Internalization and survival mechanisms of human ehrlichiosis agents ehrlichia chaffeensis and anaplasma phagocytophilum in host cells
Obligatory intracellular, human ehrlichiosis agents Ehrlichia chaffeensis and Anaplasma phagocytophilum have remarkable abilities to parasitize first-line immune defensive cells, monocytes/macrophages and granulocytes, respectively. As newly discovered pathogens, the bacterial or host components, the signaling pathways required for their internalization and proliferation, and how these bacteria inhibit the microbicidal activities in host cells remain vastly unknown. In this study, we found that the entry of E. chaffeensis into THP-1 cells rapidly induces the following essential signaling events: protein cross-linking by transglutaminase, tyrosine phosphorylation, phospholipase C (PLC)-?2 activation, IP 3 production, and an increase in cytosolic free calcium levels. The entry and proliferation of these pathogens involve caveolae-mediated endocytosis and glycosylphosphatidylinositol-anchored proteins. Furthermore, caveolar marker protein caveolin-1, tyrosine-phosphorylated proteins, and PLC-?2 are colocalized in both early and replicative nclusions. Because of the extremely fragile and pleomorphic nature of E. chaffeensis and A. phagocytophilum , we tested if cholesterol is also required for them as for mycoplasmas with similar physical characteristics. The genome of these bacteria lacks genes for biosynthesis of lipid A and peptidoglycan. Although they lack genes for cholesterol biosynthesis or modification, host cell-free E. chaffeensis and A. phagocytophilum can directly incorporate significant amount of exogenous cholesterol into their membranes, which provides them physical strength and is essential for infecting host leukocytes. We finally examine the mechanism whereby E. chaffeensis downregulates host microbicidal activities. Using lipopolysaccharides (LPS) as a model stimulant of monocytes, we demonstrate that E. chaffeensis infection disables human monocytes to kill E. chaffeensis in response to E. coli LPS. This is correlated with the downregulated expression of several pattern recognition receptors such as CD14, toll-like receptor (TLR) 2/4, and NF-?B activity from 1 d post infection (pi). This change is preceded by reduced phosphorylation of p38 MAPK/ERK in response to LPS from 3 h pi and decreased PU.1 activity, suggesting the role of p38 MAPK and PU.1 in downregulating expressions of TLR2/4 and CD14. Taken together, these results enhance our understanding of the entry and survival mechanisms of human ehrlichiosis agents, and may direct us to new approaches for prevention or treatment of these diseases.
School:The Ohio State University
School Location:USA - Ohio
Source Type:Master's Thesis
Keywords:human ehrlichiosis ehrlichia chaffeensis anaplasma phagocytophilum entry and proliferation transglutaminase protein tyrosine kinase phospholipase c calcium caveolae gpi anchored proteins cholesterol lipopolysaccharide toll like receptors mapk
Date of Publication:01/01/2003