Protective immunity in bancroftian filariasis. Selective recognition of a 43-kD larval stage antigen by infection-free individuals in an endemic area

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)

Interferon-gamma production by peripheral blood mononuclear cells from residents of an area endemic for Schistosoma mansoni

During human schistosomiasis host responses to antigens of various parasite life-cycle stages may contribute to whether the severe, hepatosplenic state develops or the patient remains relatively asymptomatic throughout infection, and may play a role in resistance. This study evaluated production of interferon gamma (IFN-gamma) in vitro by schistosome antigen-stimulated peripheral blood mononuclear cells (PBMCs) from asymptomatic patients, and by PBMCs from apparently uninfected, untreated persons living in areas endemic for Schistosoma mansoni ('endemic normals'). IFN-gamma production parallels PBMC proliferation in that schistosomal egg antigens stimulate patent patients' cells poorly, but strongly stimulate PBMCs from 'endemic normals'. This is proportionally true for antigens from adult worms and cercariae. Although asymptomatic patent patients' cells produced little or no IFN-gamma in response to the 3 schistosomal antigenic extracts, their PBMCs, and PBMCs from 'endemic normals', produced expected amounts of IFN-gamma when exposed to phytohaemagglutinin. This implies that persons with patent infections have schistosome antigen-specific defects in their ability to respond to IFN-gamma production that are not exhibited by putatively resistant 'endemic normals'.

Cloning and characterization of a potentially protective chitinase-like recombinant antigen from Wuchereria bancrofti

While there is no direct evidence demonstrating the existence of protective immunity to Wuchereria bancrofti infection in humans, the presence of individuals, in populations in areas where infection is endemic, with no clinical evidence of past or current infection despite appreciable exposure to the infective larvae, suggests that protective immunity to filarial parasites may occur naturally. Earlier work indicated that such putatively immune individuals generated antibodies to a 43-kDa antigen from larval extracts of the related filarial parasite Brugia malayi that was recognized by only 8% of the infected population. With rabbit antiserum raised against this 43-kDa antigen, this current study identified a recombinant clone, WbN43, with an insert size of 2.3 kb, from a W. bancrofti genomic expression library. The recombinant fusion protein was differentially recognized by the putatively immune individuals but not by the infected patients. The coding sequence (684 bp) from the 5' end had significant sequence similarity to chitinases from Serratia marcescens, Bacillus circulans, Streptomyces plicatus, and B. malayi. Peptide sequencing of the expressed product also defined a chitinase-like sequence. Molecular characterization indicated WbN43 to be a low-copy-number gene, with expression predominantly in infective larvae and microfilariae but not in adult parasites.

Ivermectin-facilitated immunity in onchocerciasis; activation of parasite-specific Th1-type responses with subclinical Onchocerca volvulus infection

The present study examined the quantitative and qualitative changes registered in the parasite-specific antibody response, cellular reactivity and cytokine production profile in onchocerciasis patients repeatedly treated with ivermectin over a period of 8 years. The densities of Onchocerca volvulus microfilariae (mf) in treated patients remained significantly reduced, whereas the number of permanently amicrofilaridermic patients (subclinical infection) increased with repeated treatments. In vitro cellular responses to O. volvulus antigen (OvAg) were highest (P < 0.01) in untreated control individuals exposed to infection, but negative for mf of O. volvulus (endemic normals). Cellular reactivity in repeatedly treated patients was higher at 84 than at 36 months post initial treatment (p.i.t); furthermore, the proliferative responses to OvAg, mycobacterial purified protein derivative (PPD) and streptococcal SL-O were greater (P < 0.05) at 84 months p.i.t. in amicrofilaridermic than in microfilaria-positive onchocerciasis patients. In amicrofilaridermic patients such reactivity approached the magnitude observed in endemic normals. Peripheral blood mononuclear cells (PBMC) from patients and endemic normals produced equivalent amounts of IL-2, IL-4 and interferon-gamma (IFN-gamma) in response to mitogenic stimulation with phytohaemagglutinin (PHA); in response to OvAg, however, significantly more IL-2 and IFN-gamma were produced by PBMC from subclinical amicrofilaridermic patients or endemic normals than by mf-positive patients. OvAg-specific production of IL-4 by PBMC from treated patients was lower at 84 than at 36 months p.i.t. At three months p.i.t. the titres of circulating OvAg-specific IgG1-3 had increased (P < 0.05), but they then continuously declined with repeated treatments. Only IgG1 and IgG4 bound to OvAg of mol. wt 2-12 kD at 1 month p.i.t., while recognition of OvAg of mol. wt 10-200 kD by IgG1, IgG2 and IgG4 reached a maximum intensity at 3-6 months p.i.t., with the overall intensity of binding to OvAg gradually weakening thereafter. These results suggest that onchocerciasis-associated immunosuppression is reversible following ivermectin-induced permanent clearance of microfilariae from the skin; and that a vigorous parasite-specific cellular reactivity and a sustained production of IL-2 and IFN-gamma in amicrofilaridermic individuals may contribute to controlling O. volvulus infection.

Engineered human vaccines

The limitations of human vaccines in use at present and the design requirements for a new generation of human vaccines are discussed. The progress in engineering of human vaccines for bacteria, viruses, parasites, and cancer is reviewed, and the data from human studies with the engineered vaccines are discussed, especially for cancer and AIDS vaccines. The final section of the review deals with the possible future developments in the field of engineered human vaccines and the requirement for effective new human adjuvants.

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.

The analysis of the humoral response of the BALB/c mouse immunized with radiation attenuated third stage larvae of Brugia pahangi

BALB/c mice immunized with L3 of Brugia pahangi, irradiated at 45 kRad from a 137Caesium source, are strongly immune to challenge infection (75-100% reduction in the recovery of challenge infection larvae on day 6 post-challenge). The target of immunity appears to be the post-infective L3, as challenge infection larvae are killed within 5-6 days of infection. By immunoblot analysis, serum from immune animals recognizes a limited set of somatic antigens, the majority of which are shared between different life cycle stages. Serum from immune mice also strongly recognizes larval surface antigens by immunofluorescence, some of which may be stage specific. The larval surface determinants do not appear to be protein or glycoprotein by standard immunochemical analysis. A proportion of the antibody response of the BALB/c mouse is directed towards phosphorylcholine epitopes on filarial antigens, but the limited antigen recognition cannot be explained on the basis of the mouse strain used, as CBA/Ca mice recognize a similar limited set of antigens.

Immune responses to filarial parasites

Filarial nematode parasites are long-lived organisms which are responsible for one of the major tropical diseases. Their ability to survive may be related to the antigen-specific anergic state observed in carriers of the parasite. Severe disease symptoms are associated with the breakdown of anergy. Different stages of the parasite life cycle are anti-genetically distinct, and a state of concomitant immunity is therefore possible which prevents superinfection. The evidence for these conclusions is discussed, in the context of the need for an effective vaccine against filarial parasites which will avoid any risk of increasing disease severity.

Circulating filarial antigen in cats infected with Brugia pahangi is indicative of the presence of adult worms

Using counterimmunoelectrophoresis with rabbit antisera raised against soluble extracts of adult females of Brugia pahangi parasite antigen was detected in the serum of all cats repeatedly infected with B. pahangi. Antigen was never detected in uninfected cats. The antigen was associated with the presence of adult worms. Antigen was detected consistently in a cat that was amicrofilaraemic but at autopsy harboured only two or three adult worms. Conversely, some cats showed slowly declining numbers of microfilariae and, in these, circulating antigen declined before the number of microfilariae. Eventually no antigen was detectable in circulation whereas microfilariae, although in diminishing numbers, were still present. At autopsy no adult worms were found in these cats. Antigen also appeared in several cats before they became microfilaraemic.

A recombinant clone of Wuchereria bancrofti with DNA specificity for human lymphatic filarial parasites

In order to understand the immune response to Wuchereria bancrofti and to aid in the diagnosis of W. bancrofti infections, recombinant antigens were identified from a W. bancrofti genomic expression library made in lambda gt11 using a pool of sera from infected Indian patients. One of the recombinant clones, lambda WbN1, containing a 2.5-kb insert, reacted strongly to a pool of sera from patients with lymphatic filariasis but not to normal human sera. In addition, this clone showed restricted specificity at the genomic level to the major lymphatic filarial parasites W. bancrofti and Brugia malayi but not to the closely related filarial parasite Brugia pahangi or to other filarial and non-filarial species tested. Nucleotide sequence analysis indicated the cloned DNA to have homology to myosin-like myofibrillar proteins. Polymerase chain reaction amplification initiated by specific synthetic oligomers amplified DNA in a species-specific manner from as little as 16 pg of isolated DNA or from one microfilaria.

Age-specific acquisition of immunity to infective larvae in a bancroftian filariasis endemic area of Papua New Guinea

The development of antibodies to infective stages of the filarial parasite, Wuchereria bancrofti, with age of the host human population was studied by immunofluorescence, immunoprecipitation and immunoblotting assays. Among individuals under 20 years of age, few had detectable antibodies to the infective (L3) larval surface by IFA: only 2 out of 10 scored positive. However, all adults (over 20 years) were positive in this assay although the utilization of isotypes varied between different individuals. Whilst antibodies to the L3 surface are therefore acquired after prolonged exposure to infection (greater than 20 years), recognition patterns of L3 surface labelled antigens, measured by immunoprecipitation analysis iodinated proteins on SDS-PAGE, and of somatic L3 proteins on immunoblots, were equivalent in the two age groups. Thus, a critical surface antigen, recognised in an age-dependent manner, is present on the L6 cuticle but cannot be resolved as a conventional protein or glycoprotein constituent.

Molecular parasitology: progress towards the development of vaccines for malaria, filariasis, and schistosomiasis

Advances in molecular biology have allowed for the identification of potential vaccine candidates against several parasitic diseases. Antigens from various life stages of Plasmodium and Schistosoma species and filarial worms have been cloned, sequenced and tested as vaccines. Results to date in animal models have been promising. Modest levels of protection against experimental human malaria have been obtained using both sporozoite and blood-stage antigens. However, a greater understanding of the mechanisms which lead to immunity against parasites is required before effective vaccines can be developed.

The endemic normal in lymphatic filariasis: A static concept

Residents of areas endemic for lymphatic filariasis are continually exposed to infection with mosquito-transmitted infective larvae (L3), some of which survive to become adult worms and subsequently produce micro filarial (mf) transmission stages. The question of whether naturally acquired resistance occurs in adult residents of endemic areas has recently become of interest as the development of molecular vaccines against filarial parasites is being considered(1,2). There have been two epidemiological approaches to demonstrate acquired resistance to Filariasis in human populations. In this review Karen Day examines both approaches in the context of an immunoepidemiological study of bancroftian filariasis in Papua New Guinea (PNG). The merits of each as a conceptual framework for studies of protective immunity in lymphatic filariasis will be discussed.

Immunological tolerance: The key feature in human filariasis?

Filariasis is a widespread tropical disease caused by a group of nematode parasites that can survive for many years in immunocompetent hosts. The paradox of filariasis has always been the inverse association between parasite density, in terms of circulating microfilariae in the blood, and severe pathology. In this review, Rick Maizels and Rachel Lawrence argue that microfilariae and adult parasites induce a form of immunological tolerance which prevents both parasite elimination and progression to disease. The breakdown of this unresponsiveness is seen as the critical step towards pathogenesis. However, not every exposed individual progresses through infection to disease. The authors discuss evidence for protective immunity acting on antigens from the mosquito-borne infective larva, and propose that this stage represents a vulnerable target outside the scope of tolerance and pathogenesis. Stage-specific larval antigens, to which asymptomatic hosts are known to respond, may therefore represent the most effective and safe choice for an anti filarial vaccine.

Onchocerca volvulus: molecular cloning, primary structure, and expression of a microfilarial surface-associated antigen

Although the filarial nematode parasite Onchocerca volvulus is an important human pathogen in large areas of Africa and Latin America, little is known of the molecular interactions that govern the clinical status of patients with this chronic, debilitating disease. As a step toward defining the parasite molecules important to the immunobiology of host-parasite interactions, we have identified and cloned a major surface-associated antigen expressed by O. volvulus microfilariae. Radiolabeling experiments demonstrated that O. volvulus microfilariae have a limited repertoire of peptides at the surface. Prominent among these labeled peptides is an 18-kDa component. Immunological cross-reactivity between a surface-associated component of Dirofilaria immitis microfilariae and the 18-kDa surface-associated molecule from O. volvulus was exploited in a strategy to clone this potentially important O. volvulus microfilarial antigen. The cross-reacting antibodies were used to immunoscreen O. volvulus cDNA expression libraries. One clone, M2f.e, contained an open reading frame of 495 bp encoding an 18.1-kDa protein (OVMS18). Antibodies produced against the expression product of M2f.e recognized an 18-kDa component in extracts of O. volvulus microfilariae and bound to the surface of intact O. volvulus and D. immitis microfilariae. Southern blot analyses showed that M2f.e-like sequences are present in the genomic DNA of a number of filarial nematode species, but not in DNAs from nonfilarial nematode species.

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.

Interaction of monoclonal antibodies with cuticular antigens of filarial parasites, Brugia malayi and Wuchereria bancrofti

Monoclonal antibodies (mAbs) have been prepared against excretory-secretory-metabolic (ESM) antigens of microfilariae (mf) of Wuchereria bancrofti (WbmfESM) and against third stage larvae (L3) of Brugia malayi (BmL3), and purified from ascites fluids with ammonium sulphate. Both antibodies were of the IgM type and did not react with phosphorycholine. The mAb against BmL3 (F46) reacted in ELISA with antigens of L3 of B. malayi, B. pahangi and W. bancrofti and of adults of B. malayi. The mAb raised against wbmfESM (F32) resembled F46 in this respect, though with a lower titer towards the antigens, and in addition reacted with the ESM-antigens of mf and of L3 of W. bancrofti. F46 was able to detect L3 antigens of filarial parasites in spiked serum samples with a detection limit of 8-16 ng in absolute amount. The antibody was found to label the cuticular portion of L3 and adults of the lymphatic parasites, and not the epicuticular surface, in immunoelectron microscopic studies. The antibody recognized a 36 kDa component of the beta-mercaptoethanol extracts of B. pahangi-adults in Western blot analysis.

Human T-cell stimulation, molecular characterization and in situ mRNA localization of a Brugia malayi recombinant antigen

Cellular immune responses play a major role in lymphatic filarial infections. To further our understanding of the host-parasite interaction, we investigated T-cell stimulation by purified filarial recombinant antigens in peripheral blood mononuclear cells derived from filarial-infected individuals. One of a subset of cloned Brugia malayi antigens involved in the humoral immune response to filarial infection was found to be a T-cell-stimulating antigen. The fusion protein encoded by clone lambda Bm19 induced proliferation of human T cells in a parasite-specific, antigen dose-dependent manner. The deduced amino acid sequence from this cloned region revealed 4 predicted T-cell recognition sites. The lambda Bm19 DNA sequence hybridizes to a 3-kb transcript, and in situ mRNA hybridization analyses of the adult female worm demonstrated that this gene is expressed in developing uterine microfilariae. The native parasite protein is present in several developmental stages since clone lambda Bm19 was initially identified with antiserum directed against the infective larval stage; this protein is therefore a potential target for the host's immune system.