Interleukin-15 augments superoxide production and microbicidal activity of human monocytes against Candida albicans

Abstract

Interleukin-15 (IL-15) is a newly described cytokine that shares biological activities with IL-2. We report here results demonstrating the ability of IL-15 to enhance superoxide production and antifungal activity of human monocytes. After 18 and 48 h of treatment with IL-15, human elutriated monocytes manifested enhanced superoxide production in response to either phorbol myristate acetate or opsonized Candida albicans blastoconidia. Similar results were obtained when monocytes were treated with IL-2, but to a lesser extent. Combination studies with IL-15 and IL-2 showed no additive or synergistic effects. Following incubation of monocytes with IL-15 for 18 h, there was no significant increase in mRNA transcripts for components of the NADPH oxidase complex, p40-phox, p47-phox, and gp91-phox, suggesting a posttranscriptional modulation of enhanced superoxide production. Antibodies against the gamma chain of the IL-2 receptor and, to a lesser extent, against the beta chain partially abrogated the IL-15-mediated enhanced superoxide production. Additionally, human monocytes showed enhanced killing activity against C. albicans after 18 h of incubation with IL-15 or IL-2, but this treatment did not enhance the ability of these cells to phagocytose the organism. In addition, the enhanced fungicidal activity seen after 18 h of treatment was no longer detectable after 48 h of cytokine treatment. Culture supernatants from the IL-15-treated monocytes were assayed for the presence of other proinflammatory cytokines. IL-15 treatment did not induce the release of detectable levels of tumor necrosis factor alpha, IL-1beta, or IL-12. Our results indicate that IL-15 upregulates the microbicidal activity of human monocytes against C. albicans.

FIG. 1

Enhanced O₂⁻ production by IL-15- and IL-2-treated monocytes. Human elutriated monocytes were treated for 18 h (A and B) or 48 h (C and D) with increasing concentrations of IL-15 (▨) or IL-2 (). Untreated control cells were simultaneously incubated in medium alone (▪). Cells treated for 48 h also were incubated with a combination of IL-2 and IL-15 at 50 ng of each per ml (▩). Superoxide anion release was measured in response to PMA (A and C) or opsonized C. albicans blastoconidia (B and D), as described in Materials and Methods. Data were collected from four to six experiments, performed in duplicate with different donors, and are expressed as means ± standard errors of the means. There was a significant enhancement in O₂⁻ release from cells treated with ≥50 ng of IL-15 per ml (∗∗, P < 0.01; ∗∗∗, P < 0.001) or ≥100 ng of IL-2 per ml (∗, P ≤ 0.05; ∗∗, P < 0.01) in response to both PMA and C. albicans blastoconidia.

FIG. 2

Effect of combinations of escalating concentrations of IL-15 and IL-2 on monocyte O₂⁻ release in response to PMA. Human elutriated monocytes were treated for 18 h with dose-escalating combinations of IL-15 and IL-2 or with increasing concentrations of each cytokine alone. Untreated control cells were simultaneously incubated in medium only. Superoxide anion release in response to PMA was measured as described in Materials and Methods. Data represent the means ± standard errors of the means from at least three separate experiments run in duplicate. There was no significant enhancement in O₂⁻ release with any combination of IL-15 and IL-2 compared with IL-15 alone.

FIG. 3

Fungicidal activity of monocytes against C. albicans following treatment with either IL-15 or IL-2 or a combination of IL-15 and IL-2. Monocytes were incubated with 1 to 100 ng of IL-15 or IL-2 per ml or with a combination of 50 ng of both cytokines per ml for 18 h prior to testing fungicidal activity against C. albicans. Untreated control cells were simultaneously incubated in medium alone. The figure depicts the 30-min time point at an effector-to-target cell (E:T) ratio of 10:1. Data were collected from three experiments performed in duplicate, using different donors, and are expressed as means ± standard errors of the means. There was a significant enhancement in fungicidal activity with cells treated with ≥50 ng of IL-15 per ml, 100 ng of IL-2 per ml, and the combination of these cytokines.