Wolbachia-Induced Neutrophil Activation in a Mouse Model of Ocular Onchocerciasis (River Blindness)

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Abstract
EndosymbioticWolbachiabacteria are abundant in the filarial nematodes that cause onchocerciasis (river blindness), including the larvae (microfilariae) that migrate into the cornea. Using a mouse model of ocular onchocerciasis, we recently demonstrated that it is these endosymbiotic bacteria rather than the nematodes per se that induce neutrophil infiltration to the corneal stroma and loss of corneal clarity (Saint Andre et al., Science295:1892-1895, 2002). To better understand the role ofWolbachiaorganisms in the pathogenesis of this disease, we examined the fate of these bacteria in the cornea by immunoelectron microscopy. Microfilariae harboringWolbachiaorganisms were injected into mouse corneas, and bacteria were detected with antibody toWolbachiasurface protein. Within 18 h of injection, neutrophils completely surrounded the nematodes and were in close proximity toWolbachiaorganisms.Wolbachiasurface protein labeling was also prominent in neutrophil phagosomes, indicating neutrophil ingestion ofWolbachiaorganisms. Furthermore, the presence of numerous electron-dense granules around the phagosomes indicated that neutrophils were activated. To determine ifWolbachiaorganisms directly activate neutrophils, peritoneal neutrophils were incubated with either parasite extracts containingWolbachiaorganisms, parasite extracts depleted ofWolbachiaorganisms (by antibiotic treatment of worms), orWolbachiaorganisms isolated from filarial nematodes. After 18 h of incubation, we found that isolatedWolbachiaorganisms stimulated production of tumor necrosis factor alpha and CXC chemokines macrophage inflammatory protein 2 and KC by neutrophils in a dose-dependent manner. Similarly, these cytokines were induced by filarial extracts containingWolbachiaorganisms but not byWolbachia-depleted extracts. Taken together, these findings indicate that neutrophil activation is an important mechanism by whichWolbachiaorganisms contribute to the pathogenesis of ocular onchocerciasis.