The pathogenesis of
Acanthamoeba keratitis begins when Acanthamoeba trophozoites bind specifically to mannosylated
glycoproteins upregulated on the surfaces of traumatized corneal epithelial cells. When Acanthamoeba castellanii trophozoites are grown in
methyl-alpha-D-mannopyranoside, they are induced to secrete a novel 133-kDa
protein that is cytolytic to corneal epithelial cells. Clinical isolates of Acanthamoeba spp., and not the soil isolates, were proficient at producing a
mannose-induced
protein (MIP-133) and generating disease in Chinese hamsters. The purified
protein was efficient at killing corneal epithelial cells, the first mechanistic barrier, by inducing apoptosis in a
caspase 3-dependent pathway. Subsequent steps in pathogenesis require the amoebae to penetrate and degrade
collagen. Only the clinical isolates tested were efficient at migrating through a collagenous matrix in vitro, presumably by MIP-133 degradation of both human type I and human
type IV collagen. A chicken anti-MIP-133 antiserum effectively bound to the
protein and blocked collagenolytic activity, migration, and cytopathic effects (CPE) against corneal cells in vitro. Chinese hamsters orally immunized with MIP-133 displayed a >30% reduction in disease.
Immunoglobulin A isolated from immunized animals bound MIP-133 and blocked CPE on corneal cells in vitro. Animals induced to generate severe
chronic infections displayed significant reductions in disease symptoms upon oral immunization postinfection. These data suggest that MIP-133 production might be necessary to initiate
corneal disease and that it may play an important role in the subsequent steps of the pathogenic cascade of
Acanthamoeba keratitis. Furthermore, as
antibodies produced both prior to and after
infection reduced clinical symptoms of disease, the
protein may represent an important immunotherapeutic target for
Acanthamoeba keratitis.