Interleukin-8 fails to induce human immunodeficiency virus-1 expression in chronically infected promonocytic U1 cells but differentially modulates induction by proinflammatory cytokines

Abstract

This study addresses the role of interleukin (IL)-8, a CXC-chemokine, the level of which is reported to be raised in the peripheral circulation of human immunodeficiency virus-1 (HIV-1)-infected individuals, during the induction of HIV-1 expression from latency and during cytokine-mediated HIV-1 up-regulation. IL-8 at the higher concentrations tested (> or = 100 ng/ml) was unable to induce HIV-1 expression in the chronically infected promonocytic U1 cell line, as measured by p24 antigen enzyme-linked immunosorbent assay (ELISA), whereas at lower concentrations of 1 and 10 ng/ml, constitutive HIV-1 expression was only marginally reduced. HIV-1 replication in acutely infected U937 cells was also significantly reduced by IL-8. The potent up-regulation of HIV-1 expression in U1 cells by tumour necrosis factor-alpha (TNF-alpha) remained unaffected by the addition of IL-8. HIV-1 induction by IL-1beta, IL-6 and TNF-beta, cytokines grouped here as intermediate HIV-1 inducers, was suppressed by IL-8 at concentrations of 1 and 10 ng/ml. However, IL-8 at 100 ng/ml did not significantly alter the effect of IL-1beta, synergized with IL-6 in enhancing, and marginally suppressed TNF-beta-induced HIV-1 expression. IL-8 suppressed granulocyte-macrophage colony-stimulating factor (GM-CSF) and enhanced interferon-gamma (IFN-gamma)-induced HIV-1 expression in a dose-dependent manner. Pretreatment of U1 cells with IL-8 did not alter the IL-8-mediated effects on cytokine-induced HIV-1 expression, suggesting that this chemokine exerts its effect at the time of HIV-1 induction or at a postinduction stage. Furthermore, IL-8 was itself induced by cytokines that up-regulate HIV-1 expression in U1 cells and the levels produced correlated directly with the levels of p24 antigen produced, suggesting common pathways for cytokine induction of both HIV-1 and IL-8. These results show that IL-8, typically a non-inducer, can differentially modulate HIV-1 expression in U1 cells and that this is dependent on the inducing cytokine and on the concentration of IL-8.

Figures

Figure 1

Comparison of the stimulation indices (SI values) for human immunodeficiency virus-1 (HIV-1) induction by different concentrations of interleukin (IL)-8 in U1 cells and their corresponding biological activity determined by IL-8 bioassay. IL-8 bioactivity was monitored by measuring the amount of β-glucuronidase released from cytochalasin-treated polymophonuclear neutrophils (PMNL) in a degranulation assay. Results are expressed as the mean absorbance (A) at 405 nm ± SEM from experiments using purified PMNL from four normal donors. Mean SI values for induction of HIV-1 by IL-8 in U1 cells were determined from six independent experiments.

Figure 2

U937 cell cultures were infected with either human immunodeficiency virus-1 (HIV-1)IIIB or HIV-1ADA, as described in the Materials and methods, and were untreated (UN) or treated with the following cytokines: 2·5 ng/ml of tumour necrosis factor-α (TNF-α), 500 U/ml of interleukin (IL)-1β, 100 U/ml of IL-6, 25 ng/ml of TNF-β, 100 U/ml of granulocyte–macrophage colony-stimulating factor (GM-CSF), 100 U/ml of interferon-γ (IFN-γ) and 100 ng/ml of IL-8. Supernatants were collected after 4 days of incubation for determination of HIV-1 p24 levels by enzyme-linked immunosorbent assay (ELISA). Results are from one experiment that was representative of three independent experiments (SEM in all experiments were < 15% of the mean).

Figure 3

Interleukin (IL)-8 modulation of human immunodeficiency virus-1 (HIV-1) induction in U1 cells by cytokines classified as (a) strong or intermediate inducers or (b) weak inducers. U1 cells at 2 × 105/ml were incubated with 2·5 ng/ml of tumour necrosis factor-α (TNF-α), 250 U/ml of IL-1β, 250 U/ml of IL-6, 25 ng/ml of TNF-β, 100 U/ml of granulocyte–macrophage colony-stimulating factor (GM-CSF) or 100 U/ml of interferon-γ (IFN-γ) in the presence or absence of 1, 10 or 100 ng/ml of IL-8. Values represent the average percentage suppression of HIV-1 p24 production by IL-8 calculated as: 1 – (average pg/ml for cultures of IL-8 + inducing cytokine/average pg/ml for duplicate cultures of inducing cytokine cultures). Results shown are from one experiment that was representative of four independent experiments (SEM in all experiments were < 15% of the mean).