Endothelial cells release soluble factors that support the long-term survival of filarial worms in vitro

Dirofilariasis mouse models for heartworm preclinical research

Introduction

Dirofilariasis, including heartworm disease, is a major emergent veterinary parasitic infection and a human zoonosis. Currently, experimental infections of cats and dogs are used in veterinary heartworm preclinical drug research.

Methods

As a refined alternative in vivo heartworm preventative drug screen, we assessed lymphopenic mouse strains with ablation of the interleukin-2/7 common gamma chain (γc) as susceptible to the larval development phase of Dirofilaria immitis.

Results

Non-obese diabetic (NOD) severe combined immunodeficiency (SCID)γc^-/- (NSG and NXG) and recombination-activating gene (RAG)2^-/-γc^-/- mouse strains yielded viable D. immitis larvae at 2-4 weeks post-infection, including the use of different batches of D. immitis infectious larvae, different D. immitis isolates, and at different laboratories. Mice did not display any clinical signs associated with infection for up to 4 weeks. Developing larvae were found in subcutaneous and muscle fascia tissues, which is the natural site of this stage of heartworm in dogs. Compared with in vitro-propagated larvae at day 14, in vivo-derived larvae had completed the L4 molt, were significantly larger, and contained expanded Wolbachia endobacteria titres. We established an ex vivo L4 paralytic screening system whereby assays with moxidectin or levamisole highlighted discrepancies in relative drug sensitivities in comparison with in vitro-reared L4 D. immitis. We demonstrated effective depletion of Wolbachia by 70%-90% in D. immitis L4 following 2- to 7-day oral in vivo exposures of NSG- or NXG-infected mice with doxycycline or the rapid-acting investigational drug, AWZ1066S. We validated NSG and NXG D. immitis mouse models as a filaricide screen by in vivo treatments with single injections of moxidectin, which mediated a 60%-88% reduction in L4 larvae at 14-28 days.

Discussion

Future adoption of these mouse models will benefit end-user laboratories conducting research and development of novel heartworm preventatives via increased access, rapid turnaround, and reduced costs and may simultaneously decrease the need for experimental cat or dog use.

A mouse infection model and long-term lymphatic endothelium co-culture system to evaluate drugs against adult Brugia malayi

The development of new drugs targeting adult-stage lymphatic filarial nematodes is hindered by the lack of a robust long-term in vitro culture model. Testing potential direct-acting and anti-Wolbachia therapeutic candidates against adult lymphatic filariae in vitro requires their propagation via chronic infection of gerbils. We evaluated Brugia malayi parasite burden data from male Mongolian gerbils compared with two immune-deficient mouse strains highly susceptible to B. malayi: CB.17 Severe-Combined Immmuno-Deficient (SCID) and interleukin-4 receptor alpha, interleukin-5 double knockout (IL-4Rα-/-IL-5-/-) mice. Adult worms generated in IL-4Rα-/-IL-5-/- mice were tested with different feeder cells (human embryonic kidney cells, human adult dermal lymphatic endothelial cells and human THP-1 monocyte differentiated macrophages) and comparative cell-free conditions to optimise and validate a long-term in vitro culture system. Cultured parasites were compared against those isolated from mice using motility scoring, metabolic viability assay (MTT), ex vivo microfilariae release assay and Wolbachia content by qPCR. A selected culture system was validated as a drug screen using reference anti-Wolbachia (doxycycline, ABBV-4083 / flubentylosin) or direct-acting compounds (flubendazole, suramin). BALB/c IL-4Rα-/-IL-5-/- or CB.17 SCID mice were superior to Mongolian gerbils in generating adult worms and supporting in vivo persistence for periods of up to 52 weeks. Adult females retrieved from BALB/c IL-4Rα-/-IL-5-/- mice could be cultured for up to 21 days in the presence of a lymphatic endothelial cell co-culture system with comparable motility, metabolic activity and Wolbachia titres to those maintained in vivo. Drug studies confirmed significant Wolbachia depletions or direct macrofilaricidal activities could be discerned when female B. malayi were cultured for 14 days. We therefore demonstrate a novel methodology to generate adult B. malayi in vivo and accurately evaluate drug efficacy ex vivo which may be adopted for drug screening with the dual benefit of reducing overall animal use and improving anti-filarial drug development.

The Immunological Role of Vascular and Lymphatic Endothelial Cells in Filarial Infections

Simple Summary

The endothelium is a monolayer of cells forming a thin membrane that lines the inside of blood vessels. These cells release molecules that regulate vascular relaxation, contraction, and can control blood clotting and the immune response. During infections with filarial nematodes, common parasites of humans and animals, the endothelium is believed to play a key role in the communication between the host and the parasite, since the embryonic stage of filaroids is distributed in the bloodstream. Therefore, this review aims to gather research from different scientists in order to better understand the host immune response in infections with filarial nematodes.

Abstract

The embryonic stage of filarial nematodes, or microfilariae (Mf), shows daily and seasonal periodicity that requires their migration through blood vessels into the lungs, where they are sequestered when not circulating in the peripheral blood. Therefore, Mf and the host endothelium are likely in a permanent state of hide and seek. Interestingly, filarial nematodes co-cultured in media with a murine endothelial cell line survive eight times longer than those cultured in media alone. This suggests that the endothelium is an important element of the immune response in filarial nematodes, perversely promoting their survival in the host. In this review, we will focus on potential pathways involved in the relationship between filarial nematodes and the host endothelium, including the role of endothelial ICAM/VCAM/PECAM adhesion molecules, surface markers involved in the passage of Mf through host tissue, anti-thrombolic effects caused by the presence of filarial nematodes (including plasmins), endothelial cell proliferation (VEGF), and other aspects of the immune activation of the endothelium. The aim of this review is to merge the knowledge about the cross-talk between Mf of different filarial nematode species and endothelial cells (EC), thus allowing a better understanding of the mechanism of these parasitic infections.