CD15 monoclonal antibodies and heparin sulfate abrogate Human Granulocytic Ehrlichiosis (HGE) agent infection of HL60 cells

The tick-borne infectious agent of HGE is observed mostly in granulocytes. Infection of HL60 cells differentiated into macrophages in vitro is abortive. The components involved in attachment to granulocytes are not known, but sialyl-Lewis^X (CD15) has been suggested as an important host ligand. The wide distribution of this molecule on non-granulocytic cells would contradict its role as an important adhesin prior to cell entry. We investigated whether addition of mucopolysaccharides and antibodies to Lewis^X sialomucin would abrogate ehrlichial propagation. HL60 cells were adjusted so that 1% and 5% were infected and incubated for 1 to 6 days with varying concentrations of heparin sulfate and anti-CD15 mab. Ehrlichial propagation was assessed microscopically by counting at least 100 cells in duplicate cultures. Chi-square tests for significance were performed. At 2 to 4 days, both heparin (>= 100 U/ml) and CD15 mab in high concentrations inhibited ehrlichial propagation (p<0.017 and p<0.0004, respectively) in a dose-dependent manner. Cells in heavily infected cultures retained the morphology of promyelocytes while those in cultures with high concentrations of either heparin or CD15 mab developed a morphology suggesting monocytoid differentiation. Both heparin and anti-CD 15 inhibit further ehrlichial growth potentially by abrogation of ehrlichial binding to host cell ligands; however, the mechanism of inhibition may also be related to induction of differentiation into host cells that are less permissive for continuous ehrlichial propagation.