Dirofilaria immitis: fate and immunogenicity of irradiated infective stage larvae in beagles

Examining the role of macrolides and host immunity in combatting filarial parasites

Macrocyclic lactones (MLs), specifically the avermectins and milbemycins, are known for their effectiveness against a broad spectrum of disease-causing nematodes and arthropods in humans and animals. In most nematodes, drugs in this class induce paralysis, resulting in starvation, impaired ability to remain associated with their anatomical environment, and death of all life stages. Initially, this was also thought to be the ML mode of action against filarial nematodes, but researchers have not been able to validate these characteristic effects of immobilization/starvation of MLs in vitro, even at higher doses than are possible in vivo. Relatively recently, ML receptor sites exclusively located proximate to the excretory-secretory (ES) apparatus were identified in Brugia malayi microfilaria and an ML-induced suppression of secretory protein release by B. malayi microfilariae was demonstrated in vitro. It is hypothesized here that suppression of these ES proteins prevents the filarial worm from interfering with the host's complement cascade, reducing the ability of the parasite to evade the immune system. Live microfilariae and/or larvae, thus exposed, are attacked and presented to the host's innate immune mechanisms and are ultimately killed by the immune response, not the ML drug. These live, exposed filarial worms stimulate development of innate, cellular and humoral immune responses that when properly stimulated, are capable of clearing all larvae or microfilariae present in the host, regardless of their individual sensitivity to MLs. Additional research in this area can be expected to improve our understanding of the relationships among filarial worms, MLs, and the host immune system, which likely would have implications in filarial disease management in humans and animals.

A comprehensive, model-based review of vaccine and repeat infection trials for filariasis

SUMMARY Filarial worms cause highly morbid diseases such as elephantiasis and river blindness. Since the 1940s, researchers have conducted vaccine trials in 27 different animal models of filariasis. Although no vaccine trial in a permissive model of filariasis has provided sterilizing immunity, great strides have been made toward developing vaccines that could block transmission, decrease pathological sequelae, or decrease susceptibility to infection. In this review, we have organized, to the best of our ability, all published filaria vaccine trials and reviewed them in the context of the animal models used. Additionally, we provide information on the life cycle, disease phenotype, concomitant immunity, and natural immunity during primary and secondary infections for 24 different filaria models.

Heartworm disease in animals and humans

Heartworm disease due to Dirofilaria immitis continues to cause severe disease and even death in dogs and other animals in many parts of the world, even though safe, highly effective and convenient preventatives have been available for the past two decades. Moreover, the parasite and vector mosquitoes continue to spread into areas where they have not been reported previously. Heartworm societies have been established in the USA and Japan and the First European Dirofilaria Days (FEDD) Conference was held in Zagreb, Croatia, in February of 2007. These organizations promote awareness, encourage research and provide updated guidelines for the diagnosis, treatment and prevention of heartworm disease. The chapter begins with a review of the biology and life cycle of the parasite. It continues with the prevalence and distribution of the disease in domestic and wild animals, with emphasis on more recent data on the spreading of the disease and the use of molecular biology techniques in vector studies. The section on pathogenesis and immunology also includes a discussion of the current knowledge of the potential role of the Wolbachia endosymbiont in inflammatory and immune responses to D. immitis infection, diagnostic use of specific immune responses to the bacteria, immunomodulatory activity and antibiotic treatment of infected animals. Canine, feline and ferret heartworm disease are updated with regard to the clinical presentation, diagnosis, prevention, therapy and management of the disease, with special emphasis on the recently described Heartworm Associated Respiratory Disease (HARD) Syndrome in cats. The section devoted to heartworm infection in humans also includes notes on other epizootic filariae, particularly D. repens in humans in Europe. The chapter concludes with a discussion on emerging strategies in heartworm treatment and control, highlighting the potential role of tetracycline antibiotics in adulticidal therapy.

Molecular cloning, purification and characterisation of myosin of human lymphatic filarial parasite Brugia malayi

Global efforts have been made towards development of vaccine for prevention of lymphatic filariasis. However, lack of thorough knowledge about developmental biology and pathogenesis of filarial parasite restricts us from developing an effective vaccine. A limited number of immunodominant antigens of human lymphatic filariid Brugia malayi have been characterised; however, none of these recombinant antigens so far induced significant degree of protective immunity to challenge infection. In the present study, we identified a approximately 2.0 Kb cDNA clone by immunoscreening of cDNA library of adult female Brugia malayi. The nucleotide sequence of the identified clone showed 94.3% homology with C-terminal part of myosin heavy chain gene of Brugia malayi. This cDNA insert was sub-cloned into pET28b vector and expressed in BL21(DE3). The recombinant protein was purified to near homogeneity by immobilised metal affinity chromatography (IMAC) with yield of approximately 25 mg/l. The purified protein was recognised in western blot with anti-His tag antibody as also with the antibodies present in the sera of human W. bancrofti patients of all categories and infected/immunized rodent serum demonstrating its functional role. Recombinant myosin induced marked cellular immune response as observed by lymphoproliferation assay. The present findings demonstrate the usefulness of B. malayi recombinant myosin as vaccine candidate against human lymphatic filariasis.

Th1 response in BALB/c mice immunized with Dirofilaria immitis soluble antigens: a possible role for Wolbachia?

The immune response to filarial infection has been shown to be of both the Th1 and Th2 types. Studies aimed at developing immunization strategies against Dirofilaria immitis infection in dogs have shown that protection against larval challenge is of the Th2 type and that several proteins are recognized by immunized or infected animals. The bacterial endosymbiont Wolbachia, harbored by many filarial species including D. immitis, has recently been shown to interact with the host immune system. Specific antibodies to the Wolbachia recombinant surface protein (WSPr) have been observed in cats infected with D. immitis. In this work the authors have determined cytokine production and antibody response in BALB/c mice inoculated with soluble antigens from third stage larvae or from adult worms of D. immitis. Inoculated mice first produced IFN-gamma followed by a peak in IL-4. Specific antibodies to the Wolbachia protein WSPr were exclusively IgG2a, while antibodies against peptides derived from antigens of D. immitis were in the IgG1 and IgE subclasses. The cytokine response is thus similar to that reported for other filarial infection, where Th1 response shifts towards Th2. Antibody response indicates that Wolbachia may induce preferentially a Th1 response during filarial infection, while nematode antigens may be involved in Th2 response. There is thus an overall agreement with current opinions on the role of bacterial versus nematode molecules in driving the response towards the different directions.

Immunoaffinity-isolated antigens induce protective immunity against larval Strongyloides stercoralis in mice

The objective of this study was to identify soluble protein antigens that would induce protective immunity against infective-stage larvae (L-3) of Strongyloides stercoralis in mice. Deoxycholate (DOC)-soluble proteins derived from L-3, adsorbed to aluminum hydroxide, induced protective immunity in BALB/c mice. The immunized mice generated parasite-specific IgG that could transfer passive immunity to naïve animals. The protective antibodies bound to parasite antigens found in the muscles and nerve cords of the L-3. An IgG affinity chromatography column generated with IgG from the sera of DOC-immunized mice was used to purify specific larval antigens. Proteins were eluted from the affinity column with sizes of 80, 75, 61, 57, 43, and 32 kDa. This antigen pool stimulated both proliferation and IL-5 production by splenocytes recovered from mice immunized with live L-3. Vaccination of mice with the immunoaffinity-isolated antigens led to significant protective immunity, with 83% of challenge larvae killed. This study demonstrates that IgG-isolated proteins are candidate antigens for a vaccine against larval S. stercoralis.

Development of a recombinant antigen vaccine against infection with the filarial worm Onchocerca volvulus

Efforts to control Onchocerca volvulus, the etiologic agent of river blindness, have been limited to vector control and drug treatment to eliminate microfilariae, with no means available to prevent infection. The goal of this study was to develop a vaccine against this infection using recombinant antigens that are expressed in the early larval stages of the parasite. Five recombinant antigens, Ov7, Ov64, OvB8, Ov9M, and Ov73k, were identified by screening adult and larval cDNA libraries with antibodies from immune humans, chimpanzees, or rabbits. When mice were immunized with the five individual recombinant antigens, statistically significant reductions in parasite survival were induced in mice immunized with Ov7, OvB8, or Ov64, when administered in alum but not when injected in Freund's complete adjuvant (FCA). Live larvae recovered from control and immunized mice were analyzed to determine their developmental stages. A decrease in the percentage of larvae molting from the third stage to the fourth stage was observed with mice immunized with Ov7, Ov64, or OvB8 in alum but not with mice immunized with Ov9 and Ov73k or with mice immunized with any of the five antigens in FCA. Mice immunized with a cocktail of the three protective antigens developed protective immunity equal to that seen with mice immunized with individual antigens. This study has identified, for the first time, three recombinant antigens capable of inducing protective immunity to O. volvulus. Furthermore, since the antigens functioned with alum as the adjuvant, this vaccine could potentially be used clinically to prevent river blindness in humans.

Effects of canine heartworm (Dirofilaria immitis) on body condition and activity of free-ranging coyotes (Canis latrans)

We used radiotelemetry to study relationships among canine heartworm (Dirofilaria immitis) infection, body condition, and activity of free-ranging coyotes (Canis latrans). Average body mass at death was lower for 17 coyotes in a high-intensity infected group (mean = 33.6 heartworms) than for 18 coyotes in a control group (mean = 3.6 heartworms; p < 0.01). Coyotes in the infected group lost body mass at an average rate of 20% per year relative to the control group (p < 0.01). Bone marrow fat was negatively correlated with heartworm burden (R2 = 0.27; p < 0.01). Average body mass of coyotes at initial capture (i.e., potentially before infection) did not differ between infected and control groups (p = 0.90; 1–β = 0.70). Activity was negatively correlated with heartworm burden during the last 2 months of life (R2 = 0.30; p < 0.01), but no correlation was found 2–4 months before death. Activity of the infected group (n = 13) declined over time (p = 0.01), whereas no difference in activity was observed in the control group (n = 13; p = 0.50). Our findings indicate that heartworm infection reduced body condition and activity of coyotes but that nutritional status did not significantly affect susceptibility to infection.

Acanthocheilonema viteae: characterization of a molt-associated excretory/secretory 18-kDa protein

Post-invasive third-stage larvae (pL3) of Acanthocheilonema viteae were labeled with [(35)S]-methionine in vivo, and proteins released into the culture supernatant before and during the third molt were analyzed. The molting supernatant (MSN) contained abundant proteins of 14, 18, 29, and 36 kDa. The 14- and 29-kDa proteins were exclusively found in the MSN, while the 18- and 36-kDa proteins were also produced by nonmolting pL3, albeit in much lower quantities. The cDNA for the most abundant protein in the MSN, an 18-kDa protein (Av18), was isolated by polymerase chain reaction (PCR) with reverse transcribed (RT) RNA of pL3, using information of the protein sequence. The Av18 full-length cDNA of 583 base pairs contained the 5' spliced leader sequence of nematodes, an open reading frame of 427 base pairs, and a poly(A) tail in typical distance to a polyadenylation signal. The deduced amino acid sequence encodes for a protein with a calculated size of 15.8 kDa. The N-terminus starts with a hydrophobic signal sequence and a predicted cleavage site after amino acid 20. The Av18 protein showed homologies to the deduced amino acid sequence of the larval transcripts Bm-alt-1 and alt-2 of Brugia malayi and to the Dirofilaria immitis proteins Di20/22 as well as to the Onchocerca volvulus proteins Ov-alt-1 and Ov-alt-2. Av18 is present in all parasite stages within the mammalian host, as determined by immunoblot with sera against the Escherichia coli-expressed protein and RT-PCR experiments. However, it was released into culture medium only by L3 and adult female worms. In female worms Av18 was localized in the cuticular region as demonstrated by immunofluorescent antibody tests using cryosections.

Identification of an asparagine amidohydrolase from the filarial parasite Dirofilaria immitis

The nematode cuticle is a complex extracellular structure which is secreted by an underlying syncytium of hypodermal cells. Recent studies have demonstrated that the cuticle of parasitic nematodes is a dynamic structure with important absorptive, secretory, and enzymatic activities. In addition, the cuticle serves as a protective barrier against the host. A 48-h third stage larval Dirofilaria immitis cDNA library was immunoscreened with sera raised against larval cuticles. One clone, L3MC4 that reacted strongly with the anti-cuticle antisera was sequenced. The composite cDNA sequence comprises 2073 bp coding for a full-length protein of 590 amino acids. GenBank analysis showed that DiAsp had significant similarity to a Caenorhabditis elegans gene-product (54% identity) and to other asparaginases at the amino acid level. Escherichia coli-expressed recombinant DiAsp (rDiAsp) catalysed the hydrolysis of asparagine to aspartate and ammonia. Antibodies raised against D. immitis larval cuticles reacted with rDiAsp in immunoblots. This is the first report of identification of a cDNA clone encoding an asparaginase enzyme from a parasitic nematode.

Molecular characterization of a calcium-binding protein from the filarial parasite Dirofilaria immitis

A full length D. immitis cDNA (nDiCal) encoding a protein with significant similarity to the calreticulin protein family was isolated from a 6-day fourth-stage larval cDNA expression library by immunoscreening, using serum from a rabbit immunized by repeated injection of small numbers of third-stage larvae. nDiCal is 1538 bp long and contains the 21 bp nematode splice leader sequence SL1 at the 5' end. nDiCal encodes for a protein (pDiCal) with a predicted molecular mass of 46 kDa. pDiCal sequence analysis revealed similarities with calreticulin, a protein that typically resides in the endoplasmic reticulum. pDiCal possesses three consensus sequences of the calreticulin family of proteins: a neutral N-terminal region with a putative signal sequence; a proline- and tryptophan-rich P region; and a highly acidic C-terminal region. A 45Ca2+-overlay assay showed that recombinant pDiCal (rDiCal) is a Ca2+-binding protein. Antibodies to rDiCal identified a 56 kDa native antigen in all developmental stages including the excretory-secretory products derived from larvae and adult worms. Localization studies demonstrated the ubiquitous presence of pDiCal with intense expression in the hypodermis and syncitial muscle cells in both male and female adult worms. Labeling was also seen in the developing embryos within the uterus of the female worms. Sera from immune as well as chronically-infected microfilaremic dogs contained antibodies that bind rDiCal. In addition, immunoblot analysis showed that serum from a rabbit immunized with L3 cuticles reacted with rDiCal.

Immunologic protection against canine heartworm infection

This study was conducted to evaluate protective efficiency of three different protocols for vaccination in canine heartworm infection. To evaluate the three protocols of immunization, dogs were separately immunized with living larvae; 1) immunization with gamma-attenuated infective larvae, 2) with 50 micrograms/kg ivermectin-abbreviation, and 3) with chemical abbreviation plus Freund's complete adjuvant (FCA). Each group was composed of two dogs. All dogs used for this study were subcutaneously challenged with 100 intact third-stage larvae (L3) various days after the last immunization, and the worms in the pulmonary arteries and the right ventricle of the heart were recovered 17 to 25 weeks post-infection. The numbers and the sexes of the worms were determined. A mean of 38 worms was burdened in the group immunized with irradiated L3, 36 worms in the chemically-abbreviated group, but 15.5 worms in the group with chemical abbreviation plus FCA. The percentages of the protection in the former two groups were nearly 50%, but 72.3% in the group with ivermectin plus FCA. The adjuvant enhanced the protective immunity against L3 challenge. Obvious eosinophilia was observed in both immunized and control dogs except for two dogs. There was no correlation between the suppression of eosinophilia and the protective immunity in the present study.

Molecular cloning, expression and enzymatic activity of a thioredoxin peroxidase from Dirofilaria immitis

A Dirofilaria immitis cDNA clone encoding a nucleic acid homolog of thioredoxin peroxidase (nDiTPx) was isolated from a fourth-stage larval cDNA library, using serum from dogs vaccinated by chemotherapeutically-abbreviated D. immitis larval infections. The protein encoded by nDiTPx had a predicted molecular mass of 22.1 kDa and the deduced amino acid sequence was homologous to thioredoxin peroxidase-like sequences described in other filarial nematodes, yeast, bacteria and mammals. As is true for other members of this peroxiredoxin family, the nDiTPx-encoded protein had the conserved cysteine near the amino terminus, considered to be essential for enzyme activity. nDiTPx was expressed in E. coli and the resulting recombinant fusion protein was shown to have thioredoxin peroxidase (TPx) activity, by its ability to protect DNA from oxidative-nicking in a metal-catalyzed oxidation system. A polyclonal antibody to the DiTPx fusion protein detected a 22-kDa native protein in D. immitis larval and adult parasite extracts.

OvB20, an Onchocerca volvulus-cloned antigen selected by differential immunoscreening with vaccination serum in a cattle model of onchocerciasis

A cDNA of Onchocerca volvulus has been isolated by differential immunoscreening of an adult worm expression library using sera raised in cattle against the related species, O. lienalis. It was selected because of its recognition by antibodies from cattle immunized with irradiated third-stage (L3) larvae and not by antibodies from animals infected with non-irradiated larvae. The original 311-bp clone was used to isolate a 1478-bp cDNA. Designated OvB20, this codes for 460 amino acid residues, hybridizes with a approximately 1.6 kBp transcript and appears to be transcribed from a filarial-specific, single copy gene. It is expressed in developing stages from embryo to L4 larva, but not in the adult. The product of OvB20 appears to undergo co- or post-translational processing: in vitro transcription and translation give rise to a polypeptide consistent with the deduced amino acid sequence (approximately 52 kDa), whilst products of 52 and 65 kDa are detected in larvae by immunoblotting and following in vitro translations to which exogenous microsomes have been added. A 42-kDa protein was also detected in all in vitro translations. No homologous genes were found in the computer databases, although there are regions of weak sequence similarity with C-reactive proteins. The functional role of OvB20 may warrant further attention, as it has recently been shown that the recombinant protein confers host protection against a related rodent filaria following active immunization (Taylor, M.J., Abdel-Wahab, N., Wu, Y., Jenkins, R.E. and Bianco, A.E. (1995) Onchocerca volvulus larval antigen, OvB20 induces partial protection in a rodent model of onchocerciasis. Infect. Immun. 63, 4417-4422).

Molecular cloning of a developmentally regulated protein isolated from excretory-secretory products of larval Dirofilaria immitis

Three proteins isolated from the excretory-secretory products (ES) of larval Dirofilaria immitis have been previously characterized and termed the 20, 22L and 22U kDa proteins. Two of the proteins (20 and 22L) were produced and released around the time of the third molt and were specifically recognized by immune dog sera. An amino acid sequence common to both proteins was used to synthesize a DNA probe to molecularly clone these molecules from a 48-h third stage larval cDNA library. The DNA sequence of the isolated clones encoded a 17.5 kDa protein with a 21 amino acid hydrophobic leader sequence that when removed yielded a 15.3 kDa protein starting with the N-terminal sequence obtained from the 20 kDa protein and containing all sequences obtained from tryptic peptides of the 20 and 22L kDa proteins. It was hypothesized that the 20 and 22L kDa proteins were the same, differing only by a 21 amino acid hydrophobic leader sequence which was later cleaved. The calculated molecular masses were consistent with those determined by reducing Tris-tricine SDS-PAGE. Expression of the protein without the leader sequence was accomplished in Escherichia coli. Antiserum raised against the expressed protein demonstrated the presence of the protein in L3 and L4, but not in adults or microfilariae. Expression of the protein with the leader sequence using a baculovirus system demonstrated processing of the signal sequence at the same time as found in larval D. immitis ES. Sera from dogs immune to infection were reactive with the D. immitis proteins expressed in either E. coli or insect cells.

Purification and characterization of three larval excretory-secretory proteins of Dirofilaria immitis

Two proteins were previously described in the excretory-secretory products (ES) collected from Dirofilaria immitis during the molt from the third stage to the fourth stage in vitro. The two proteins were purified using cation exchange and reverse phase HPLC. During the purification of these two proteins, a third protein was identified that co-migrated with one of the others during previous gel analysis. All three had molecular masses of 20-23 kDa as determined by Tris-glycine SDS-PAGE and have been designated 20, 22L and 22U kDa proteins. The three proteins were digested with trypsin. Amino acid sequences were subsequently determined for four peptides and the N-terminus of the 20 kDa protein, five peptides of the 22L kDa protein and three peptides of the 22U kDa protein. The 20 and 22L kDa proteins were quite similar based on sequence and purification characteristics. The 22U kDa protein, but not the 20 and 22L kDa proteins, was also identified in adult worms using tryptic mapping and amino acid sequencing techniques. Immunoblot analysis demonstrated that the 20 and 22L kDa proteins were specifically recognized by sera from dogs immune to infection by D. immitis but not by sera from infected non-immune dogs. The 22U kDa protein was weakly recognized by the same immune sera but not by the infected non-immune dog sera. Since the 20 and 22L kDa proteins appear to be larval specific, associated in time with the molt from L3 to L4 and are specifically recognized by immune dog sera, they are good vaccine candidates.

Onchocerca volvulus larval antigen, OvB20, induces partial protection in a rodent model of onchocerciasis

OvB20 is an antigen of Onchocerca volvulus preferentially recognized by sera from cattle vaccinated with irradiated infective larvae of Onchocerca lienalis. Antibodies raised against the recombinant protein were used to characterize the expression of the native protein in different developmental stages of O. volvulus and the rodent filaria Acanthocheilonema viteae. In O. volvulus, antibodies reacted to a polypeptide of 42 kDa in microfilariae and with proteins of 52 and 65 kDa in third-stage larvae. No products were detected in adult stages. Immunogold electron microscopy localized the native protein to discrete patches of the hypodermis and cuticle of infective larvae. Characterization of a homologous protein in A. viteae confirmed the stage-specific expression in infective larvae of the 65-kDa protein, which was secreted during in vitro culture. Vaccination of rodents against A. viteae with a B20-maltose-binding-protein fusion protein resulted in a 49 to 60% reduction in adult worm recoveries with a corresponding 97% reduction in microfilaremia.

Cloning of an early immunodominant filarial antigen: a member of the Brugia malayi myosin heavy chain gene family

Partial protective immunity to filariasis can be achieved in animals by vaccination with irradiated infective larvae. A Brugia malayi cDNA expression library was screened with serum pools from vaccinated and infected jirds to select clones that expressed potentially protective recombinant antigens that were preferentially recognized by sera from vaccinated animals. Bmmyo-2, the largest of a group of related clones, was studied in detail. Jirds produced strong antibody responses to the protein product of Bmmyo-2, Bmmhc-B, as early as 1 month after vaccination with irradiated larvae. Antibody responses to Bmmhc-B in infected jirds were weaker than those of vaccinated jirds, and they developed somewhat later. Antibodies produced to Bmmhc-B were reactive with a 200 kDa native B. malayi antigen by immunoblot and with muscle bands in the body wall of microfilarial by indirect immunofluorescence. Sequence analysis of the 1454 bp cDNA insert of Bmmyo-2 showed that it codes for a portion of the rod region of a B. malayi myosin heavy chain isoform. The deduced amino acid sequence of Bmmyo-2 is 74.6% identical with that of the corresponding region of Caenorhabditis elegans myosin heavy chain B, but only 64.6% identical with a recently described B. malayi myosin heavy chain, Bmmhc-A.

Vaccine research and development for the prevention of filarial nematode infections

The development of vaccines for the prevention of filarial nematode infections is in a state of relative infancy in comparison to vaccines for other parasitic diseases, such as schistosomiasis and malaria. There are many reasons for this slow start. Some of the principal problems are: (1) the lengthy and complex life cycle of these organisms with attendant complex immune responses, (2) the unique characteristics associated with a relatively large number of different pathogens, (3) the lack of suitable model systems for study of medically important infections, (4) the paucity of parasite material for antigen discovery and recombinant library construction, (5) the lack of substantial evidence suggesting the natural occurrence of protective immune responses, and (6) the limited data on mechanisms responsible for protective immunity. As technical hurdles are considered, it is also critical to focus on the characteristics of a vaccine necessary for its eventual utility. In the case of a vaccine for D. immitis a completely successful product will need to approach a 99+% efficacy. This is because of the 99+% efficacy of competitive chemotherapeutic products and the fact that microfilaremia observed on blood examination, resulting from as few as two worms, would present as a vaccine failure. Although very low worm burdens in large dogs could be perceived as success in the context of protection from clinical disease, because of the option of virtually complete chemoprophylactic protection, the typical veterinary practitioner would probably fail to appreciate less than complete vaccine protection. In contrast, a vaccine that produced a reduction in adult worm burdens without complete protection in either lymphatic filariasis or onchocerciasis would be very important. Highly effective chemoprophylactic agents are not widely available for prevention of the human filariases, and dramatically reduced clinical disease provided by less than a completely effective vaccine could occur as the result of fewer adult worms. The importance of developing these vaccines has outweighed the obstacles to this research. There has been a great deal of epidemiological and experimental evidence to suggest a vaccine is feasible and antigen discovery has progressed relatively rapidly within just the past few years. Efforts to generate appropriate larval cDNA libraries are beginning to yield dividends and a variety of fascinating vaccine candidates have been cloned. Additional antigen discovery, research on appropriate modalities for overexpression of genes from these parasites, and the complex tasks associated with vaccinology remain as significant research and development obstacles.(ABSTRACT TRUNCATED AT 400 WORDS)

An analysis of the humoral immune response of dogs following vaccination with irradiated infective larvae of Dirofilaria immitis

In this study, dogs were immunized with irradiated L3 larvae of Dirofilaria immitis. Following challenge with non-irradiated L3, vaccinated dogs had an average of 71% fewer adult worms compared to non-vaccinated animals. A comparative analysis of eosinophil and antibody responses of these two groups of dogs is presented. Vaccinated dogs preferentially recognized several larval (14, 20, 30, 34, 39 kDa), adult worm (20 kDa) and microfilarial (36, 38, 71, 84 kDa) antigens. To characterize these antigens, the extent of glycosylation was assessed. The data suggest that an earlier response to these antigens may be important in the protection induced in dogs by administration of irradiated L3 of D. immitis.

Experimental onchocerciasis in chimpanzees: cellular responses and antigen recognition after immunization and challenge with Onchocerca volvulus infective third-stage larvae

Immunization of chimpanzees with radiation-attenuated infective 3rd-stage larvae (L3) of Onchocerca volvulus did not induce strong protective immunity against a subsequent challenge infection; only 1 out of 4 immunized animals remained non-patent (i.e. microfilariae-negative) after challenge, and may have been protected. However, during immunization and before challenge, a broad range of adult O. volvulus-derived antigens (OvAg) and also uterus-derived OvAg were recognized by circulating antibodies; moreover, the repertory of antigens recognized increased further in subsequently patent animals after challenge, particularly in the range of M(r) 12-42 kDa. In the immunized and non-patent chimpanzee, by contrast, serological recognition of uterus-derived OvAg with M(r) 14 kDa and 105 kDa disappeared by 19 months post-challenge (p.c.). During immunization, Acanthocheilonema viteae L3 antigens of M(r) 11-12 kDa were strongly recognized only by the non-patent animal, suggesting that recognition of these antigens may have supported resistance to the subsequent challenge infection. In immunized chimpanzees, a substantial increase in the cellular reactivity to OvAg was induced; this, however, declined by 19 months p.c. to levels similar to those seen prior to immunization. At that time, 3 out of 4 immunized animals were patently infected. The effect of exogenous cytokines on in vitro-reactivity of PBMC to OvAg was examined. Addition of exogenous IL-2 alone, IFN-gamma alone, and IFN-gamma in combination with IL-2, did not augment net cellular responses to OvAg by PBMC from infected and control chimpanzees. In the presence of IL-4 alone, IL-6 alone, IL-2 with IL-4, IL-2 with IL-4 and IFN-gamma, or IL-2 with IL-4 and IL-6, the net cellular reactivity to OvAg increased significantly in patent chimpanzees and reached levels similar to non-patent animals. Thus, non-patent chimpanzees maintain high cellular reactivity to OvAg and in vitro cellular unresponsiveness to OvAg on the part of patent chimpanzees is reversible after addition of several cytokines which act individually or synergistically.

Molecular characterization of a Dirofilaria immitis cDNA encoding a highly immunoreactive antigen

Dirofilaria immitis, a filarial nematode, is the causative agent of canine and feline heartworm disease. Previous research has demonstrated that immunity to D. immitis can be induced in dogs by repeated chemical abbreviation of infections while the parasite is a fourth-stage larva. Sera obtained from dogs immunized in this manner has been effective in passively transferring larval killing and stunting. These immune sera, by comparison to nonimmune sera from infected cohorts, recognize a number of unique D. immitis antigens, some of which are larval specific. In this study immune dog sera were used to screen a D. immitis larval cDNA expression library. Three overlapping cDNA clones, Di22, Di18 and Di16, were obtained that encode a portion of a large molecule, greater than 150 kDa, that is composed of multiples of a 399-bp repeat. This protein when immunoblotted with antibody against a recombinant expressed Di22 fusion protein is found in larval as well as adult extracts and excretory-secretory products, and is seen as a series of ascending subunits, each approximately 15 kDa larger than the previous one. This antigen is highly immunogenic, as evidenced by the strong reactivity of the recombinant expressed Di22 fusion protein with sera from immune dogs, microfilaremic dogs and infected amicrofilaremic dogs. While the function of this antigen is unknown it has significant sequence similarity with an allergen found in Ascaris.

Cloning of a cuticular antigen that contains multiple tandem repeats from the filarial parasite Dirofilaria immitis

An unusual antigen composed of tandemly repeated protein units was cloned from the filarial parasite Dirofilaria immitis. The antigen was initially identified by screening a lambda gt11 cDNA library with serum from dogs immunized with irradiated D. immitis third-stage larvae. DNA sequence analysis of the cDNA clone, Di5, revealed a continuous open reading frame composed of two 399-base-pair repeats arranged in tandem. Southern blot analysis of genomic D. immitis DNA showed that the gene coding for Di5 is composed of a tandem array of 25-50 copies of this same 399-base-pair repeat. Antiserum raised against recombinant Di5 protein detected a protein "ladder," from about 14 to greater than 200 kDa with steps approximately 15 kDa apart, on immunoblots of D. immitis extract. Metabolic labeling of adult parasites with [35S]methionine showed that Di5 is synthesized as a large precursor that is subsequently cleaved to produce the ladder-like array. These results suggest that the characteristic ladder is created by proteolytic cleavage of the precursor at the same site in each monomer. The Di5 antigen was localized to the cuticle and hypodermis of adult D. immitis by immunoelectron microscopy. Both male and female parasites were found to release Di5 when cultured in vitro. DNA hybridization analysis demonstrated that Di5 is a member of a gene family present in many filarial parasites that infect both animal and human populations.

Brugia malayi: antibody responses to larval antigens in infected and immunized jirds

Vaccination with irradiated third stage Brugia malayi larvae (L3) has been reported to induce partial protective immunity to L3 challenge in jirds. The purpose of this study was to identify antigens that may be targets of protective immunity in this model. Jirds were immunized by s.c. injection of irradiated L3 and challenged either s.c. or i.p. Necropsy was performed 11 wk after challenge. Partial protection was achieved in s.c. challenged animals; worm recovery was only 41% of that observed in unvaccinated controls, and worms recovered from immunized animals were stunted. Worm recoveries in immunized animals that were challenged i.p. did not differ from those of unimmunized controls. Group differences in parasite antigen levels in sera collected 2-11 wk after larval challenge were consistent with parasitological findings obtained at necropsy. Antibody studies compared prechallenge sera from immunized animals to sera from infected (unimmunized) controls. Antibody responses to L3 surface antigens (assessed by IFA) were much stronger after immunization than after infection. Immunoblot studies showed preferential recognition of several L3 antigens (97, 54, 48, and 40 kDa) by antibodies in sera from immunized animals. Additional studies are needed to determine whether immunization with such preferentially recognized antigens can induce protection to larval challenge comparable to or better than that observed with live vaccines.

Onchocerca volvulus: immunization of chimpanzees with X-irradiated third-stage (L3) larvae

To provide a theoretical basis for the potential development of vaccines against Onchocerca volvulus (Ov) a trial has been conducted to assess the protective efficacy of immunization of chimpanzees with X-irradiated L3 larvae. Approximately 1000 larvae were injected at 0, 1, and 7 months. The immunized animals, and unimmunized controls, were then challenged with 250 live L3. In order to provide possibly protective exposure to the immunologically distinct L4 epicuticle, a radiation dose (45 krad) was chosen which preserved about 50% of the molting ability of unirradiated larvae. Despite the presence of a strong immune response to crude adult worm extracts, and to cloned Ov antigens, at the time of challenge little or no significant protection against patent infection was observed: three of four immunized animals developed patent infection as compared to four of four controls. One immunized animal failed to become patent or to manifest the late antibody response to adult worm antigens seen in both subpatent and patent infections in this model, and may have been protected from infection. The implications of these studies for future attempts to immunize against O. volvulus are discussed.

Surface-associated antigens of Brugia malayi L2 and L3 parasites during vector-stage development

Surface and metabolic labeling procedures were used to characterize the composition and the time of expression of Brugia malayi L2 and L3 surface-associated molecules as the larvae develop within the mosquito vector. Larvae were harvested from mosquito tissues at 5 (early L2), 8 (late L2) and 11 (L3) days post-infection and labeled with 125I-Iodo-Gen. The results of one-dimensional analysis showed that there is a progressive increase in the complexity of peptides associated with the surface of developing larvae, culminating in the expression of 7 major labeled components on L3s. Both L2 and L3 parasites have surface-associated components of 42, 35, 33, 19 and 17 kDa. Between days 8 and 11 of development in the insect vector, Brugia malayi undergoes the L2 to L3 molt and acquires additional major immunogenic peptides of 40 and 22 kDa. Two-dimensional analyses of extracts from 125I-labeled L2s and L3s revealed that the major 35-, 33-, 19- and 17-kDa molecules are part of a peptide complex that forms a 'ladder' between 17 and 150 kDa. To gain information on the times during which the major surface-associated molecules are produced by the parasite, larvae were labeled with [35S]methionine either in situ as they developed within the mosquito or during culture after exiting the vector. For in situ labeling, [35S]methionine was introduced into the hemolymph of infected mosquitoes by micro-injection at days 2, 5 and 8 post-infection and the larvae were allowed to develop for an additional 3 days. The results of 1- and 2-dimensional analyses of [35S]methionine-labeled extracts from vector-stage or post-vector-stage larvae indicate that the molecules associated with the surface of B. malayi L3s are synthesized between day 5 and day 11 of development in the insect host. Immediately after the larvae exit the vector, the synthesis of the 40 and 22-kDa peptides is drastically reduced or terminated.

Changes in the surface composition after transmission of Acanthocheilonema viteae third stage larvae into the jird

This study describes the dynamics and the biochemical nature of changes in the surface of the filarial nematode Acanthocheilonema viteae after its transmission into the vertebrate host. Vector-derived third-stage larvae (mL3) were inoculated into naive Meriones unguiculatus and recovered from the tissues at different times post-infection until their moult to fourth-stage larvae (L4). Surface-specific labelling with fluoresceinated lectins revealed that the larvae are covered by a carbohydrate envelope. Although the mL3 envelope was strongly reduced one day after transmission, new surface carbohydrates appeared until the onset of moulting, some of which could also be identified on the surface of L4. In general, surface carbohydrates were partially shed by moving larvae, suggesting a loose association of these components in the epicuticle. The fate of cuticular lipids and proteins of L3 and L4 was monitored by external 125I-labelling and differential extraction of the components. Thin-layer chromatography of surface-labelled lipids revealed only minor changes 1 day after parasite transmission. Afterwards the number of lipids accessible to label decreased further until moulting was complete. Two-dimensional sodium dodecyl sulphate-polyacrylamide gel electrophoresis of surface-labelled proteins showed a consistent surface exposure of mL3 specific proteins until 1 day post-infection. Thereafter, the composition of surface-labelled proteins changed rapidly, resembling that of the L4 as early as several days before moulting. During this period individual differences in the composition of surface proteins were evident.

Animal models for the study of immunity in human filariasis

A major challenge to the development of vaccines against human lymphatic filariasis and onchocerciasis is to direct the immune response toward elimination of the early, prepathogenic larval stages and away from responses that mediate pathology. In this review, James Lok and David Abraham discuss the various animal models that have been used to investigate the pathways leading to immunity, immunological tolerance and chronic pathology in these diseases. Owing to the strict host specificities of the human-dwelling filariae, no single model serves to duplicate exactly all these aspects. Nevertheless, it has been possible to demonstrate a protective immune response invoked by and directed against incoming third-stage larvae of both lymphatic and skin-dwelling filariae. The fact that subsets of the sequelae of human filarial infection can be duplicated in animal systems should also aid in unravelling the mechanisms determining the course of infection and in ensuring that vaccine candidates do not produce an inappropriate immunopathological response. A proposed scheme for using animal models in screening candidates for a vaccine against Onchocerca volvulus is presented.

Inhibition of carbohydrate uptake of Onchocerca volvulus infective larvae (L3) by effector cell constituents

The in vitro killing of Onchocerca volvulus infective third stage larvae (L3) by components of their human hosts' defence mechanisms is not well documented, as no suitable assay exists. Motility is inappropriate as a criterion of larval viability because of the unsteady winding movements of L3. In the present study, a metabolic parameter for larval viability, the uptake of [3H]2-deoxy-D-glucose, was evaluated. To demonstrate the reproducibility and validity of this test, the oxygen radical hydrogen peroxide (H2O2) was applied to viable L3 and the death of L3 demonstrated by a 90% reduction in glucose uptake. The incorporation of glucose by the filarial larvae was also determined after in vitro exposure to lysates of the putative effector cells, i.e. eosinophilic and neutrophilic granulocytes and monocytes. Effector-cell-derived components led to a 30-80% dose-dependent decrease in deoxy-glucose uptake, with a half-maximal effect at about 500 micrograms ml-1. These experiments demonstrate, for the first time, the deleterious impact of effector cell constituents on the metabolic activity of O. volvulus L3. The assay could be used to evaluate the effect of distinct natural or recombinant effector molecules on the viability of O. volvulus infective larvae and to investigate the effect of parasite molecules which interfere with effector mechanisms.

Dose-dependent recovery of adult Acanthocheilonema viteae (Nematoda: Filarioidea) after single and trickle inoculations in jirds

Increasing single doses of 5, 15, 30, 60 and 90 third-stage Acanthocheilonema viteae larvae per animal were inoculated into jirds. The adult worm load increased steadily, whereas the recovery rate decreased significantly, i.e. the correlation between dose and recovery was negative (rs, -0.90; n = 5; alpha, 0.05). The same inoculation doses were given as trickle inoculations of 5 L3 each (3 x 5, 6 x 5, 12 x 5, 18 x 5) at intervals of 2-6 days throughout the prepatency period. Irrespective of the number of repeated inoculations, a rather constant but low load of 7-10 worms/animal was reached. The recovery rate decreased drastically (rs, -1.0; n = 5). When trickle inoculations were carried out in animals exhibiting patient infections, the superinoculated larvae seemed to be destroyed almost completely; thus, a parasite-host equilibrium was guaranteed in all cases. The immunological background is discussed.

Histochemical localization of gene expression in Onchocerca volvulus: in situ DNA histohybridization and immunocytochemistry

We report here the development of in situ hybridization and immunohistochemistry protocols which permit the histological identification of gene expression of a cloned antigen of Onchocerca volvulus, OI5, in the parasite. Skin nodules containing female adult worms were fixed in a modified Carnoy's fixative and embedded in paraffin. Histological staining of tissue sections revealed uniformly excellent morphology and RNA preservation. To localize mRNA by in situ hybridization, tissue sections were incubated with biotin-labeled pOI5, the plasmid containing the genomic sequence of the antigen, and hybridization signals were histochemically visualized using a streptavidin-enzyme conjugate and chromogenic substrates. The protein antigen was localized immunohistochemically by incubating the sections with specific antibodies prepared against a recombinant fusion protein containing the OI5 sequence (OI3), and visualized via a secondary antibody-biotin-enzyme conjugate procedure. The results reported here showed distinct localization of the OI5 mRNA and OI3 antigen in specific cellular and tissue regions of the adult parasite, and in microfilariae located within the uteri and in the surrounding host tissue. The specificity and high sensitivity of these histological detection methods should be generally applicable for the characterization of gene expression in the filarial parasite, particularly the insect-borne, infective filarial larvae, which are severely limited in quantity.

Acanthocheilonema viteae: vaccination of jirds with irradiation-attenuated stage-3 larvae and with exported larval antigens

Jirds (Meriones unguiculatus) were immunized with irradiated (35 krad) stage-3 larvae (L3) of Acanthocheilonema viteae. The induced resistance against homologous challenge infection and the antibody response of the animals were studied. Immunization with 3, 2, or 1 dose of 50 irradiated L3 induced approximately 90% resistance. Immunization with a single dose of only 5 irradiated L3 resulted in 60.8% protection while immunization with a single dose of 25 L3 induced 94.1% protection. The protection induced with 3 doses of 50 irradiated L3 did not decrease significantly during a period of 6 months. Sera of a proportion, but not all resistant jirds, contained antibodies against the surface of vector derived L3 as defined by IFAT. No surface antigens of microfilariae or adult worms were recognized by the sera. Vaccinated animals had antibody responses against antigens in the inner organs of L3 and in the cuticle and reproductive organs of adult worms as shown by IFAT. Immunoblotting with SDS-PAGE-separated L3 antigens and L3-CSN revealed that all sera contained antibodies against two exported antigens of 205 and 68 kDa, and against a nonexported antigen of 18 kDa. The 205-kDa antigen easily degraded into fragments of 165, 140, 125, and 105 kDa which were recognized by resistant jird sera. Various antigens of adult worms, but relatively few antigens of microfilariae, were also recognized. To test the relevance of exported antigens of L3 to resistance, jirds were immunized with L3-CSN together with a mild adjuvant. This immunization induced 67.7% resistance against challenge infection and sera of the immunized animals recognized the 205- and 68-kDa antigens of L3.

Characterization of an Onchocerca volvulus cDNA clone encoding a genus specific antigen present in infective larvae and adult worms

The isolation and characterization of a recombinant cDNA clone (OV7) expressing an antigen present in Onchocerca volvulus infective larvae and adult stages is described. Using chimpanzee antiserum generated against irradiated infective larvae, we isolated a cDNA clone from a lambda gt11 cDNA expression library derived from adult O. volvulus mRNA. The open reading frame encodes 131 amino acids corresponding to a 15.2-kDa protein. Affinity purified antibodies which bound specifically to OV7 fusion polypeptide recognized a single antigen with an apparent molecular weight of 17,000 in extracts of L3, L4 and adult worms. Immunoelectron microscopy established that the antigen encoded by this clone is present in the hypodermis and the basal layer of the cuticle of L3 and female adult worm, and in the egg shell around developing microfilariae. Since the OV7 fusion polypeptide is onchocerca-specific and is recognized specifically by sera from onchocerciasis patients, and sera from non-patent but infected chimpanzees, and not by sera from patients with other filarial parasites, it may have potential as an antigenic component in a test for detection of non-patent and patent infections of O. volvulus. The OV7 amino acid sequence contains residues that have a probable homology with the cysteine proteinase inhibitor superfamily.

Surface-associated antigens of second, third and fourth stage larvae of Dirofilaria immitis

The surface-associated molecules of the second (L2), third (L3) and fourth (L4) larval stages of Dirofilaria immitis were characterized employing radiolabeling techniques and SDS-PAGE analysis. Major labeled components of 35 kDa and 6 kDa were present in extracts from IODO-GEN-labeled L2 and L3 parasites. The results of lactoperoxidase-catalyzed reactions also demonstrated that L2 and L3 stages of D. immitis have a similar repertoire of surface-associated antigens. However, in contrast to the results obtained with IODO-GEN, lactoperoxidase reactions labeled components with apparent molecular weights of 66, 48, 25, 16.5 and 12 kDa. The similarities in the molecular weights of the L2 and L3 surface-associated components and the results of immunoprecipitation experiments which demonstrated that antibodies produced against the 35 kDa molecule from D. immitis L3s also recognize the 35 kDa component from L2 parasites suggest that synthesis of the molecules found at the surface of mature infective larvae begins as early as day 6 of development in the mosquito, D. immitis L4s emerged from the molting process with a repertoire of surface-associated antigens distinct from those found on L2s and L3s. IODO-GEN labeling of D. immitis L4s showed major surface-associated molecules with apparent molecular weights of 57, 40, 25, 12 and 10 kDa when analyzed under non-reducing conditions. In addition to molecules of 57, 40, 25, 12 and 10 kDa, extracts of D. immitis L4s labeled with lactoperoxidase contained additional major bands at 45, 43 and 8 kDa. Metabolic labeling experiments demonstrated a shift in the amount and complexity of the excretory/secretory products released by D. immitis during L3 to L4 development.

Isolation and characterization of expression cDNA clones encoding antigens of Onchocerca volvulus infective larvae

The isolation of recombinant cDNA clones expressing antigens found in Onchocerca volvulus infective larvae is described. To isolate such clones, an expression cDNA library constructed from adult O. volvulus RNA was screened with antiserum raised against infective larvae. One clone, designated lambda RAL-1 was characterized further. The recombinant antigen produced by lambda RAL-1 stimulates proliferation of peripheral blood mononuclear cells from O. volvulus infected humans. Lambda RAL-1 is derived from a 1450 bases message that encodes a protein with an apparent molecular weight of 42,000 in adult O. volvulus. The inserted DNA of lambda RAL-1 contains an open reading frame of 1008 bp. The amino acid sequence predicted by this open reading frame contains three repeats of the sequence KKPEDWD. The identification of clones such as lambda RAL-1 will provide quantities of purified antigens sufficient to begin to study the immune response to and explore the development of immunity against the infectious form of the parasite.

Dirofilaria immitis: surface properties of third- and fourth-stage larvae

The objective of this study was to analyze surface properties of larval Dirofilaria immitis with potential relevance to protective immunity. Comparisons were made between third (L3)- and fourth-stage larvae (L4) based on their net surface charge, surface carbohydrate and antigen composition, ability to nonspecifically absorb host proteins, complement activation, and nonspecific cellular adherence. It was determined that L3 had a net negative surface charge, whereas L4 had either a neutral or weakly positive surface charge. The lectin Con A, but not any of the other lectins tested, bound only to the surface of L4, and not to that of L3. Monoclonal antibodies were prepared which reacted with the surface of L3 or with the surface of L4, but never both. L4 were found to nonspecifically adsorb host protein to their surfaces, whereas L3 did not. Both L3 and L4 were found to activate complement through the alternate pathway. Finally, nonspecific cellular adherence was found on L3 both in vitro and in vivo but not on L4. The surfaces of L3 and L4 were thus shown to be significantly different and, potentially, in ways which would have great impact in the generation and effectiveness of a protective immune response.

Heartworm infection

Dirofilaria immitis is an epizootic filarial parasite that most often infects domestic dogs. This article provides information about the life cycle of Dirofilaria immitis as well as the epidemiology, diagnosis, clinical signs, and treatment of heartworm infection.

Brugia pahangi in nude mice: protective immunity to infective larvae is Thy 1.2+ cell dependent and cyclosporin A resistant

Mechanisms of protective immunity to larvae of Brugia pahangi were studied in congenitally athymic nude C3H/HeN mice and their syngeneic heterozygous littermates. An average 11% of subcutaneous larval inocula was recovered from control nudes 28 days after inoculation. No worms were recovered from nude recipients of viable splenic Thy 1.2+ T lymphocytes from heterozygotes which had killed a priming dose of B. pahangi larvae. Primed T lymphocytes, depleted of either Lyt 1.1+ or Lyt 2.1+ cells or incubated with anti-Thy 1.2 monoclonal antibody and complement, failed to protect nude mice against a larval challenge. Nor were primed B lymphocytes depleted by Thy 1.2+ T cell contaminants protective. Treatment with cyclosporin A (CsA) did not increase the numbers of worms recovered from heterozygotes nor did CsA treatment of heterozygous cell donors abolish the ability of primed Thy 1.2+ T lymphocytes to transfer protection to nude mice. IgG but not IgM antibody titres to B. pahangi antigens were depressed in all CsA-treated mice. CsA treatment of nude mice had no direct effect upon development of B. pahangi larvae. These results show that protective immunity to larvae of B. pahangi in mice depends upon small numbers of Thy 1.2+ T cells which are CsA-resistant.

Dipetalonema viteae: resistance in Meriones unguiculatus with multiple infections of stage-3 larvae

The jird, Meriones unguiculatus, infected with 80 normal infective larvae of Dipetalonema viteae, revealed a recovery rate of 27.9% 12 weeks after infection. A pretreatment by three injections of 50 normal larvae each and challenge by 80 larvae resulted in a recovery rate of 10.7%. The recovered worms were longer than those from the challenge control animals. When three times 50 irradiated larvae (35 krad) were inoculated, the recovery rate of the challenge decreased to 2.6%, representing a protection of 90.7%. The surviving adult worms were stunted and derived exclusively from the 80 normal larvae given for challenge, since absolutely no adult worms were recovered in eight animals inoculated three times with 50 irradiated larvae only. Sera of all pretreated jirds contained IgG and IgM antibodies which bound in immunoblotting experiments bound predominantly to three proteins of larvae with molecular masses of 68,140, and 165 kDa, respectively. Enzymatic surface iodination revealed that the three antigens were exposed on the larval surface. The coincidence of a partial resistance to a challenge infection and of an antibody response against surface proteins of infective larvae suggests an importance of these antigens for the rejection of D. viteae mediated by an acquired immunological resistance of M. unguiculatus.

Attempted vaccination of jirds (Meriones unguiculatus) against Brugia pahangi with radiation attenuated infective larvae

Jirds were vaccinated by three to five subcutaneous (SC) injections of infective larvae of Brugia pahangi which had been irradiated at 25, 45 or 90 krads from a 60Co source. They were challenged either SC or intraperitoneally. Vaccination with four doses of 50 larvae irradiated with 25 krads produced 49.3% resistance to IP challenge worms and 39.8% against SC challenge worms. Five doses of larvae irradiated with 45 krads produced 62% resistance to SC challenge. Three doses of larvae irradiated with 90 krads produced 74.9% resistance to SC challenge and five doses produced 76.2% resistance. The reasons why irradiated larvae produce resistance whereas normal larvae do not are discussed.

Surface components of Onchocerca volvulus

A technique employing Sephadex G25 gel filtration has been developed for the rapid isolation and purification of live microfilariae of Onchocerca volvulus from subcutaneous nodules and skin samples. Microfilariae, adult worms and L3 larvae have been surface radiolabelled using the Iodogen technique. Two proteins have been characterised on the surface of uterine microfilariae: these have apparent molecular weights of 14,800 and 15,000. A MW 15,000 protein was the only molecule labelled on the surface of skin microfilariae. Ten proteins were labelled on adult male worms: these have molecular weights of 15,000, 17,500, 20,000, 22,000, 24,000, 29,000, 32,000, 37,000, 42,000, and 50,000. Some, if not all, of these proteins were also identified on female worms. Seven proteins were labelled on the surface of L3 larvae: these have molecular weights of 17,500, 48,000, 50,000, 52,000, 54,000, 57,000, and 105,000. Three of the adult surface proteins were precipitated by selected human infection serum: these are the MW 17,500, 32,000 and 42,000 molecules. The microfilarial surface proteins were not precipitated by human infection serum. The antiserum used in these experiments was shown by Western blot analysis to contain high levels of antibody with specificity for microfilarial and adult antigens. Indirect immunofluorescent assays showed these sera to contain antibody which bound to the surface of adult worms and eggs but not microfilariae. The possibility that skin microfilariae absorb host serum albumin was investigated: Western blot analysis and surface immunofluorescence assays using a specific anti-human albumin serum gave negative results. Fluorescent lectin binding studies revealed the presence of stage-specific carbohydrate moieties exposed on the surface of adult worms and eggs. Microfilariae do not have surface carbohydrate determinants.

Suppression of microfilaraemia in Litomosoides carinii infections in cotton rats by vaccination with adult worm homogenate

Twenty cotton rats were injected with a homogentae of adult male and female Litomosoides carinii prepared by freeze-thawing the worms in phosphate-buffered-saline followed by mechanical treatment in a Potter-Elvejhma homogenizer. Intramuscular injections including Freund's complete adjuvant were made into rats which were subsequently quantitatively exposed to infection with L carinii in Liponyssus bacoti. There was no development of a microfilaraemia between 50 and 100 days post-infection in any of the 'vaccinated' cotton rats despite the presence of living adult male and female worms and microfilariae in the pleural cavity. In all the control animals and microfilaraemia developed which persisted for up to 100 days after infection.