Published online before print August 12, 2009
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,
,1
* Immunology Graduate Program and Departments of
Cell Biology and Physiology and
Immunology, University of Pittsburgh School of Medicine, Pittsburgh, Pennsylvania, USA; and
Department of Pathology and Comprehensive Cancer Center, University of Michigan Medical School, Ann Arbor, Michigan, USA
1. Correspondence: University of Pittsburgh School of Medicine, E1052 BST, 200 Lothrop St., Pittsburgh, PA 15213, USA. E-mail: rds{at}pitt.edu
ABSTRACT
CDC are exotoxins secreted by many Gram-positive bacteria that bind cholesterol and oligomerize to form pores in eukaryotic cell membranes. We demonstrate that CDC TLO induces caspase-1 cleavage and the rapid release of IL-1β from LPS-primed murine BMDM. IL-1β secretion depends on functional toxin pore formation, as free cholesterol, which prevents TLO binding to cell membranes, blocks the cytokine release. Secretion of the mature forms of IL-1β and caspase-1 occurs only at lower TLO doses, whereas at a higher concentration, cells release the biologically inactive proforms. IL-1β release at a low TLO dose requires potassium efflux, calcium influx, and the activities of calcium-independent PLA2, caspase-1, and cathepsin B. Additionally, mature IL-1β release induced by a low TLO dose is dependent on the NLRP3 inflammasome, and pro-IL-1β release induced by a high TLO dose occurs independently of NLRP3. These results further elucidate a mechanism of CDC-induced IL-1β release and suggest a novel, immune evasion strategy in which IL-1β-containing macrophages might release primarily inactive cytokine following exposure to high doses of these toxins.
Key Words: bacterial • cytokines • inflammation
