Optimizing 3-dimensional human skin models to facilitate long-term development of Onchocerca volvulus, a debilitating obligatory human parasite

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 therapies kill microfilariae but fail to kill the adult parasites, which reside within subcutaneous nodules. To support a more target-driven drug development that can also cure and thus support the elimination of this disease, 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 3-dimensional (3-D) culture systems that simulated the human in vivo niche using in vitro engineered skin and adipose tissue. We show that an optimized indirect co-culture of in vitro skin tissue with fourth-stage larvae supported a significant increase in growth until 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 improve the long-term development of adult worms in vitro, and thus 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. Author summary The filarial nematode Onchocerca volvulus is an obligatory human parasite and the causative agent of onchocerciasis, better known as river blindness. In 2017, more than 20 million infections with O. volvulus were estimated worldwide, 99 % of the patients live in Africa. Current international control programs focus on the reduction of microfilaridermia by mass drug administration of ivermectin. However, to meet the elimination goals, additional therapy strategies are needed that also target the adult worms. As this parasite is obliged to humans, there are no small animal models that sustain the full life cycle of the parasite, thus greatly impeding the research on this filarial nematode. To overcome these drawbacks, we developed co-culture systems based on engineered human skin and adipose tissue that represent the in vivo niche of O. volvulus in which we were able to establish improved conditions of culturing to the pre-adult stages of the parasite. Furthermore, our new culture approach could significantly reduce the use of animal models currently used for drug testing of surrogate larvae.