The agent of human granulocytic ehrlichiosis (HGE) induces the production of myelosuppressing chemokines

HGE is characterized clinically by cytopenias, the pathogenesis of which is unknown. Because few cells are seen to be directly infected, we hypothesized that factors elaborated by HGE-infected cells might be important. Chemokines are powerful leukocyte chemoattractants capable, in minute concentrations, of suppressing hematopoiesis. We recently cultivated the HGE agent in HL-60 cells and further have shown that granulocytic differentiation of these cells by DMSO enhances infection (Klein, J Inv Med. 45:199A, 1997). Cytokines in HL-60 culture supernatants were measured in triplicate using modified sandwich ELISA assays on samples obtained sequentially after inoculation with the HGE agent. Production of both α and β Chemokines by infected cells was significantly elevated compared with uninfected controls in 3 independent experiments (table). In contrast, infected cells did not secrete TNF-α or IL-6. Chemokine levels peaked 48h after inoculation with HGE and correlated with the degree of susceptibility to infection (DMSO-treated > unstimulated HL-60 cells). Cells inoculated with heat-killed HGE did not secrete significant levels of chemokines. No endotoxin was detected using a Limulus assay. Mean Chemokine level (ng/10⁶ cells): 48 h after inoculation with HGE MIP-1α MIP-1 β MCP-1 RANTES IL-8 Infected HL-60 8.74 11.68* 8.50* 3.58* 2.70 Uninfected HL-60 0.08 1.51 0.08 0.70 0.31 infected DMSO-tx 16.55 61.54 50.29* 28.4 39.11* uninfected DMSO-tx 0.22 2.51 3.95 5.04 0.31 (*p <0.05 vs. uninfected controls). The striking production of chemokines by HGE-infected cells is likely to be of pathogenic importance not only in the observed cytopenias, but also in mediating inflammation and host defenses.