.FIG. 1.FIG. 1.FIG. 1.
Cell-type-specific variations in inhibitor activity. IC50s and IC90s were observed for CD4-IgG2 (A), PRO 140 (B), T-20 (C), and RANTES (D) against HIV-1JR-FL (open circles), HIV-1SF162 (filled squares), and HIV-1Case C 1/85 (filled triangles) on the indicated ...

Although the entry inhibitors were broadly active, cell type differences in potency were observed. CD4-IgG2 was ninefold (P = 7.9 × 10−7) and sixfold (P = 0.0039) more potent when assayed on macrophages and CBMC, respectively, than on PBMC. Notably, macrophages and CBMC express comparatively low levels of CD4 (6, 23). Immature DC required higher ICs, with the mean IC90s of T-20 and PRO 140 being ninefold (P = 8.8 × 10−4) and sixfold (P = 5.7 × 10−4) higher than their respective PBMC values. These findings may reflect the higher concentration of virus inoculum required to infect immature DC.

Correlation between the potencies of entry inhibitors.

We next performed linear regression analysis of the log IC90s observed for a pair of inhibitors within a given experiment, keeping the virus, cell type, and cell donor constant. Data obtained with different cell types were pooled to increase the statistical power. An r2 of >0.25 was deemed indicative of a strong correlation, and in those cases, P values were calculated by using JMP software (SAS Institute, Inc., Cary, N.C.).

Significant correlations were observed in the cases of PRO 140 and RANTES (r2 = 0.26, P = 0.0005) and PRO 140 and T-20 (r2 = 0.34, P < 0.0001). Both correlations were positive; i.e., a virus culture that was sensitive to one inhibitor was sensitive to the other. The correlation observed for PRO 140 and RANTES is perhaps unsurprising, given both are CCR5 inhibitors. The correlation between PRO 140 and T-20 may reflect the fact that these agents act at sequential, interdependent stages of fusion.

CD4-IgG2's potency did not correlate with that of any other inhibitor and thus may be influenced by an independent set of viral and host determinants. Distinct resistance determinants are desirable for therapy.

Regression analysis was used to examine other potential correlates of activity. However, for a given cell type and inhibitor, no significant correlation could be established based on the virus isolate, cell donor, or degree of viral replication or “fitness.” These variables may have complex combinatorial effects on inhibition, and further studies are needed to dissect the relative contributions.

DC-mediated infection in trans.

We next examined the inhibition of trans infection. Briefly, CD14+ and CD14 cells were isolated as described previously (11, 36). The CD14 cells were cryopreserved, while the CD14+ cells were differentiated into mature DC (11). DC were incubated overnight at 37°C with 10,000 tissue culture infective doses/ml of virus, washed five times, resuspended in PBMC culture medium (36), seeded at 2 × 105 cells/well in 96-well plates, and incubated with CD4-IgG2 or T-20 for 1 h at 37°C. CD14 cells were thawed, stimulated as described previously (36), and incubated with PRO 140 or RANTES. CD14 cells (1.4 × 105 cells/well) were then added to DC, and HIV-1 replication was determined as described above.

Each inhibitor effectively blocked trans infection. In no case did the ICs for a given inhibitor vary by more than threefold between direct and trans infection of PBMC (Table (Table11 and Fig. Fig.1).1). Thus, during trans infection, DC-associated virus is sensitive to inhibition throughout the fusion cascade.

Thus DC-mediated trans infection is inhibited by CD4-IgG2 as well as HIV-1-neutralizing monoclonal antibodies (13, 17). In contrast, follicular DC-mediated trans infection reportedly is unaffected by neutralizing antibody (15). The divergent results could reflect differences both in how DC versus follicular DC process immune-complexed virus and in assay methodology.

Similarly, we observed that T-20 efficiently blocks DC-mediated trans infection. Our findings stand in apparent contrast to those reported for trans infection mediated by intestinal epithelial cells, where T-20 was inactive (22). However, the different outcomes may again reflect methodology differences. In the earlier study, T-20 was removed from the epithelial cells prior to the addition of CD4+ target cells (22), whereas T-20 was present in our DC-T-cell cocultures.

In the present study, DC were added in excess of T cells such that most of the T cells could be infected in trans during the first round of infection. To explore the potential importance of secondary rounds of infection, inhibitors were washed away prior to the combination of DC and T cells. CD4-IgG2 was fully active in the washout setting (data not shown), indicating that the agent blocks trans infection. These findings support the notion that the inhibitors block trans infection.

Our findings shed light on the mechanism of HIV-1 infection in trans. Notably, during this process, virus is sensitive to each of the known classes of entry inhibitors. This result is surprising for two reasons. Firstly, lentiviruses are internalized in prominent vacuoles in mature DC (11) and internalization may be critical for infection (20). Secondly, trans infection may occur at polarized interfaces between the cells (19) and internalized virus moves to this immunologic synapse within minutes of cell contact (D. McDonald, D. Unatmaz, V. KewalRamani, and T. Hope, Keystone Symp. HIV Pathog., abstr. 121, 2002). Nonetheless, our findings indicate that virus remains vulnerable to inhibition by membrane-impermeable drugs.

In summary, we report that the main classes of HIV-1 entry inhibitors block the direct and trans infection of multiple primary cell types. Although minor cell-type-specific differences in activity were observed, CD4-IgG2, PRO 140, and T-20 were effective in each setting. Our findings also offer new insights into the nature of the DC/T-cell synapse during trans infection.


M.P. is an Elizabeth Glaser Scientist of the Pediatric AIDS Foundation. J.P.M. is a Stavros S. Niarchos Scholar.

This work was funded by NIH grants AI46871, R01 AI43084, R01 AI40877, R01 AI41420, and P01 AI52048; The Rockefeller Foundation; the Elizabeth Glaser Pediatric AIDS Foundation; and Progenics Pharmaceuticals, Inc. The Department of Microbiology and Immunology at the Weill Medical College gratefully acknowledges the support of the William Randolph Hearst Foundation.


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