Differential Stimulation of Monocytes to Secrete Secretory Leukocyte Protease Inhibitor by Lipopolysaccharide of Periodontal Pathogens

Lipopolysaccharide (LPS), or endotoxin, is the major component of the outer surface of gram-negative bacteria. LPS is a potent activator of cells of the immune and inflammatory systems, including macrophages, monocytes, and endothelial cells, and contributes to systemic changes known as septic shock. Each LPS molecule has three regions: oligosaccharide, core, and lipid A. Oligosaccharide is exposed on the cell surface, and lipid A anchors LPS in the outer membrane. Recent studies from our lab demonstrated that LPS of periodontal pathogens suppresses neutrophil\u27s ability to secrete super oxide ions and also shown it to interfere with neutrophil chemotaxis. These factors are believed to aid the pathogens to remain stealthy and avoid killing by the mononuclear phagocyte cells. Secretory Leukocyte Protease Inhibitor (SLPI) is a single-chain serine protease inhibitor secreted from secretary and inflammatory cells that protects tissue from damage caused by the inflammatory response. The present study examined the interaction of LPS prepared from periodontal pathogens with monocytes to stimulate them to secrete SLPI. The hypothesis of this study is that (1) LPS molecules of more pathogenic periodontal pathogens are better inducer of monocots to secrete SLPI, and (2) Secreted SLPI in turn inhibits the production of inflammatory cytokines produced by monocytes in response to LPS. The limited stimulation of monotypic cells to secrete super oxide ions as shown previously along with the proposed hypothesis that pathogenic organisms produces more SLPI to suppress the inflammatory cytokines may aid the pathogens to remain undetected by mononuclear phagocytes. The LPS from three different periodontal pathogens, Porphyromonas gingivalis, Prevotella denticola, and Fusobacterium nucleatum were isolated for this study under identical conditions. Cultured human monocytic cells were incubated with LPS and the amounts of SLPI secreted were measured. Monocytic cells were also incubated with endogenous SLPI along with LPS of the selected periodontal pathogens and the secretion of the cytokines IL-1β and TNF-α was measured by ELISA. The results demonstrated that LPS of periodontal pathogens differ in their ability to stimulate inflammatory cytokines. Pathogenic strains appear to induce secretion of greater amounts of cytokines than the less pathogenic organisms. Pathogenic organisms tested also showed to induce the cultured THP-1 cells to secrete SLPI. In the presence of exogenous SLPI, the amounts of inflammatory cytokines secreted by THP cells when stimulated with LPS appeared to be diminished. The long-term objective is to understand the mechanisms, by which the periodontal pathogens escape detection by human defensive cells, colonize the gingival tissue and cause severe periodontitis. Understanding the evasive mechanisms by which pathogenic microorganisms escape detection or stimulate the host cells to secrete proteins, such as SLPI, which suppresses production of inflammatory cytokines, may aid the pathogenic organisms to survive in the oral cavity and continue their pathogenic effects