Abstract
Onchocerciasis also known as river blindness is a neglected tropical disease and the world's second-leading infectious cause of blindness in humans; it is caused by Onchocerca volvulus. Current treatment with ivermectin targets microfilariae and transmission and does not kill the adult parasites, which reside within subcutaneous nodules. To support the development of macrofilaricidal drugs that target the adult worm to further support the elimination of onchocerciasis, an in-depth understanding of O. volvulus biology especially the factors that support the longevity of these worms in the human host (>10 years) is required. However, research is hampered by a lack of access to adult worms. O. volvulus is an obligatory human parasite and no small animal models that can propagate this parasite were successfully developed. The current optimized 2-dimensional (2-D) in vitro culturing method starting with O. volvulus infective larvae does not yet support the development of mature adult worms. To overcome these limitations, we have developed and applied 3-dimensional (3-D) culture systems with O. volvulus larvae that simulate the human in vivo niche using in vitro engineered skin and adipose tissue. Our proof of concept studies have shown that an optimized indirect co-culture of in vitro skin tissue supported a significant increase in growth of the fourth-stage larvae to the pre-adult stage with a median length of 816-831 μm as compared to 767 μm of 2-D cultured larvae. Notably, when larvae were co-cultured directly with adipose tissue models, a significant improvement for larval motility and thus fitness was observed; 95% compared to 26% in the 2-D system. These promising co-culture concepts are a first step to further optimize the culturing conditions and improve the long-term development of adult worms in vitro. Ultimately, it could provide the filarial research community with a valuable source of O. volvulus worms at various developmental stages, which may accelerate innovative unsolved biomedical inquiries into the parasite's biology.
Highlights
The filarial nematode Onchocerca volvulus is an obligatory human parasite and the causative agent of onchocerciasis
We have developed co-culture systems based on engineered human skin and adipose tissue that represent the in vivo niche of O. volvulus adult worms that improved the culturing conditions and the development to the pre-adult stages of the parasite
We described a novel 2-D in vitro culturing system based on a feeder layer of human umbilical vein endothelial cells (HUVEC) that has successfully supported the development of O. volvulus fourth stage larvae (L4) to the early pre-adult stage once they have molted to the fifth-stage larvae, L5 [15]
Summary
The filarial nematode Onchocerca volvulus is an obligatory human parasite and the causative agent of onchocerciasis This neglected tropical disease known as river blindness predominantly occurs in sub-Saharan Africa [1,2]. The larvae migrate to subcutaneous tissues, where they further develop to adult worms within subcutaneous highly vascularized nodular tissues consisting of extracellular matrix (ECM) and various immune cells, such as macrophages, neutrophils and eosinophils [3,4,5]. Inside these nodules, the reproduction of male and female worms results in the release of large numbers of microfilariae that migrate into the surrounding skin. When microfilariae reach ocular tissues, e.g. the cornea and the conjunctiva, they induce an immunopathologic reaction, which can result in blindness after many years of chronic infection [6]
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