Actinobacillus actinomycetemcomitans produces a
leukotoxin that selectively kills human leukocytes. Recently, we reported that macrophages are highly sensitive to
leukotoxin and that their lysis involves activation of
caspase 1. In this study, we show that
leukotoxin also induces the production and release of proinflammatory
cytokines from human macrophages. The macrophages were challenged with
leukotoxin or
lipopolysaccharide (LPS) from A. actinomycetemcomitans or LPS from Escherichia coli, and the production and secretion of
interleukin-1beta (IL-1beta),
IL-6, and
tumor necrosis factor alpha (
TNF-alpha) were determined at the
mRNA and
protein levels by reverse transcription-PCR and
enzyme-linked
immunosorbent assay, respectively.
Leukotoxin (1 to 30 ng/ml) induced abundant production and secretion of IL-1beta, while the effects on
IL-6 and
TNF-alpha production were limited.
Leukotoxin (1 ng/ml) caused a 10-times-higher release of IL-1beta than did LPS (100 ng/ml). The secreted IL-1beta was mainly the bioactive 17-kDa
protein. At higher concentrations (>30 ng/ml),
leukotoxin caused secretion of mainly inactive
cytokine, the 31-kDa pro-IL-1beta. The presence of specific
antibodies to IL-1beta or of a
caspase 1 inhibitor blocked the secretion and production of the
cytokine. Supernatants of
leukotoxin-challenged macrophages stimulated
bone resorption when tested in a mouse calvarial model. The activity could be blocked by an
IL-1 receptor antagonist or specific
antibodies to IL-1beta. We concluded that A. actinomycetemcomitans
leukotoxin can trigger abundant production and secretion of bioactive IL-1beta by human macrophages, which is mediated by activation of
caspase 1.