Development of a serum-free system for the in vitro cultivation of Brugia malayi infective-stage larvae

Development of a novel rodent model for dog heartworm microfilaremia using the severe-combined immunodeficiency mouse

Dirofilaria immitis is a mosquito-borne parasitic nematode that causes fatal heartworm disease in canids. The microfilariae are essential for research, including drug screening and mosquito-parasite interactions. However, no reliable methods for maintaining microfilaria long-term are currently available. Therefore, we used severe combined immunodeficiency (SCID) mice to develop a reliable method for maintaining D. immitis microfilaria. SCID mice were injected intravenously with microfilariae isolated from a D. immitis-infected dog. Microfilariae were detected in blood collected from the tail vein 218 days post-inoculation (dpi) and via cardiac puncture 296 dpi. Microfilariae maintained in and extracted from SCID mice showed infectivity and matured into third-stage larvae (L3s) in the vector mosquito Aedes aegypti. L3s can develop into the fourth stage larvae in vitro. Microfilariae from SCID mice respond normally to ivermectin in vitro. The microfilariae in SCID mice displayed periodicity in the peripheral circulation. The SCID mouse model aided in the separation of microfilariae from cryopreserved specimens. The use of SCID mice enabled the isolation and sustained cultivation of microfilariae from clinical samples. These findings highlight the usefulness of the SCID mouse model for studying D. immitis microfilaremia in canine heartworm research.

An automated, high-resolution phenotypic assay for adult Brugia malayi and microfilaria

Brugia malayi are thread-like parasitic worms and one of the etiological agents of Lymphatic filariasis (LF). Existing anthelmintic drugs to treat LF are effective in reducing the larval microfilaria (mf) counts in human bloodstream but are less effective on adult parasites. To test potential drug candidates, we report a multi-parameter phenotypic assay based on tracking the motility of adult B. malayi and mf in vitro. For adult B. malayi, motility is characterized by the centroid velocity, path curvature, angular velocity, eccentricity, extent, and Euler Number. These parameters are evaluated in experiments with three anthelmintic drugs. For B. malayi mf, motility is extracted from the evolving body skeleton to yield positional data and bending angles at 74 key point. We achieved high-fidelity tracking of complex worm postures (self-occlusions, omega turns, body bending, and reversals) while providing a visual representation of pose estimates and behavioral attributes in both space and time scales.

Establishment of an in vitro culture system to study the developmental biology of Onchocerca volvulus with implications for anti-Onchocerca drug discovery and screening

Background

Infections with Onchocerca volvulus nematodes remain a threat in Sub-Saharan Africa after three decades of ivermectin mass drug administration. Despite this effort, there is still an urgent need for understanding the parasite biology especially the mating behaviour and nodule formation as well as the development of more potent drugs that can clear the developmental (L3, L4, L5) and adult stages of the parasite and inhibit parasite reproduction and behaviour.

Methodology/Principal findings

Prior to culture, freshly harvested O. volvulus L3 larvae from dissected Simulium damnosum flies were purified by centrifugation using a 30% Percoll solution to eliminate fly tissue debris and contaminants. Parasites were cultured in both cell-free and cell-based co-culture systems and monitored daily by microscopic visual inspection. Exhausted culture medium was replenished every 2–3 days. The cell-free culture system (DMEM supplemented with 10% NCS) supported the viability and motility of O. volvulus larvae for up to 84 days, while the co-culture system (DMEM supplemented with 10% FBS and seeded on LLC-MK₂ feeder cells) extended worm survival for up to 315 days. Co-culture systems alone promoted two consecutive parasite moults (L3 to L4 and L4 to L5) with highest moulting rates (69.2±30%) observed in DMEM supplemented with 10% FBS and seeded on LLC-MK₂ feeder cells, while no moult was observed in DMEM supplemented with 10% NCS and seeded on LEC feeder cells. In DMEM supplemented with 10% FBS and seeded on LLC-MK₂ feeder cells, O. volvulus adult male worms attached to the vulva region of adult female worms and may have mated in vitro. Apparent early initiation of nodulogenesis was observed in both DMEM supplemented with 10% FBS and seeded on LLC-MK₂ and DMEM supplemented with 10% NCS and seeded on LLC-MK₂ systems.

Conclusions/Significance

The present study describes an in vitro system in which O. volvulus L3 larvae can be maintained in culture leading to the development of adult stages. Thus, this in vitro system may provide a platform to investigate mating behaviour and early stage of nodulogenesis of O. volvulus adult worms that can be used as additional targets for macrofilaricidal drug screening.

River blindness affects people living in mostly remote and underserved rural communities in some of the poorest areas of the world. Although significant efforts have been achieved towards the reduction of disease morbidity, onchocerciasis still affects millions of people in Sub-Saharan Africa. The current control strategy is the annual mass administration of ivermectin which has accumulated several drawbacks over time, especially the action of the drug is solely microfilaricidal, very long treatment period (15–17 years) and reports of ivermectin losing its efficacy; thus, raising the urgent need for new adulticidal compounds. Our study has established an in vitro platform capable of supporting the growth and development of Onchocerca volvulus for up to 315 days, enabling the observation of parasite developmental processes: moulting (from the infective L3 stage to adults), increase in morphometry, the attachment of adult male and female worms and the potential initiation of nodulogenesis. Moreover, the platform might provide more insight into O. volvulus adult worms behavioural pattern in vitro. Also, our findings provide more avenues for mass production of different parasite stages, the investigation of parasite developmental biology and the identification of targets for drug discovery against different developmental stages of this filarial parasite within 315 days.

Evaluation of in vitro culture systems for the maintenance of microfilariae and infective larvae of Loa loa

Background

Suitable and scalable in vitro culture conditions for parasite maintenance are needed to foster drug research for loiasis, one of the neglected tropical diseases which has attracted only limited attention over recent years, despite having important public health impacts. The present work aims to develop adequate in vitro culture systems for drug screening against both microfilariae (mf) and infective third-stage larvae (L3) of Loa loa.

Methods

In vitro culture conditions were evaluated by varying three basic culture media: Roswell Park Memorial Institute (RPMI-1640), Dulbecco’s modified Eagle’s medium (DMEM) and Iscove’s modified Dulbecco’s medium (IMDM); four sera/proteins: newborn calf serum (NCS), foetal bovine serum (FBS), bovine serum albumin (BSA) and the lipid-enriched BSA (AlbuMax® II, ALB); and co-culture with the Monkey Kidney Epithelial Cell line (LLC-MK2) as a feeder layer. The various culture systems were tested on both mf and L3, using survival (% motile), motility (T₉₀ = mean duration (days) at which at least 90% of parasites were fully active) and moulting rates of L3 as the major criteria. The general linear model regression analysis was performed to assess the contribution of each variable on the viability of Loa loa L3 and microfilarie. All statistical tests were performed at 95% confidence interval.

Results

Of the three different media tested, DMEM and IMDM were the most suitable sustaining the maintenance of both L. loa L3 and mf. IMDM alone could sustain L3 for more than 5 days (T₉₀ = 6.5 ± 1.1 day). Serum supplements and LLC-MK2 co-cultures significantly improved the survival of parasites in DMEM and IMDM. In co-cultures with LLC-MK2 cells, L. loa mf were maintained in each of the three basic media (T₉₀ of 16.4–19.5 days) without any serum supplement. The most effective culture systems promoting significant moulting rate of L3 into L4 (at least 25%) with substantial maintenance time were: DMEM + BSA, DMEM + NCS, DMEM-AlbuMax®II, DMEM + FBS all in co-culture with LLC-MK2, and IMDM + BSA (1.5%), DMEM + FBS (10%) and DMEM + NCS (5%) without feeder cells. DMEM + 1% BSA in co-culture scored the highest moulting rate of 57 of 81 (70.37%). The factors that promoted L. loa mf viability included feeder cells (β = 0.490), both IMDM (β = 0.256) and DMEM (β = 0.198) media and the protein supplements NCS (β = 0.052) and FBS (β = 0.022); while for L. loa L3, in addition to feeder cells (β = 0.259) and both IMDM (β = 0.401) and DMEM (β = 0.385) media, the protein supplements BSA (β = 0.029) were found important in maintaining the worm motility.

Conclusions

The findings from this work display a range of culture requirements for the maintenance of Loa loa stages, which are suitable for developing an effective platform for drug screening.

Electronic supplementary material

The online version of this article (10.1186/s13071-018-2852-2) contains supplementary material, which is available to authorized users.

Screening of microfilaricidal effects of plant extracts against Dirofilaria immitis

Canine dirofilariasis is a common tropical parasitic disease of companion animals, caused by infestation of Dirofilaria immitis filarids within the pulmonary arteries and extending into the right heart. Increased reports of adverse reactions elicited by current microfilaricidal agents against D. immitis such as neurological disorders, circulatory collapse and potential resistance against these agents, warrant the search for new agents in forms of plant extracts. The use of plant extracts in therapeutic medicine is commonly met with scepticism by the veterinary community, thus the lack of focus on its medical potential. This study evaluated the presence of microfilaricidal activities of the aqueous extracts of Zingiber officinale, Andrographis paniculata and Tinospora crispa Miers on D. immitisin vitro at different concentrations; 10mg/ml, 1mg/ml, 100 microg/ml, 10 microg/ml and 1 microg/ml within 24h, by evaluation of relative microfilarial motility as a measure of microfilaricidal activity. All extracts showed microfilaricidal activity with Z. officinale exhibiting the strongest activity overall, followed by A. paniculata and T. crispa Miers. It is speculated that the microfilaricidal mechanism exhibited by these extracts is via spastic paralysis based upon direct observation of the microfilarial motility.

The Influence of Host Sex on the L3 to L4 Molt of Brugia malayi

Immunocompetent male mice are more susceptible to experimental infection with Brugia spp. than are females. Because permissive male SCID mice (severe combined immunodeficient mice), which lack T and B cells, also possess higher worm burdens, the mechanism is not solely immune mediated. Recovery of fewer adult worms from the female SCID mouse suggests that females do not provide sufficient nutrients for larval growth. This study assessed the potential of the female SCID mouse to support the L3 to L4 molt of Brugia malayi. Unexpectedly, worms grown in females molted at earlier time points of recovery than those harvested from males. This suggests that the early stage of development of B. malayi is delayed in the male murine host. To determine whether the effect of host sex on molting may be similar in humans, worms were cultured in media supplemented with serum from male or female donors. Worms grown in serum obtained from female donors exhibited a significantly higher percentage of complete molts over those cultured with serum from males. Host-derived molecules required for the L3 to L4 molt may be more abundant in the female, perhaps allowing the worms to survive a vulnerable developmental stage in a less permissive environment.

Ascorbic acid is a requirement for the morphogenesis of the human filarial parasite Brugia malayi

The nematode parasites Wuchereria bancrofti, Brugia malayi, and B. timori cause a disease in humans known as lymphatic filariasis, which afflicts approximately 120 million people worldwide. The parasites enter the human host from the mosquito either as L3 or as infective larvae and subsequently differentiate through 2 molts. In this article, we show that B. malayi depends on an exogenous source of vitamin C to complete the L3 to L4 molt, a critical morphogenic step in its life cycle. Brugia malayi apparently belongs to a small group of living organisms that depend on an exogenous source of vitamin C. This group includes only primates (including man) and guinea pigs among mammals.

Production of eicosanoids and other oxylipins by pathogenic eukaryotic microbes

Oxylipins are oxygenated metabolites of fatty acids. Eicosanoids are a subset of oxylipins and include the prostaglandins and leukotrienes, which are potent regulators of host immune responses. Host cells are one source of eicosanoids and oxylipins during infection; however, another potential source of eicosanoids is the pathogen itself. A broad range of pathogenic fungi, protozoa, and helminths produce eicosanoids and other oxylipins by novel synthesis pathways. Why do these organisms produce oxylipins? Accumulating data suggest that phase change and differentiation in these organisms are controlled by oxylipins, including prostaglandins and lipoxygenase products. The precise role of pathogen-derived eicosanoids in pathogenesis remains to be determined, but the potential link between pathogen eicosanoids and the development of TH2 responses in the host is intriguing. Mammalian prostaglandins and leukotrienes have been studied extensively, and these molecules can modulate Th1 versus Th2 immune responses, chemokine production, phagocytosis, lymphocyte proliferation, and leukocyte chemotaxis. Thus, eicosanoids and oxylipins (host or microbe) may be mediators of a direct host-pathogen "cross-talk" that promotes chronic infection and hypersensitivity disease, common features of infection by eukaryotic pathogens.

Update on Lymphatic Filarial Infections

Filarial infections remain significant causes of disability in tropical areas worldwide. However, insights into the developmental and molecular biology of the parasite and the immunobiology of the host response to infection have advanced our understanding, even as progress is being made towards implementing eradication programs. This article summarizes some of the recent advances in the understanding of filarial biology and parasite immune evasion mechanisms, and reviews those newer aspects of diagnosis and treatment most relevant to clinicians.

Lymphatic filariasis: new insights and prospects for control

Although lymphatic filariasis remains among the major causes of disability among the tropical infectious diseases, dramatic advances have been made in the approach to its diagnosis, epidemiology and treatment, in our understanding of the molecular composition of the parasites that cause these infections, and in the factors underlying the pathology seen. Superimposing the tools of modern epidemiology, immunology, and molecular biology on field-based clinical trials has allowed the emergence of the concept of elimination of lymphatic filariasis. Much of the important new research emphasizes parasite development in the context of the host response, the importance of both the adult worm and other factors in the pathogenesis of lymphatic filarial disease, the role the Wolbachia endosymbiont holds as both a target for drug treatment and in inducing post-treatment reactions, and the various principles underlying the implementation of control programs.

Inhibitors of the lipoxygenase pathway block development of Brugia malayi L3 in vitro

Brugia malayi L3 molt to the L4 stage in serum-free cultures supplemented with arachidonic, linoleic, or linolenic acids and the basidiomycetous yeast Rhodotorula minuta. These fatty acids are capable of entering the eicosanoid pathway of arachidonate metabolism, the pathway responsible for generating a number of biologically active mediators, including prostaglandins, leukotrienes, and lipoxins. To determine whether this pathway was required for L3 development, we added dual inhibitors of cyclooxygenase and lipoxygenase to in vitro cultures containing B. malayi L3. These compounds significantly inhibited L3 molting. To evaluate whether 1 or both of these pathways of arachidonate metabolism were involved in molting, we tested drugs inhibiting either cyclooxygenase or lipoxygenase. Lipoxygenase inhibitors blocked L3 molting, whereas cyclooxygenase inhibitors did not. To assess whether enzymes operating downstream of lipoxygenase were also involved in L3 molting, we added inhibitors of enzymes involved in leukotriene synthesis and found they were also capable of preventing development. We tested the same inhibitor panel on Dirofilaria immitis L3. A single lipoxygenase inhibitor and inhibitors of 2 different enzymes operating downstream of lipoxygenase disrupted D. immitis development. These results demonstrate that a lipoxygenase pathway product is required for molting of the infective stage larvae of filarial parasites.

Tetracycline inhibits development of the infective-stage larvae of filarial nematodes in vitro

In recent years, studies have linked tetracycline treatment of filaria-infected animals with reduced adult worm burdens and decreased levels of microfilaremia. These observations are believed to be attributable to clearance of Wolbachia, intracellular rickettsial-like organisms found within filarial tissues. Although maximal worm reductions were observed when treatment was initiated early in infection, it is not known whether tetracycline inhibits development of infective-stage larvae. To address this issue, we studied the effect of tetracycline on three different species of filarial nematodes, Brugia malayi, Brugia pahangi, and Dirofilaria immitis, in a serumfree in vitro system supporting molting to the fourth larval stage. Tetracycline was capable of inhibiting L3 to L4 molting within a dosage range similar to that reported for susceptible rickettsial organisms. However, Wolbachia DNA could still be detected in nematodes from tetracycline-treated cultures. In addition, three other antibiotics with anti-rickettsial and anti-chlamydial activity (chloramphenicol, erythromycin, and ciprofloxacin) failed to inhibit L3 to L4 molting. Although tetracycline is capable of completely blocking molting of infective-stage larvae, it remains possible that this effect is due to pharmacological activities unrelated to its anti-rickettsial functions.