Short report: A practical technique for the cryopreservation of Dirofilaria immitis, Brugia malayi, and Wuchereria bancrofti microfilariae

Cryogenic preservation of Dirofilaria immitis microfilariae, reactivation and completion of the life-cycle in the mosquito and vertebrate hosts

BACKGROUND

The cryopreservation of filarial nematodes has been studied for nearly 70 years. Largely, these studies examined the effectiveness of cryopreservation methods by using the post-thaw survival of microfilariae (mf) and the development to third-stage larvae (L3s) following inoculation into a competent insect vector. Only one study reported complete reestablishment of a filarial nematode (Brugia malayi) life-cycle in a competent vertebrate host from cryopreserved stock. Expanding on this previous research, a cryopreservation method was developed to cryopreserve the mf of the dog heartworm, Dirofilaria immitis.

METHODS

A combination of cryoprotectants, dimethyl sulfoxide (DMSO) and polyvinyl pyrolidone (PVP) at 6% and 4 mM, respectively, provided acceptable post-thaw survival of mf that developed into L3s in Aedes aegypti. L3s developed from cryopreserved and freshly collected mf in mosquitoes were inoculated into ferrets and dogs and were assessed after a sufficient duration post-inoculation for development into adult heartworms.

RESULTS

Fewer adult heartworms derived from cryopreserved stocks of mf were recovered from ferrets compared to adult heartworms derived from freshly collected mf, and the former were smaller by weight and length. The onset of patency (circulating mf) occurred at similar post-inoculation time points and at similar mf densities in dogs infected with L3s sourced from cryopreserved stocks or freshly collected mf. Adults derived from cryopreserved mf have survived and produced viable mf for more than 3 years in dogs. Approximately 60% of inoculated L3s were recovered as adults from dogs at 2 and 3.5 years post-inoculation.

CONCLUSIONS

The results from these direct comparisons demonstrate that cryopreserved mf can develop into L3s in vector mosquitoes and that these L3s are infective to both dogs and ferrets, where they undergo normal development into adult worms. These worms are able to mate and produce viable mf and complete the heartworm lifecycle in dog.

Identification and validation of a commercial cryopreservation medium for the practical preservation of Dirofilaria immitis microfilaria

BACKGROUND

Dirofilaria immitis is a parasitic nematode transmitted by mosquitoes and the cause of heartworm disease in dogs and dirofilariasis in humans and other mammals. The parasite is endemic worldwide. Vector stage research requires a reliable supply of D. immitis microfilariae (mf). It is believed that cryopreserved mf would retain viability and provide a powerful tool for vector stage research. However, reports on cryopreservation of D. immitis mf are limited. Therefore, this study aimed to validate commercial cryopreservation media to establish a practical, convenient and reproducible storage procedure for D. immitis mf.

METHODS

Six different commercially available cryopreservation media were compared with the traditional polyvinylpyrrolidone-dimethyl sulfoxide (PVP-DMSO) preservation solution. In vitro viability of purified D. immitis mf and mf-infected total blood was analyzed using a motility assay and propidium iodide staining. In vivo infectivity of Aedes aegypti mosquitoes with cryopreserved mf was assessed using a mosquito survival test and quantifying the number of third-stage larvae (L3) after 13 days post-infection.

RESULTS

Purified mf cryopreserved in CultureSure showed the best viability when compared to mf cryopreserved in the remaining five commercially available media and PVP-DMSO. Viability of mf in mf-infected total blood cryopreserved in CultureSure varied with the ratio of infected blood to CultureSure. Optimum results were obtained with 200 µl mf-infected blood:800 µl CultureSure. CultureSure was also the optimum medium for cryopreserving mf prior to infectivity of A. aegypti. The number of L3 was approximately the same for CultureSure cryopreserved mf (3× concentrated solution) and non-cryopreserved fresh mf.

CONCLUSIONS

CultureSure is an optimal commercial cryopreservation solution for the storage of D. immitis purified mf, mf-infected total blood, and mf used for in vivo mosquito experiments. Furthermore, this study describes an easy preservation method for clinical D. immitis-infected blood samples facilitating vector stage studies, as well as the study of macrocyclic lactone resistance in heartworms and the education of veterinarians.

In vivo imaging of transgenic Brugia malayi

Background

Studies of the human filarial parasite have been hampered by the fact that they are obligate parasites with long life cycles. In other pathogenic infections, in vivo imaging systems (IVIS) have proven extremely useful in studying pathogenesis, tissue tropism and in vivo drug efficacy. IVIS requires the use of transgenic parasites expressing a florescent reporter. Developing a method to produce transgenic filarial parasites expressing a florescent reporter would permit IVIS to be applied to the study of tissue tropism and provide a non-invasive way to screen for in vivo drug efficacy against these parasites.

Methodology/Principal findings

We report the development of a dual luciferase reporter construct in a piggyBac backbone that may be used to stably transfect Brugia malayi, a causative agent of human filariasis. Parasites transfected with this construct were visible in IVIS images obtained from infected gerbils. The signal in these infected animals increased dramatically when the transgenic parasites matured to the adult stage and began to produce transgenic progeny microfilaria. We demonstrate that the IVIS system can be used to develop an effective method for cryopreservation of transgenic parasites, to non-invasively monitor the effect of treatment with anti-filarial drugs, and to rapidly identify transgenic F1 microfilariae.

Conclusions

To our knowledge, this represents the first application of IVIS to the study of a human filarial parasite. This method should prove useful in studies of tissue tropism and as an efficient in vivo assay for candidate anti-filarial drugs.

The human filarial parasites are responsible for two major debilitating diseases; lymphatic filariasis (elephantiasis) and onchocerciasis (river blindness). Both have been identified as diseases that can be eliminated as a public health problem. However, the current elimination programs rely upon prolonged distribution of a limited number of drugs, a process which is logistically difficult to accomplish and may encourage the development of resistance. Thus, more effective drugs are needed. In other infectious diseases, in vivo imaging systems (IVIS) have proven to be very effective tools to study the pathogenesis of infection and to develop rapid and non-invasive assays for new drugs. Here we report the adaptation of IVIS to a human filarial parasitic infection and demonstrate in principal that it may be used to non-invasively monitor the efficacy of anti-filarial treatment in vivo. This system should prove useful as an in vivo screen for new anti-filarial compounds, as well as studies of the basic biology of these parasites.

Filarial worms reduce Plasmodium infectivity in mosquitoes

BACKGROUND

Co-occurrence of malaria and filarial worm parasites has been reported, but little is known about the interaction between filarial worm and malaria parasites with the same Anopheles vector. Herein, we present data evaluating the interaction between Wuchereria bancrofti and Anopheles punctulatus in Papua New Guinea (PNG). Our field studies in PNG demonstrated that An. punctulatus utilizes the melanization immune response as a natural mechanism of filarial worm resistance against invading W. bancrofti microfilariae. We then conducted laboratory studies utilizing the mosquitoes Armigeres subalbatus and Aedes aegypti and the parasites Brugia malayi, Brugia pahangi, Dirofilaria immitis, and Plasmodium gallinaceum to evaluate the hypothesis that immune activation and/or development by filarial worms negatively impact Plasmodium development in co-infected mosquitoes. Ar. subalbatus used in this study are natural vectors of P. gallinaceum and B. pahangi and they are naturally refractory to B. malayi (melanization-based refractoriness).

METHODOLOGY/PRINCIPAL FINDINGS

Mosquitoes were dissected and Plasmodium development was analyzed six days after blood feeding on either P. gallinaceum alone or after taking a bloodmeal containing both P. gallinaceum and B. malayi or a bloodmeal containing both P. gallinaceum and B. pahangi. There was a significant reduction in the prevalence and mean intensity of Plasmodium infections in two species of mosquito that had dual infections as compared to those mosquitoes that were infected with Plasmodium alone, and was independent of whether the mosquito had a melanization immune response to the filarial worm or not. However, there was no reduction in Plasmodium development when filarial worms were present in the bloodmeal (D. immitis) but midgut penetration was absent, suggesting that factors associated with penetration of the midgut by filarial worms likely are responsible for the observed reduction in malaria parasite infections.

CONCLUSIONS/SIGNIFICANCE

These results could have an impact on vector infection and transmission dynamics in areas where Anopheles transmit both parasites, i.e., the elimination of filarial worms in a co-endemic locale could enhance malaria transmission.

Reproductive costs of the immune response of the autogenous mosquito Culex pipiens molestus against inoculated Dirofilaria immitis

Culex pipiens molestus is an autogenous mosquito, a vector of Dirofilaria immitis in Madeira Island, and it mounts a melanotic encapsulation response when inoculated intrathoracically with D. immitis microfilariae (mf). Because Cx. p. molestus is autogenous, this mosquito is a good model from which to gain a better understand the relationship between oviposition and melanization, independent of the signaling pathways related to blood feeding. The present work assessed the impact follicle growth might have on melanization of intrathoracically inoculated mf. The ovaries from mosquitoes undergoing melanotic encapsulation developed more eggs than those that could not melanize the mf. Possible explanations are discussed in this article.

The role of phenylalanine hydroxylase in melanotic encapsulation of filarial worms in two species of mosquitoes

Melanin formation has a significant influence on mosquito vector competence by limiting the development of metazoan parasites. Tyrosine, the rate-limiting substrate of melanin production, can be obtained exogenously or derived from phenylalanine by phenylalanine hydroxylase (PAH). The characteristics of this defense mechanism, such as temporal expression of constituent enzymes involved in the biosynthetic pathway, can vary considerably between mosquito species. We investigated the functional role of PAH in the melanotic encapsulation response in Aedes aegypti and Armigeres subalbatus, two mosquito species with markedly different melanization responses. We used double-stranded RNA (dsRNA) to knock down PAH and observed the phenotypic effects on melanin formation. PAH transcripts were dramatically reduced in both mosquito species after gene knock down. The abundance of PAH proteins was decreased in gene knockdown mosquitoes that were inoculated with Dirofilaria immitis microfilariae (mf) as compared to inoculation controls. A significant reduction of mf melanization also was observed in these knockdown mosquitoes as compared to inoculation controls. Our data suggest that PAH is required for a fully functional melanotic encapsulation response in both mosquito vectors.

Reassessing the role of defensin in the innate immune response of the mosquito, Aedes aegypti

Defensin is the predominant inducible immune peptide in Aedes aegypti. In spite of its activity against Gram-positive bacteria in vitro, defensin expression is detected in mosquitoes inoculated with Gram-positive or negative bacteria, or with filarial worms. Defensin transcription and expression are dependent upon bacterial dose; however, translation is inconsistent with transcription because peptide is detectable only in mosquitoes inoculated with large doses. In vitro translation assays provide further evidence for post-transcriptional regulation of defensin. Clearance assays show that a majority of bacteria are cleared before defensin is detected. In gene silencing experiments, no significant difference in mortality was observed between defensin-deficient and control mosquitoes after bacteria inoculation. These studies suggest that defensin may have an alternative function in mosquito immunity.