Wolbachia endosymbiont of Onchocerca volvulus: driver of immunopathology and target for therapy

Onchocerciasis affects an estimated 37 million people in Sub-Saharan Africa and Latin America, causing debilitating skin and eye disease and accounting for a global loss of 1 million Disability-Adjusted Life-Years. The discovery of Wolbachia bacterial endosymbionts in filarial nematodes, including Onchocerca volvulus, has revolutionised the understanding of the parasite’s biology and immunopathogenic mechanisms of disease, and has offered a novel approach to its treatment and control with anti-Wolbachia antibiotics. Treatment with doxycycline is effective at sterilising and killing adult O. volvulus worms, proving superior to standard microfilaricidal treatment with ivermectin and of great potential as an alternative strategy for the control of onchocerciasis. Although the length of the required treatment has raised concerns about the use of doxycycline in Mass Drug Administration (MDA) strategies, a recent trial in Cameroon demonstrated the feasibility of a six week course of doxycycline MDA delivered with a community-directed approach. In the work presented here we found a significant reduction in microfilaridermia prevalence and loads four years after doxycycline MDA distribution, demonstrating its long-term effectiveness and supporting its implementation in existing control strategies. Wolbachia peptidoglycan-associated lipoprotein stimulates innate and adaptive immune responses, contributing to disease pathogenesis through the induction of pro-inflammatory cytokines and recruitment of neutrophils. These features together with the cytokine milieu induced by filarial nematodes could support the development of a pro-inflammatory Type-17 immune response. In this work, PBMC from patients with onchocerciasis were found to produce only minimal levels of IL-17 in response to filarial extracts. On the contrary, a rich IL-17+ cell infiltrate was found surrounding adult worms in Wolbachia positive onchocercomas using immunohistochemistry (IHC), which was depleted from onchocercomas following doxycycline treatment. Although a high percentage of Th17 cells were present in this infiltrate compared to other diseases, the majority of IL-17 producing cells in nodules were neutrophils, within an extracellular trap-like structure. This unexpected result was consistent with the reported IL-17 production by human neutrophils using IHC in the literature, but could not be confirmed at the protein or the transcription level in vitro in this work. Wolbachia is responsible for an abundant neutrophil infiltration in Onchocerca-infected tissues. However, the role of neutrophils in the immune response to filarial parasites and their interaction with Wolbachia is poorly known. The work presented here showed that Wolbachia lipoprotein activates human neutrophils in vitro, supporting previous studies demonstrating the link between Wolbachia and neutrophils in the host inflammatory responses to O. volvulus infection. These results also indentify Wolbachia lipoprotein as a key molecule driving human neutrophil recruitment and activation