Summary
Human granulocytic ehrlichiosis (HGE) is a recently recognized rickettsial tick-borne
febrile illness that may occasionally be complicated by coagulopathy. The agent of
HGE (aHGE) is an obligate intracellular pathogen, which replicates in endosomes within
neutrophils and their precursors. We hypothesized that aHGE might cause DIC via induction
of monocyte tissue factor procoagulant activity (TF PCA). Peripheral blood mononuclear
cells (PBMNC) and HL-60 cells were used to model the effect of aHGE infection on monocytes/macrophages.
Mononuclear cells inoculated with aHGE in vitro demonstrated approximately a 12-15-fold increase in TF PCA, with peak activity occurring
at 8-12 h. HL-60 cells inoculated with aHGE also manifested a 4-6 fold induction of
TF PCA, with maximal activity occurring at about 8 h. By comparison, E. Coli lipopolysaccharide (LPS) also induced an increase in TF PCA of an equivalent magnitude,
and with a similar time course. Induction of TF did not require inoculation of HL-60
cells with live organism, since heat-inactivated aHGE still stimulated TF PCA expression
in the target cells. Furthermore, filtered supernatants from heat-inactivated organisms
induced TF PCA suggesting that the effect is due to a soluble mediator produced by
the organism. Although aHGE is a gram negative organism, the soluble mediator did
not appear to be classic endotoxin in that the supernatants tested negative for endotoxin
by the Limulus Amoebocyte assay, and polymixin had no inhibitory effect on aHGE supernatants.
We conclude that aHGE induces cells of the myelo-monocytic lineage to synthesize TF,
which may contribute to the clinical coagulopathy that can be observed in this condition.
An atypical soluble mediator or cellular component of the organism appears to be critically
important in TF induction by aHGE.
Key words
Human granulocytic ehrlichiosis - monocyte - tissue Factor - HL-60 - lipopolysaccharide
