Acanthamoeba spp. secrete a mannose-induced protein that correlates with ability to cause acanthamoeba keratitis
Acanthamoeba spp. are ubiquitously distributed in the environment. The trophozoite
form can infect the cornea and cause sight-threatening corneal inflammation known as
Acanthamoeba keratitis. The pathogenic cascade of Acanthamoeba keratitis begins when
Acanthamoebae bind to mannose expressed on traumatized corneas. Published reports
indicate that mannose is upregulated on the corneal surface during wound healing.
Experiments in laboratory animals have shown that corneal abrasion prior to infection is
essential for generating Acanthamoeba keratitis. Furthermore, supernatants from
trophozoites grown in medium supplemented with mannose were found to be more toxic to
corneal cells than from those without mannose. After binding to mannose ligands, the
amoebae desquamate the corneal epithelial cells, perforate the Bowman’s membrane, and
invade the corneal stroma. Once inside the stroma, the amoebae secrete collagenolytic
factors that dissolve the stromal matrix. Collectively, these data suggest that binding to
mannose induces Acanthamoeba trophozoites to secrete cytotoxic factors necessary for
The data showed that mannose induced A. castellanii to secrete a novel 133 kDa
protein that is cytolytic to corneal epithelial cells. Cytolytic activity of the mannose-induced
protein (MIP-133) was abrogated with the addition of serine protease inhibitors, indicating
that MIP-133 is a serine protease. Examination of the cytolytic mechanism revealed that cell
death was due to the activation of the caspases-3 and -10-dependent apoptotic cascade.
Incubations of MIP-133 with artificial Bowman’s membrane and stromal matrix
demonstrated that MIP-133 degraded the collagen comprising the bulk of these layers.
Analysis of Acanthamoeba spp. revealed that production of MIP-133 correlated with
ability to cause keratitis. Soil isolates neither made MIP-133 nor produced significant
keratitis. All clinical isolates examined produced MIP-133 and keratitis in laboratory
Anti-MIP-133 antibodies neutralized the cytolytic and collagenolytic activity of MIP-
133 in vitro. Additionally, incubations of anti-MIP-133 with whole trophozoites inhibited
their migration through an extracellular matrix.
Oral immunization with MIP-133 resulted in reduced severity and duration of
Acanthamoeba keratitis. Mucosal antibodies isolated from orally immunized animals
inhibited cytolytic activity in vitro. Furthermore, oral immunization in animals with existing
chronic Acanthamoeba keratitis (mimicking human disease) displayed immediate reduction
in disease severity and mitigation of corneal disease.