Inhibition of binding, entry, or intracellular proliferation of Ehrlichia risticii in P388D1 cells by anti-E. risticii serum, immunoglobulin G, or Fab fragment.

The effects of equine antiserum, immunoglobulin G (IgG) specific for Ehrlichia risticii, and its Fab fragment on E. risticii binding to, internalization into, and proliferation in P388D1 cells were studied by immunofluorescence flow cytometry. Anti-E. risticii equine serum or IgG inhibited E. risticii at a stage beyond binding and internalization. In contrast, monovalent anti-E. risticii equine Fab fragments inhibited E. risticii binding and internalization into P388D1 cells. In the presence of control equine serum, IgG, or its Fab fragment, E. risticii cells were bound, were internalized and subsequently grew within P388D1 cells, and eventually destroyed the host cells as effectively as was the case without equine serum, IgG, or Fab fragments. Anti-E. risticii IgG but not normal horse IgG inhibited L-[14C]glutamine metabolism in Percoll gradient-purified E. risticii. These findings suggest that the Fab fragment of intact anti-E. risticii IgG blocks the ligands on E. risticii responsible for non-IgG-mediated internalization and diverts them to bind via the Fc receptor. Following Fc-mediated entry of E. risticii, the antibody interfered with the metabolic activity of E. risticii cells, rendering them incapable of proliferation in P388D1 cells and resulting in the eventual destruction of the organisms.