Interleukin-16 stimulates the expression and production of pro-inflammatory cytokines by human monocytes

Abstract

Interleukin-16 (IL-16) acts as a chemoattractant for CD4+ cells, as a modulator of T-cell activation, and plays a key role in asthma. This report describes the cytokine-inducing effects of IL-16 on total peripheral blood mononuclear cells (PBMC) and PBMC subpopulations. While CD4+ T lymphocytes did not secrete cytokines in response to rhIL-16, CD14+ CD4+ monocytes and maturing macrophages secrete IL-1beta, IL-6, IL-15 and tumour necrosis factor-alpha (TNF-alpha) upon rhIL-16 stimulation. The mRNA species for these four cytokines were detected as early as 4 hr post-stimulation, with protein being secreted by 24 hr. Secretion of IL-1beta and IL-6 by total PBMC was dose dependent, with maximal secretion being observed using 50 ng/ml rhIL-16. However, for IL-15 or TNF-alpha maximal secretion by total PBMC occurred with all concentrations between 5 ng/ml to 500 ng/ml rhIL-16. Purified monocytes/macrophages secreted maximal concentrations of all four cytokines in the presence of 500 ng/ml rhIL-16, except for monocytes where maximal secretion of IL-15 was, interestingly, observed with only 50 ng/ml rhIL-16. The use of higher concentrations of rhIL-16 (1000 ng/ml) inhibited secretion of all four cytokines. While these IL-16-induced cytokines are likely to be involved in the immune system's response to antigen, the data suggest that IL-16 may play a key role in initiating and/or sustaining an inflammatory response.

Figure 1

Cytokine specific RT–PCR for IL-1β, IL-2, IL-6, IL-15, proIL-16, TNF-α and GAPDH after rhIL-16 stimulation (0, 5 or 50 ng/ml rhIL-16) for 4 or 8 hr.

Figure 2

Concentrations of (a) IL-1β (b) IL-6 (c) IL-15 and (d) TNF-α secreted by total PBMC stimulated with varying concentrations of rhIL-16 or LPS (5 µg/ml) for 24 hr. The data represent the mean± SD and are representative of 20 donors. An asterisk (*) denotes statistically significant increases in cytokine concentration (P < 0·05).

Figure 3

Concentrations of (a) IL-1β (b) IL-6 (c) IL-15 and (d) TNF-α secreted by CD14⁺ monocytes stimulated with varying concentrations of rhIL-16 for 24 hr. The data represent the mean± SD and are representative of 20 donors. An asterisk (*) denotes statistically significant increases in cytokine concentration (P < 0·05).

Figure 5

Concentration of IL-6 secreted by CD14⁺ monocytes stimulated with 0 or 500 ng/ml rhIL-16 for 24 hr, 4, 6, 8 and 10 days post incubation with GM-CSF. The data represent the mean ± SD and are representative of 20 donors. An asterisk (*) denotes statistically significant increases in cytokine concentration (P < 0·05).

Figure 4

Expression of CD4 on monocytes/maturing macrophages on days 0, 4, 6, 8 and 10 after culture with GM-CSF. The dashed line represents the negative control.

Figure 6

Concentrations of (a) IL-1β (b) IL-6 (c) IL-15 and (d) TNF-α secreted by CD14⁺ macrophages stimulated with varying concentrations of rhIL-16 for 24 hr. The data represent the mean± SD and are representative of 20 donors. An asterisk (*) denotes statistically significant increases in cytokine concentration (P < 0·05).